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Radiation and Reason The Impact of Science on a Culture of Fear

Authors:
Radiation
and Reason
The Impact of Science on
a Culture of Fear
Wade Allison
Published by
Wade Allison Publishing
© Wade Allison 2009
All rights reserved.
No part of this publication may be reproduced,
stored on a retrieval system, or transmitted, in
any form or by any means, without explicit
permission from the author.
Enquiries should be sent to
w.allison@physics.ox.ac.uk
Published as an e-book (June 2009)
ISBN 978-0-9562756-0-8
and in paperback (October 2009)
ISBN 0-9562756-1-3 978-0-9562756-1-5
Website http://www.radiationandreason.com
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York, UK YO31 7ZQ
Last saved 7 June 2011
For Alfie, Alice, Joss, Minnie,
Edward, George
and those who come after,
may they understand one day.
v
About the author
Professor Wade Allison, MA DPhil, is a Fellow of Keble College
and a Professor Emeritus at the University of Oxford where he
has studied and taught physics for over 40 years. His earlier
research work was in elementary particle physics, in particular
the radiation field of relativistic particles, but his interests and
expertise have spread much wider. He recently published
Fundamental Physics for Probing and Imaging, an advanced
textbook for his course at Oxford on medical physics, including
radiation and its use in clinical medicine and the wider
environment. Of the safety of radiation and nuclear technology,
he says
I have no axe to grind, I have no links with the industry, I
just want to see the truth out there. So many people have
been under a misapprehension for so long.
Of the conclusions of this book he says
It brings good news – but are the people of the world
ready to re-examine past assumptions in the light of
current science? It is important that they do, because,
without nuclear energy, the future for mankind looks
bleak.
Contents vii
Preface......................................................................................... 1
Chapter 1 Perceptions..................................................................5
A mistake – Personal risk and knowledge – Individual and collective
opinions – Confidence and decisions – Science and safety
Chapter 2 Atmospheric Environment........................................15
Size and composition of the atmosphere – Atmospheric change
– Energy and agriculture
Chapter 3 The Atomic Nucleus.................................................21
Powerful and beneficial – Size scales – Atoms and electrons
– The nuclear atom – The quiescent nucleus – Energy for the Sun
Chapter 4 Ionising Radiation.....................................................35
The spectrum of radiation – Damage from radiation – Nuclear stability
– Measuring radiation – Natural environment
Chapter 5 Safety and Damage...................................................55
Proportionate effects – Balancing risks – Protection of man
– Damage and stress – Time to repair – Collective dose
– Safety margins – Multiple causes – Beneficial and adaptive effects
– Surprise at Chernobyl
Chapter 6 A Single Dose of Radiation.......................................77
What happens to molecules – What happens to cells
– Evidence at high dose – Repair mechanisms – Low and intermediate doses
– Survivors of Hiroshima and Nagasaki – Radiation-induced cancers
– Medical diagnostic scans – Nuclear medicine – People irradiated at
Chernobyl – Thyroid cancer – Other cancers at Chernobyl
Chapter 7 Multiple Doses of Radiation...................................111
Distributed doses – Cancer therapy – Fractionation
– Doses in the environment – Radon and lung cancer
– Radiation workers and dial painters – Biological defence in depth
Chapter 8 Nuclear Energy........................................................133
Realising nuclear energy – Explosive devices
– Civil power from fission – Energy without weapons – Waste
Chapter 9 Radiation and Society.............................................161
Perceiving radiation – Public concern – Testing and fallout
– Deterrence and reassurance – Judging radiation safety
Chapter 10 Action for Survival................................................177
Relaxed regulations – New power stations – Fuel and politics
– Waste strategy – Decommissioning – Proliferation and terrorism
– Fusion power – Costs and the economy – Fresh water and food
– Education and understanding
Chapter 11 Summary of Conclusions......................................199
Epilogue: Fukushima................................................................201
....Instability and self destructioni - Explanation or appeasement
8
Further Reading and References 208
Index......................................................................................... 217
1
Preface to the e-book edition
What happened at Fukushima has not changed what is known of
the benefits and dangers of nuclear radiation. However, it has
highlighted many of the arguments and I have added an epilogue
that discusses these; otherwise the text is largely unaltered from
the first print edition. As was the case for earlier accidents some
reactors at Fukushima were destroyed but the impact of the
released radiation on the population has been overstated with
significant consequences for all those affected. Initial reactions
around the world to Fukushima and its implications for nuclear
technology have varied from one nation to another, depending in
part on its historical experience. Nuclear technology can do
much for our lives and our view of it should be based on science
-- and that is the same in every country. Political and geological
instabilities affect many aspects of a nation's life, and nuclear
questions should not be exceptional.
It is natural that when there is an accident the question should be
asked 'who is to blame?' but this question may have no answer
even when many must pay for the consequences. I hope that this
book with its epilogue will provide a welcome and accessible
account of the science and a basis for understanding, mutual trust
and optimism for the future.
I have taken the opportunity to clarify the section 'Doses in the
environment' in chapter 7.
Wade Allison, Oxford, June 2011
Preface to the first edition
The human race is in a dilemma; it is threatened by economic
instability on one hand and by climate change on the other.
Either of these could lead to widespread unrest and political
turmoil, if the right choices are not made now. In particular,
prosperity without carbon emission implies a comprehensive
switch in our sources of energy. With luck, the activity generated
by the process of switching will also contribute to prosperity in
2 Preface
the short and medium term. There are many solutions wind,
tidal, solar, improved efficiency but the most powerful and
reliable source is nuclear. However, it is widely supposed that
this presents a major problem of safety. Is this long-held concern
about radiation and nuclear technology fully justified? Straight-
forward questions should have simple answers, and the simplest
answer is No. Explaining and exploring the question and this
answer in accessible terms is the subject of this book.
Over the years I have taught and studied many areas of physics,
including nuclear physics and medical physics, although I have
never had a close link with the nuclear industry. While it always
seemed clear to me that radiation safety was somewhat alarmist
and unbalanced, in earlier decades the apparent freedom to opt
for fossil fuel as the primary source of energy meant that there
was no special reason to confront public perceptions of the issue.
But now the situation has changed, and it is time to address the
whole question.
But how, and with what voice? A discussion in popular terms
that would appeal to the native common sense of the reader is too
easily dismissed by the science. But scientific answers are
impenetrable to many readers, and so fall on deaf ears. A way
forward is to vary the tone, sometimes scientific but still
accessible, and sometimes with illustrations and examples that
appeal to general experience. Nevertheless, I shall probably tax
each reader's tolerance in places, one way or the other, and for
that I apologise. While ways of avoiding the use of equations
have been found except in some footnotes, use is made of the
scientific notation for very large and small numbers.1 Finding
passages that seem either trivial or impenetrable, the reader is
encouraged to skip forward to rejoin further on. The passages
that discuss recent scientific results are supported with references
labelled in square brackets in the text and listed in full at the
back. Most references may be found on the Web at the address
given but the text is self-contained and does not suppose that
1 Thus 106 means one million, 1 followed by six noughts. Similarly 10-6
means one millionth part.
Preface 3
these are consulted. Also at the back, there is a short list of books
and papers, headed Further Reading.
The story starts with the physical science, much of which has
been established for decades the atmosphere, the atomic
nucleus and radiation. And then it moves on to the effect of
radiation in biology, most of which was not so well known 30
years ago. Often, popular science is written to amaze and inspire
and that is important. But here the target is more prosaic and
practical, namely a clear understanding of the scientific
background to some of the threats and decisions that are likely to
determine our environment and thence our survival. The central
question is this: how significant are the health risks due to
radiation and nuclear technology? In Chapters 6 and 7 the current
evidence is shown with the relevant ideas in modern biology.
Not all questions can be answered completely yet, but they can
be answered quite well enough. The conclusions are rather
surprising, and do not match well with currently enforced
radiation safety levels. This challenge by modern radio-biology
to radiation safety regulation is well aired in scientific papers,
but has not been explained to the community at large, who have
a significant interest in the matter. The costs of nuclear
technology are very high, in part because of the exceptional
radiation safety provision that is made. Scaling back such
provision by a large factor would have a major beneficial effect
on the financial viability of an extensive nuclear power
programme.
These scientific findings do not depend on climate change,
although that is what makes the question important at this time.
But why, in the past, did most of the human race come to hold
extreme views about the dangers of radiation and nuclear
technology? The last part of the book describes what nuclear
technology now offers, a large-scale supply of carbon-free
electric power, with further options for the supply of food and
fresh water.
E M Forster wrote
4 Preface
I suggest that the only books that influence us are those
for which we are ready, and which have gone a little
farther down our particular path than we have yet gone
ourselves.
I hope that for some readers the message of this book is timely.
To keep the discussion focussed on a few main points, many
important topics have been omitted or just noted in passing – in
particular, the subject of micro-dosimetry is treated rather
briefly, in spite of its importance for future understanding. No
doubt mistakes have been made too, and credit not given where it
was due. Such choices, mistakes and lapses are mine, and I
apologise for them.
I have benefited from conversations with many colleagues during
the writing of this book. It has been a privilege to have had the
opportunity for quiet reflection and study, undisturbed by the
pursuit of grant funding that distorts so much academic study
today. This work would not have reached fruition without the
contributions of many people. Former students and members of
their families, members of my own family too, have spent long
hours, reading and providing feedback on my efforts to produce
an accessible account. In particular, I should like to thank Martin
Lyons, Mark Germain, James Hollow, Geoff Hollow, Paul
Neate, Rachel Allen, John Mulvey and John Priestland for their
reading of the text and important comments. Chris Gibson and
Jack Simmons have provided me with invaluable comment and
information. Throughout, I have relied heavily on the
encouragement of Elizabeth Jackson and my wife, Kate their
advice and persistence were essential. I thank Kate and all
members of my family for their love and tolerance over the past
three years while I have been rather absorbed.
Finally I would like to thank Paul Simpson of LynkIT and Cathi
Poole of YPS for their enthusiastic ideas and can do reaction to
the task of printing and promoting this book and its message.
Wade Allison,
Oxford, September 2009
5
Chapter 1 Perceptions
Science is the great antidote to the poison of enthusiasm
and superstition.
Adam Smith, economist (1723–1790)
A mistake
Radiation is seen as a cause for exceptional concern and alarm,
though few people have any personal experience of its dangers.
Is this view justified, and how did it come to be held?
Prior to the Second World War there was a degree of relaxed
public acceptance of radiation, principally because few knew
anything to suggest otherwise. That changed with the arrival of
the Nuclear Age.
The destruction of the Japanese cities of Hiroshima and Nagasaki
by nuclear bombs in 1945 was a military and political success
that avoided a land invasion of Japan, which would have been
immensely costly in lives for both sides. As a technical scientific
enterprise, it was a triumph – no project depending on
fundamentally new physical developments on such a scale had
ever been attempted before.
As an exercise in the public understanding of science, it was a
disaster whose consequences still persist. The message that came
through was very clear – what happened was both extraordinarily
dangerous, and incomprehensible to all but a few. The extreme
apprehension generated in the population was self-sustaining.
Sources of fear inhibit free enquiry, and few in the population
ever questioned the extent of the danger. In the decades of the
Cold War that followed, this fear was a useful weapon in
international politics, and its basis was not doubted, even by
those in a position to do so. And then there was Chernobyl a
further failure of public understanding. In the public mind the
fear of nuclear war had infected views on civil nuclear power.
6 Chapter 1 Perceptions
Most people simply wanted to distance themselves from
anything nuclear.
More questions should have been asked, although some of the
answers could not have been given in earlier decades. There are
three concentric concerns, related like the layers of an onion, as
it were. The first and innermost is to understand the effect of
radiation on human life. This is a scientific question, not
dependent on the other two. The second task is to educate public
opinion and formulate safety regimes in the light of the solution
to the scientific question. The final problem is to discourage
nation states and terrorists from exploiting radiation as a source
of fear by threatening and posturing. This depends critically on
the second task, establishing robust public opinion and a
regulation regime that can face up to international arm twisting.
In the last 50 years these problems have been confused. During
the Cold War era, international politics exploited public fear and
ignorance of radiation, while only recently has the scientific
evidence and understanding become established to answer the
prior scientific question. In the absence of a clear picture of the
biology and of adequate human-based evidence, radiation safety
guidelines and legislation became established on a reactive basis.
Public concerns were handled by imposing draconian regulation
on radiation and nuclear technology, in the expectation that this
would provide the necessary reassurance. But the very severity
of the restraints only increased public alarm and people were not
reassured.
But now in the new century there have been two changes. Firstly,
the scientific answers that were lacking previously are now
largely available. Secondly, new nuclear power plants are
urgently needed so that the use of fossil fuel can be reduced
this does not change the safety of radiation but it does affect the
importance of setting matters right as soon as possible. So the
purpose of this book is to explain the science in fairly accessible
terms, together with some of the evidence, and to offer a rough
but justified estimate of the level of new safety regulation.
A mistake 7
Consequences for public policy and international diplomacy may
then follow.
Personal risk and knowledge
Making decisions to reduce the risk of accidents involves
everybody in society, what they believe to be the level of risk, as
well as what is actually the level of risk. People may be alarmed,
when they do not need to bethey may be fearless, when they
should be more cautious.
What level of risk is tolerable in exceptional circumstances? We
should not say zero a risk-free society is utopian and
unachievable. Although personal fear may feel absolute and
unquantifiable, it should be controlled any risks involved
should be compared with those of alternative courses of action.
Even the duration of life on Earth will have its term, hopefully
not caused by early escalating climate change. But for us as
individuals, the end is closer and more certain, for finally we all
die life expectancy may be 70 to 80 years, depending on
standard of living, health and diet. So what is the average effect
on a life of an accident that carries a 1% risk of death? For an
average age of 40, that means a life expectancy reduced by an
average of 0.4 years, or 5 months. If the lifelong risk is 0.1%, the
reduction in life expectancy is just 2 weeks. This is at the same
level as many risks and choices that people incur as they go
about their daily lives. Many people would, willingly, give up 2
weeks of life for the benefit of their children or grandchildren if
that would really benefit the large-scale prospect for the planet.
Well, wouldn't they? So, thinking straight, a lifetime risk of
death at the level of one in a thousand is sensible – if undertaken
for good reason, of course. As we shall see, the evidence shows
that only under quite exceptional conditions is any nuclear risk at
such a high level.
In general, those who make decisions need to be sure that they
themselves understand the relevant situation. If their information
is picked up from others on the basis of a collective idea that
everybody knows, there is a chance that wrong decisions will be
8 Chapter 1 Perceptions
made. The greater the number of people relying on the opinion of
others, the longer it takes for them to realise if something is
wrong. So, the bigger a blind spot in understanding, the greater
the chance that basic questions go unasked and unanswered. At a
practical level, a hard question may be beyond the immediate
field of an individual and so be passed to an expert for a
specialised opinion, perhaps without reference to other problems.
In this way the true picture may become distorted in the form of
a collection of separate narrowly defined opinions.
An example was the flow of intelligence and decision-making in
the conduct of the First World War. A consequence was the
extreme loss of life, for example, on the Somme in July 1916.
Decisions on the course of action were taken by commanders,
who did not know or appreciate the actual situation in the field.
And those on the battlefield were not permitted to use their own
intelligence to modify the plan. It was assumed that the heaviest
possible artillery bombardment would destroy the barbed wire
and overcome the machine-gun posts but it did not. The
commanders did not find out, and the men on the ground were
required to obey instructions. The result was an avoidable
massacre.
A more recent example was the effect on the stability of the
world financial system of various trading and insurance practices
employed in the first few years of the 21st century. Financial
regulators and senior managers of corporations, who, in the years
leading up to 2008, encouraged their dealers to negotiate and
exchange contracts of risk for money, were not able to grasp the
instability of the structures that they were building. These were
described as complex and sophisticated words that should
themselves be a warning. Used to impress, they invite acceptance
without question. When the financial structures collapsed,
nobody was able to determine the ownership and the worth of
their holdings. The absence of anyone with the ability to see the
consequences of what was happening was as serious as on the
Somme in 1916. The financial dislocation, which played a
Personal risk and knowledge 9
dramatic part in the collapse of 2008, was foreseen eight years
earlier by Wilmott [1], who wrote as follows.
The once 'gentlemanly' business of finance has become a
game for 'players'. These players are increasingly
technically sophisticated, typically having PhDs in a
numerate discipline. ... Unfortunately, as the
mathematics of finance reaches higher levels so the level
of common sense seems to drop. ... It is clear that a
major rethink is desperately required if the world is to
avoid a mathematician-led market meltdown.
When decisions are scientific, the availability of adequate first-
hand understanding can be a major hurdle, because such
understanding is sparse in the population. This is especially true
for decisions involving nuclear radiation. To the general
population and those who make decisions for society, the words
and ideas that describe the science do not have familiar
meanings. Apprehension of anything nuclear, or concerned with
radiation, is deeply engrained in popular culture, and few
scientists have pursued the broader inter-disciplinary field.
For reasonable decision-making, it is essential that the truth
underlying the fears of nuclear material and radiation are
properly exposed and that the science is more widely understood.
This is more urgent now because new dangers affect the survival
of the environment as a whole, not just the lives of individuals.
Individual and collective opinions
Should decisions on major dangers be made individually or
collectively? Many creatures concentrate on collective survival
at the expense of the individualthe herd or the swarm comes
first. But man is different he places special value on the
importance of individual rights, as well as the collective
agreements that are essential to society and its survival. This
dynamic relationship between individuals and society is what
being human is about. But what happens if a collective
understanding takes a wrong turn, leading to a consensus that
10 Chapter 1 Perceptions
threatens survival? Then the problem needs to be re-examined,
which is most difficult if it is largely scientific.
What people understand of the world depends on their previous
experience, including education and upbringing. Even what they
think that they see is shaped and filtered by their background.
Through the character of the Professor in his children's book,
The Lion, the Witch and the Wardrobe, C S Lewis advises that
we should listen to evidence from others, assess their reliability
and sanity, and then await further developments. Recent
scientific reports [2] relate how, even today, the experience of
our own bodies can be distorted alarmingly by suggestion and
supposition, in a way dating back to ancient witchcraft. In
modern physics, too, there are serious questions concerning
reality in its different manifestations [3].
So reality is tricky. It is not just an academic matter for
philosophers, but a practical matter that is the source of everyday
disagreements. If differing views are reconciled, plans of action
can then be agreed and decisions taken that lead to success and
increased confidence. So, decisions need an acceptable collective
picture of reality, and this only becomes established through
repeated observations at different times and by different people,
and is confirmed when expectations based on it turn out to be
correct. This is most credible when scientific observations are
found to confirm precise mathematical predictions – although we
cannot account for the unreasonable relevance of such
mathematics in the world. There is no logic that requires that,
when I wake tomorrow morning, the world as I know it will still
be there. A chicken, accustomed to being fed by the farmer each
morning, is unprepared for his day of slaughter, although that
was the ultimate purpose of each morning feed.2 So we become
accustomed to the continuity and predictability of our
experience. But could it be otherwise? We need always to be
alert to the possibility that our collective understanding is quite
wrong. It is the task of the following chapters to try to unpick the
2 Remarked by Bertrand Russell.
Individual and collective opinions 11
dangers of radiation and nuclear technology and to explain how
we were previously mistaken.
Philosophers and physicists may mull over evidence for the
existence of parallel streams of reality. Some may follow the
ideas of Descartes by looking at which properties of the Universe
are necessary, simply to allow us to be here now asking
questions. This is called the Anthropic Principle and it turns out
to have significant consequences, if you accept its premise. But
our task is different, though related in a practical and local sense.
We are re-opening our attitude to radiation and nuclear
technology in order to help answer the larger question: what kind
of world and choice of life style will permit the possibility that
mankind will be here in the future to ask questions? This local
anthropic prospective is also restrictive. If no solution is found,
human life on Earth as we know it will die out.
Confidence and decisions
Consider an example. In earlier centuries exploration and the
transport of people and goods depended on the confidence and
safety of navigation. Observations and sightings had to be
agreed, a ship's course calculated and steered – the arrival of the
ship at its destination was the demonstration that these decisions
were not just matters of opinion. The calculation of the position
of the ship relied on measurements and the known apparent
orbits of the Sun, Moon and stars, the magnetic field of the
Earth, the tides and other quantities. With every improvement in
navigation came an uplift in world communication; better
accuracy gave improved confidence, leading to more ambitious
voyages and better trading. Conversely, whenever confidence in
the natural world fails, human activity gets choked off and
prosperity declines.
If there is disagreement, the observations and preconceptions
have to be talked through to reach a consensus. But it is an
important concession to the variety of human experience that
individuals have the right of choice. Except when it is
unavoidable, we do not exercise choice on behalf of others – and
12 Chapter 1 Perceptions
then only with a degree of caution that we would not exercise for
ourselves. And so it is in matters of safety, especially where
apprehension is high.
The dangers of radiation and nuclear technology have been a
matter of vocal public concern for half a century, mainly among
the currently middle-aged and elderly who remain confused and
apprehensive. The younger generation never experienced the
Cold War and are more relaxed. In the past many scientists kept
away from the long-running debate of nuclear politics.
Meanwhile, radiation safety remains subject to exceptionally
stringent conditions, although few people appreciate the related
expense and no one seems to feel safer as a result. In the 21st
century the agenda has changed and decisions are needed for the
future of the environment where the choice of primary energy
source is between nuclear power with the dangers of its waste
and the combustion of fossil fuel with its waste.
Science and safety
The astronomer who first predicted an eclipse and announced it
to the political masters of his day discovered the influence that
scientific knowledge can bring. His ability was held in awe by all
around him. Today physics and astronomy have given the human
race control over much of the natural world. In earlier times and
in the absence of scientific interpretation, darkness, fog, thunder,
lightning and other variations in nature tended to generate
superstition and thoughts of divine intervention, even
punishment. Such feelings suppress confidence and discourage
initiative and enterprise.
The scientific enlightenment from the 17th to mid 20th centuries
showed man how to overcome fear of the unknown by empirical
study. Through universal education, training and communication
this encouraged prosperity and better standards of living and
health. However, misapprehension of the natural world is still the
background of life for many.
Science and safety 13
Scientists, too, suffer misapprehensions, but these are overcome
by continual re-measurement, re-thinking and re-calculation, like
the helmsman steering the wrong course who, by making further
observations, discovers and corrects his error. If this is not done,
confidence may fail and unguided imagination and panic fill its
place. Then careful observation and calm thought are at risk, and
the opportunity to correct errors is reduced.
This is particularly true for those dangers that cannot be sensed.
The prospect of a threat, unseen but lethal, makes people worry,
even panic. Trivially, in the dark, when sources of danger cannot
be seen, people can be frightened until the light is turned on. This
case is instructive to give people confidence they need to see
for themselves using a basic instrument, like a flashlight or torch.
Just telling them that they should not be frightened is not
effective. Equally, consulting people for their opinion about
safety, when they do not know or understand, may simply
accelerate an implosion of confidence decisions taken in
everybody's best interest cannot emerge in this way. Regulation
and legal restraint do not give people confidence either. Only
real education of a sizeable fraction of the community can
reassure, and this should be based on an objective understanding
of the issues.
For the confidence of those on board, the ship should be on the
right course, and be known to be on the right course. The two
aspects of safety actual and apparent are different, though
equally important. Once actual safety has been established,
apparent safety becomes a matter for education, communication
and information. If an appearance of safety is given priority over
actual safety, real danger can follow, as reassured passengers on
board the Titanic learned to their cost.
15
Chapter 2 Atmospheric Environment
Size and composition of the atmosphere
The environment comprises the Earth's crust, the oceans and the
atmosphere. The depth of the crust that affects us on a regular
basis is between a few hundred and perhaps a thousand metres,
and the oceans have a similar mass. But the atmosphere is much
smaller although it reaches to an effective height of about
10,000 metres, its density is a thousand times less than water. So
it is equivalent to a layer of water on the Earth just 10 metres
thick less than 1% of the mass of the oceans or the Earth's
crust. So it is easily polluted and, being composed of gas, any
pollution is quickly dispersed into the whole.
The composition of the atmosphere today is 78% nitrogen, 20%
oxygen and 1% argon with smaller amounts of carbon dioxide
and water vapour. Oxygen and water are fiercely reactive but
nitrogen, carbon dioxide and argon are less reactive or totally un-
reactive. Until two and a half billion years ago there was little
atmospheric oxygen. Its concentration was increased by
photosynthesis in early plant life powered by the Sun. This
break-up of carbon dioxide into free oxygen and carboniferous
plant life 'charged the battery' for all life processes. Oxygen
remains a powerful chemical, not only when this battery is
discharged in the burning of plant matter, fossilised or not, but
also in related oxidative processes in living cells. These may be
benign, as in the oxidation of sugars that provides the energy for
living creatures; they may also be malign, as when oxidation
disrupts cellular processes and leads to cancer. Fortunately life
has evolved ways in which to protect itself against such oxygen
damage that are effective most of the time. Coincidentally, these
same protection mechanisms turn out to be equally effective
against the damage caused by radiation, as we shall see later.
16 Chapter 2 Atmospheric Environment
Atmospheric change
The average surface temperature of the Earth is critically
dependent on the composition of the atmosphere, and a small
release of pollution can have a relatively large effect on the
climate. The reason for this is explored in Chapter 4 in terms of
the spectrum of thermal radiation absorbed and emitted by the
Earth. Pollution released into the oceans would also have an
environmental effect, but a much diluted one that would not
impact directly on the temperature. In the case of the Earth's
crust dangerous materials – suitably buried can stay put for
many millions of years. So care of the environment is concerned
first and foremost with the atmosphere.
Since man started to employ fire and organised agriculture to
raise his standard of living, he has released an increasing mass of
contaminants into the atmosphere, although only recently has the
extent of their effect been appreciated.
Figure 1 The concentration of carbon dioxide in the
atmosphere for three separate epochs. Left: prehistoric
variation (measured from Antarctic ice cores). Centre:
historic data (also from ice cores). Right: modern
measurements (direct from the atmosphere).
Atmospheric change 17
For example, the growth in the concentration of carbon dioxide
in the atmosphere is shown in Figure 1. The left part of the
diagram shows the concentration for most of the past 160,000
years, going up and down within the range 200–280 parts per
million (ppm) and spanning various states of the world's ice
sheets. The central part of the plot shows that it was fairly
constant at 280 ppm from 1000 AD until the industrial
revolution, with its rapid increase in population and pollution.
Since then it has risen remorselessly as shown on the right – the
latest data say that it has risen by 40 ppm in 25 years and
currently stands at 360 ppm. Note the large change in timescale
for the three parts of the plot.
A plot for methane would show a similar rapid increase. These
effects come from the increased burning of fossil fuels and the
destruction of forests, exacerbated by the rising world population
of humans and animals. These gases are called greenhouse gases
because they have the effect of causing a rise in the average
world temperature, as explained in Chapter 4. The temperature
change is expected to be self-reinforcing for several reasons
whose relative importance is still uncertain.
Firstly, the water vapour in the atmosphere naturally increases as
the air gets warmer, and, since water vapour is also a greenhouse
gas (as explained later on page 39), it is expected to contribute a
further rise in temperature.
Secondly, as the temperature rises the extent of the polar ice caps
is reduced, and, without the reflection of the snow and ice, the
surface of the Earth becomes darker to sunlight. The extra solar
absorption in polar regions is responsible for another increase in
the surface temperature.
Thirdly, as the temperature rises, plant material that was
previously preserved and locked in the 'deep freeze' of the
permafrost starts to rot and decompose, emitting further
greenhouse gases, specifically methane.
Any increased incidence of forest fires accompanying the
temperature rise releases yet more gases. As living plant life
18 Chapter 2 Atmospheric Environment
absorbs carbon dioxide and releases oxygen, any reduction in
forestation is harmful on both counts. The re-absorption of
carbon dioxide from the atmosphere by sea water and through
plant growth is slow. In fact, on average, it takes about a hundred
years for any carbon dioxide, once released, to be re-absorbed.
So, even if all emissions were stopped immediately, climate
change would continue for a century or so before possibly
stabilising. If emissions continue, the climate will continue to
change. The population that the world can support may be
reduced and, as deserts expand, large migrations of people
towards more temperate regions may be expected. To reduce
greenhouse gas emission, other ways of providing sufficient
energy and food for the world population must be found, and all
available solutions pursued simultaneously.
Much energy can be saved with care and by investment in new
technology, for example efficient power supplies and LEDs
(light-emitting diodes). For the energy production itself, wind,
tidal, solar, geothermal and hydroelectric sources provide electric
power without gas emission. Each is appropriate to a particular
kind of locality. Some are intermittent, some are expensive and
many are limited in scale. Intermittent sources need to be
coupled with energy storage, but there are no easy options there.
Energy for transport also needs storage, but battery technology
and hydrogen storage have significant limitations.
Energy and agriculture
Increased populations with rising standards of living expect more
fresh water and food. The shift from a basic, mainly vegetarian,
diet to a regular meat-eating lifestyle requires more water. But
the extra water consumption of ruminants and their added gas
releases are both significant. Meanwhile many parts of the world
suffer increased desertification and depletion of ground water
supplies. Unlimited clean water can be obtained from sea water
by the process of desalination but this requires significant
amounts of energy.
Energy and agriculture 19
Much food goes to waste though traditionally its deterioration
may be reduced by refrigeration, but this also requires energy,
both to power the refrigeration and to transport the refrigeration
units. Alternatively food may be preserved by irradiation, a
method that requires no ongoing energy supply but is little used.
Food waste and an affluent diet increase the demand for more
agricultural land, which leads in turn to further deforestation.
These observations motivate a re-examination of society's
attitude towards radiation and the nuclear option, as the major
source of energy for almost all purposes.
The word energy is used frequently in the following chapters and
it might be helpful to explain what it means. Energy is measured
in joules, and 100 joules of energy is what it takes to power a
100 watt light bulb for 1 second. Energy is conserved that
means it does not get lost and it is inter-convertible between
different forms, to some extent. Forms of energy include heat,
sunlight, chemical, nuclear, electrical, hydro and many others.
In a waterfall the same quantity of energy may be carried by a
large mass of water that drops a small height, or a smaller mass
of water that drops through a larger height. But the difference
can be important. There is a similar distinction between nuclear
and fossil fuel energy sources. The same total energy may come
from a small number of atoms each releasing a large energy, or a
large number of atoms (or molecules) releasing a small energy.
The former is what happens in a nuclear power station and the
latter in a fossil fuel one. Usually in the following chapters the
word energy will refer to the energy per atom. It should be
understood that many, many atoms may deliver much energy,
but the amount of fuel required and the waste generated for each
joule produced increases if the energy per atom is small.
This energy per atom is five million times smaller for fossil fuel
than for nuclear, as explained in footnote 6 on page 29. So, for
the same amount of electricity, the amount of fossil fuel required
(with its waste) is five million times the amount of nuclear fuel
(with its waste). This is the crux of the story.
21
Chapter 3 The Atomic Nucleus
His enormous head bristled with red hair; between his
shoulders was an enormous hump…
The feet were huge; the hands monstrous. Yet with all
that deformity was a certain fearsome appearance of
vigour, agility and courage…
'It's Quasimodo, the bell ringer. Let all pregnant women
beware!' cried the students.
'…Oh that hideous ape! ... As wicked as he is ugly
…it's the devil.'
The women hid their faces.
Victor Hugo, writer (1802–1885)
Powerful and beneficial
In his novel, The Hunchback of Notre Dame, Victor Hugo
introduces the central figure with these words. While the citizens
of mediaeval Paris are repelled by his ugliness and afraid of his
strength, no one cares to discover his true nature. As the story
unfolds, Quasimodo reveals a natural gentleness and kindness
towards Esmeralda, the beautiful gypsy girl, who is condemned
to death on the gallows. The people's fear prevents them from
appreciating him until he uses his strength in the attempt to save
Esmeralda's life.
Such is the public image of radiation. Like Quasimodo, it is seen
as ugly, strong and dangerous. Like him it engenders an almost
universal reaction of fear and rejection. Many do not want to be
near anything to do with radiation or even to understand such
things. This is unfortunate, because the human race has survived
through the power of thought and understanding. The suspension
of that power is not good news for the future.
The following descriptive but scientifically robust account shows
how radiation and the atomic nucleus fit into the natural physical
world.
22 Chapter 3 The Atomic Nucleus
Size scales
The stage on which the science of radiation, radioactivity and
fundamental life processes is set requires a broad range of scales
very small distances as well as larger ones, and very small
energies and much larger ones too. Despite their differences
these distances and energies are inter-related through
fundamental science.
Figure 2 The scales of the different structures relevant to the
interaction of radiation with life, from man through cells,
molecules and atoms to nuclei.
Figure 2 gives an idea of these spatial scales, starting from a
human on the scale of a metre, Figure 2a. Roughly speaking the
biological structure of each human is realised in the form of a
large ensemble of cells, each on a scale of about 10-5 metres,
Figure 2b, although some cells are very much smaller and some
larger. This means that some are just about visible with the naked
eye but for many a microscope is needed. Cells vary as much in
function as in size. Each is composed of about 70% water and a
large number of biological molecules.
Figure 2c is a sketch of a section of a biological molecule
typically these form long chains that fold up within cells. Such
are the working proteins and the double-helical DNA that holds
Size scales 23
the genetic records. Each molecule is a particular sequence of
chemical atoms. Simple diatomic molecules, like the oxygen and
nitrogen in the atmosphere, have just two atoms. The polyatomic
ones, like carbon dioxide, methane and water, have three or
more, so that they can stretch, turn and wriggle about which
gives them their greenhouse gas properties. Big biological
molecules are composed of hundreds of thousands of atoms.
Whereas there is a multitude of different molecules, there are
only a small number of different types of atom. The information
and variety of molecules lies in the arrangement of these atoms
and their chemical connections. Biological molecules are
composed of hydrogen, carbon, nitrogen and oxygen atoms only,
with special additional roles for calcium, phosphorus, sodium
and potassium. Within less than a factor two all atoms have the
same size, about 10-10 metres across. In other words, each atom is
as about 100,000 times smaller than a typical cell, roughly the
same factor by which a cell is smaller than a man.
Figure 2d shows an atom as made of a tiny nucleus surrounded
by a cloud of electrons. The number of electrons in this cloud is
known as the atomic number Z and this alone determines the
atom's chemical behaviour – the nucleus with a balancing charge
Ze makes the atom electrically neutral overall but takes no part in
the 'social' behaviour between atoms. This is because the scale of
the nucleus is 100,000 times smaller than the atom itself,
coincidentally the same ratio as an average cell is to a man and
as an atom to an average cell. All types of nuclei are of a similar
size, about 10-15 metres across.
What do we know about these atoms and nuclei, and how were
they discovered?
Atoms and electrons
To the eye and to the touch most materials are smooth and
continuous. A few are grainy but the grains vary and are not
fundamental in any sense. Only the occurrence of crystals with
their highly regular facets gives a clue of hidden structure. But
that was not why the Greeks, Leucippus and Democritus,
24 Chapter 3 The Atomic Nucleus
suggested that matter is composed of atoms. Their arguments
were not really based on observation at all, it seems. They were
simply unhappy in principle that matter should be indefinitely
divisible. Based on such a vague argument, perhaps it is not
surprising that the atomic idea fell into disfavour in classical
times, not least because Aristotle was not impressed by it.
Only at the start of the 19th century did the atomic theory
reappear, this time to account for observations. These were that
the proportions of pure chemicals taking part in reactions,
burning for example, are related by simple whole numbers.
Altogether there are over 90 different types of atom the
elements. These atoms themselves do not change changes to
their mutual groupings are all that is needed for a simplified
account of chemistry. As these patterns change, the stored energy
may increase or decrease, and when this energy is released, the
material becomes hotter.
Some chemical changes do not happen unless the atoms are hot
in the first place. This can lead to a runaway process, in that, the
hotter the material becomes, the more heat is released. This is the
unstable process that we all know as fire, a chain reaction that is
often highly destructive. It was an early and crucial stage in
human development when man learnt how to control fire, to use
it for warmth and to cook with it. He came to accept its risks in
exchange for the better life that it brought. Even today much
expense is incurred in protecting against its dangers and many
still die every year through fire accidents. In spite of this no
civilisation has banned fire on safety grounds – it is too valuable
a technology to lose.
But there are lessons that early man did not learn about fire. The
waste products are solid ash and gas, predominantly carbon
dioxide and water vapour, released into the atmosphere. Once in
the atmosphere, if the temperature is sufficiently low, the water
condenses out in a few hours or days in the form of rain, but the
carbon dioxide persists. Only now has mankind started to
appreciate the danger of releasing the waste of this chain
Atoms and electrons 25
reaction. Unfortunately he did not understand this when he first
started making use of fire in prehistoric times.
But it was discovered that there is more to the behaviour of
atoms than simply rearranging them to make different molecules.
Towards the end of the 19th century with advances in glass and
vacuum technology it became possible to make sealed glass
tubes of low pressure gas through which electric currents could
be passed between metal electrodes if one of these was heated.
These currents emit light and this technology is the basis of the
sodium and mercury lights commonly used in street lighting, the
neon tube used in signs, and energy-saving fluorescent tubes. If
fully evacuated, such a tube is called a cathode ray tube
familiar today as an old TV tube, now largely replaced by flat
panel displays. In the science laboratory two early fundamental
physics discoveries were made with such tubes.
Firstly, the current, as it passes through a cathode ray tube, is
composed of a stream of charged particles of very small mass.
Remarkably these electrical particles are of the same type,
whatever the atomic composition of the electrodes or gas. These
particles, present in all atoms, are electrons, a new universal
particle discovered in 1897 by J J Thompson. In a TV tube the
picture is 'painted' by a beam of these electrons striking the
inside of the front face of the tube and lighting up the different
coloured phosphors there.
Secondly, if these electrons are given enough energy and then
strike a metal plate, invisible uncharged radiation is emitted.
These X-rays were found to be electrically neutral and highly
penetrating, unlike the parent electron beam. This discovery was
made by Röntgen in 1895. Very quickly the value of the
penetrating power of this radiation was appreciated for medical
imaging and therapy. The relationship between electrons, atoms
and the electrically charged ions, as they appear in the workings
of electric cells and batteries, was explained ions are formed
when an uncharged atom gains or loses one or more electrons.
However, knocking such small parts off an atom an electron
forms less than one thousandth of the weight of an atom did
26 Chapter 3 The Atomic Nucleus
not reveal much about the composition or structure of the rest of
the atom. There was more to be discovered, deeper within.
The nuclear atom
The first evidence of activity inside the atom, beyond the
addition or loss of electrons, came with the discovery of
radioactivity in 1896 by Henri Becquerel, whose work was
followed later by the discoveries made by Pierre and Marie
Curie.3 They found that all chemical salts of certain heavy
elements emitted radiation and that this energy was not
dependent on the ionised state of these elements, or on their
physical and chemical state. Evidently the energy was coming
from the deeper inner atom and not from the surrounding
electrons. Through careful work the Curies showed that chemical
elements were being transformed, so that new ones appeared
whenever an atomic nucleus emitted radiation connected with its
radioactivity. Three types of this radiation were identified, alpha,
beta and gamma – quite often in physics discoveries are given
such enigmatic names, because, initially at least, not enough is
known to name them in terms of their true characteristics. Later
it was shown that alpha, beta and gamma radiation are in fact
streams of helium ions, electrons and electromagnetic radiation4,
respectively.
Radioactive atoms are very unusual and heavy – the implications
for the structure of ordinary elements that are not radioactive
were quite unclear initially. Some years later Ernest Rutherford
showed by experiment that, for every atom, all of the mass
(except for the electrons) and all the balancing positive charge
are concentrated in a tiny volume at the centre of the atom – the
nucleus. With its surrounding atomic electrons, this is the atom
3 The discoveries of 1895, 1896 and 1897 were so unexpected and came in
such short succession that less careful experimenters felt encouraged to come
forward with claims based on fantasy. In particular the magical powers
attributed to so-called N rays were only shown to be false after much
publicity.
4 Described in Chapter 4.
The nuclear atom 27
as we understand it today. The arrangement has been compared
with the Sun and its solar system of planets. But this is deceptive
– the proportions are wrong. The Sun is a thousand times smaller
than the solar system while the nucleus is a hundred thousand
times smaller than the atom. Seen from the edge of the atom, the
nucleus would be far too small to be seen with the naked eye – if,
for a moment, you can imagine yourself on such a scale. The rest
of the atom is quite empty apart from the thin cloud of electrons.
Since the 1920s quantum mechanics, the radical shift in our
understanding of the physical world, has explained in full
precisely why molecules, atoms and nuclei have the structure and
behaviour that they do. Recently, as computers have become
faster, it has been possible to extend such explanations and
predictions to the properties of larger and larger chemical and
biological molecules.
For reasons explained below, nuclear change is very energetic
compared with chemical change and it powers the Sun upon
which life depends. Earlier in the history of the Universe all the
chemical elements were formed by nuclear change from the
primordial hydrogen and helium that remained after the Big
Bang. However, since the Earth was formed roughly six
thousand million years ago, only one nucleus in a million has
undergone any change at all. Within a small range all materials
are 99.975% nuclear by weight electrons only account for
0.025%. So nuclear material is very common but substantial
nuclear change is quite remarkably rare.
In the early 1930s it was shown that every nucleus is composed
of a certain number of protons and neutrons. The proton is the
positively charged nucleus of simple hydrogen, and the neutron
is its uncharged counterpart. The proton and neutron are almost
identical in size and weight, and their properties differ only on
account of electrical charge. Elements are characterised by their
chemistry – that is by the number of surrounding electrons. This
is the same as the characteristic number of protons to ensure
electrical neutrality. However, a given element may exist in
several forms called isotopes – the only difference between these
28 Chapter 3 The Atomic Nucleus
is the number of neutrons each contains. Apart from the variation
in mass, different isotopes behave identically, except on the rare
occasions when nuclear change is involved. They are named by
their element and then their atomic mass number A this is just
the total number of protons and neutrons that each contains.
Examples are uranium-235, lead-208 and oxygen-16.
Whereas the number of neutrons that an atom contains has little
influence on its external behaviour, the internal structure and
stability of the atomic nucleus are significantly affected,
including whether it rotates. In fact each element has only a
small number of isotopes, of which only a few are stable. Most
unstable ones have decayed away long ago and are no longer to
be found in nature. If a nucleus rotates, it behaves like a tiny
magnet.5 In a large magnetic field these rotating nuclei tend to
line up like iron filings or compass needles. Their alignment can
be controlled and measured using radio-waves without invoking
any nuclear change. This is called nuclear magnetic resonance
(NMR) and is the basis of magnetic resonance imaging (MRI). In
clinical use the description nuclear has been dropped from the
name, in deference to the risk of worry that this label might
cause! In fact the magnetic energy of a nucleus in MRI is about
one millionth of a typical chemical energy.
On the other hand the typical energy of a proton or neutron
inside a nucleus is large about a million times larger than the
energy of an electron inside an atom, that is normal chemical
energy. The reason for this is a universal basic feature of
quantum mechanics. The simple two-line calculation given in
footnote 6 at the bottom of the next page gives a factor of about
five million. This ratio does not change much if calculated more
precisely and sets the scale of the enhancement of nuclear energy
over chemical energy. So roughly speaking, a nuclear power
station gives a million times as much energy per kilogram of
fuel, and per kilogram of waste, as a fossil fuel power station
delivering the same electrical energy.
5 Every rotating charge behaves as a magnet – this is a universal relationship.
Conversely, all magnets are due to rotating or circulating charge.
The quiescent nucleus 29
The quiescent nucleus
Each nucleus remains really rather isolated at the centre of its
atom. Other than flipping its spin under the influence of radio-
waves, as in MRI, it can do nothing. It may be moved passively
within its deep shield of surrounding electrons, but only as an
inert mass. What prevents it taking a more active role? There are
several reasons for this remoteness.
Like the electrons the behaviour of a nucleus within an atom is
described by quantum mechanics – in both cases there are only a
certain number of states that can be occupied. For the electrons
these states are not far separated in energy. As a result atoms
change electronic state frequently, emitting or absorbing light
and generally taking part in chemical or electrical activity. But
nuclei cannot do this because the separation of their states is
typically five million times greater.6 So nuclei are effectively
'frozen' into their lowest state unless extreme energy is available.
The second reason for the remoteness of the nucleus is that the
electrons ignore it, except through the electrical attraction. As a
result the electrons on the one hand and the protons and neutrons
on the other keep their own company and the nuclear core
remains separated at the atomic centre.
Even if nuclei do not interact with electrons much, why do they
not interact with each other? Actually this is not possible
because, being all highly positively charged, they are kept well
apart by their strong mutual electrical repulsion. This acts like a
6 In quantum mechanics a proton or electron of mass m contained in a region
of size must have a momentum P of about ħ/, where ħ is Planck's constant.
In simple mechanics, for a mass m with velocity v, there is a relation between
the momentum P = mv and the kinetic energy E = ½mv2. So that
E = P2/2m = ħ2/(2mℓ2).
Using this formula we can compare the energy of an electron in an atom with
that of a proton or neutron in a nucleus. The ratio of the size of the region is
100,000; the ratio of mass m is 1/2000. The formula tells us that typical
nuclear energies are larger than atomic (that is chemical) energies by the ratio
of mℓ2, that is about 5 million.
30 Chapter 3 The Atomic Nucleus
powerful spring between them and it takes enormous energy to
drive them close enough together to touch, effectively.
Nuclei are not excited when illuminated by beams of radiation
either, unless the energy of the radiation is quite exceptionally
high or the radiation is composed of neutrons. If the radiation is a
beam of protons or alpha particles, these are repelled before they
ever get close to any nucleus. A beam of electrons or
electromagnetic radiation is equally ineffective because these do
not react with nuclei except electrically, as just discussed. The
only way in which the outside world can effect any change in a
nucleus is through collision with a neutron. Having no electrical
charge, neutrons are not repelled and can enter a nucleus with
ease. But free neutrons are unstable, decaying with a half-life7 of
15 minutes, and so their presence in the natural environment is
exceptional.
If the influence of the environment on a nucleus is very rare, how
about the other way around? When do nuclei affect the
environment? On its own, all that a nucleus can do is decay, if it
is unstable, thereby releasing a certain energy into the
environment. Most naturally occurring nuclei are stable and
cannot do this. For the handful of naturally occurring nuclei that
do decay, the process is very slow and rare, and this is why
unstable nuclei were not discovered until 1896. Most varieties of
nuclei that could decay, already did so within a few million years
of being formed, more than six thousand million years ago.
When a nucleus decays, the total energy of the emitted radiation
and the residual nucleus (including its mass) must equal the total
energy of the initial nucleus (including its mass). This is because
in a decay no energy is lost – and no electric charge is lost either.
7 In a group of neutrons the decay of any particular one occurs quite
randomly in time – except that each can decay only once. So the number that
remain falls naturally with time, and consequently so does the rate at which
these decay. If the time for half of the nuclei to decay is T (called the half-
life), then the number left is reduced by a further factor of a half with every
successive time interval T. This is called an exponential decay and describes
any unstable atom and nucleus.
The quiescent nucleus 31
The same is true for the atomic mass number A, the sum of the
number of neutrons plus protons, that is N+Z. The total A present
before the decay equals the combined number afterwards.8 Table
1 explains how the alpha, beta and gamma decays first studied by
the Curies match with these rules.
Table 1 The usual types of natural radioactivity, alpha, beta
and gamma, where N and Z are the numbers of neutrons and
protons in the initial nucleus.
Type Residual nucleus Radiation
Neutrons Protons Charge Form Charge
Alpha N2Z
2Z
2helium nucleus +2
Beta N
1Z+1 Z+1 electron
1
Gamma N Z Z electromagnetic 0
In alpha decay both N and Z of the residual nucleus decrease by
two and an alpha particle, a helium ion composed of the four
nucleons, is emitted. In beta decay a neutron becomes a proton
with emission of an electron to balance electric charge. There is a
second type of beta decay in which a proton is changed into a
neutron and a positive anti-electron (or positron) such decays
are of great importance in nuclear medicine. Actually in all types
of beta decay another particle is emitted too. It is called the
neutrino. But we are not interested in neutrino radiation in this
context because it effectively disappears, without depositing any
energy or doing any damage.9
In fission decay the nucleus splits into two fairly equal halves
with the emission of a few extra neutrons. Such decays are
exceedingly rare 'in the wild', even for radioactive isotopes,
8 The rules for charge and energy conservation are deeply embedded in the
principles of physics, although the rule for A is empirical.
9 Neutrinos interact so seldom that they can pass right through the Sun or the
Earth, although after 50 years of experiments they are now well understood.
32 Chapter 3 The Atomic Nucleus
which are themselves rare. However, in the artificial
circumstance in which a nucleus has just absorbed a neutron,
fission can occur efficiently and quickly. This induced fission
process requires a flux of such neutrons for instance inside a
fission reactor. Each fission releases further neutrons that may
then be absorbed by other nuclei, thus building up a neutron-
induced chain reaction. This is like a chemical fire which is
stimulated by its own heat production. A difference in the
nuclear case is that remarkably few materials are 'combustible',
as it were, and the 'fire' is very difficult to ignite.
Energy for the Sun
The Sun provides the energy that drives all aspects of life and the
environment. Without the energy of the Sun, the Earth's surface
would cool towards minus 270ºC, the temperature of inter-stellar
space. Only the dull heat of the radioactive energy released
within the Earth would raise the temperature at all.
Viewed over geological periods, fossil fuels act as chemical
batteries that absorb the Sun's energy in one geological period
and then give it back in another. The problem for mankind is that
these batteries, charged over millions of years, are being
discharged on the timescale of a century.
It is no surprise that the Sun was worshipped as a god in ancient
times as the source of heat and light – a rather sensible choice of
deity. An important question is, where does the Sun gets its
energy? If this came from a chemical fire, it would have run out
of fuel after about 5,000 years, but it has been shining for a
million times longer than that already. The Sun is made of
hydrogen and a small quantity of the element helium.10 However,
10 The name, helium, comes from the Greek name for the Sun. It is scarce on
Earth but abundant in the Sun where its presence was first discovered. It is a
very light gas that escapes upwards in the atmosphere on Earth – indeed it is
used to fill fairground and party balloons for that reason. Fortunately plentiful
supplies of helium for these and other uses come from the emission of alpha
radiation in the decay of naturally radioactive atoms in the rocks of the Earth's
crust.
Energy for the Sun 33
there is no air or oxygen with which to support chemical
combustion of the hydrogen.
The source of the Sun's energy is nuclear – it is a large reactor in
which hydrogen 'burns' by fusion to form helium. The increase in
energy relative to a chemical fire will enable the Sun to shine for
many more thousand million years yet. This fusion reaction can
only happen in the centre of the Sun where the temperature
reaches several million degrees. Just as a chemical fire has to be
started by a source of heat to get the reaction going, so to ignite a
nuclear fusion fire, the hydrogen atoms must be given enough
energy that, when they collide head-on, the nuclei can fuse
together. At lower temperatures they simply bounce off one
another without touching because of their mutual electrical
repulsion. The visible surface of the Sun is at 5,800ºC but the
temperature rises with depth, and towards the centre it gets hot
enough for this fusion to occur, a process that is now well
understood. The energy released near the centre then finds its
way slowly outwards towards the solar surface.
The Sun burns 600 million tons of hydrogen every second and
yields 596 million tons of helium in its place. This curious loss
of mass is balanced by the energy that streams out in all
directions, such as towards the Earth. The rate at which the Sun
loses energy E is related to the rate at which it loses mass m, the
four million tons per second, by the equation E = mc2 where c is
the velocity of light. It is sometimes suggested that nuclear
physics has a special connection with Einstein's Theory of
Relativity, but this is not true – energy of all kinds is connected
to mass in this way. One kilogram of mass is equivalent to
9×1016 Joules, or about 2×1010 kilowatt-hours. This is so large
that it is only in the nuclear case that the mass change is
noticeable. In the case of hydrogen fusing to helium it is just
under 1%. This vast solar energy flux spreads as it radiates away.
By the time it reaches the radius of the Earth's orbit it is a
pleasantly warm 1.3 kilowatts per square metre.
Pleasant, maybe, but such a nuclear energy source deserves to be
respected by modern man, as it was by the ancients. It is unwise
34 Chapter 3 The Atomic Nucleus
to lie out in its radiation for long periods. However, the majority
of people take a sensible attitude, enjoying a modest exposure
without expecting that the risk of sunburn or skin cancer can be
completely eliminated. By applying ultraviolet blocking cream
and by limiting the duration of exposure, the warmth of sunshine
at longer wavelengths may be enjoyed. No one seeks absolute
safety from the Sun's rays – otherwise summer vacations taken in
total darkness deep under ground would be keenly sought after!
As with fire, mankind has learnt to live with the Sun, enjoying its
benefits and avoiding its hazards. In both cases straightforward
education and simple rules play their part. A similar measured
attitude to other kinds of radiation would be beneficial.
35
Chapter 4 Ionising Radiation
The spectrum of radiation
So what exactly is radiation? The simplest answer is that it is
energy on the move – and there are many kinds. Sunshine, music
and waves on the surface of water are examples. At low levels
many are quite harmless and even beneficial to life. Extreme
levels can cause damage in almost every case – very loud music
can damage hearing, and too much sun causes sunburn.
However, a little sunshine is positively good for the skin by
promoting the production of important vitamins. Similarly music
that is not too loud may be positive and uplifting.
There is an important point here. It is not that gentle music
causes only a little damage, but that it causes no damage to
hearing whatever. When compared with the damage due to
excessively loud sounds, the effect is not proportionate.
Technically such a relationship is termed non-linear and this will
be an important idea in subsequent chapters. In the case of music
and damage to hearing the non-linearity may be obvious, but for
other forms of radiation the distinction between proportionate
and non-proportionate response will need to be looked at using
both experimental data and an understanding of what is
happening.
Most of the radiation from the Sun comes in the form of
electromagnetic waves this includes light and other parts of a
wide spectrum. Each such wave involves entwined electric and
magnetic fields. It has a frequency and an intensity just as a
sound wave has a pitch and a volume. Our understanding of
electromagnetic waves dates from the work of James Clerk-
Maxwell in the 19th century, who built on the work of Michael
Faraday and others. As for any wave, the speed at which it
moves is equal to the frequency times the wavelength. Since the
speed is essentially constant, the wave may be labelled by its
36 Chapter 4 Ionising Radiation
Figure 3 The frequency spectrum of electromagnetic waves.
wavelength instead of its frequency, but either will do. On a
radio receiver, for example, some stations are labelled by their
frequency in MHz (mega-hertz, millions of waves per second),
while for others the wavelength in metres is used. The product of
the two is the speed of radio-waves, 300 million metres per
second, the same as that of light.
How a wave is received is determined largely by the frequency
not the intensity. For example, a radio receiver selects a station
The spectrum of radiation 37
by choosing its frequency rather than its loudness. In the same
way that for sound there are frequencies that cannot be heard by
the ear, so for light there are frequencies that are invisible to the
eye. In fact only a tiny range of frequencies of electromagnetic
waves is visible. The whole spectrum is represented in Figure 3
with a logarithmic frequency scale running up the page and
covering more than 15 powers of 10, as shown in the second
column in oscillations per second (Hz). The first column gives
the corresponding wavelength. Visible light with its
characteristic spectrum of rainbow colours is the narrow cross-
hatched band half way up the diagram. The point is that there
really is no fundamental difference between these waves, from
radio through light to X-rays, except the frequency. At the
highest frequencies (and shortest wavelengths) the powers of 10
become harder to cope with and a third scale based on the
electron volt (eV) is often used.11 This is shown on the right of
Figure 3 with the usual prefixes for powers of 10.12
Much benefit has been brought to everyday life through enabling
mankind effectively to see using these other frequencies [4].
Lower in the diagram are radio-waves up to 109 Hz, used for
example in MRI to see inside the human body and in radar to see
ships and planes in fog and darkness. Slightly higher is thermal
imaging, used to see warm bodies accidentally buried or
concealed. Just below the visible frequencies is a region called
the infrared absorption band, shown as shaded in the diagram.
At these frequencies many materials are opaque because the
rotation and vibration of molecules are in tune and resonate with
electromagnetic waves. Above the visible there is another band,
the ultraviolet absorption band. Here it is the more nimble
atomic electrons that are in tune and the cause of the absorption.
So here too materials are opaque, as marked by the shading.
11 The electron volt is 1.6×10-19 joules. This is a useful scale in the atom. The
electron in the hydrogen atom has an energy of 13.6 eV while typical nuclear
energies are in MeV.
12 μ or micro, one millionth. m or milli, one thousandth.
k or kilo, one thousand. M or mega, one million. G or giga, one billion.
38 Chapter 4 Ionising Radiation
Heavier elements with their more tightly bound electrons have an
ultraviolet absorption band that extends to much higher
frequencies than light elements. This is the frequency range of
the X-rays. Here, metals like copper and calcium absorb
radiation whereas carbon, hydrogen and oxygen are transparent.
Medical images of a patient's teeth or bones (calcium)
illuminated with such radiation show clearly any fracture or
disease because the enveloping tissue (carbon, hydrogen and
oxygen) is transparent.
Above about 100 keV atomic electrons, even those that are most
tightly bound in the heavier elements, cannot move fast enough
to follow the oscillating wave.13 Consequently there is no
resonance and all materials are largely transparent. This region is
called the gamma ray region. Historically the distinction between
X-rays and gamma rays depended on the sourceelectrons and
nuclei, respectively. This distinction is deceptive because their
effect does not depend on the source, only on their energy (or
frequency). Today this switch of name is usually made at about
100 keV, but the distinction is really only a convention. Gamma
rays are very penetrating, being only weakly absorbed, which is
why they are used in radiotherapy to target energy into a cancer
tumour, deep within a patient's body. This energy may then be
absorbed in the tumour with sufficient intensity that its cells are
killed and it ceases to function. There are practical difficulties in
doing this, as discussed later in Chapter 7.
Damage from radiation
So understanding light, and then learning to see with radiation in
other parts of the spectrum, is really useful. But what of the
risks? The spectrum can be divided roughly into two halves
separated at about 10 eV. Radiation of greater frequency or
13 At such high frequencies the radiation appears less like waves and more
like rays, or particles. In quantum mechanics this distinction has no real
substance, and electromagnetic waves of any frequency f come in bundles of
energy called photons, E = hf, where h is Planck's Constant. Each atom or
nucleus emits one such bundle or particle when it decays.
Damage from radiation 39
energy is called ionising radiation, that below, non-ionising
radiation. The distinction is that ionising radiation can ionise and
break molecules apart – this is the radiation with which this book
is primarily concerned.
Public concern about weak levels of non-ionising radiation, for
instance from overhead power lines or mobile phones, is
misplaced. The only known way in which such radiation can
cause damage is by heating.14 Put briefly, these radiation sources
are safe if heat is not sensed – even then, benefits may dominate
over any reasonable risk. Warmth from sunshine or a domestic
fire is brought by the same kind of radiation as that in a
microwave oven. While the radiation levels in such an oven can
certainly be dangerous, the heat radiated by a glowing fire on a
cold winter's day is a quite acceptable source of radiation hazard
for most people in spite of the fact that its heat level can be
sensed, indeed because of it.
But non-ionising radiation still has a crucial environmental
impact. On the right hand side of Figure 3 are two boxes labelled
sunshine and earthshine. Very hot materials like the Sun emit
light in the visible region, but cooler materials also emit, though
predominantly in the infrared frequency range. The sunshine box
indicates the range of frequencies that the Sun emits. Because
this is centred on the visible region for which the atmosphere is
largely transparent, much of this radiation reaches the surface of
the Earth for the benefit of all, including plant life. (Actually the
spectrum of the Sun extends a bit into the infrared and
ultraviolet, too – the infrared part provides warmth, the
ultraviolet causes sunburn, if not filtered by barrier cream and
the small concentration of ozone present in the upper
atmosphere.) The earthshine box indicates the frequency band of
radiation that the surface of the Earth emits with its lower
temperature – but not all of this radiation succeeds in getting out
14 This important statement can be scrutinised but the effect of radio-waves
and microwaves on living tissue is well understood and they are widely used.
For instance, they are used in MRI, safely below the level at which any
significant heating occurs.
40 Chapter 4 Ionising Radiation
of the atmosphere because of infrared absorption by polyatomic
gases,15 in particular carbon dioxide, water vapour and methane.
With an atmosphere containing more of these the Earth is not
able to cool itself nearly as effectively as it is able to absorb the
sunshine. So energy is trapped in the atmosphere and the
temperature increases. Crudely, this is how the Greenhouse
Effect works. If the concentration of these gases rises, the Earth
gets hotter and the climate changes. An extraordinary example is
close at hand – Venus has a surface temperature of 460ºC, thanks
in part to an atmosphere with 97% carbon dioxide.
Like electromagnetic waves, beams of charged particles such as
alpha and beta radiation can also damage molecules, so that they
are classified as ionising radiation and beams of neutrons and
other ions too, although these are less common in the natural
environment.
Nuclear stability
But what makes a nucleus decay? Or rather, what holds it
together in the first place? The mutual electrical repulsion of the
protons makes large nuclei more unstable than small ones.
Stability only comes from the nuclear force that attracts
neighbouring protons and neutrons together. This nuclear force
overwhelms the electrical repulsion, but only at short distances
within about 10-15 metres. As a result it favours small nuclei for
which the protons and neutrons can huddle close together. The
result is a balance between the preferences for nuclei to be not
too large and not too small, which gives rise to the nuclear
stability curve, Figure 4. The most stable atoms are those with
nuclei at the highest point on the curve, the tightest average
binding. These are in the region of iron, A = 56.
While quantum mechanics prefers nuclei with roughly equal
numbers of protons and neutrons, the disruptive electrical force
15 Molecules like oxygen and nitrogen with just two atoms each do not vibrate
and rotate with the same readiness that most polyatomic molecules do with all
their many modes of internal movement. So they do not absorb much.
Nuclear stability 41
Figure 4 The average binding energy per proton or neutron
as it depends on the atomic mass number, A.
makes nuclei with too many protons unstable. The result is that
all stable nuclei, except the largest, have roughly equal numbers
of protons and neutrons, so that iron (Z = 26) has 30 neutrons. As
shown in Figure 4, for smaller values of A the binding effect of
the nuclear force is reduced; at larger values of A the disruptive
influence of the electrical effect is increased either way the
binding is less. Above iron the compromise favours nuclei with
more neutrons than protons because the disruption only acts on
the protons. So for example, the most abundant isotope of lead,
lead-208, has 82 protons but 126 neutrons. There are no naturally
occurring elements above uranium (Z = 92) those above
actinium (Z = 89) are collectively referred to as the actinides.
The curve shows that in principle nuclei with small A could fuse
together to release energy due to the nuclear force, as shown by
the arrow on the left. This is nuclear fusion and the source of
stellar energy, including that of the Sun. In addition, nuclei with
large A can in principle release energy by splitting apart and
moving towards greater stability as shown by the arrow on the
42 Chapter 4 Ionising Radiation
right. This is nuclear fission.16 Because, like lead, the parent
nucleus has more extra neutrons than its stable fission products,
there are excess free neutrons emitted in the fission process. The
liberation of these extra neutrons is crucial to the nuclear chain
reaction mechanism.
In practice fission is very rare. Alpha decay in which a heavy
nucleus splits into helium and a smaller nucleus is more
common. This is the source of much of the natural radioactive
energy in the Earth's crust the energy source of natural
geothermal power, in fact. In alpha decay nuclear energy is
released by moving to the left along the curve in steps of four
units in A. As A reduces, the excess proportion of neutrons has
also to be reduced, and this occurs by beta decay in which a
neutron in the nucleus decays emitting an electron and leaving
behind an extra proton within the nucleus.
Table 2 The four distinct primordial radioactive series with
their head members and half-lives (T1/2), and also end
members. T1/2 is given in G-year, a thousand million years.
4n series 4n+1 series 4n+2 series 4n+3series
Head thorium-232 neptunium-237 uranium-238 uranium-235
T1/2 14.1 G-year 0.002 G-year 4.5 G-year 0.70 G-year
End lead-208 bismuth-209 lead-206 lead-207
The natural radioactivity of heavy nuclei consists of a sequence
of alpha and beta decays in which energy is released as the
nucleus moves to lower A along the stability curve (Figure 4).
There are four distinct series of nuclei, depending on whether A
is of the form 4n, 4n+1, 4n+2, or 4n+3, where n is a whole
number. Within each series nuclei may decay, one into another,
by alpha or beta decay. Each series has a long-lived primordial
head member and an end member which is effectively stable
16 It is curious to note that in nuclear fission it is stored electrical energy that
is released. Energy due to strong nuclear binding is absorbed, not released, in
the fission process.
Nuclear stability 43
these are given in Table 2. The 4n+1 neptunium series has
already died out, but the other three are still active in the natural
environment. The successive members of the 4n+2 series, with
their decays and half-lives, are shown in Table 3, as an example.
Table 3 Members of the uranium-238 series (the A = 4n+2
series). Some half-lives are measured in thousands of years
(k-year).
Element-A Z N Decay Half-life
uranium-238 92 146 alpha 4.5 G-year
thorium-234 90 144 beta 24.1 day
proactinium-234 91 143 beta 1.17 minute
uranium-234 92 142 alpha 240 k-year
thorium-230 90 140 alpha 77 k-year
radium-226 88 138 alpha 1.6 k-year
radon-222 86 136 alpha 3.82 day
polonium-218 84 134 alpha 3.05 minute
lead-214 82 132 beta 26.8 minute
bismuth-214 83 131 beta 19.8 minute
polonium-214 84 130 alpha 164 microsecond
lead-210 82 128 beta 22.3 year
bismuth-210 83 127 beta 5.01 day
polonium-210 84 126 alpha 138.4 day
lead-206 82 124 metastable
44 Chapter 4 Ionising Radiation
Measuring radiation
To speak usefully of the effect on human life of different doses
of ionising radiation, these must be measured, somehow. But
how exactly?
The first step in quantifying a radiation exposure is to measure
how much energy is absorbed per kilogram of living tissue
during the exposure. This energy may cause chemical damage by
breaking molecules apart that leads to biological (cellular)
damage and finally to clinical damage, such as cancer or other
disease. Such clinical damage turns out to be more difficult to
relate to the exposure, especially as it may manifest itself in
different ways, and on long or short timescales, from days to
years.
In earlier decades knowledge of cell biology was too primitive to
provide confident understanding, and adequate evidence of the
effect of radiation on humans was not available to corroborate
any particular view. In their absence, and for lack of anything
better, the knowledge gap was bridged by a rule of thumb a
model in science-speak. This is the Linear No-Threshold model,
abbreviated LNT. This assumes that clinical damage is in simple
proportion to the initial radiation energy dose. No justification
was given for it, but it was a reasonable working hypothesis at
the time. Despite the poor state of knowledge, a start had to be
made somewhere.
However, given modern biological knowledge and extensive
records of human data, this model is now redundant and many of
its more cautious implications can be ignored. The details are for
discussion in later chapters. First, we return to the questions of
the quantification of radioactivity and absorption of radiation
energy in materials.
The rate at which energy is emitted by a radioactive source
depends on the number of radioactive nuclei N, the energy of the
decay, and the half-life T of the nucleus. The value of N is
reduced by half with every successive time interval T and the
average activity is proportional to N/T. Activity is measured in
Measuring radiation 45
decays per second, called becquerel and abbreviated Bq.
Sometimes the activity may be measured in a cubic metre of
material, thus Bq m-3.
So what does this mean in practice? Contamination by
radioactive nuclei with a short half-life results in high activity for
a short time; the same contamination with a longer half-life
results in a lower activity, but it continues for longer. Half-life
values vary between a small fraction of a second and many times
the age of the Earth. So sources of radioactive contamination
with short half-lives fade away while others with longer half-
lives continue on. This is in contrast to most chemical pollutants,
such as heavy metals like mercury or arsenic, that remain
hazardous indefinitely. A slightly different situation arises when
a dose of ionising radiation energy comes from an external beam
produced by an accelerator (such as an X-ray machine) or from
an external radioactive source.
Either way the important question is, how far does the radiation
travel in material before being absorbed? Some radiation is so
strongly absorbed in air, or any thin material, that it never
reaches human tissue unless the source is on the skin or inside
the body. Other radiation is weakly absorbed and can pass
through the body. So what is important is not the intensity of the
radiation, but the amount that is absorbed, for instance, per
kilogram of tissue.17 The extent to which it is absorbed depends
on the kind of radiation and its energy (or frequency).
Alpha radiation is stopped even by air, and so the decay energy
is deposited very close to the site of the radioactive
contamination itself, with no dose at all only a little further away.
An example is the energetic, but short range, alpha radiation
emitted by the decay of the radioactive isotope polonium-210. A
large internal dose of this was used allegedly by Russian agents
to kill Alexander Litvinenko in London in 2006. No energy
17 Radiation that just passes through and does not deposit any energy is
necessarily harmless – like the neutrino radiation mentioned on page 31.
46 Chapter 4 Ionising Radiation
escaped the immediate location of the poison but there the tissue
received the full radiation energy dose.
Beta decay produces electrons that travel further in material and,
therefore, the deposited energy dose is more diffusely distributed
around the radioactive source. Gamma rays go further still. So
for a radioactive source in rock, for example, any alpha and most
beta radiation is absorbed internally within the rock, and only the
gamma radiation escapes to give an external energy deposition.
In general a deposited energy dose is quantified as the number of
joules of energy absorbed per kilogram of material, such as
patient tissue. One joule per kilogram is called a gray (Gy).
Typically doses are measured in milligray, with a milligray
(mGy) being one thousandth part of a gray.
The clinical damage caused to living tissue by this deposited
radiation develops as a result of a number of steps.
1. The immediate molecular mayhem left by the radiation.
2. Biological damage in which living cells are put out of
action – this changes with time as the tissue responds to
the radiation dose.
3. The incidence of cancer (and other possible delayed or
heritable effects) related to the exposure, perhaps decades
later.
4. The reduction in life expectancy as a result of such
cancers (this effect on life expectancy is called the
radiation detriment of the exposure).
5. The chance of death shortly after exposure due to acute
radiation sickness brought on by cell death and the
shutdown of the normal biological cycle in one or more
vital organs.
The two lasting consequences for life are described by the
sequences 1-2-3-4 and 1-2-5, and later we will discuss how each
of these outcomes relates to the initial radiation energy dose.
There are other causes of cancer, unrelated to radiation. Some
causes – we shall refer to them generally as stresses – are natural,
others are imposed by choice of lifestyle. Following decades of
study much is known about how these stresses are related to the
Measuring radiation 47
occurrence of cancer to the detriment in fact. An important
question is how the outcome is influenced when there is more
than one stress. These stresses may be quite independent, as in
smoking and radiation, but the result may not be. There remain
some unanswered questions. But the point is that the range of
residual uncertainty is too small to prevent mankind from taking
decisions now about how radiation can be used with effective
safety.
For a single acute dose the damage is related to the size of the
dose and the type of radiation. The effects of X-rays, γ-rays and
electrons are found to be roughly the same for the same energy
dose in milligray. However, for other types of ionising radiation
the biological damage is different. Quantitatively, the measured
ratio of damage relative to X-rays is called the relative biological
effectiveness (RBE). So the RBE of a radiation dose indicates
how much more clinical damage it causes than is caused by the
same number of milligray of energetic gamma rays. Essentially
these RBE factors are measured quantities.
RBE factors vary with the clinical end point that is with the
cancer or disease concerned. Timing effects are important and
we look at these later. The variation with radiation type is
particularly interesting although not too large. For most practical
applications of radiation safety, which we are thinking about in
this discussion, we need to watch the factors of ten, a hundred
and a thousand. RBE factors close to one are less important.
Only in radiotherapy are the effects of radiation very finely
balanced – but in that case gamma rays are usually used and so
RBE is 1.0 anyway. So for this simplified discussion it is
sensible to ignore the RBE factor in the first instance.
Nevertheless the International Commission for Radiological
Protection (ICRP) has felt it necessary to include RBE in some
way. In their radiation safety standards they multiply each energy
dose in gray by a weighting factor, wR, which plays the role of a
broad-brush averaged RBE. [They define wR for protons to be
two; for alpha, fission fragments and other heavy ions to be 20;
for neutrons it depends on the energy; for electrons and photons
48 Chapter 4 Ionising Radiation
it is just one, by definition.] The result they define to be the
equivalent dose, measured in units of sievert (Sv) – or
millisievert (mSv). In ignoring RBE initially we treat doses
measured in milligray and millisievert as equivalent, and come
back later to the distinction when a variation in the type of
radiation has something special to say about how radiation
damage occurs.
These measures of energy deposited (and equivalent dose) may
be for a single acute exposure. It is observed that cell damage is
different if the dose is spread over a period of time, either as a
series of repeated exposures, or as a continuous chronic rate of
exposure. The question is why? What is the radiation detriment
resulting from a chronic rather than an acute radiation exposure?
How does the effect of a single dose of so-and-so many milligray
compare with the effect of a continuous dose rate of a number of
milligray per day or per year? The matter is not simple,
because dose and dose rate are quite different measures. This is
the subject of Chapter 7.
Natural environment
Figure 5 Origins of the average annual radiation exposure of
the UK population, total 2.7 millisievert per year [5].
Natural environment 49
The radiation dose rate experienced by the population of the UK
varies from one place to another. The average is 2.7 millisievert
per year, and a breakdown of this radiation by source is
summarised in Figure 5.
The slice labelled cosmic is for radiation that originates from
space. The radon and gamma slices describe natural radioactive
sources in nearby materials such as water, soil and rock. The
internal radiation slice relates to the decay of radioactive atoms
that occur naturally within the human body. The artificial part of
the exposure is predominantly medical the average due to all
other man-made sources amounts to less than 0.5%.
The ionising radiation incident on the Earth from space is made
up of electromagnetic radiation, protons and electrons. Some of
the charged particle radiation comes from the Sun where the
erupting magnetic fields of sunspots act as accelerators. At the
top of the atmosphere this radiation causes ionisation, and the
resulting discharges may be seen as the Northern Lights or
aurora. Charged particles with low energy are deflected by the
Earth's magnetic field, except in the magnetic polar regions,
which is why the aurora are seen there. The resulting increased
ionisation of the upper atmosphere affects satellite and radio
communications, and when there is a magnetic storm this
ionisation is high. None of these phenomena has any effect on
health and the ionisation radiation does not reach the ground.
Cosmic radiation also includes protons that are more energetic
and come from outside the solar system, and even outside the
galaxy. These suffer nuclear collisions in the upper atmosphere.
Some collisions create neutrons that then hit nitrogen nuclei high
in the atmosphere to form the famous isotope, carbon-14.
Although only 7.5 kg is created in total in the atmosphere each
year, this is sufficient to maintain the proportion of carbon-14 in
the natural biosphere (1 part in 1012), which provides the basis of
radiocarbon dating. This isotope decays with a half-life of 5,700
years, and its concentration starts to fall as soon as material,
animal or vegetable, dies that is, stops refreshing its carbon
from the air or digesting other living tissue. By measuring its
50 Chapter 4 Ionising Radiation
concentration, materials can be dated. Famous examples are the
Turin Shroud, the Ice Man from 3,300 BC found in the Otztal
Alps in 1991, and bottles of fake 'old' whisky.
The most energetic protons from space create showers of sub-
atomic particles, most of which decay or are absorbed by the
atmosphere. The only radiation that reaches the ground is a flux
of muons18 and this is responsible for the cosmic slice in Figure
5. At sea level this delivers about 0.6 millisievert per year in
polar latitudes. In equatorial regions the flux is three times
smaller because of the shielding effect of the Earth's magnetic
field, which sweeps incoming protons away into space. The
radiation rises rapidly with height above sea level because of the
reduced absorption by the atmosphere.
In the very distant past the flux of radiation was much greater.
The Universe itself started from a simultaneous explosion,
known as the Big Bang, 13.8 billion years ago. The early stages
were dominated by particle physics of the kind studied on a
small scale at modern research accelerators. After a few minutes
the explosion had cooled sufficiently for the distinct nuclei of
hydrogen and helium to emerge. But, until 300,000 years later it
remained so hot that electrons and nuclei were not bound
together. As it cooled further, the heat radiation became non-
ionising and atoms of hydrogen and helium appeared for the first
time.
Over the next few billion years galaxies and stars formed. These
evolved through nuclear fusion in massive stars, creating the
heavier atoms that we see around us today, a process called
nucleosynthesis. Slowly, as the Universe began to settle down,
systems of planets formed in the neighbourhood of rotating stars,
often composed of lumps of nuclear ash made spherical by
gravity. Interplanetary fragments collided with the larger planets
and their moons, leaving craters on the surfaces of those without
an atmosphere.
18 The muon is an unstable subatomic particle with the properties of a heavy
electron, which decays with a half-life of 1.4 microseconds.
Natural environment 51
This all happened before about 4.5 billion years ago when the
Earth was formed and activity became quieter. Ionising radiation
still reaches the Earth from hot stars in the form of heat radiation
and from exceptional acceleration processes elsewhere in the
Universe.
Meanwhile nuclei of the four radioactive series (described in
Table 2 on page 42) created during the period of nucleosynthesis
continued to decay, although the neptunium series died out long
ago. The other three are still going. The abundance of thorium in
the Earth's crust is 3 to 10 parts per million by weight. For
uranium-238 it is 1.8 to 2.7 parts per million. These values vary
depending on the rock formation. There are significant quantities
of uranium in sea water because its salts are soluble, unlike those
of thorium. Within all natural uranium ores the ratio of uranium-
235 to uranium-238 is currently 0.7%. This varies very little as
the physical and chemical properties of the two isotopes are
almost identical (see page 27) and their relative proportion does
not naturally become diluted or enriched except through decay.
Highly refined materials may be free of radioactivity but they are
exceptional. Wood, concrete, glass and metals are all radioactive
to some degree because they contain traces of natural
radioisotopes.
A few primordial radioactive nuclei are not members of the four
radioactive series. The most abundant is potassium-40 with a
half-life of 1.27 billion years. It decays by beta decay, either to
calcium-40 or to argon-40, both of which are stable. Potassium is
a common element in the Earth's crust (2.1% by weight) and in
sea water (0.044%). The regular stable isotope, potassium-39, is
the most common and the unstable potassium-40 is only a tiny
proportion (0.01117%). Potassium is essential to the electro-
chemistry of living cells and forms about 0.15 kg of human body
weight. Other radioactive isotopes, such as carbon-14, with
shorter lives are found in the environment too, being created
afresh by cosmic radiation. Thus carbon-14 and potassium-40
between them account for 7,500 radioactive decays per second in
52 Chapter 4 Ionising Radiation
an adult human body. The annual dose from such internal
radiation is 0.25 millisievert (see Figure 5).
Two billion years ago the radiation from these nuclei was much
as it is today, except that the proportion of uranium-235 in
natural uranium was higher. In fact, from the measured half-lives
(see Table 2) it is straightforward to calculate that at that time
natural uranium contained 3.5% of the faster decaying
uranium-235. Today, some nuclear fission reactors use uranium
fuel artificially enriched to this proportion, with ordinary water
acting as coolant and moderator, in order to maintain a steady
nuclear chain reaction. Two billion years ago such enriched fuel
and water were available naturally, so that a similar nuclear
reactor could occur by itself under the right circumstances. Clear
evidence that this actually happened has been found in Gabon,
West Africa. This natural nuclear fission reactor, known as the
Oklo Reactor [6, 7], ran on its own for up to a million years. In
our own time the extraordinary evidence came to light with the
discovery that the relative abundance of uranium-235 in this
particularly rich uranium deposit lay outside the narrow range
found elsewhere in the world. It has been shown that the missing
uranium-235 was consumed in the natural reactor cores and that
the remains of the resulting fission products are still to be found
there. This is significant because this reactor was not
decommissioned and buried in a specially selected underground
site at great cost. The residue of the uranium fuel and its fission
products were left where they lay and have not moved in two
billion years. This is an important demonstration of the stability
that nuclear waste deposits can have over extremely long
periods.
Radiation from radioactive sources in materials such as water,
soil or rock reaches the external environment mainly in the form
of gamma radiation and radon alpha and beta radiation are
mostly absorbed. Radon is a noble gas with little chemical
activity, like helium, neon, argon, krypton and xenon. The
isotope radon-222 has a half-life 3.82 days and is formed in the
uranium-238 series (Table 3). This radioactive gas, once it has
Natural environment 53
been released into the air, can be inhaled into the lungs where it
may be absorbed and decay by alpha emission leaving polonium-
218, a non-volatile isotope which decays with a sequence of
further alpha emissions. Such alpha radiation has a short range
and deposits all its energy in the lungs. On this basis radon
would be expected to be a significant source of risk to the health
of those who ingest it every day where they live and work.
Figure 5 shows radon as a large component of the average
human radiation exposure. In fact, it is larger by factors of five or
more in certain places, depending on the local geology. A
significant question, discussed in Chapter 7, is whether this large
variation is reflected in the local incidence of lung cancer.
55
Chapter 5 Safety and Damage
Poison is in everything, and no thing is without poison.
The dosage makes it either a poison or a remedy.
Paracelsus, physician and botanist (1493–1541)
Proportionate effects
We appear to live in a causal world where what happens next is
determined by what is happening now. This relationship of cause
and effect is easiest to follow if each element of the cause
determines its own part of the effect. Such a relation between
cause and effect is called linear in mathematical physics. In fact,
linearity is really a rather basic and simple idea that does not
require fancy mathematics to appreciate.
Here is a simple example. If you are selling me apples and pears,
the amount of money that I hand over depends on the number of
apples and pears that I buy. Normally, the amount of money that
you charge me will simply equal the number of pears times the
cost per pear, plus the number of apples times the cost per apple.
That is linear – the cost of an extra apple does not depend on the
number that I have already bought, or on the number of pears.
But it could be otherwise. You could say that extra apples after
the first dozen are half price, or that pears are more expensive
unless I buy apples too – or that you will pay me to take the first
dozen pears, but then charge for further ones. Such pricing is
non-linear, and modern supermarkets have certainly learned how
to use it to encourage us to buy!
The standard test of linearity is the Superposition Principle. If the
total cost is the same as the sum of the cost of buying each apple
and pear separately, the pricing is linear. It is true that, if
linearity applies, a graph of cost against the number of apples is a
straight line – but the reverse is not true. If the slope of the graph
56 Chapter 5 Safety and Damage
for apples changes depending on the number of pears, the pricing
is non-linear.
Many aspects of the world described by modern physics are
linear or nearly so. Indeed the scientific method is most useful if
we can disassemble a problem into pieces and then add the
contributions of each back together and still have the right
answer.19 This is the feature that makes telecommunications and
audio systems valuable linearity makes it possible to work
backwards and reconstruct the input signals from the output – for
example, to hear the strings as separate from the wind
instruments when listening to a piece of music. It is linearity that
makes it possible to solve problems in quantum mechanics, and
that allows light waves and radio-waves to cross through one
another without any effect. If what is transmitted on one TV
station affected what was received on all the others, that would
be non-linear and not much use either! Similarly, if what we
see when we look at one object was influenced to some extent by
light from objects that we are not looking at, that would be non-
linear too. Fortunately that is not the case for light and
electromagnetism. A linear world is like a world of LEGO®,
easy to work with scientifically because it is built up of separate
bricks.
But not all causes generate effects independently in a linear
fashion. Take social behaviour, for instance. The way in which
people interact one-to-one gives no information on how they
behave as a crowd. So, for example, most aspects of economics
are non-linear. Non-linearity occurs in physics too, most
obviously in the turbulent flow of fluids.
On page 44 we explained how the relationship between radiation
dose and clinical damage has been assumed to be linear the
LNT model. So the question is whether this linear assumption is
correct, or not. It will be seen later, that some data fit with the
Superposition Principle and some do not, but that linearity is not
19 Much use is made of approximations and changes of mathematical
perspective in the description of modern physics, all with the aim of making
problems linear, since then they are far easier to solve and understand.
Proportionate effects 57
what we should expect from an understanding of modern
biology. The science is about understanding what is occurring at
the biological level, not about fitting straight lines to data, or
even curves. There was a similar situation in early planetary
science. The real reason for discarding Earth-centric cosmology
in favour of the Copernican theory was simplicity the Earth-
centric cosmology may have contrived to fit the data, but its
calculations lacked simple explanation.
Analysing non-linear systems is possible but not as simple as
linear ones. To go back to the apples and pears if I have
accepted your offer of Buy one get one free or One pound off
apples when you buy pears, I may be content with my purchase,
but I cannot say how much the apples cost me because the
question does not have a simple answer.
Linearity is about independent causes. The superposition test
asks two questions. If the response to cause A is α and the
response to cause B is β, is the response to 2A just 2α; and is the
response to A and B together just α+β? If either of these is
untrue, the response is non-linear. We already had an example on
page 35 – the response to different volumes of music. It can be
dangerous to impress linearity on our view of a problem, just
because it makes the assessment easier. A more pragmatic view
is suggested by the words of Paracelsus in the early 16th century
quoted at the start of the chapter. He understood that the hazard –
or benefit – associated with any given action or dosage is often
non-linear. Whether administering a drug at a certain dose level
is beneficial or harmful is a matter for experimental evidence. In
popular parlance we say you can have too much of a good thing.
Looking at the same situation from the other end, it may be that a
little of a bad thing will do no harm, and may even do some
good.
Balancing risks
Life presents choices, whether to individuals or to society as a
whole. Any choice carries a certain risk and these have to be
balanced. Two of these choices involving ionising radiation are
58 Chapter 5 Safety and Damage
illustrated in Figure 6. When a malignant tumour is diagnosed a
patient must choose between the likely course of the cancer and a
dose of radiotherapy with its radiation side effects (if not a
different treatment). Medical advice may guide but there may be
mortal dangers either way. Nevertheless, a decision in favour of
radiotherapy often results in an extension of enjoyable life, in
spite of the high doses that treatment involves. This is a decision
for the individual.
Figure 6 Choices involving ionising radiation. (a) Balancing
risk between the effects of radiotherapy and cancer. (b)
Balancing risk between nuclear waste and carbon dioxide
emission.
An equally significant choice faces society collectively –
whether to minimise the impact of climate change by opting for
Balancing risks 59
nuclear power with its attendant waste and perceived radiation
risk or to avoid any effect of radiation while incurring
significant greenhouse gas emission. Altho