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Pocket Dosimeters Personnel Monitoring: Pocket Dosimeters

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Abstract

Pocket Dosimeter (PD) is a personnel monitoring Instrument.it based on ionization of gases in small chamber called free air ionization chamber or Air Wall. Providing an immediate reading of any types of Exposure Radiation is considered to be unique feature for PD. There are two types of Pocket Dosimeter, Indirect Reading Dosimeter (IRD) and Direct Reading Dosimeter (DRD). In this Report, Principles of PD, the two types, Mechanism of Action and Construction will be discussed.
Pocket Dosimeters
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Personnel Monitoring: Pocket Dosimeters
By\ Mahm oud R a`fa t M.
1
Abstract
Pocket Dosimeter (PD) is a personnel monitoring Instrument.it based on ionization of gases
in small chamber called free air ionization chamber or Air Wall. Providing an immediate
reading of any types of Exposure Radiation is considered to be unique feature for PD. There
are two types of Pocket Dosimeter, Indirect Reading Dosimeter (IRD) and Direct Reading
Dosimeter (DRD). In this Report, Principles of PD, the two types, Mechanism of Action and
Construction will be discussed.
Introduction
The International Commission on Radiological Protection (ICRP) recommends monitoring individual
radiation workers for external exposure unless it is clear that their doses will be consistently low. The
International Atomic Energy Agency (IAEA), in its Basic Safety Standards, interprets this to mean that
workers whose occupational dose may exceed 30% of the regulatory limit should be monitored. The most
widely used personal monitoring devices include film badges, thermoluminescent dosimeters,
opticoluminescent dosimeters, electronic dosimeters and pocket dosimeters that will be discussed in this
Report.
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Measuring Radiation by Ionization Methods
The oldest accurate technique for measuring radiation involves measuring the charge produced by the
radiation. This can be done in two different ways: If the radiation is more or less constant, it is possible to
measure the ionization current. This is a dose rate meter. The results will be given in R/hour or a similar
unit. If the exposure is short, as in the case of an” X-ray exposure”, all of the ionization charge is collected
and measured. This is called an "integrating dosimeter." A simple dosimeter of this type is a pocket or pen
dosimeter.
Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays
and gamma rays. As the name implies, they are commonly worn in the pocket.
To measure radiation dose, the response of the instrument must be proportional to absorbed energy. A basic
instrument for doing this is the free-air ionization chamber.
1
B.Sc. Student at Biophysics Group. Physics Department, Faculty of Science A.S.U
Pocket Dosimeters
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To Understand the Concept of Free Air Chamber
the exposure unit can be satisfied by the instrument shown in Figure 1. The X-ray beam enters through the
portal and interacts with the cylindrical column of air defined by the entry port diaphragm.
- All the ions resulting from interactions between the X-rays and the volume of air (ABCD), which is
determined by the intersection of the X-ray beam with the electric lines of force from the edges of the
collector plate C, is collected by the plates, causing current to flow in the external circuit.
- Most of these collected ions are those produced as the primary ionizing particles lose their energy by
ionizing interactions as they pass through their. (The primary ionizing particles are the Compton electrons
and the photoelectrons resulting from the interaction of the x-rays ‘photons’ with the air).
- The guarding G and the guard wires W help to keep these electric field lines straight and perpendicular to
the plates. The electric field intensity between the plates is high enough to collect the ions before they
recombine but not great enough to accelerate the secondary electrons produced by the primary ionizing
particles to ionizing energy.
-The guard wires are connected to a voltage-dividing network to ensure a uniform potential drop across the
plates. The number of ions collected because of X-ray interactions in the collecting volume is calculated
from the current flow and the exposure rate, in roentgens per unit time, is then computed.
Fig 1: Schematic diagram of Free Air Ionization
Direct Reading Dosimeter (DRD):
Also called Capacitor \ Self Reading Dosimeter or Quartz Fiber Electroscope which has the
advantage of being reusable.
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Construction:
As shown in Fig (2), a direct reading pocket ionization dosimeter is generally of the size and shape of a
fountain pen. The dosimeter contains a small ionization chamber with a volume of approximately two
milliliters. Inside the ionization chamber is a central wire anode, and attached to this wire anodes a metal
coated quartz fiber.
Fig 2: Capacitor-type Dosimeter schematic.
Mechanism of Action for DRD:
- When the anode is charged to a positive potential, the charge is distributed between the wire anode
and quartz fiber.
- Electrostatic repulsion deflects the quartz fiber, and the greater the charge, the greater the deflection of
the quartz fiber.
-Radiation incident on the chamber produces ionization inside the active volume of the chamber.
- The electrons produced by ionization are attracted to, and collected by, the positively charged central
anode.
-This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the
direction of the original position.
-The amount of movement is directly proportional to the amount of ionization which occurs.
Or on another words, amount of Discharge, consequently the change in position of fiber proportional to
radiation exposure.
-By pointing the instrument at a light source, the position of the fiber may be observed through a system of
built-in lenses.
-The fiber is viewed on a translucent scale which is graduated in units of exposure (mR). Typical industrial
radiography pocket dosimeters have a full scale reading of 200milliroentgens but there are designs that will
record higher amounts.
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Advantages and Disadvantages:
The main advantage of this type is that, Doesn`t have to be re-charged, so self-Reading allow user to check
exposure at any time.
Care should be taken against erroneous readings due to electrical leakage, but, a High degree of accuracy is
not important, Reliability is. Potential for discharging and reading loss due to dropping or bumping are a
few of the main disadvantages of a pocket dosimeter.
Indirect Reading Dosimeter (IRD):
Also called Condenser \ Non-self-Reading Dosimeter the indirect reading type (Fig 3); an
auxiliary device is necessary in order to read the measured dose. This device, which is, in
reality, an e lectrosta t i c voltme t er that is calibra t ed in roentgens, i s called a charger -reader” .
And can measure X-ray also Gamma-ray. So the main disadvantage of Condenser type is it
MUST be charged after each reading.
Fig 3: Condenser-type pocket dosimeter
-Non-self-Reading Dosimeter are usually used for measuring any level of Exposure in range 5-200 mR or
Up to this range, also these dosimeters can be used to measure Beta radiation whose energies exceed 1
MeV.
-By coating the inside of the chamber with boron, the pocket dosimeter can also be made sensitive to
thermal neutrons (~ 0.025 eV) and emits Alpha particles that causes Ionization, or Coating with Plastic,
made it sensitive to fast neutron (<8 keV), in which the neutron is slowed down by elastic scattering with H-
atoms which are Light Nuclei that, in turn, cause ionization.
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Construction & Mechanism of Action for IRD:
Fig 4: Non-self-reading condenser-type
As shown in fig the instrument consists of an outer cylindrical wall, made of electrically conducting plastic.
Coaxial with the outer wall, but separated from it by a very high-quality insulator, is a center wire. This
center wire, or central anode, is positively charged with respect to the wall. When the chamber is exposed to
X-radiation or to gamma radiation, the ionization, which is produced in the measuring cavity as a result of
interactions between photons and the wall, discharges the condenser, thereby decreasing the potential of the
anode. This decrease in the anode voltage is directly proportional to the ionization produced in the cavity,
which in turn is directly proportional to the radiation exposure.
Conclusion
The main advantages and disadvantages, corresponding to the other Personnel
Monitoring as shown before, can be conclude in the following table:
Advantages
Disadvantages
-Useful for immediately reading for
any Types of exposure Radiation.
-Most Sensitive.
-Ideal for use in High Radiation areas
which required periodically
Examination.
-High cost, especially Quartz fiber,
lensesetc., Which are Expensive.
- Fragile, Easy broken or damaged
- Charge leakage, or drift, can also affect
the reading of a dosimeter.
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Resources
-Attix, F. H. Introduction to Radiological Physics and Radiation Dosimetry. John Wiley and Sons,
New York, 1986.
-Radiation Dosimetry, by John Cameron, Environmental Health Perspectives Vol. 91, pp. 45-48, 1991
- Cember, Herman, Introduction to Health Physics, Third Edition, McGraw-Hill Health
Professions Division, New York, 1996.
- Burnham, J.U., Radiation Protection, Point Lepreau Generating Station, New Brunswick
Power Corporation, Rev. 4, 2001.
Figures:
-Fig 1: From Design of Free Air Ionization Chamber. Washington, DC: National Bureau of
Standards;
1957. NBS Handbook 64.
-Fig 2&3: Canadian Nuclear Safety Commission, General Nuclear Safety and Control Regulations,
January 2000.
-Fig 4: Canadian Nuclear Safety Commission, Radiation Safety Officers Handbook Info-0718.
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