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Historical Note: Bibha Chowdhuri – Her Cosmic Ray Studies in Manchester

Historical Note
Bibha Chowdhuri – Her Cosmic Ray Studies in Manchester
S C Roy* and Rajinder Singh**
(Received 26 February 2018)
The name Biva/Bibha Chowdhuri/Chaudhury (1913-1991) appears in literature whenever there
is a talk of the experimental discovery of mesotrons/mesons and there are enough reasons for it. She can
be seen as the first Indian woman in the field of high energy particle physics. She obtained her Ph.D.
degree from the University of Manchester on cosmic rays, in the laboratory of the Nobel Laureate PMS
Blackett. However, very little is known about her Ph.D. work at Manchester. The present article focuses
on her work at Manchester leading to her Ph.D. degree and discusses the acceptance of her work by the
international scientific community.
Key words: Bibha Chowdhuri, Bose Institute, D M Bose, Extensive air showers, Mesons.
*Science and Culture, Indian Science News Association, 92 A.P.C. Road, Kolkata-7000009, Email:
**Research Group – Physics Education and History of Science, University of Oldenburg, Germany, Email: Rajinder.singh@uni-
We found a variation in the spelling of her
name: Biva Chaudhuri, Bibha Chowdhuri, B
Chaudhury. We have followed the spelling Bibha
Chowdhuri (abbreviated as BC) in this article as
we found the same on the title page of her Ph.D.
thesis (Fig. 1), which can therefore be considered
as her authentic spelling.
Bibha Chowdhuri was born at a time when
higher studies in science in India was at its
beginning. BC did her M.Sc. in physics from the
Calcutta University in the year 1936 and joined
Calcutta University immediately after under
Professor D M Bose for her Ph.D. degree. When
D M Bose joined Bose Institute as its Director
after the death of Sir J C Bose in 1937, his students
including BC, followed him to join Bose Institute.
At that time very few women ventured to
undertake postgraduate studies in physics and this
was more true in India. Seen in Indian context,
Indian Journal of History of Science, 53.3 (2018) 356-373 DOI: 10.16943/ijhs/2018/v53i3/49466
Fig. 1. Cover page of Bibha Chowdhuri’s Ph.D. thesis
submitted in 1949 at Manchester University, UK. Note that
the thesis was typed in a manual typewriter. (Courtesy:
Manchester University, UK)
the courage exhibited by BC to undertake physics
research for a Ph.D. degree is legendary. An
extensive study by R M Godbole and R
Ramaswamy shows that until 2012 only 3.3% of
INSA Fellows in physics were women whereas in
Chemistry this number is 0.08%. The total number
for all subjects together is 5%. “The percentage
of women in the fellowship for the IASc is 7%,
for INSA it is 5%, the numbers being fairly similar
in other academies. In TWAS as well, the
percentage of women Fellows is around 7%.”
(Godbole and Ramaswamy, 2015, pp. 67-84).
History is replete with many instances
where achievements made by women in science
and other fields had, in general, been
underestimated and marginalised and due credit
had been denied. Therefore, it is not surprising
that very little has been written on Indian women
scientists.1 Rohini M Godbole, Centre for High
Energy Physics, Indian Institute of Science (IISc),
Bangalore, in a conference said that
‘there was no such thing as women’s science. ….
The participation of women in science in India is
meagre. I will look forward to a day when people
don’t say, ‘I’m a woman scientist, instead they say
I’m a scientist’ (Basu, 2016).’
Not long ago, the Indian Academy of
Sciences, Bangalore brought out ‘Lilavati’s
Daughters: The Women Scientists of India
(Godbole and Ramaswamy, 2009, pp. 68-69). The
book deals with 100 women scientists from
Victorian era to present-day. S Raychaudhuri,
TIFR, in his writing on ‘The early days of particle
physics in India’ states: ‘Even the landmark
volume on Indian women scientists, Lilavati’s
Daughters …, does not feature her [B.
Chowdhuri’s] name (Raychaudhuri, 2016, pp.
148-172).’ None of the three Science Academies
of India elected her to be a Fellow.2 Recently, in
Scientifically Yours - Selected Indian Women
Scientists’, life history of 13 Indian women
scientists has been explored (Bhattacharya, 2017,
pp.351-352). This also does not include the name
of BC.
To the best of our knowledge almost
nothing has been written on Bibha Chowdhuri who
was the first Indian woman to work on cosmic
rays or rather high energy particle physics. Her
life and scientific work will be explored by the
present authors in a book to be published soon
(Singh and Roy, 2018). The present article focuses
on her scientific work done in the U.K. and the
reception of her work by the western scientific
community. We have also tried to explore the
reasons for her courageous choice of profession.
Almost nothing is known or available
about Bibha Chowdhuri’s life, except a brief life
sketch published in a Bengali magazine by
Ranatosh Chakrabarti (2005, pp.159-161), Roy
and Singh (2018, pp. 159-164). Most of the
information about Bibha Chowdhuri and her
family presented in this article has been obtained
from Ms. Rita Sarkar (nee Palit), wife of my
(Roy’s) physicist friend Dipak Sarkar. Rita Sarkar
had been in close association with the family of
Bibha Chowdhuri for a long time. The Palit family
and Chowdhuri family lived in two adjacent
houses on Upper Circular Road (now Acharya
Prafulla Chandra Road) and then lived in the same
premises on Broad Street, Kolkata (erstwhile
Calcutta). Bibha Chowdhuri died in her Broad
Street residence on 2nd June, 1991.
Bibha Chowdhuri was born in 1913. Her
father Banku Behari Chowdhuri was a doctor by
profession. They were zamindars of Bhandarhati
in the district of Hooghly in Bengal. Her mother
1For the study of three Indian women physicists, who worked in C.V. Raman’s laboratory and Raman’s reaction to them, see, Sur
A., Dispersed Radiance: caste, gender and modern science in India, Navayana Publishing, New Delhi 2011. Sur A. Dispersed
Radiance - Women Scientists in C. V. Raman’s Laboratory, Meridians 1, 95-127, 2001. Other worth reading book is by Krishna
S., Chadha G., Eds., Feminists and Science: Critiques and Changing Perspectives in India, Sage Publications India Pvt. Ltd.,
New Delhi 2017.
2We are thankful to Professor Arnab Rai Chaudhuri, Indian Institute of Sciences, Bangalore, for this information.
was Urmila Devi, daughter of Girish Chandra
Mazumdar. Urmila’s sister Nirmala was married
to Sir Nilratan Sircar, a famous doctor, after whose
name the Campbell Medical School at Sealdah,
Kolkata was renamed. Urmila Devi’s family was
Brahmo (who followed the doctrines of Brahmo
Samaj). Brahmo Samaj discarded the multi-god
Hindu beliefs, idolatory and rituals and focussed
on education especially among women. Her
upbringing, as we understand, was influenced by
the doctrines and attitudes of Brahmo Samaj.
In order to marry Urmila, Banku Behari
had to convert to the Brahmo doctrine and became
a member of Brahmo Samaj. This change of
religious following costed him badly and he had
to lose the right to the property of his parents.
Banku Behari had five daughters and one
son. Eldest daughter Roma Chowdhuri was a
teacher at Brahmo Balika Vidyalaya (Brahmo
Girls’ School) in Kolkata. After retirement she
started a nursery school “Southern Nursery and
Infant School” on Ritchie Road (now known as
Pankaj Mallik Sarani) in Kolkata. The school is
still running. His second daughter died at an early
age. His third daughter was Bibha Chowdhuri born
in 1913. The fourth daughter of Banku Behari was
Leela Chowdhuri who worked in Tea Board in
Kolkata for a while. Later she went to England to
be trained in the Montessori system of education.
She retired as Principal of Jadavpur Vidyapith
(Junior Section) in Kolkata. The youngest
daughter of Banku Behari was Uma Chowdhuri
who did post-graduation in psychology from Calcutta
University and did her Ph.D. in social psychology
from the USA. After her return from the USA,
she took a job in the Health Department of West
Bengal Government. She worked on the social
aspects of aborigines (like jarwās in Andaman),
santhāls etc. Ranjit Chowdhuri was the only son
and youngest of all, who obtained his engineering
degree from Jadavpur University in Kolkata. He
was Sales Manager in English Electrics.
All her siblings and she herself remained
un-married. One probable reason, as we feel, is
that Brahmos were a small community and they
preferred to make matches within their own
community. As because they were alienated from
the main stream Hindu society, getting a proper
match within the Brahmo community was
difficult. In general, “The proportion of women
scientists who never married (14%) is higher than
that of similar male scientists (2.5%), …” . 3
Immediately after obtaining her M.Sc.
degree in physics from Calcutta University in
1936, Bibha Chowdhuri started working under D
M Bose, Palit Professor of Physics, at Calcutta
University. When D M Bose joined Bose Institute
as its Director in late 1937, all his research
students, including BC, also joined Bose Institute.
According to M S Sinha, her colleague at Bose
Institute and batch-mate of M.Sc. class, she
worked at Bose Institute during the period 1939-
42 (Sinha, 1986, pp.359-367). However, we find
a report titled ‘Studies in nuclear disintegration
by cosmic rays using photographic plate method’
published in the Annual Report of Bose Institute
for the year 1938-39 which indicates that she was
in Bose Institute before 1939.
She made a significant contribution
towards the discovery of mesons using
photographic plates while in Bose Institute and
published three papers consecutively in Nature
(Bose and Chowdhuri, 1941, pp. 240-241; 259-
260; 1942, 302). She was involved in a work in
which several batches of Ilford half-tone plate
were exposed to cosmic rays at two stations
located at different altitudes: Sandakphu (12000
ft.) and Pharijong (15000 ft). After developing and
processing these plates, BC scanned them under
the microscope and found that many of the
recorded tracks were curved. Such curvature was
not observed in α-tracks from Po or proton tracks
obtained from elastic scattering of neutrons from
3, Feb. 16, 2018.
a radium-beryllium radioactive source recorded
in the same emulsion. The curvature in these tracks
was due to multiple Coulomb scattering of these
particles of various momenta by nuclei present in
the emulsion and the variable grain spacing along
the tracks was due to variable velocity of the
charged particle as it slows down in the emulsion.
A careful analysis of the mean Coulomb scattering
and mean grain spacing of measured track lengths
in the emulsion led to a division of the tracks into
two groups having the same mean grain spacing
(5 – 6) µ (same velocity) but different mean angles
of scattering. These estimates finally gave the
average mass of the first group (almost all recorded
at Sandakphu) as 221 me and the average mass of
the other group (all recorded at Pharijong) as 278
me (me is mass of electron). Furthermore, the
particles of higher mass were recorded at a higher
altitude leading to a tentative conclusion that those
recorded at lower altitude (Sandakphu) were the
decay products of those recorded at the higher
altitude (Pharijong). These investigations could
not be continued due to the non-availability of
more sensitive emulsion plates during the war
years. Seven years after this discovery of mesons
by D M Bose and Bibha Chowdhuri, C F Powell
made the same discovery of pions and muons and
further decay of muons to electrons using C2 and
G5 electron sensitive plates using the same
technique as was used by D M Bose and Bibha
Chowdhuri and won the Nobel Prize.
Bibha Chowdhuri left Bose Institute and
joined the cosmic ray research laboratory of P M
S Blackett at the University of Manchester in 1945.
It is unclear why BC, in spite of her significant
work, did not go for a Ph.D. degree from the
University of Calcutta. BC started working on the
extensive air showers in cosmic rays at
Before describing Bibha Chowdhuri’s
work on extensive air showers in cosmic rays at
Manchester, it is imperative to understand the
extensive air showers phenomenon and its
importance in particle physics at that time. Cosmic
rays are high-speed energetic particles accelerated
outside the solar system. These possess the highest
known energy of individual particles in the
universe. The energy spectrum of primary cosmic
rays extends from 1 GeV (109 eV) to above 1020
eV. At energies above 1 GeV most of the cosmic
rays are accelerated in our galaxy. In this energy
range the cosmic ray flux is dominated by H
nuclei, i.e. protons. The best way to study cosmic
rays is to detect them outside the atmosphere. This
is the reason why cosmic ray studies are made at
high altitudes. This is not, however, always
possible since the flux of cosmic rays is proportional
to E-2.7. At energies above 100 TeV their flux is so
small that no precise measurement is possible even
in a detector size that fits in a balloon or a space
craft. However, at this energy sufficient particles
are produced in the atmosphere as secondary to
the incoming primary cosmic rays. Some of the
secondaries reach mountain altitudes and as the
energy of the primary increases, they can even be
found at sea level. The transverse momentum
acquired by secondary particles at production and
the scattering which the shower particles, in
particular, undergo through interactions with the
material of the atmosphere are such that the
secondaries are spread over significant areas at
the observational level. The phenomenon of the
almost simultaneous arrival of many particles over
a large area is called an Extensive Air Shower
(EAS). At energies around 1015 eV about 106
particles cover approximately 104 m2 while at 1020
eV some 1011 particles are spread over about 10
km2 (Kampert and Watson, 2012).
In order to understand the nature of
primary particles it is important to study the nature
of air shower. Till the discovery of extensive air
showers, the maximum energy of primary particles
was conceived to be 10 GeV. The discovery of
extensive air showers in 1938, however, radically
changed this situation with the highest energy
being pushed up by about 5 orders of magnitude,
probably the single largest advance to our
knowledge of energy scales ever made. It is now
known that the energy spectrum extends to beyond
1020 eV but it has taken over 60 years to
consolidate this picture (Kampert and Watson,
It is worth mentioning that when the
shower phenomenon was initially observed, only
the muon was known, in addition to the proton,
neutron, electron and positron, so that a realistic
understanding of shower development had to wait
until the discovery of the charged pion and its
decay chain in 1947 and of the neutral pion in
1950. Initially it was thought that showers were
initiated by electrons and/or photons. Once it was
recognised that the initiating particle was almost
always a proton or a nucleus, the first steps in
understanding the nuclear cascade focussed on
such matters as whether a proton would lose all
or only part of its energy in a nuclear collision or
how many pions were radiated in such a collision.
A combination of observations in air showers
made using Geiger counters and cloud chambers
from studies in nuclear emulsions and of early
accelerator information was used to resolve the
The technical development of significant
importance in the study of EAS in cosmic rays
was the invention of coincidence technique by
Walter Bothe in late 1920s for which he was
awarded the Nobel Prize in 1954 (Bothe, 1929,
pp. 1-5). In 1929, W Bothe and W Kolhörster,
Berlin, observed that when two Geiger-Müller
counters are placed above each other, a part of the
counts/pulses was observed in both the
instruments which (counters) were due to the same
particles. They concluded that the cosmic rays
have a particle nature. They also observed the
decrease of coincidences because of absorption
by materials (Bothe and Kolhörster, 1929, pp.751–
In 1931, J N Hummel, Germany,
developed a circuit which enabled registering
coincidence-impulse in more than one Geiger-
Müller-counters simultaneously (Hummel, 1931,
pp. 765-781). Bruno Rossi from Italy, who worked
with German physicists observed that with
increase of thickness the rate of coincidence of
such particles first increases in lead with the
increase of thickness, and then starts decreasing
with increase of the thickness of the absorber
(Rossi, 1933, pp.151-178). This led to the famous
Rossi curve. In 1934, M Ackemann, Germany,
observed a second maximum in Rossi-curve. He
attributed this effect to the hard component of the
radiation (Ackemman, 1934, pp.169-170).
In 1937, K Schmeiser and W Bothe
observed a small number of coincidences in an
unshielded counter array, which were separated
by a few metres (Schmeiser and Bothe, 1938,
pp.61-177). A year later, i.e. in 1938, H Euler and
Werner Heisenberg published on the absorption
of the components of the cosmic rays. They gave
a theory on the behaviour and the secondary effects
of the fast protons and neutrons, and the heavy
electrons. The theoretical results were then
compared with the experimental results in which,
first the spectra of the individual particles and their
transformation in the atmosphere, and then its
secondary effects like smaller showers, bigger
showers, collisions and nuclear transformation
were compared (Euler and Heisenberg, 1938, pp.
Bibha Chowdhuri joined the cosmic ray
laboratory of would-be Nobel Laureate P M S
Blackett in 1945 (Blackett was awarded Nobel
Prize in 1948) at a time when studies on extensive
air showers in cosmic rays were one of the most
important investigations in particle physics.
For instance, a local newspaper (Fig. 2)
The Manchester Herald reported her work under
the title: ‘Meet India’s New Woman Scientist –
She has an eye for cosmic rays.’ The article was
written by Birgit Maxwell based on an interview
taken on her. BC said:
Women are terrified of physics - that is the trouble.
It is a tragedy that we have so few women physicists
today. In this age when science, and physics
particularly, is more important than ever, women
should study atomic power; if they don’t
understand how it works, how can they help decide
how it should be used? I can count the women
physicists I know, both in India and England, on
the fingers of one hand. At school scientifically-
inclined girls choose Chemistry; perhaps because
a really sound grasp of Higher Mathematics is one
essential of any Physicist’s equipment.
The reporter wrote:
Mathematics presented no problem to Miss
Chowdhuri; she took to the study ‘like a duck to
water’. … If you ask her why she chose to
specialise in Physics, she says simply: ‘I followed
my inclination.’
About her research work, it was noted:
‘Bibha Chowdhuri’s particular programme is the
study of extensive air showers caused when cosmic
rays enter the earth’s atmosphere from the
interstellar spaces. A cosmic ray touching a nuclear
particle produces a shower, which extends itself
by scattering in lower altitudes.” “Miss
Chowdhuri is trying to discover the how, why
and wherefore of this process; ultimately she
hopes to estimate the exact extent of scattering
and the number of particles involved.The
extensive air showers Miss Chowdhuri is
investigating, contain both kinds, though mainly
soft ones; she hopes to find out in exactly what
proportions.” (emphasis in original).
The journalist said that cosmic ray research
is closely related to atomic physics; and is an
abstruse subject to understand, but under Miss
Chowdhuri’s expert guidance “I began to
understand something of the question.”
BC was asked how many pictures were
taken by her during the last three years. ‘Literally
thousands’, she replied. As far as her future plan
was concerned, we read: ‘As soon as her thesis
on ‘Extensive Air Showers’ is completed, some
time in the next few months, Miss Chowdhuri will
take her Degree of Philosophy. She then intends
to return to Calcutta and continue her research
The journalist was of the opinion: ‘I am
certain that she will undertake another task too
when she gets home; she will try to persuade more
Indian girls to follow in her own distinguished
footsteps.’ As we shall see later, her dream never
turned to reality.
She submitted her PhD thesis in early
1949. She ‘was examined by Lajos Janossy
(external) and J G Wilson (internal) in March
1949. The minutes of the General Board of
Faculties confirm that she successfully defended
her thesis.’ What puzzled us is the long gap of
about three years between the submission of her
thesis and obtaining her Ph.D. degree. When
enquired about this we got the following reply
from the University of Manchester: ‘It is clear that
she successfully completed her thesis in March
1949. The gap between this and the actual
graduation on 3 December 1952 is not obviously
Fig. 2. Bibha Chowdhuri’s interview and her work at
Manchester, UK was reported in the The Manchester Herald
by Bright Maxwell. (Courtsey: Prof. A K Ganguli)
explicable but may have been due to demands of
“employment/ residence. There is nothing to
suggest this was due to problems with the thesis
(Peters, 2017).’
While Homi J Bhabha was looking for
young scientists for the newly established TIFR
(Tata Institute of Fundamental Research,
Bombay), he asked J H Wilson, who was one of
BC’s Ph.D. examiners, about his opinion of Bibha
Chowdhuri. It seems that his opinion was positive,
as BC was offered a job at the TIFR.
B V Sreekantan of National Institute of
Advanced Studies, Bangalore, who worked at the
TIFR, wrote to us that BC joined the Institute in
1949 and was there till 1957 or so. After leaving
TIFR she spent a year abroad and then spent
several years at Physical Research Laboratory,
Ahmedabad before going back to Kolkata4. In an
article he mentioned that B Peters, M G K Menon,
S Biswas, (Miss) Bibha Chowdhuri, Appa Rao,
Gaurang Yodh joined TIFR at about the same time
and gave a big boost to its cosmic ray activity.5
To find out more about her stay abroad,
we contacted the University of Michigan where
she worked with Prof. J G Wilson. Miss Caroline
Barraco - Reference Assistant, Bentley Historical
Library, communicated to us:
I am responding to your reference request regarding
Bibha Chowdhuri. I wasn’t able to find much more
on her time at the university. There’s one other note
about her in the Regents Proceedings, from the
March 1957 meeting. It says she was given a
stipend of $2,500 in her role as a Physics
Department Visiting Lecturer”. 6
India’s renowned Plasma Physics scientist,
Yogesh C Saxena, Sr. Prof. (Retired), Institute for
Plasma Research, Gandhinagar, Gujarat, who did
his Ph. D. under the guidance of BC (technically
he did his Ph.D. under Vikram Sarabhai because
BC was not a registered guide at that time)
My first meeting with Bibha Chowdhuri was during
the course work, which I was taking as a Research
Scholar, at Physical Research Laboratory (PRL),
Ahmedabad in year 1964. She gave us a course on
Interaction of High Energy Particle and Matter and
I was highly impressed by the way she taught.7
Saxena recalls:
I was more inclined to do experimental work and I
learnt that Bibha Chowdhuri was planning a new
experiment on ‘Detection of High Energy Muons
Associated with Extensive Air Showers of Cosmic
Rays’ and I decided to join her as a student, to the
surprise of many of my colleagues and seniors. At
that time there was a feeling in the Research
Scholars at PRL that she was a tough person to
work with, which turned out to be totally misplaced
as I discovered during my work with her for next
several years.
As far as her teaching qualities are
concerned, Saxena writes:
She was a very good teacher and took care that I
kept up with the required studies while doing the
experiment and collecting data and she provided
the books and papers for that. She had a very good
understanding of the subject and was of great help
in analyzing and interpretation of the experimental
data. In addition to the Cosmic Rays and High
energy Physics, she taught me French language,
which was part of the Ph.D. Curriculum at Gujarat
University at that time.
Unfortunately, she did not care much about
publishing her work in international journals. One
4Prof. B.V. Sreekantan, Private communication, Jan. 2, 2018.
5Sreekantan B.V., Sixty Years of the Tata Institute of Fundamental Research 1945-2005; The role of young men in the creation
and development of this institute, Lecture delivered on 18th August 2005 at the Tata Institute of Fundamentals Research as one
of JRD Tata Birth Centenary Events organized by TIFR Alumni Association.
6Caroline Barraco - Reference Assistant, Bentley Historical Library, 150 Beal Avenue Ann Arbor, Michigan, U.S.A. Private
communication, Oct. 23, 2017.
7Yogesh Chandra Saxena, Sr. Prof. (Retired), Institute for Plasma Research, Gandhinagar, Gujarat. Private communication, Dec.
28, 2017
of her students recalled: ‘While the research work
she carried out was of high quality, she was a bit
conservative in her approach to publication and
she submitted most of her papers from this work
to Indian Academy Journal for publication.’
She was intensely involved with the Kolar
Gold mine experiment using an indigenously
developed detector to be placed at a depth of 700
ft. and to be operated in conjunction with the
Extensive Air Shower array of TIFR on the
ground. She worked out the modalities of the
experiment with TIFR group and arranged the site
underground as well as rented a bungalow for her
and her assistants.
After the Kolar Gold Mine experiment, she
had a plan to set up a new experiment at Mount
Abu on radio frequency emissions associated with
Extensive Air Showers. She had discussed these
plans in detail with Vikram Sarabhai before his
untimely demise. ‘Changes at PRL afterwards
results in change of directions and the program
and PRL did not permit her to take up that
experiment. She took a voluntary retirement from
PRL and moved to Kolkata to continue her pursuit
of High Energy Physics.’
She continued to be associated with
physics research at Saha Institute of Nuclear
Physics in Kolkata till her death and also
participated in seminars and conferences. She
published several papers during this time jointly
with other scientists of different institutes of
Bibha Chowdhuri started her thesis with
the historical survey of extensive air showers, a
short review on ‘General structure of extensive
showers’, ‘Spread of showers, ‘Origin of showers’
and ‘Penetrating components in extensive
showers.’ The experimental set up used for the
study of density spectrum of air showers is shown
in Fig. 3. About the recording technique she wrote:
A coincidence is recorded between the shielded
counter tray and the four other counters, if an air
shower selected by the four-fold array is
accompanied by a penetrating particle. These
penetrating counts were recorded on the cloud
chamber photographs by means of an indicator
Fig. 3. A, B, C and D are four counters to select samples of
Extensive Air Showers. The three counters are placed on
the three sides of an equilateral triangle, whereas the 4th
between the two side counters. The area of each counter
was 200 cm2. Cc’ is the position of the cloud chamber, which
has an effective area approximately 70 cm2. P- counter is
shielded by lead
To study the distribution of particle
densities a large number of photographs by
triggering arrangement were taken. The number
of tracks on each photograph and effective
collecting area of cloud chamber enabled her to
calculate the densities of extensive showers. Out
of thousands of photographs, she selected 1460
photographs, which fulfilled particular conditions
such as, they did not contain tracks which were
either too old (too broad photographic lines) or
too new (too thin). The photographs were
classified into group A and B. In group ‘A’ ‘The
particle densities in these showers were observed
to have no definite relationship to the density of
penetrating events.’(Fig. 4).
In group ‘B’, ‘all the photographs showing
tracks which do not meet in any local point were
included.’ For this group she produced a table on
‘total number of shower photos with a particular
number of tracks and number of photos
accompanied by penetrating events. From her
results she concluded that ‘the distribution of
particle densities in Extensive Showers obeys a
power law.’ She determined the exponent of the
integral density spectrum (Fig. 5) (Chowdhuri,
1948, pp. 21-25).
Further, she studied the three-fold, four-
fold and five-fold coincidences with iron and lead
plates as absorbers. In one series, coincidences of
the penetrating set with the four-fold counters were
recorded with the shielded counters under a 15
cm. lead shield. In the second series, the top 5 cm
of lead was exchanged for 16 cm. of iron. She
found that the density spectrum as well as the
counting rates under the two different conditions
were the same (Chowdhuri, 1948, pp. 25-26). She
was of the opinion that according to the cascade
At lower altitude, due to the effect of cascade
multiplication the density of particles in a shower
will be increased, so the probability of recording
the dense part of a shower will increase with
decreasing altitude. Thus the whole spectrum will
be slightly biased towards large showers at lower
altitudes and thus the value of the exponent will
be slightly reduced (Chowdhuri, 1948, pp. 40-41).
She stated that even if all these results and
her experimental results are taken into account:
‘we find no clear evidence whether this variation
in exponent from 1.7 – 1.5 is due to the effective
altitude or is a mere fluctuation in the experimental
results (Chowdhuri, 1948, pp. 40-41).’ She
proposed – before definite conclusion can be
reached, more experiments at different altitudes
should be performed.
As far as the density spectrum of
penetrating Extensive Showers was concerned she
stated that due to particular experimental
Fig. 4. A large shower produced in the material of cloud
chamber, which is not accompanied by a penetrating event.
(Credit: University of Manchester).
Fig. 5. Left: Density distribution of all observed Showers. The broken line represents the curve B on the right side. Right:
Density distribution of Showers with penetrating events (right). (Credit: University of Manchester)
conditions: (i) ‘The exponent of extensively
penetrating Showers was found to be of the same
order as for all particle extensive Showers.
However, due to the effect of shielding, the values
of the counting rates of the penetrating extensive
Showers should be relatively lower than those of
all particle Showers.’(ii) ‘It was observed that
penetrating extensive Showers were 300 times less
frequent than all particle extensive Showers of the
same density (Chowdhuri, 1948, p. 45).’
BC also studied ‘the probability of a
Shower particle being accompanied by a
penetrating event, its dependence on the density
of the shower particles and on the atomic number
of the local absorber.’ For finding ‘the probability
of a Shower particle being accompanied by a
penetrating event’ she calculated the ratio Cp/Cs,
that is, the ratio of the penetrating shower counting
rates to the total shower counting rate; which were
determined by her experimentally. With this ratio
it was possible to find the probability (k) of a soft
particle being accompanied by a penetrating event.
The values for five-fold and four-fold coincidences
vs. ε (which is the product of probability k and
area of shielded counters) determined by her are
shown in Fig. 6.
BC determined the ‘dependence of ‘k’ on
the particle densities of an Extensive Shower.’
From her experimental arrangement and results,
she concluded that ‘the probability of a shower
particle being accompanied by a penetrating event
is approximately constant for the whole range of
particle densities of the shower recorded
(Chowdhuri, 1948, p 50).’ She emphasized the
importance of her results as follows:
It shows that the density of penetrating events is
approximately proportional to the density of
shower particles. The most probable conclusion
from this is that they are created locally. If this is
not the case, it is difficult to understand how the
penetrating particles, like mesons or nucleons,
coming along with other soft particles of the shower
obey the same law of scattering as the soft particles.
One could expect the penetrating particles to be
distributed differently from the general shower
particles. Thus, the penetrating particles in
Extensive Showers are mostly locally produced by
the electronic component of the Extensive Showers
(Chowdhuri, 1948, pp. 51-52).
Fig. 6. Left - Ratio of Cp5/Cs4 vs. ks, where s is the area of shielded counter. From the intersection of the two curves the
value of k can be determined. Right – The ratio of the four-fold penetrating shower counting ratio to the three-fold shower
ratio. From the point of intersection of the observed and calculated curves, the k comes out to be 0.025 ± 0.003. This value
found by BC was in agreement with the results of the other investigators. (Credit: University of Manchester)
She also studied the ‘dependence of ‘k’ on
‘Z’, that is, the atomic number of the local
absorber.’ She was of the opinion that if
penetrating particles are produced by electrons and
photons, then the production cross-section should
be strongly dependent on the atomic number of
the local absorber (Chowdhuri, 1948, p. 56).
However, before her, W B Fretter had suggested
that the cross-section is proportional to Z2 (Fretter,
1948, pp.41-46). He studied the penetrating
showers in lead plates with cloud chamber
technique. He found various examples of mesons
accompanying air showers. He wrote:
but no production of penetrating particles in lead
by electronic radiation was observed. The pictures
taken give general support to recent theoretical
speculation that the primary radiation produces
penetrating particles and electronic radiation in the
same event (Fretter, 1948, pp.41-46).
For the study of ‘Production of the
penetrating particles in groups’, BC, took
photographs under two different conditions. In the
first case the experimental arrangement was the
same as before, the only difference was that instead
of a single indicator lamp for all the six counters
joined together, a separate neon indicator lamp was
used for each counter. She wrote:
Thus, if any one of the six shielded counters was
discharged simultaneously with the main shower
set then it was recorded on the Cloud Chamber’
photograph by means of an indicator lamp. So by
this procedure one is able to find whether a single
counter or more than one was discharged
simultaneously with the main shower set. Thus, it
indicates whether a, penetrating event contains a
single particle or a group of particles (Chowdhuri,
1948, p. 26).
The above procedure was intended to find
out, whether penetrating events are of a simple or
complex nature, that is, whether they are produced
singly or in groups. The experiments were
performed without- and with absorber. It was
shown that ‘penetrating particles are mostly
created in the local absorber, not coming from air
along with other soft shower particles (Chowdhuri,
1948, p. 61).’ Her conclusion was supported by
Italian physicists G Salvini and G Tagliaferri,
University of Milan (Salvini and Tagliaferri, 1948,
pp. 261-262).
At the University of Cornell, Ithaca,
U.S.A., G Cocconi and K Greisen were working
on the penetrating showers. They were of the
opinion that ‘In the accompanied showers the
penetrating particles are very likely to pre-exist
in the air; in the unaccompanied showers they are
created in the absorbers surrounding the counters
by a nucleonic component of the cosmic radiation
(Cocconi and Greisen, 1948, pp. 62-65).’ BC while
discussing their results stated that her experiments
show that the ‘penetrating events mostly consist
of groups of particles and must be produced locally
(Chowdhuri 1948, p. 64).’ She discussed the
differences between the two experimental set ups
and showed that Cocconi-Greisen experimental
setup ‘is such that it cannot record particles of the
same group, unless they have spread out at a very
wide angle and escaped absorption in lead. This
also indicates that these particles are not very
penetrating (Chowdhuri, 1948, p. 68).’ Further she
stated that J Daudin’s experiment supports her
view, whose result is intermediate between her and
Cocconi’s. He (Daudin) showed that 40% of the
particles are absorbed in 0.8 cm. lead.
In the foregoing, she argued in favour of
production of penetrating particles in absorbers.
Under the section ‘On the investigations with a
thin roof at a certain height above the penetrating
set’ she gave the following three possibilities of
their production:
(1) groups of mesons may be produced in local
absorber by nucleons present in the air shower, by
some nuclear interaction as in the production of
local penetrating showers. (2) Penetrating particle
groups may consist of protons or other heavy
nuclear particles due to some sort of star-like
nuclear process generated in local absorber either
by nucleon or by other fast shower particles. (3)
Penetrating particles can be produced in the local
absorber by soft particles, i.e., electrons and
photons present in the Air Shower (Chowdhuri,
1948, pp. 70-79).
With her own experiments and that of L
Janossy and G D Rochester8; and D Broadbent
and L Janossy (1948, pp.364-385), she came to
the conclusion: ‘we are justified in rejecting the
suggestion that the nucleons are responsible for
the production of penetrating particles in
Extensive Showers (Chowdhuri, 1948, pp. 73-74).’
A paper of L Janossy and D Broadbent is of interest
as it gives the analysis of the experimental work
done by them and other scientists on the causes of
the ‘production of penetrating particles in
extensive air showers’, but more importantly ‘use
is also made of unpublished experimental
material obtained by Miss B Chowdhuri
(emphasis added). The authors wrote in abstract,
and they acknowledge: ‘We are … indebted to
Miss B Chowdhuri for communicating
unpublished results (Broadbent and Janossy, 1948,
About the angular distribution of particles
her view was:
The Italian workers – Salvini and others (Salvini
and Tagliaferri, 1948, pp.261-262) (…) observed
that the frequency of two close counters
discharging simultaneously is much higher than
that of two distant ones. The differences that they
observed is quite significant, so considering our
lack of material in this case, we think we are not
justified in drawing any definite conclusions on
the angular distribution of these particles
(Chowdhuri, 1948, pp. 77-76).
Also, she observed the ‘Absorption of
penetrating particles in lead.’ About the
experimental set up she described that:
Six shielded counters were spit up into two groups
of three counters. The two groups were separated
vertically by 10 cm. of lead. The top trays of
counter were shielded by 15 cm. lead. Both top
and bottom trays of counters were surrounded by
8 cm. lead on all sides. In order to allow for the
geometry of the arrangement, a series of
photographs was first taken with only 10 cm. air
space [and then with 10 cm. lead absorber] between
the two counter trays (Chowdhuri, 1948, p.30).
Her experimental results are shown in Fig.
7 and Fig. 8.
8Janossy, L; Rochester, G D. The transition effect of penetrating showers, Proc. R. Soc. Lond. pp. 183, 181-185, 1944. The
authors studied the transition effect of penetrating showers in lead and aluminium and found to agree with theories of J Hamilton,
W Heitler and H W Peng as extended by Janossy.
Fig. 7. Results with 10 cm. air space between two counter
trays. (Credit: University of Manchester)
Fig. 8. Results with 10 cm. lead absorber between two
counter trays. (Credit: University of Manchester)
She observed: ‘the nature of the
distribution of tracks with corresponding number
of indicator lamps on is the same as we observed
previously (Chowdhuri, 1948, p.33).’
Other part of her Ph.D. thesis deals with
‘The distribution of the Angular deviations of
Shower particles from the vertical direction.’ She
measured the projected angles of tracks in each
shower from the vertical direction. For showers
with two or more tracks, she took average direction
of all tracks from the vertical. The showers were
grouped in four classes; and plotted in intervals
of 10 degrees. She produced tables with ‘Single
track showers’, ‘2-4 track showers’, ‘5-15 track
showers’ and ‘›15 track showers.’ For each table
the values for ‘Angles from the vertical’, ‘Total
number of photos’ and ‘Number of photos
accompanied by penetrating events’ were given.
An example of such table is shown in Fig. 9
(Chowdhuri, 1948, pp. 33-36).
decrease with increasing density of shower
particles. From that she concluded: ‘It is possible
that sometimes the low density parts of showers
contain some very low energy particles which are
scattered at large angles from the vertical direction
(Chowdhuri, 1948, p. 81).’
She observed a prominent maximum for
single track showers at 0-10°angles; which was
not the case in other groups. She had no
explanation for it. She stated that M Deutschmann
made similar experiment and found similar results,
however, he recorded only 100 number of
showers, thus his statistics is poor. Here she refers
to Martin Deutschmann, University of Freiburg,
Germany. He wrote an article, which was based
on his Ph.D. thesis. With a big cloud chamber, he
studied 200 extensive showers. He also studied
the frequency distribution of the radiation
densities; and found that heavy particles were
detectable only in very small numbers. He also
measured the directions of incidence of the
showers (Deutschmann, 1947, pp.61-69).
In 1943, G Cocconi et al. in a series of
experiments studied the particle density of the
extensive showers in air. They found a correlation
between the average density of the extensive
showers and the total recording surface area
(Cocconi, Loverdo and Tongiorgi, 1943, pp.314-
324). A year before, that is, in 1942, P Auger and
J Daudin from France observed:
The production of multiple secondary particles by
cosmic rays under thick layers of lead has been
studied with coincidence counters and with a
cloud chamber. Part of the coincidences obtained
under 15 cm of lead is attributed to groups of
particles of atmospheric origin associated with
extensive showers. The other part is due to a local
effect produced in the lead by single penetrating
particles. Among the particles emitted in these
processes, some have the penetrating power of low
energy mesons (Auger and Daudin, 1942, pp.549-
Fig. 9. Experimental result –Distribution of the angular
deviations of tracks in vertical direction. (Credit: University
of Manchester)
Her measurement showed that the shower
particles in single tracks as well as showers with
large number of particles were concentrated within
30 degree of the vertical. In every group there was
a small percentage making an angle greater than
30 degree with the vertical. For instance, single
track, 2-4 track and 5 track were 16.8%, 13% and
8.7% respectively. This percentage showed a
BC’s study of the number of showers
accompanied by penetrating events showed that
the distribution of these showers was similar to
the general distribution. It confirmed that the
extensive showers are ‘generally of an electronic
nature, the electronic part of the showers can give
rise to penetrating events locally (Chowdhuri,
1948, p. 82).’ She concluded: ‘Extensive showers,
accompanied by penetrating events are of the same
type as the all particle extensive showers. This
was also confirmed by Cocconi’s et al. (Cocconi,
Loverdo and Tongiorgi, 1943, pp.314-324) and
Daudin’s experiments. They had observed that the
exponent of the density spectrum of extensive
penetrating shower is the same as that for all
particle showers.
From the summary of BC’s PhD thesis we
see that she showed (Chowdhuri, 1948, pp. 82-
87) that the penetrating particles are mainly
produced in the local absorbers. They do not come
from air along with other soft shower particles.
With various arguments she ruled out:
They are groups of mesons produced by nucleons
present in Air Showers, in approximately the same
way as in local penetrating showers.’ The
experimental results cannot be accounted for ‘some
sort of nuclear explosion or star-like process taking
place in the local absorber and produced by the
fast soft particles of Air Showers.
Her final conclusion was:
The soft particles in Air Showers are the origin of
the penetrating events. These events, however, are
not due to meson showers produced by soft
particles, because the large production cross-
section – which means a strong interaction between
electrons and mesons – would also require a reverse
process, such a process has not been observed
experimentally. And also the Z2 [where is atomic
number] dependence of cross-section of production
cannot be explained by such process (Chowdhuri,
1948, pp. 82-87).
And further:
We are led to the conclusion that the majority of
penetrating events are due to soft particles of Air
Showers, and that they are not mesons or other
heavy ionising particles but may be some ionizing
particle having a lighter mass than mesons,
produced in thin layers of local absorber. In order
to fit the cross-section of production their mass may
be 5 – 7 times the electron mass, as suggested by
Auger and Janossy and others (Chowdhuri, 1948,
pp. 82-87).
She continued:
“…, from the evidence of the roof experiment we
can further conclude that these particles are
unstable and their life period may be of the order
of 10-9 sec (Chowdhuri, 1948, pp. 82-87).”
In general, scientists were not sure about
the nature and production of particles. For
instance, A. Mura et al. wrote:
The existence of a penetrating component in
extensive atmospheric showers has been
investigated by several workers, using different
methods. Results obtained with cloud-chambers do
not seem to justify a final conclusion. Cocconi and
co-workers, using counters, decided in favour of
the presence of meson showers simultaneous with
extensive showers; more recently, Rogozinski,
using counters driving neon lamps, concludes that
mesons are probably associated with extensive
showers (Mura, Salvini and Tafliaferri, 1947,
How far work of BC and other associates
played role in winning the Physics Nobel Prize
for P M S Blackett in 1948 is unknown. The fact
is, in 1932, he and ‘Giuseppe Occhialini connected
the cloud chamber to a Geiger counter, which
detects the passage of a particle. In this way a
picture could be captured precisely when a particle
passed by. Patrick Blackett showed, among other
things, that with the application of high energy,
pairs of electrons and positrons could be formed
out of light particles, photons.’9 In 1948, he was
awarded the Nobel Prize in Physics ‘for his
development of the Wilson cloud chamber method,
and his discoveries therewith in the fields of
nuclear physics and cosmic radiation.’
9, Dec. 31, 2017.
Evidently BC was a privileged physicist
to work in the laboratory of the future Nobel Prize
laureate. BC acknowledged the following in her
‘I wish to record my sincere thanks to Professor
Blackett for the encouragement; and facilities he
has given for conducting experiment this
experiment. I am also greatly indebted to Professor
L Janossy [Hungarian born physicist] and Dr J G
Wilson [U.S.A.] for many helpful suggestions and
discussions during the progress of the work
(Chowdhuri, 1948, p. 96).’
As far as we know, BC published two
papers in British journals based on her Ph.D.
thesis. In her article in Nature she showed that (1)
The density of penetrating events associated with
extensive showers is proportional to the total
particle density in the region of the shower. (2)
The average probability of a single ionising
shower particle associated with a subsequent
penetrating count is 0.020 ± 0.002, and almost
independent of shower density over the whole
range of densities. (3) No significant change in
the rate for penetrating events took place when
part of the shielding lead plate was replaced by
iron of equal thickness in the units of cascade
theory (Chowdhuri, 1948 p.680; 1950, pp.165-
172). In her publication in Proceedings of Physical
Society, London, she wrote that she extended her
experiments in such a way that the discharge of
individual counters in the shielding layer were
distinguished and recorded on cloud chamber
photographs. This was meant to get information
concerning the nature of the events arising under
15 cm. of lead absorber when extensive showers
fall upon the apparatus. Her experimental set-up
is shown in Fig. 10.
One of her conclusions was:
It seems, …, plausible to regard the penetrating
particles in extensive showers as single particles
capable of producing large numbers of secondaries
in the local shielding material, which are the
immediate agency for discharging the shielded
counters. This conclusion … agrees well with the
‘hypothesis …’ suggested by Broadbent and
Janossy (1948, pp.364-385), where penetrating
particles are supposed to come from the air above
capable of discharging the shielded counter set by
means of a triple knock-on showers. But it has been
shown from all existing measurements that the
probability of production of knock-on secondaries
by ì-mesons is too small to account for our result
(Chowdhuri, 1950, pp.165-172).
And further:
‘…, if the above conclusion is valid, it is not easy
to associate the required behaviour of the
penetrating particles carrying energy into the
shielded cavity with the known behaviour of
ordinary ì-mesons. We have not distinguished,
however, between the operation of known
mechanisms for new particles of appropriate
Fig. 10. Above: The six counters kept horizontally in a single
cavity in the lead shield, with 15 cm. lead above the counter
and 8 cm. lead at the sides and below the counters. Below:
The counters are placed in two layers, with three counters
spaced 1 cm. apart in each layer and ‘the layers spaced
vertically about 15 cm. lead’ and are ‘shielded on the
remaining side by 8 cm. lead. Measurements were made
with and without 10 cm. lead in the cavity between the two
counter layers. When the lead was not in position the total
material between the two counter layers were two thin iron
sheets of thickness 1.5 cm. each.’ (Credit: Proc. Phys. Soc.).
properties and the occurrence of some hitherto
unexplored process taking place for familiar
particles(Chowdhuri, 1950, pp.165-172) .’
Shortly after her publication, S M Mitra
and W G V Rosser, who were also working in the
same laboratory as BC, wrote on the ‘Momentum
spectrum of the particles in extensive air showers.’
While discussing density distribution of the
particles in showers they stated that due to the
work of various authors such as B Chowdhuri it
is known that the distribution of shower densities
follow a power law of particular from. They wrote:
Following Chowdhuri (…), the density distribution
obtained in the present experiment can be
compared with the results of Cocconi, Loverdo and
Tongiorgi by comparing the observed rate of the
occurrence of showers showing n track above the
lead plate with the rate calculated according to the
formula … (Mitra and Rosser, 1949, pp.364-369).
G Cocconi and K Greisen, Cornell
University, New York, while writing on ‘λ-Mesons
in air showers’ wrote that L Janossy and C B A
McCusker who studied the penetrating particles
in lead did not mention the thickness of lead plates,
‘but we conclude from their sketch of the apparatus
and from their statement that the experiment was
a repetition of a similar one by Miss Chowdhuri,
that the thickness was 15 cm. or less.’ They were
of the opinion that ‘at least 20 cm. of lead is
required to reduce the soft component of the
showers to a level of intensity that is small
compared with that of the penetrating particles
(Cocconi and Greisen, 1949, p. 810).’
While discussing the nature of penetrating
particles in air showers, L Janossy and C B A
McCusker stated that air showers mainly contains
electrons and photon, but in addition a small
percentage of particles which penetrate more than
electrons. The work of various scientists shows
that they cannot be mesons. A group of physicists
call them λ-mesons, having mass about 3 times
the mass of electron. Other group (that is
Broadbent and Janossy) suggested them to be
‘heavy electrons’ of the mass about 10 me. The
recent experimental results presented in a
symposium held at Bristol in September 1948 give
additional support to these hypothesis. He reported
about Miss Chowdhuri’s experimental work,
which was presented in the symposium, who
showed that a considerable change of the rate of
fourfold coincidences in the presence of the roof
was noticed. For four-fold the actual rate observed
by her was as follows:
With roof 0.263±0.037
No roof 0.474±0.053
Difference 0.211±0.064
The authors stated that her experiment was
repeated and confirmed by them in Dublin. They
also performed experiment with bricks. Their
conclusion was that the two experiments show that
thin lead roof reduces the number of penetrating
extensive showers recorded, while a mass-
equivalent brick roof show no considerable effect
(Janossy and McCusker, 1949, pp. 181-183).
K Greisen et al., while referring to BC’s
article (Chowdhuri, 1950, pp.165-172) opined:
The high energy N-component, capable of
producing penetrating showers, has been found to
have an intensity near the cores of air showers about
60 percent as great as that of the non-interacting
penetrating particles, and about ½ percent as great
as that of the electrons. …. It is suggested that all
of the N-component in the lower atmosphere
belongs to extensive showers and contributes to
the shower development. Near the core, the lateral
density distribution of the N-component is found
to be as strong as that of the electrons (Greisen,
Walker and Walker, 535-1950, p.545).
‘Reports on Progress in Physics’ while
writing review on ‘The nuclear interactions of
cosmic rays’ stated:
Counter work by Chowdhuri (1950), Sitte (1950b)
and McCusker (1950) shows that below 20 cm. of
lead the penetrating component of extensive
showers consists of some 80% mesons and 20%
of nuclear-interacting particles. It is thus
abundantly clear that there exists a very close
connection between the soft and hard components
of showers (Rochester and Rosser, 1951, pp.227-
What we see from the foregoing is that B
Chowdhuri was an integral part of the western
scientific community and her work had received
a glowing acceptance in the scientific community.
It is an enigma why a scientist of Bibha
Chowdhuri’s calibre, who did such significant
research on contemporary modern physics at a
time when very few women ventured to take
physics research as a profession, did not attract
any attention from her country of origin.
Surprisingly enough her involvement and
contribution to the Kolar Gold mine experiment
had been underplayed and remained practically
unmentioned in any literature. That she was a
victim of gender discrimination in the male
dominated scientific society may not be ruled out.
We are grateful to many persons for
providing information about Bibha Chowdhuri in
some form or other. Acknowledgements are due
to Ms. Rita Sarkar (wife of Prof. Dipak Sarkar),
Rana Biswas, Noopur Biswas and Reba Biswas
(respectively the son, daughter and wife of Late
Prof. Sukumar Biswas of TIFR), Dr. A K Ganguly,
former professor of Calcutta University, Prof. B
V Sreekantan of National Centre of Advanced
Studies, Bangalore, Prof. Y C Saxena of Indian
Plasma Research Institute, Ms. Bhavya
Ramakrishnan of TIFR Archives and James Peters
of the University of Manchester. One of us (R.
Singh) thanks Prof. Dr. Michael Komorek, Head
– Physics Didactic and History of Science,
University of Oldenburg for providing research
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Full-text available
An experiment has been carried out at a vertical depth of 580 m.w.e. at Kolar Gold Fields, to investigate various characteristics of energetic muons (E mln ⋍ 150 GeV) associated with extensive air showers (EAS). Double parallel penetrating particles with narrow separations (<1m) have an exponential decoherence distribution withe-folding separation of ⋍ 25 cm.
The high energy N-component, capable of producing penetrating showers, has been found to have an intensity near the cores of air showers about 60 percent as great as that of the non-interacting penetrating particles, and about 1/2 percent as great as that of the electrons. The ratio of charged to neutral N-component is about 1.5. Large penetrating showers are more frequently observed to be associated with air showers than are small penetrating showers. It is suggested that all of the N-component in the lower atmosphere belongs to extensive showers and contributes to the shower development. Near the core, the lateral density distribution of the N-component is found to be as strong as that of the electrons. It is shown that this results from purely geometric considerations, provided one accepts the concept of a core containing N-component of great energy, continually feeding out lower energy particles to the side.
Production of penetrating showers in lead has been observed in a cloud chamber containing eight ½-inch thick lead plates. In over half of the 53 examples of penetrating showers photographed high, energy electronic radiation was simultaneously produced. Many examples of mesons accompanying air showers were observed, but no production of penetrating particles in lead by electronic radiation was observed. The pictures taken give general support to recent theoretical speculation that the primary radiation produces penetrating particles and electronic radiation in the same event.
The production of multiple secondary particles by cosmic rays under thick layers of lead has been studied with coincidence counters and with a cloud chamber. Part of the coincidences obtained under 15 cm of lead is attributed to groups of particles of atmospheric origin associated with extensive showers. The other part is due to a local effect produced in the lead by single penetrating particles. Among the particles emitted in these processes, some have the penetrating power of low energy mesons.
IN a previous communication1 we gave an account of star-like tracks obtained in vertically placed Ilford Halftone plates exposed to cosmic rays at Sandakphu (3660 m.) and Darjeeling (2130 m.). We suggested that they were due to secondary mesotrons. Since then we have carried out a more detailed examination of these plates and also of other similar plates kept at Sandakphu for 202 days (i) under 20 cm. of water in a galvanized iron box, (ii) in a lead box with 5.5 cm. lead on the top of the vertically placed plates. The last-named plates on development were found to be fogged. Fortunately, the observations of Heitler and his co-workers2 on similar plates exposed under different thicknesses of lead at Jung-fraujoch supplement our observations and are in agreement with our results.
The present paper contains an analysis of the experimental material presented in two previous communications (Broadbent & Janossy 1947 a,b, referred to as I and II) and also the important papers of Cocconi, Loverdo & Tongiorgi (1943, 1944, 1946 a,b), and those of Daudin (1942, 1943). Use is also made of unpublished experimental material obtained by Miss B. Choudhuri.