TECHNOLOGY EXPERTS GROUP

BASIC PRINCIPLES OF

RADIATION PROTECTION

FOR RPO

Prepared by

Prof. Dr. M. FAROUK AHMAD

RIYADH

APR. 2006

2

FOREWORD

The use of man-made ionizing radiation and radioactive sources

are now a day widespread, and continue to increase around the world.

Nuclear techniques are in growing use in industry, agriculture, medicine,

well logging, and research benefiting the society as a whole. Irradiation

is used around the world to preserve foodstuffs. Sterilization techniques

have been used to eradicate diseases, and ionizing radiation are widely

used in diagnosis and therapy of different diseases. Industrial

radiography is widely used to examine welds and detect cracks and

microscopic bubbles in metallic pipes, tanks and other devices, and help

prevent the failure of engineered structures.

It has been recognized that exposure to a an acute dose of ionizing

radiation causes clinical damage to the tissues of the human body. In

addition, long term studies of populations exposed to ionizing radiation

have demonstrated that this exposure has a potential for the delayed

induction of malignancies. Due to these risks all activities involving

radiation exposure shall be subjected to certain national and international

safety standards, in order to protect radiation workers, general public and

environment from exposure to ionizing radiation.

One of the requirement of the national and international safety

standards is that any installation, that is acquiring any of the radiation

sources shall appoint a radiation protection officer, RPO, (or officers), to

oversee the application of the requirements of the radiation protection

and safety of radiation sources. According to the Saudi national and

international regulations, this individual shall be technically competent

in radiation protection scientific and organizational matters, relevant for

a given type of practice. In Accordance with Saudi national regulations

shall be licensed by the national regulatory authority through passing a

qualification exam, which is held periodically by this authority.

For successfully passing this qualification exam, one should study

different scientific and organizational topics, which are existing in

different English books, and are specialized very deep in the subjects of

interest. It may be very difficult for individuals non specialized in

radiation physics to follow this subjects.

3

For this reason this booklet is prepared, and will be issued, by the

technology experts group, to cover the fundamentals and all scientific

and organizational topics that are necessary for any radiation protection

officer to be qualified as a RPO. Together with the included topics in

this booklet the practical lessons are essential part of the qualification of

the RPO. This practice in the different relevant fields may be gained

easily through these practical lessons.

We hope that the booklet will be helpful in acquiring the necessary

knowledge in the field.

Technology experts Group

and the author

4

PATRT 1

SCIENTIFIC AND TECHNOLOGICAL ASPECTS

OF RADIATION PROTECTION

5

CONTENTS

Part 1: Scientific and technical aspects of radiation protection.

Chapter 1: Radioactivity and radioactive decay.

1-1 Some properties of atomic nuclei. 1-2 Some properties of alpha decay and alpha particles. 1-3 Some properties of beta decay and beta particles. 1-4 Some properties of gamma disintegration. 1-5 The x-rays. 1-6 The neutrons and their sources. 1-7 Calculation of the source activity 1-8 The units of activity. 1-9 The physical half-life time. 1-10 The biological and effective half-life times. 1-11 The radioactive decay law.. 1-12 The relation between the decay constant and the half-life time. 1-13 Some important multipliers.

Chapter 2: Interaction of radiation with matter.

2-1 Introduction. 2-2 Interaction of heavy charged particles with matter. 2-3 Interaction of beta particles with matter. 2-4 Interaction of x and gamma radiation with matter. 2-5 Interaction of neutrons with matter.

Chapter 3: Radiation detectors, survey meters and monitors.

3-1 General. 3-2 The gas detectors. 3-3 The scintillation detectors. 3-4 The semi-conductor detectors. 3-5 The survey meters. 3-6 The contamination monitors. 3-7 Devices for personal dosimetry.

Chapter 4: Some radiation measurement techniques and statistical

fluctuations.

4-1 Introduction. 4-2 The solid angle. 4-3 The detector intrinsic efficiency.

6

4-4 Relation between the counting rate and source activity. 4-5 Other factors affecting the measurements. 4-6 Dead time correction. 4-7 The statistical fluctuation of radiation measurements.

Chapter 5: Dosimetry quantities and their units.

5-1 The exposure. 5-2 The absorbed dose. 5-3 The equivalence between the Roentgen, the rad and Gray. 5-4 The Kerma 5-5 The radiation weighting factor. 5-6 The equivalent dose. 5-7 The tissue weighting factor. 5-8 The effective dose. 5-9 The committed equivalent or effective dose.

Chapter 6: Biological effects of radiation.

6-1 Direct and indirect action of ionizing radiation on cell. 6-2 Radiation effects. 6-3 Deterministic and stochastic effects. 6-4 Acute deterministic effects. 6-5 The stochastic effects. 6-6 Hereditary effects.

Chapter 7: Dose calculation.

7-1 Dose calculation from point sources. 7-2 Dose calculation for beta emitters. 7-3 Dose calculation from external gamma sources. 7-4 Dose calculation from neutron sources. 7-5 The inverse square low for external exposure 7-6 Dose calculation from internal exposure. 7-7 The annual limit on intake. 7-8 The derived air concentration.

Chapter 8: Radiation shielding.

8-1 Shielding of sources of alpha particles. 8-2 Shielding of sources of beta particles. 8-3 Shielding of x and gamma ray sources. 8-4 Shielding of the neutron sources.

7

Part 2: Organizational aspects of radiation protection.

General framework and requirements for radiation protection.

1- Introduction.

2- Administrative requirements.

3- Management requirement for radiation protection.

4 - The principle requirements.

5- Verification of safety.

6- Condition of service.

Responsibilities of parties.

1- Responsible parties for radiation protection.

2- Responsibilities of the licensee.

3- Cooperation between licensees and employers.

National (SA) dose limits.

1- The terms limit and level.

2- Radiation exposures.

3- The occupational dose limits.

4- The dose limits for general public.

5- The dose limits for medical exposures.

6- The dose limits for emergency exposures.

The radiation Protection Program (RPP).

1- Introduction.

2- The structure of the RPP.

The safe transport of radioactive material.

1- Introduction.

2- Definitions.

3- General provisions.

4- Determination of the transport index.

5- Categories of packages.

6- Marking and labeling.

7. Storage in transit.

8

CHAPTER 1

RADIOACTIVITY AND RADIOACTIVE DECAY

1-1 Some properties of the atomic nuclei:

- Any atom is composed of the atomic nucleus, around which

electrons are orbiting in elliptical shells.

- The radius of the atom is in the order of 10-10

m, while the radius

of the nucleus is in the order of 10-15

m, so that the volume of the

nucleus is smaller than that of atom by about thousand trillions times

(trillion = 1012

). Due to these dimensions, the atom is similar to the solar

system, with its inter- planetary distances.

- Any atomic nucleus consists of nucleons, which are protons or

neutrons. The proton mass is, approximately, higher than that of the

electron by about 1836 times, while the neutron mass is higher by about

1838 times. So, the neutron and the proton may be considered as

particles with the same mass. From these data the atomic mass is

concentrated in the atomic nucleus, and the nuclear density is,

approximately, constant and equals 1017

kg/m3 (about 100 millions

ton/cm3).

- The charge of the proton equals to the electron charge in

magnitude (1.6x10-19

Coulomb), but it is positive in sign, while the

neutron is neutral (e.g. its total charge equals zero). So, in a neutral atom

the number of the protons in the nucleus equals the number of the orbital

electrons.

- The number of the protons in a nucleus is called its atomic

number Z, while the total number of protons and neutrons, in it, is

called the mass number A. So the number of neutrons N in a nucleus is

N = A Z. Symbolically, any atom is represented by the first letter written in capital, or by the first one in capital and other one written in

small. The atomic number is written in the lower left corner, while the

mass number is written in the upper left one. Example of that is C126 (or

carbon-12), Cl3517 (or chlorine-35), Cr51

23 (chrome-51) and Cd114

48 (or

cadmium-114).

- The nucleus of any element is composed of the same number of

protons Z, but it may have different numbers of neutrons N. these

9

different forms of the same element are called isotopes of the element.

For example, hydrogen exists in three forms (the nucleus of each

contains one proton), H11 without any neutron, H2

1 (or deuterium) with

one neutron and, H31 (or tritium) with two neutrons. The isotopes of the

element are characterized by the same chemical properties while they

have different physical properties. Some Elements have more than 40

isotopes.

- Some nuclides are stable, while some others are unstable and

they may, spontaneously, decay to daughter nuclides through the

emission of alpha or beta particle, or may disintegrate through the

emission of gamma radiation. These nuclides are called radio-nuclides

and there atoms are called radio-active isotopes. So, there are three types

of the radioactive decay, which are:

a) alpha decay ( decay) b) beta decay ( decay), and c) gamma disintegration ( disintegration)

1-2 Some properties of -decay and -particles:

- In decay of a nucleus, an alpha particle (), which is the

nucleus of a helium-4 atom ( He42 ), is emitted. This particle is composed

of 2 protons and 2 neutrons. So, in an decay of a parent radionuclide the mass number of the daughter nuclide is reduced by 4 while the

atomic number is reduced by 2. An example of alpha decay is the decay

of uranium-238 to thorium-234 with the emission of an alpha particle , which is symbolically represented as:

U23892 Th234

90 + He4

2

Another example is the decay of polonium ( Po21084 ) to the stable

lead-206 ( Pb20682 ) which is symbolically represented as:

Po21084 Pb206

82 +

- Alpha particles emitted from a certain radionuclide are

characterized by, so called, discrete spectrum. This means that all alpha

particles emitted from that radionuclide will have the same energy value

or separated but fixed values. So, by measuring the energy value or

values of particles the radionuclide can be easily identified. In other

10

words, it is known that U23892 (for example) emits particles with two

energy values which are 4.196 and 4.149 MeV. So, if these two energy

values for any alpha emitter are detected, then it mean that this emitter is

U23892 .

1-3 Some properties of -decay and particles:

- There are three types of beta decay, which are:

1-3-1 Electron or -negative decay:

- in this type of decay one of the neutrons n of the parent nucleus decays, spontaneously, to a proton p, negatron - (which is a -negative particle i.e. electron) and a third particle, named anti-neutrino -. This is represented symbolically as;

n p + - + -

- One example of - (or electron decay) is the decay of

Co6027 (Cobalt-60) to Ni60

28 (Nickel-60) with the emission of - particle and

anti-neutrino -(see fig. 1-1), which is expressed symbolically as:

Co6027 Ni60

28 + - + -

- Other example is the decay of cesium-137 to barium-137 with the

emission of the same two particles (see fig. 1-2). This is expressed as:

Cs13755 Ba137

56 + - + -

- It should be mentioned that the decay energy which is a fixed

amount for each parent radionuclide to decays to a daughter one is

distributed randomly between the two emitted particles, - and -. In some decays of the parent radionuclide the majority of the fixed decay

energy is acquired by beta particle, and the remaining small amount of

energy is acquired by the anti-neutrino. In other decays of the same

parent radionuclide the beta particles acquire a medium or a small

amount of the decay energy, and hence the anti-neutrino will get a

medium or a large amount of the decay energy. That is the reason of

emission of beta particles from the same radionuclide with energies

varying from zero up to the maximum decay energy. This is expressed,

in other words, in that the beta spectrum of any beta emitter is a

11

continuous one for different types of beta decay, and by studying beta

spectra it is impossible to identify the beta-emitting radionuclide.

- In beta-negative decay the mass number A of both parent and

daughter radio-nuclides remains constant and does not change, while the

atomic number Z of the daughter nuclide is increased by one with

respect to that of the parent one, since a neutron is converted into a

proton in the nucleus.

1-3-2 Positron or beta positive decay:

- In this type of decay one of the protons of the parent nucleus decays spontaneously to a neutron, + (which is a -positive particle i.e. positron) and a third particle, named neutrino . This is represented symbolically as;

p n + + +

- One example of + (or positron decay) is the decay of Na-22 (Sodium-22) to Ne-22 (Neon-22) with the emission of + particle and neutrino (see fig. 1-1), which is expressed symbolically as:

Na2211 Ne

2210 +

+ +

- In beta-positive decay the mass number A of both the parent and

daughter radio-nuclides remains constant and does not change, while the

atomic number Z of the daughter nuclide is decreased by one with

respect to that of the parent one, since one proton of the parent nucleus is

converted into a neutron.

1-3-3 The electron capture:

- In this type of decay one of the protons of the parent nucleus captures an orbital electron from the shells, which are very close to the

nucleus, forming a neutron and a neutrino is emitted during this process. This is represented symbolically as;

p + e- n +

- One example of the electron capture is the capture of an orbital

electron by Na-22 (Sodium-22) nucleus to form a Ne-22 (Neon-22)

nucleus with the emission of a neutrino . This is expressed symbolically as:

12

e- +

Na2211 Ne2210 +

- In the electron capture no beta particle is emitted, but the only

emitted particle is the neutrino. Moreover the mass number A of both the

parent and daughter nuclides remains constant and does not change, as in

all other types of beta decay, while the atomic number Z of the daughter

nuclide is decreased by one with respect to that of the parent one, since a

proton is converted into a neutron, by the analogy to the beta positive

decay.

1-4 Some properties of gamma disintegration:

- If an atomic nucleus is formed in, so called, excited energy state

(i.e. in a state with excess energy) it may disintegrate to a state with a

lower excitation energy or to the so called, the ground state (i.e. to the

state with zero excitation energy). This disintegration is accompanied

with the emission of a gamma () photon, that carries an amount of energy equal to the difference between the excitation energies of the

initial and final states. So, the energy E of the emitted photon is given as:

E = Ei - Ef

where Ei and Ef are the excitation energies of the initial and final states

of the gamma emitting nucleus, respectively.

- Each photon is an electromagnetic wave (with zero rest mass) with an ultra-high frequency f of a given value, which is, in its turn, a

characteristic value for this disintegration.

- An example of gamma disintegration is the disintegration of

*6028 Ni nucleus, which is formed in an excited state, as a result of beta

decay of the Co6027 , with an excitation energy equal to 2505 KeV, and then

it disintegrates, promptly, to a lower excited state with an excitation

energy equal to 1332 KeV, which, in its turn, disintegrates promptly to

the ground state with zero excitation energy. This means that the *6028 Ni

emits two photons, one with energy E1 = 2505 1332 =1173 KeV, and the second with energy E2 = 1332 0 = 1332 KeV. These two gamma ray photons are characteristic lines (i.e energies) for the gamma

disintegration of *6028 Ni , and hence for the decay of the Co60

27 to *60

28 Ni .

13

So, the detection of two gamma ray lines with energies 1173 and 1332

KeV is an indication that the original radio-nuclide is Co6027 .

Fig (1-1): decay of Co-60 and gamma disintegration of Nickel-60

- Other example of gamma disintegration is the disintegration of

*13756 Ba nucleus, which is formed in an excited state, as a result of beta

decay of the Cs13755 , with an excitation energy equal to 662 KeV, and then

it disintegrates, promptly to the ground state with zero excitation energy.

This means that the *13756 Ba nucleus emits one photon with energy E

= 662 0 = 662 KeV. This gamma ray photon is a characteristic line for

the gamma disintegration of *13756 Ba , and hence for the decay of the

Cs13755 to *137

56 Ba . So, the detection of one gamma ray line with energy

662 KeV is an indication that the original radio-nuclide is Cs13755 .

- Gamma ray photons emitted from a certain radionuclide are

characterized by, so called, discrete spectrum. This means that all

photons emitted from that radionuclide will have the same energy value,

2505 KeV

1332 KeV

1173 KeV photon

1332 Kev photon

Ni6028

Co6027

14

as in the case of Ba-137, where the energy of the emitted photons is 662 KeV, or

separated but fixed values, as in the case of Co-60 where photons are

emitted with two discrete energies 1173 and 1332 KeV. So, by measuring

the energy value or values of gamma rays the radionuclide can be easily

identified. In other words, if photons with energy equal to 662 KeV (for

example) are detected, then this means that this emitter is Cs-137, and if

photons with energies 1173 and 1332 KeV are detected it means that the

emitter is Co-60

Fig (1-2): decay of Cs-137 and gamma disintegration of Barium-137

- It should be noticed, that in gamma disintegration, neither the atomic number Z nor the mass number A change. This is

expressed symbolically by the following gamma

disintegration:

CoCo 602860

28 *

BaBa 13756137

56 *

- It should be also mentioned, that gamma emitters can be obtained

as a result of alpha or beta decays, when the daughter nuclei are formed

in their excited states. Gamma emitters may be obtained, too, by forming

excited states of nuclides during different nuclear reactions. If the half-

life time of the excited states is extremely short then the gamma

Cs13755

*13756 Ba

Ba13756

662 KeV

KeV

line662 KeV o

1

0 KeV

15

disintegration will be prompt. In case, if the half-life time of the excited

states is long, then this state is called metastable, and the gamma

disintegration occurs during relatively long time. An example of the

metastable radio-nuclides, which is widely used in medicine as a gamma

emitter, is technicium-99 (Tc-99).

1-5 The x-rays:

- The x-rays are electromagnetic radiation, emitted either: a) as a

result of the interaction of the charged particles (mainly light particles

such as the electrons) with the negative orbital electrons or the positive

atomic nuclei or, b) as a result of the transfer of an orbital electron from

an orbit with higher energy to another one with lower energy. So, based

on the origin of x-ray there are two types which are bremstrahlung

and characteristic x-rays. The frequencies of these rays lay in the

region from about 1x1017

up to about 1x1022

Hz and even higher. So, the

x and gamma radiation are widely overlapping with respect to their

energies.

- An example of the bremstrahlung x-rays, is the x-rays which

are emitted from x-ray tubes as a result of acceleration of the electrons

by a voltage difference, and then braking these electrons by high Z

elements (e.g. in the electric field of the orbital electrons and nuclei).

These bremstrahlung rays are characterized by a continuous energy

spectrum, (e.g energies of the photons may vary from zero up to the

maximum energy of the accelerated electrons). With some

approximation, the average energy of the x-ray photons may be

considered equal 0ne third of the energy of the accelerated electrons.

- An example of the characteristic x-rays, is these x-rays which

are emitted as a result of the transfer of an electron from an orbit with

higher energy to another one with lower energy, when there is an

electron vacancy in the lower shell. Since electronic orbits have definite

discrete energy values for each element, there will be a characteristic x-

ray discrete spectrum for each element. This means that x-ray will be

emitted from all atoms of same element with the same definite energy

values, which are characteristic values for this element.

1-6 The neutrons and their sources:

- As it has been mentioned, the neutron is a neutral particle (e.g.

with total charge equal zero and with rest mass, very slightly, higher

16

than that of the proton. There are no naturally occurring radionuclides

that can emit neutrons. There is only one artificial (man-made)

radionuclide which can partially decay through the emission of a neutron

or with the emission of alpha particles. This is the californium-252 (Cf-

252) which is an alpha and neutron emitter with a half-life time of 2.64

years

- The most commonly used neutron sources in industrial and other

applications are: the americium-beryllium (Am242-Be9) source, the

californium- 252 and the neutron generators. The nuclear reactors are

used as a very powerful neutron sources with a neutron density ranging

from 1013

up to 1018

per cm3. These reactors are used for energy

production, as well as for thermal neutron irradiation for production of

different artificial radioisotopes.

- Neutrons emitted from all neutron sources, generators and even

reactors are fast neutrons, and their energies varies about zero up to

about 14 MeV.

1-6-1 The americium-beryllium neutron sources:

- The (Am242-Be9) neutron source is made by mixing a certain amount of a very fine powder of americium-242 with a certain

weight of a very fine powder of beryllium-9. The Am-242 is a

source of alpha particle, which interacts with a beryllium

nucleus and produces a neutron, in accordance with the

following nuclear reaction:

He4

2 + Be9

4 C12

6 + n1

0

- This reaction is expressed in other form of writing as (, n)

reaction on beryllium, where denotes the projectile alpha particle and n denotes the resultant neutron emitted in the

reaction, while beryllium denotes the target atom. Activity of

one Curie (1Ci) of Am-242 with about one gram of Be-9

produces a neutron source, with a neutron yield of about,

2.2x106 neutrons / second. Earlier, neutron sources were

made of radium-226 or Po-210, (as alpha emitters) with

beryllium-9. However, but the production of such sources has

been stopped due to the explosion hazards of Ra-226 or

relatively short half life time of Po-210. In all alpha beryllium

17

neutron sources, fast neutrons are emitted with energies

varying between zero and about 10 MeV

1-6-2 The californium-252:

The californium-252, which is an isotopic neutron sources, is

produced in nuclear reactors. 1 microgram (1 g) of Cf-252 produces about 2.3x10

6 fast neutrons per second. Neutron sources with different

yields ((up to more than 10 milligrams, e.g. 2.3x1010

neutrons/second)

are available in the market. Energies of the emitted neutrons from this

source vary from about zero up to more than 8 MeV.

1-6-3 The Photo-neutron source:

- In this type of neutron sources a gamma source which can emit

photons with energy higher than 1.67 MeV is used to interact with

beryllium-9 and split it to two alpha particles and a neutron according to

the following photonuclear reaction:

+ Be94 2 He4

2 + n1

0

- The most commonly used gamma emitter in the photo-neutron

sources is sodium-24 (Na-24), which emits gamma photons with energy

of 2.76 MeV. The fast neutrons emitted from this source are

characterized by a mono-energetic value (e.g. all emitted neutrons

will have the same energy) instead of the continuous energy

spectrum which is obtained from all alpha-beryllium sources.

1-6-2 The neutron generators:

- These devices are small accelerators in which deuterons (denoted

as d, H2

1 or D2

1 , which is an isotope of the hydrogen) are accelerated

using a potential difference of about 150 Kilo- Volt (KV), to gain energy

of about 150 KeV, and then they collide a tritium (denoted as H3

1 or T3

1 )

target (tritium is another isotope of the hydrogen) to yield an alpha

particle and fast neutrons in accordance with the following nuclear

reaction:

D2

1 + T3

1 He4

2 + n1

0

which is known as (deuteron, neutron) reaction on tritium, and

which can be written as (d, n) reaction on tritium.

18

- The neutrons are emitted from this reaction with a fixed energy

value of 14.1 MeV. Neutron generators of this type are produced with

different neutron yields, varying from about 106 up to 10

12

neutrons/second.

1-6-3 The nuclear reactors:

The nuclear reactor is a facility in which neutrons are obtained as

a result of the fission of a fissile material, such as U-235 or Pu-239, in

sustained chain reactions. The emitted neutrons from the nuclear fission

are fast. However, they are moderated (slowed down) to thermal

neutrons by a moderators which ,usually, is light or heavy water or

graphite. Most of the reactors used for different applications are operated

with thermal neutrons. The neutron density in the reactor core varies

from about 1013

up to 1018

neutrons/cm3, depending on the reactor

power.

1-7 Calculation of the source activity A:

- The activity A (in decay per second) of a certain radioactive

source or sample is defined as the number of decays (or disintegrations)

that occur in this source or sample in a unit of time. In the SI system

units the time is expressed in seconds (s). If the source contains at a

certain moment N radioactive atoms, and if the probability for a single

atom of this type, to decay per second is (1/s) then the activity of this source is equal N decays/second: e.g:

A = N (1-1)

1-8 The specific activity:

- The specific activity is the activity of a unit of mass, volume,

area or length. It represents the amount of activity existing in any of

these massive, volumetric, surface or line samples or species.

1-9 The decay (or disintegration) constant :

The probability for a single atom of a certain radionuclide to

decay per second is called the decay constant of this nuclide and its unit in SI system is (1/s) i,e s

-1.

19

1-10 The units of Activity, The Becquerel and the Curie:

- In the SI system of units the activity A is measured in Becquerel

(Bq), which is one decay (disintegration) per second. So, in a sample

with 15 Bq activity, 15 decays occur per second from the parent nuclide

to the daughter one.

- In the old system of units source activity was expressed in

Curie (Ci). One Ci was defined as the activity of one gram of pure

radium-226. Later, it has been determined that one Ci is equal to 3.7 x

1010

decays/second. So, the relation between the Ci and the Bq is:

1 Ci = 3.7 x 1010

Bq

- The SI units of the specific activity are:

* Bq/Kg for massive species, such as food, soil and other

samples

* Bq/m3 for volumetric samples, such as air, water and

other samples

* Bq/m2 for surface samples such as surface contamination.

* Bq/m for line samples such as long pipes or rods.

- In other systems of units the specific activity may be expressed

in Curies/gm, Bq/liter, Ci/m3, Ci/cm

2, Ci/cm, or many other units. One

should be able to transfer from these units to those of the SI system and

vice verse.

1-11 The physical half-life time T1/2:

- The physical half-life time Tp1/2 of a radio-nuclide, or simply the

half-life time T1/2 is defined as the time period during which one half of

the total number of that nuclide decays (disintegrate) and the other half

remains without decay (disintegration). So, if (for example) the T1/2 of a

certain radio-nuclide is 5.27 years, and if at a certain moment we have a

sample of that nuclide containing 4000 radioactive atoms, then during

5.27 years 2000 atoms decay and the other 2000 remain without decay.

During the second 5.27 years one half of the remaining atoms decays

(e.g 1000 atoms decay and the other 1000 remain without decay).

During the third 5.27 years 500 atoms decay and the other 500 remain

without decay etc.

20

1-12 The biological and effective half-life times:

- When a human being is ingesting or inhaling, any radio-active

isotope (or radio-nuclide) by injection or through a wound, then the

amount of the radio-nuclide in the body will be reduced as a function of

time due to two different effects, which are:

a) The physical decay of the radionuclide, with the physical half-life time T1/2, which is not affected by any physical,

chemical or biological factors.

b) The different biological excretion processes, such as urine and other excreta, with biological have life-time Tb1/2

- The biological half-life time Tb1/2 is defined as the time period

during which one half of the total number of that ingested, inhaled or

injected radio-nuclide will be excreted out from the human body,

through all excretion processes, and the other half remains inside the

body. It should be mentioned that although the Tb1/2 is considered

constant, it may vary in limited way, from man to other, depending on

the human dietary food habits.

- The effective half-life time Te1/2 is defined as the time period

during which one half of the total number of that ingested, inhaled or

injected radio-nuclide will be decayed or excreted out from the human

body, through the physical decay process and all excretion processes,

and the other half will remain inside the body without decay. The

effective half-life time Te1/2 is related with both the physical half-life

time Tp1/2 and the biological half-life time Tb1/2 by the following simple

relation:

(1/ Te1/2) = (1/Tp1/2) + (1/Tb1/2) (1-2)

1-13 The radioactive decay law:

- This law relates the number of remaining atom without decay N

with respect to its initial number N0 as a function of the time t. This

relation is expressed as:

N = N0 e t

(1-3)

- The same law is used to express the exponential decrease of a

sample activity A with respect to its reference activity A0 at a certain

21

reference moment t = 0, as a function of time t. It is expressed in the

following form:

A = A0 e t

(1-4)

1-14 The relation between decay constant and the half- life time T1/2:

- Using the radioactive decay law and the definition of the half-life

time T1/2 it is easy to show that the decay constant is related with the half-life time T1/2 by the following simple relation:

= ln2 / T1/2 or

= 0.693 / T1/2 (1-5)

- The biological decay constant b is related with the biological half-life time Tb1/2 with a relation of the similar form e.g:

b = 0.693 / Tb1/2

and the effective decay constant e is related with the effective half-life time Tb1/2 with a relation of the form:

e = 0.693 / Te1/2

- The effective decay constant e is related with the effective the physical decay constant and the biological decay constant as:

e = p + b (1-6)

22

1-15 Some important multipliers

Subscripts Notation The multiplier

1 deci 1 d 1 x 10-1

1centi 1 c 1 x 10-2

1 milli 1 m 1 x 10-3

1 micro 1 1 x 10-6

1 nano 1 n 1 x 10-9

1 pico 1 p 1 x 10-12

1 femto 1 f 1 x 10-15

Superscripts

1 Deco 1 D 1 x 101

1 Hekto 1 H 1 x 102

1 Kilo 1 K 1 x 103

1 Mega 1 M 1 x 106

1 Gega 1 G 1 x 109

1 Tera 1 T 1 x 1012

1 Exa 1 E 1 x 1015

23

24

CHAPTER 2

INTERACTION OF RADIATION WITH MATTER

2-1 Introduction

From the view point of interaction between particles or radiations

and matter, particles and radiations are divided into four different

groups. These are:

a- Heavy charged particles, such as alpha particles, deuterons, and

protons.

b- Light charged particles, such as beta particles (which are

electrons and positrons).

c- Electromagnetic radiations, such as x-rays and gamma radiations.

d- neutral particles such as neutrons.

2-2 Interaction of heavy charged particles, with matter:

- When a parallel beam of heavy charged particles, such as (alpha) particles or protons is incident on a matter, these particles

interact, mainly, with the orbital electrons of the atoms, which form this

matter, through the Coulomb forces that arise between the charge of the

incident particle and the orbital electrons. The interaction between the

incident particles and the atomic nuclei of the matter is too limited, from

the point of view of radiation protection. This Coulomb interaction

(due to Coulomb force between the incident charged particle and the

orbital electrons) results in transferring a portion of the energy from the

incident particle to the orbital electrons. If the transferred energy is

relatively low (within some eV), then the affected electron can be

removed from its orbit to another one in the same atom with higher

orbital energy, in a process called "excitation". If the transferred

energy is relatively large, then the affected electron will be kicked

out from its mother atom, in a process called "ionization", where

the electron (with its negative charge) becomes free and the atom

becomes ionized with positive charge, e.g. positive ion. In other words

the energy transfer will lead to formation of the so called electron-ion

pair. In case, if the transferred energy is larger enough (within some

hundreds of eV) then the kicked electron, in its turn, may ionize a

25

neutral atom forming a new electron-ion pair or pairs. In this case

electrons are called delta () electrons. The main properties of the interaction between heavy charged particles and matter can be

summarized in the following:

- The main processes by which alpha particles with relatively low

energies (5-10 MeV) transfer their energy to the matter is the ionization

and excitation.

- The track of any heavy charged particle in the matter is a straight

line (due to the large mass of the incident particle with respect to the

electron mass).

- The energy is transferred from the incident heavy charged

particle to the electrons in relatively very small portions. This means

that the energy of the incident heavy charged particle is reduced

gradually as it penetrates through the matter. At the end of the track, the

alpha particle will capture two electrons from the neighbor atoms

forming an inert atom of helium-4.

- The average energy w, which is required to form one

electron-ion pair in air or human tissue is about 34 eV, so that, the

average number of electron-ion pairs formed in the whole range of 5 MeV

alpha particles is about 150000 pairs.

- The delta electrons represent about 70 % of the total number

of free electrons, while the primary electrons represent about 30 %

only.

- Different particles with the same incident energy will have

slightly different rang inside the matter. This effect is called :stragling".

- the range of 5 MeV alpha particles is about 35- 40 mm in air at

standard temperature and pressure, and about 40 micrometers in water or

human tissues.

- The specific ionization s of alpha particles with about 5 MeV

energy in air, which is defined as the number of electron - ion pairs,

formed in 1 mm of their track, varies from about 2000 pairs/mm at the

beginning of the track to more than 6000 pairs/mm at the end of the track.

Fig. (2-1) shows the variation of s as a function of penetration distance

in the matter.

- The stopping power (dE/dx) of alpha particles in a matter, which

is defined as the amount of energy transferred per unit length of the track

26

is given as the product of the energy w needed to form one electron- ion

pair by the specific ionization s, e.g:

dE/dx = w . s (MeV/ cm) (2-1)

Fig. (2-1): Dependence of the specific ionization s of alpha particles

on the depth x in the stopping material.

- One can conclude that while a parallel beam of mono-energetic particles are penetrating a matter their energy is decreased gradually

while their number remains constant up to the end of the track, where

they are converted into inert helium gas.

2-3 Interaction of beta particles with matter:

- Beta particles, which are electrons or positrons emitted in beta

negative or positive decay of some radio-nuclides, are lighter than alpha

particles by a factor of about 7360 times. So, the speed of beta particles

is higher than that of alpha particles with the same energy by a factor of

about 86 times. So, the speed of a beta particle with 1 MeV energy is close

to the speed of light (which is 3x108 m/s). These high speed of beta

particles together with their small mass lead to that they may loose a

considerable part of their energy not only through ionization and

excitation but also by completely different mechanism, due to the very

high de-acceleration of these particles near the atomic nuclei of the

s

R

27

matter. This mechanism is the emission of electromagnetic radiation (x-

ray) known as bremstrahlung radiation.

- As the velocities of beta particles are very high comparing with

alpha particles with the same energies, the interaction time between the

incident beta particle and the orbital electrons and the nuclei of the

atoms is very small, in comparison with the interaction time of an alpha

particle. Moreover, the beta particle and orbital electrons are of the same

mass. So, all these factors strongly affect the character of interaction

between beta particles and matter. The main discrepancies between beta

and alpha interaction with matter can be summarized in the following:

- Beta particles transfer their energy to the matter via two

mechanisms which are: ionization and excitation, and emission of

bremstrahlung radiation. At comparatively low energy of particles (few hundreds KeV) the main process for energy loss is the ionization

and excitation. As the energy of these particles increases the contribution

of emission of bremstrahlung radiation increasesd ant at very high

energies, this contribution becomes the predominant process of energy

loss. Moreover, the role of emission of bremstrahlung radiation is

strongly dependent on the atomic number Z of the matter, where it

increases with the increase of Z. For this reason high Z material should

not be used for shielding sources. The best material that can be used to

shield sources are the light solid material, such as plastic or aluminum to reduce the emission of bremstrahlung radiation (x-ray).

- The energy percentage f of beta particles, which is lost via the

emission of bremstrahlung radiation as a function of both beta particles

maximum energy Emax and the atomic number Z is determined as:

f = 0.035 Emax Z %

- The track of any beta particle in the matter takes the form of a

broken line (due to the similar mass of the two interacting particles).

- The energy transferred from the incident beta particle to the

orbital electron in a single collision varies from a very low portion of the

particle energy up be very high portion of this energy, so that the complete energy of the incident particle may be transferred in a single

collision. This means that the delta electrons are predominant in interaction with matter.

28

Fig. (2-2): The broken track of particles in the material

- The specific ionization s in beta interaction is much less than that

for alpha interaction (by a factor of about one hundred due to the smaller

interaction time). So the range of beta particles is much larger than that

of alpha particles. The range of 1 MeV particles is about 4- 5 m in air, 6- 8 mm in water, plastic or human tissue, and about 2- 3 mm in aluminum.

- Both particles (e.g. the electron and the positron) behaves in the matter in accordance with the previously mentioned two

mechanisms, although they have different sign of the charge. However,

there is an essential difference between the two particles at the end of the

track. When the energy of the positron becomes very low, it annihilates

with one of the electrons of the matter, where they completely vanishes

as a mass, and these two masses are converted into electromagnetic

energy in the form of two photons, each with energy of 511 KeV. This last

process is known as the annihilation process and the two photons with

511 KeV are called annihilation photons.

- It is important to conclude that while a parallel beam of particles are penetrating a matter, not only their energies are decreased

as a function of depth in the matter, but also their number will be

decreased, due to two facts which are: (a) the continuous energy

spectrum of particles, so that low energy particles will loose their energy through, relatively, a very thin layer of the matter while high

energy particles can penetrate to much higher depth, (b) a large number

of particles will be deflected from their initial direction due the their broken track.

29

- Due to the above mentioned factors, the number of particles which penetrate a certain thickness of matter x is decreased

exponentially, in accordance with the following (2-2) relation:

N = N0 e x

(2-2)

where N is the number of particles penetrating the thickness x,

N0 is the number of particles reaching the same point in the absence of

the absorber, and is known as the attenuation factor. This factor is strongly dependent on both atomic number Z of the absorber and energy

E of the particles.

2-4 Interaction of x-ray and gamma radiation with matter:

- When a beam of x-ray or mono-energetic gamma radiation fall

on a matter, its photons may interact with this matter via one of the

following mechanisms, depending on the photon energy as well as on

the atomic number of the matter:

a- The photo-electric effect,

b- Compton scattering, and

c- The pair production.

- Other types of interaction between incident photons and the

matter, such as the interaction with the atomic nuclei, is considered

negligible from the point of view of radiation protection.

2-4-1 The photo-electric effect:

- In this process, the incident photon interacts with one of the

strongly bound orbital electrons of the atom (e.g. with any of electrons

belonging mainly to K or L shells, which are the closest shells to the

nucleus). In this type of interaction the photon delivers its total energy

E to the orbital electron and completely vanishes, and correspondingly, the electron will be knocked out from the atom, carrying an amount of

energy Ee equal to:

Ee = E B (2-3)

where, B is the binding energy of the electron in the corresponding shell,

defined as the amount of energy that should be delivered to the electron

just enough to liberate it from this shell (it varies from less than 1 to

about 100 KeV depending on the atomic number Z of the matter). If E < B, then the process will not occur. Correspondingly, the photo-electric

30

effect will yield one electron which carries approximately the photon

energy.

- The cross- section ph (sigma) of the photo-electric effect, which is defined as the probability of occurrence of this effect, when a single

photon is incident on a unit area (1 cm2) containing a single atom,

strongly depends on the photon energy E as well as on the atomic

number of the matter Z. This probability ph decreases very fast with increasing the photon energy E, while it increases very rapidly with

increasing Z, as Z4 up to Z5. The unit of ph is barn(1 barn = 10-24

cm2).

- Dependence of the photo-electric cross section ph on photon energy E is shown 0n figure (2-3) where the photon energy is expressed

in a logarithmic scale.

K-edge

ph

ln E

Fig: (2-3): Dependence of the photo-electric cross section on photon

energy

2-4-2 Compton scattering:

- In this process, the incident photon interacts with one of the

very loosely bound orbital electrons of the atom, or with a free electron (e.g. with any of electrons belonging to the outermost shells,

which are far away from the nucleus). In this type of interaction the

photon delivers a part of its energy E to the electron and the photon well be deviated (scattered) from its original direction, carrying the

remaining amount of energy. Correspondingly, the Compton scattering

31

of a photon will yield a photon with lower energy and a free Compton

electron, that carries the remaining amount of energy.

c

ln E

Fig: (2-4): Dependence of the Compton cross section on photon energy

- the cross-section c of Compton scattering decreases approximately slowly with increasing of the photon energy, while it

depends linearly on Z of the matter.

2-4-3 The pair production:

- In this process, the incident photon interacts with the strong

electric field of the atomic nucleus, when approaching it very closely

(e.g. interaction between the incident photon and the atomic nucleus),

and if the photon energy is higher than 1022 KeV. In this type of

interaction the photon vanishes completely, and one electron-positron

pair with rest mass equivalent to 1022 KeV is produced. If the energy of

the incident photon E is higher than 1022 KeV, then the excess energy is delivered to the produced electron and positron, in approximately equal

portions. Correspondingly, the pair production will yield two particles

which are the electron and the positron.

- The electron and the positrons behave inside the stopping matter

in the same way as beta particles, e.g. they loose there energy on

ionization and excitation of the atoms of this matter as will as on

emission of bremstrahlung radiation, depending on the atomic number of

the atoms of the absorbing matter. When its energy becomes very low

each positron annihilates with one of the orbital electrons, (e.g. this

positron and electron vanish as a mass converting into two photons, each

32

with energy of 511 KeV). These two photons may interact with matter via

photo-electric process or Compton scattering, or they both may escape

out from the matter without interaction, in a process known as a double

escape, or one photon may interact while the other may escape in a

process known as a single escape.

- The cross-section p of the pair production process increases with the photon energy increase. This increase is relatively slow after the

threshold value of 1022 KeV and becomes fast with increasing the energy.

This probability p depends on the atomic number of the matter as Z2.

p

1022 KeV ln E

Fig: (2-5): Dependence of the pair production cross section on photon

energy

- Due to the formation of energetic electrons and positrons,

resulting from the three processes of interaction between gamma

radiation or x-rays and the matter this radiation, is known as indirectly

ionizing radiation.

2-4-4 The total gamma cross section :

- The total gamma cross-section is defined as the total probability for a single incident photon to interact with one atom

existing in a target of 1 cm2 when it collide this area via any of the three

processes, e.g:

= ph + c + p

33

- The unit of the total cross section is the barn (1 barn = 10-24 cm

2).

2-4-5 The linear attenuation coefficient :

- By definition, the linear attenuation coefficient for a certain matter and at a certain photon energy, is defined as the probability of the

interaction of a single photon that have this energy with all atoms

existing in a cube of 1 cm3 (1 cm

2 area and 1 cm depth) of this matter, on

which it falls by all the three processes. So, if the number of atoms in 1

cm3 is n, and the total interaction cross-section is , then it is clear that:

= n

1022 KeV ln E

Fig: (2-6): Dependence of the total cross section on photon energy

- The unit of the linear attenuation coefficient is cm-1 (e.g. per cm). It is also clear from the behavior of as a function of the energy that depends strongly on the atomic number Z of the attenuating material, specially for both low and high energy photons. Moreover, is strongly dependent on the photon energy E.

2-4-6 The mass attenuation coefficient m:

- In different references another physical quantity, known as the

mass attenuation coefficient m is used instead of the linear attenuation

34

coefficient . This new quantity m is defined by dividing the linear attenuation coefficient by the density of the attenuator, e.g:

m = /

- It is seen that the unit of the mass attenuation coefficient m is (cm

2/ gm). The reason for using m instead of is that its value may be

considered, approximately, constant for different attenuating materials,

for the same photon energy.

2-4-7 The exponential attenuation of x and gamma radiation:

When a narrow beam of mono-energetic x-ray or gamma

radiation falls on a matter of thickness x cm, a part of the incident

number of photons No from this beam will interact with the matter via

any of the three known processes, resulting in the reduction of this

incident number as a function of the thickness x of the matter. Number

of the photons N, that will penetrate the thickness x without any

interaction with the matter will proceed in the same direction and do not

loose any part of their energies. This is expressed, mathematically, by

the following exponential law:

N = No e - x

- The exponential attenuation (e.g. exponential reduction of the

number of photons) is valid when specific conditions are applied. These

conditions are:

a) A very narrow beam consisting of parallel mono-

energetic photons.

b) A very small thickness x of the attenuator, so that,

multiple Compton scattering is negligible.

- In all other cases this exponential law is not valid due to

Compton scattering of photons from the broad beam as well as the

multiple Compton scattering of some photons due to the thick layer of

the attenuator. This will be discussed, in details, in a later chapter on

build-up.

- If the linear attenuation coefficient is used (in cm-1) then the thickness x of the attenuator should be expressed in (cm), to get non-

dimensional value of the product x. However, when the mass attenuation coefficient m is used (in cm

2/gm), then the thickness of the

attenuator should be expressed in the so called mass-thickness xm, which

35

is obtained as the product of the linear thickness x of the attenuator and

its density , e.g:

xm = x

The unit of the mass-thickness xm is (gram/cm2).

- The exponential attenuation of x-rays and gamma radiation

makes the concept of the range for this type of electromagnetic radiation

is not valid. A definite portion of the incident beam will penetrate

through the attenuating matter, even when its thickness is too large. For

example, if a Co-60 source is shielded (surrounded) by more than 2 m

thick concrete wall some emitted photons from this cobalt will penetrate

through this shield, without suffering any kind of interaction.

2-4-8 The half value layer (HVL):

- The half value layer (HVL), or half value thickness, of a matter

at a certain gamma energy, is defined as the thickness of that matter,

which is necessary to attenuate the original number of the incident

photons No, with this energy, to its half value ( e.g. to N = 1/2 No). The

HVL is related with the linear attenuation coefficient with the following simple relation:

HVL = 0.693 /

- Since is dependent on the radiation energy E and the material of the attenuator Z, the HVL is also dependent on these factors.

- The unit of the HVL is cm when the is expressed in cm-1, and its unit is (gm/ cm

2), when is expressed in cm2/ gm.

2-4-9 The tenth value layer (TVL):

- The Tenth value layer (TVL), or Tenth value thickness, of a

matter at a certain gamma energy, is defined as the thickness of that

matter, which is necessary to attenuate the original number of the

incident photons No, with this energy, to one tenth of this value ( e.g. to

N = 1/10 No). The TVL has the same units as the HVL, and it is related

with last value with the following relation:

TVL = 3.32 HVL

36

2-4-9 The energy absorption coefficient a:

- The energy absorption coefficient represents the portion of

energy absorbed from x-ray or gamma radiation in a definite volume of

the matter. This coefficient is used to account for the so called "kerma"

or absorbed dose from x or gamma radiation into the interacting matter,

(e.g. in dose calculations). It should be mentioned that authors of some

references are using, by fault, this coefficient to express the attenuation

coefficient . These Two coefficient (a and , both linear and mass) have different values, specially at medium and high photon energies, and

should not replace each other, except at very low photon energies (less

than few hundreds of KeV) where they are very close to each other.

- The reason of the discrepancy between a and is the Compton scattering and the pair production. In Compton scattering the photon is

deviated from its original direction, transferring only undefined part of

its energy to the matter, and the scattered photon may escape out from

this matter, so that although it has been omitted out from the beam, it

does not transfer its complete energy to the matter. In the pair production

the energy may not be transferred completely to the matter, since one or

even the two photons, resulting from the annihilation of the positron

with one electron may escape out of the matter.

- Due to the above mentioned reasons is almost higher than a , specially with increasing the photon energy

2-5 Interaction of the neutrons with the matter:

- Since the neutrons are neutral particles (e.g. uncharged particles),

they do not interact neither with any of the orbital electrons nor electro-

statically with the atomic nuclei. They may interact only with nuclei via

nuclear forces, when they very closely approach any of them. This is the

reason of the high penetrating power of neutrons in the matter.

- the most important and efficient mean for energy transfer from

neutrons to the matter is the elastic scattering of the neutron on light

nuclei, such as hydrogen (in wax, water, polyethylene, or plastic),

deuterium (in heavy water) beryllium, carbon, and oxygen. With

decreasing the mass number of the interacting nucleus, the average

energy, transferred from the neutron to this nucleus, in a single collision,

increases. For this reason the hydrogen nuclei are considered the best

moderator for neutrons, and the materials which contain high

37

concentration of hydrogen, such as wax, water, Polyethylene, and plastic

are extensively used for effective slowing down of the fast neutrons. In a

single collision with a hydrogen nucleus, the neutron loses, in average,

63 % of its energy. This portion of energy is transferred to a proton,

which is the hydrogen nucleus.

- Since the recoil protons are heavy charged particles, they ionize

the matter. So, the neutrons are considered as indirectly ionizing

particles.

2-5-1 The neutron moderation:

- The neutron moderation means the slowing down of fast

neutrons (e.g. decreasing their energies from the MeV range to about

0.025 eV. Neutrons with such low energies are called thermal neutrons,

since their motion is controlled by the prevailing temperature.

- For slowing down of the fast neutrons (with energy of about

several MeV) to thermal neutrons, these neutrons should be subjected, in

average, to about 18-19 collisions with hydrogen nuclei. This number of

collisions requires a thickness of a hydrogen rich material, such as wax

or water of about 15- 25 cm.

- The thickness of the wax or water may be increased over the

mentioned values for radiation protection purposes, since these materials

absorb thermal neutrons with a certain probability forming deuterium

atoms which are stable.

- The role of inelastic scattering of neutrons for neutron

moderation is negligible.

2-5-2 The neutron capture:

- when a neutron approach very closely to a nucleus it may be

captured in it, forming a new isotope of the same element, with the

emission of a prompt gamma photon. An example of the neutron capture

reaction is:

no1 + Cd11447 Cd

115

47 +

- The probability of the neutron capture is strongly dependent on

the neutron energy. The reaction cross-section (which represents the probability of the neutron capture) increases strongly with the decrease

of the energy, reaching very high values for thermal and slow neutrons

38

(the slow neutrons are those with energies just higher than that of

thermal neutrons). Moreover, at certain energy values for the slow and

thermal neutrons, and for some nuclides the probability of the neutron

capture reaches very high values, known as a resonance neutron capture

or absorption. The energy values at which the resonance neutron capture

occurs depend on the absorbing nuclide. For example for Cd11447 , it has

been found that the resonance capture occurs at thermal and low

energies, and the capture probability at resonance reaches extremely

high values. For this reason Cd11447 is considered one of the best absorber

for thermal and slow neutrons.

- One of the most effective method to shield a neutron source and

to reduce effective doses around it is to put three layers of different

materials in the following consequence from the source: a) About 20 cm

of wax, plastic or any other solid (or liquid) material, rich with hydrogen

content to moderate fast neutron and convert them into thermal or slow

neutrons, then b) A thin sheet of Cd11447 (with about 1 mm thickness) to

absorb thermal and slow neutrons, and finally c) a certain thickness of

lead to attenuate the prompt gamma radiation emitted in the neutron

capture in Cd11447 .

- There are other materials that can be used practically to reduce

the neutron doses arising from different neutron sources, by moderation

and absorption of these neutrons, such as water (normal or light water),

boron and others

- In the absence of all of the mentioned materials one can use other

commonly existing materials in the field, such as the sand and other

types of soil. Although their shielding properties is too limited in

comparison with other materials, a large thickness of these sand or soil

may reduce neutron doses to lesser values due to the presence of some

light elements such as oxygen and carbon.

39

CHAPTER 3

RADIATION DETECTORS, SURVEY METERS

AND CONTAMINATION MONITORS

3-1 General:

- The main two processes which are used for detection of different

types of ionizing radiation are based on the use of:

a) Ionization of the detector material and formation of

electron-ion pairs, or electron hole pairs, and collection of this

charges or their current.

b) Excitation of the detector material and then measurement

of the emitted light during the de-excitation process, and

collection of this light or their current.

- There are other processes, which are used for detection and

counting of ionizing radiation. For example, one of these processes is the

use of activation of a certain nuclides by irradiation of certain material

by neutrons and then by measurement of the induced activity due to the

neutron capture.

- The type of the detector that should be used for detection and

counting and identifying of ionizing radiation depends strongly on:

a) The type of the radiation (e.g. heavy or light charged particles,

neutrons, x, or gamma radiation.

b) The energy of the measured particles or photons.

c) The intensity of the radiation field (e.g. the particle or photon

flounce).

d) The purpose of detection and measurement.

3-2 The gas detectors:

- In all gas detectors, detection of directly and indirectly ionizing

radiation is done through the ionization of some mixture of a gas

contained in a vessel with certain shape and volume.

- For directly ionizing radiation, such as heavy charged particles or

beta particles, the ionization of the gas atoms or molecules occurs inside

40

the detector vessel. The average number of the resulting primary

electron-ion pairs in the detector is defined by dividing the particle

energy (in eV) by 34 eV, which is the average energy needed to form

one electron- ion pair. For detection of heavy charged particles (such as

alpha), the detector wall should be equipped with a very thin window of

low Z material (less than 40 gm/cm2 of a light material) to permit the entrance of these particles inside the detector, without loosing a

considerable part of its energy in this window. For the detection of beta

particles the window can be done from a thicker material, since the

range of these particles is much higher than that of alpha particles.

- For the indirectly ionizing radiation, namely x and gamma

radiation, ionization of the detectors gas is done by the primary charged electrons and positrons, emitted as a result of the interaction of the

incident photons with a very thin layer of a heavy material, such as lead,

fixed inside the wall of the detector. For detection of x and gamma

photons, There is no need to make a window in the detector wall due to

the very large range of photons.

- For neutrons, which are indirectly ionizing radiation too, the

ionization is done by charged particles such as protons emitted as a

result of the elastic scattering of the incident fast neutrons with hydrogen

nuclei existing in a very thin layer of polyethylene fixed inside the

detector wall, or by alpha particles, which are emitted as a result of the

neutron capture of thermal neutrons in certain gas materials with high

reaction cross-section, which is filling the detector, such as BF3 gas

(Boron tri-Fluoride) or others. Due to the high penetrability of neutrons,

there is no need to make any window in neutron detectors.

- There are three types of gas detectors which are:

a) the ionization chamber,

b) the proportional counter, and

c) the Geiger- Muller (GM) counter.

- For all types of gas detectors, the intrinsic detection efficiency is 100 % only for all heavy charged particles. For beta particles the

efficiency is slightly less than 100 %, due to their continuous energy

spectrum, so that a part of the low energy particles will be absorbed

inside the window thickness. The efficiency of all gas detectors for

measuring photons or neutrons is extremely low, and strongly dependent

on their energy. For example the intrinsic efficiency of these detectors

41

for photons may vary from few percents (2-4 %) to very low values (less

by many orders of magnitude) with increasing the energy of photons.

Remark: the intrinsic efficiency of a detector, for a certain type of indirectly ionizing radiation at a certain energy, is defined as the ratio of

the number of particles or photons with the mentioned energy detected

by the detector from a given source, in a certain time period to the total

number of these particles or photons, with the same energy, incident

from the source on the detector surface, during the same time period. To

get the efficiency in percent this ratio should be multiplied by 100. For

example, if the intrinsic detector efficiency for photons with 662 KeV

energy is 2.5 % then this detector will detect only 2.5 % of photons

incident on its sensitive surface with this energy.

3-2-1 The ionization chamber:

- It is a detection device (see fig. (3-1), which consists of::

a- Two electrodes (anode a and cathode c) connected to a

moderate potential difference V (about 50- 100 volts depending

on the chamber volume and pressure) to secure collection of the

majority of the electrons and ions, which are generated by the

ionizing radiation inside the chamber on the anode and the

cathode respectively.

b- A guard grid g between the anode and the cathode to

secure independency the collected current, or consequently

voltage of the output pulse signal, resulting due to the passage of

this current through a high Ohmic resistance R, on the track

position of the incident particle.

- The ionization chambers can be used in a current regime (e.g. to measure the very small average electric current, resulting by ionization

by a large number of incident particles or photons, and the chamber is

then known as a current type ionization chamber. They, also, can be

used to measure consequence pulses resulting from individual ionization

events (particles or photons), and hence to determine the number and

energies of these particles or photons, and in this case the chamber is

known as a pulse type ionization chamber.

- Since the collected current in the ionization chamber is too low

(in the range of pico-Ampers), the ionization chamber should be

42

connected with a direct current amplifier (or pulse height amplifier) with

a very high amplification gain (thousands or more).

a C

g c V R

Fig (3-1): A diagram of an ionization chamber

- Ionization chambers are characterized by certain characteristics.

Some of these characteristics are:

a) The multiplication gain of any chamber equals 1, which

means that there is no multiplication of the electric current

resulting by ionizing radiation.

b) Relatively, high energy resolution r, which means that it

can be used to differentiate between particles or photons with

relatively close energies. The energy resolution of the ionization

chambers r varies between about 2.5 and 7 %, depending on its

volume and on the gas pressure.

Remark: the energy resolution r is defined as the ratio of the

energy fluctuation E caused by the detection process, to the energy E of the particle multiplied by 100 (to get it as a percent)

e.g:

r = (E/E)x100 %.

43

c) Relatively, a constant energy response curve in a wide

range of energies, comparing with all other detectors, when the

chamber is used as a detector in dose or dose-rate survey meters.

A constant energy response means that the ratio of the

measured dose (or electric current) from ionizing radiation with a

given energy E to that at a reference one Er remains constant in a

wide range of energies when the radiation field is homogeneous.

This is a very important property of ionization chambers.

d) In some cases the wall of the chamber is made from a

material having a similar composition as air to correct for energy

absorption in different materials, for more accurate determination

of doses or dose rates. In these cases the chamber is known as

air-wall ionization chamber.

e) For measurement of relatively high energy beta particle

or photons, it is necessary to increase the gas pressure inside the

chamber to secure full stopping of the ionizing beta particles

within it. In This case the chamber is known as a pressurized

ionization chamber. Such cambers are important for dose

measurements in a radiation field with a wide energy range.

- The shape of the output pulse from a pulse type ionization

chamber, which represents the detection of a single particle or

photon with a given energy value is demonstrated in fig.(3-2).

The polarity of th pulse on this figure is inverted, since it is

originally negative. The vertical axis shows the output voltage

amplitude of the pulse which is proportional to the energy of the

particle or photon, while the horizontal axis shows the time

duration of the pulse and dependence of its amplitude on time.

The voltage amplitude of the output pulses lies in the range of

less than one microvolt up to about one hundred microvolts,

depending on the particle energy. The pulse durations lies

between less than a 100 microseconds up to more than 1000

microseconds depending on the geometrical dimensions of the

chamber as well as on its internal capacitance and resistance. The

values of the used electronic devises such as the input impedance

and capacitance of the of this circuit strongly affect the duration

of the output pulses

44

The pulse amplitude

The time (microsecond)

Fig (3-2): The pulse shape at the output of an pulse type

ionization chamber

3-2-2 The proportional counter:

- The proportional counter, (see fig 3-3) is a gas detector of a

cylindrical form, where a metallic cylinder is acting as the detector

cathode, while a very thin coaxial metallic wire with a regular diameter

is used as the anode.

- The applied voltage difference between the anode and the

cathode for the proportional counter is much higher than that used in an

ionization chamber with the same dimensions. This increase in the

applied voltage difference leads to the acceleration of ions and electrons,

so that they become capable to ionize new atoms, while they are moving

to the cathode and anode respectively. This yields in a high increase of

the electric current caused by ionizing radiations. So, the proportional

counter is acting as a detector and a current multiplier.

45

V

Fig. (3-3): A diagram of a proportional counter

- The multiplication gain of the gas in the proportional counter

varies between about 100 to more than one thousand, depending on the

magnitude of the applied potential difference between its anode and

cathode.

- As a result of the multiplication the energy resolution r of the

proportional counter is much poorer than that of the ionization chamber.

Its values vary from about 10 to 30 %.

- Although the energy resolution of the proportional counters is

relatively poor, there is still some proportionality between the energy of

the detected particle or photon and the obtained current or pulse height

from this detector. This makes the accuracy of this detector for dose

measurements acceptable and this detector comes, directly, in the next

category after the ionization chamber, concerning the accuracy point of

view, as well as from the constancy of the energy response at relatively

wide range of photons energy.

- in spite of the relatively high multiplication gain in the

proportional counter, it still needs to be connected at the output to a

current or voltage amplifier, but with a lower amplification gain than

that used with the ionization chambers.

3-2-3 The Geiger- Muller (GM) counter:

- From the construction point of view the GM counters are exactly

similar to the proportional counters. The main difference is that the GM

46

counter is operated at relatively higher potential difference between the

anode and the cathode.

- With increasing the applied voltage the current multiplication in

the gas of the tube becomes very high and almost reaches infinity. When

an ionizing particle or photon inters the GM tube, and when it interacts

with the detector material causing even one electron ion pair a series of consequent ionization occurs making avalanche multiplication. This will

cause occurring of electric discharge of the detector gas.

- The gas discharge will continue unless, it will be stopped by

internal or external reason in a process called quenching. The external

quenching is secured by inserting a large Ohmic resistance R in series

with the high voltage source, while the internal quenching is secured by

the addition of a certain ratio of a mono-atomic gas. The second

technique of quenching is preferred, since the first one leads to a serious

increase in the detector dead time, due to the increase of the magnitude

of the resistance.

- As a result of infinite amplification of the GM tubes, particles or

photons with different energies will give the same electronic signals with

the same pulse amplitude, so that, it can be measured without further

amplification.

- Due to the complete discharge through the detector tube, the

proportionality between the energy of the particle and the pulse height of

corresponding signal is completely lost. In other words the GM counter,

completely, does not differentiate between different energies, and it can

be only used to count the number of pulses (detected particles or

photons) independent of their energies.

- The dead time of a pulse type detector is defined as the time

period through which the electrons and ions are collected and treated as

a pulse. During the dead time the detector will not detect any other

ionization event, so If the time separation between two sequent ionizing

events (e.g. two consequent registered particles or photons) is less than

the detector dead time, then they will be detected as a single particle or

photon, and hence there will be some loss of the detected number of

particles or photons.

- The energy response curve of the GM counter is, comparatively,

worse than that of the proportional counter. For this reason, special

47

filters are used with the GM counters to correct for the non-constancy of

the response curve.

- It should be mentioned that dose survey meters that use GM

counters as a detector, should not be used in any place containing high

radio-frequency (rf) source, such as linear accelerators, since they are

very sensitive to high frequencies and they almost give full scale reading

in these fields without the presence of any type of the ionizing radiation.

3-3 The scintillation detectors:

- In all scintillation detectors, detection of directly and indirectly

ionizing radiation is done through the excitation of some atoms, which

are consisted in a solid crystalline or liquid scintillator. So, any

scintillation detector, (see fig 3-4), consists, mainly, of, at least, two

components, which are:

- The scintillation crystal or liquid (the scintillator)

- The Photo-Multiplier Tube (PMT).

Fig. (3-4: The components of a scintillation detector

- Sometimes, there is a third component, which is the so called

light pipe. This pipe is made of a highly transparent type of silicon glass,

which is acting as a light conductor to transfer light photons emitted

from the crystal (or liquid scintillator) to the photo-cathode of the PMT.

The PMT

The light pipe The scintillator

48

- All the components are matched together, without any air voids

or bubbles by putting a small drop of silicon oil between any of these

components and pressing so that no air bubbles are existing in between.

The detector components are enclosed inside a hermetically sealed

metallic enclosure, so that no light can penetrate through it.

- The function of the scintillator is to emit photons of visible light,

The number of these photons is linearly dependent on the energy of the

incident particle. As these emitted photons fall on the photo-cathode of

the PMT, a limited number of electrons will be emitted from this photo-

cathode. The number of these photo-electrons is linearly dependent on

the number of the incident photons on the photo-cathode, and

consequently, on the energy of the incident particle on the scintillator.

- The role of the photo-multiplier tube (PMT) is to multiply the

number of emitted electrons from the photo-cathode, by a very large

factor (at least some thousands times and much more). For this purpose

the PMT contains a large number of dynodes (about 9- 13 dynodes),

each of which is covered with a material with high coefficient of the secondary emission. The emitted photo-electrons are accelerated toward

the first dynode by a positive voltage difference V, so that they gain an

amount of kinetic energy equal V electron volts, and become capable to

induce secondary electron emission from the dynode, so that their

number will be multiplied by a factor equal to the coefficient of

secondary emission . This coefficient is strongly dependent on the voltage difference V and may reach, relatively, high values (up to 3 and

more) with the increase of V. Electrons emitted from the first dynode

are, again, accelerated toward the second dynode by another positive

voltage difference V, giving rise to another step of a secondary emission

from this second dynode, and yielding second multiplication . Then the consequent acceleration processes toward the next dynodes with a

multiplication factors of on each one of these dynodes will yield a total

multiplication factor of n (if the value of is the same for all dynodes), where n is the number of dynodes in the PMT. After multiplication a

huge number of electrons are emitted from the last dynode and they are

collected on the anode of the PMT, giving a negative pulse on the output

of this anode due to the presence of a high ohmic resistance.

- The anode pulse represents the registration of a single particle in

the detector, and the amplitude of this pulse is proportional to the energy

of the particle. So, the number of the registered pulses is proportional to

49

the number of the incident particles or photons, while the amplitude of

each pulse represents the energy of the registered particle or photon.

Output pulses on the anode of the PMT have a similar form of the pulses

from an ionization chamber shown on fig. (3-2), but the time duration of

the pulse may be more less than that of the ionization chamber for some

types of scintillation crystals.

- It should be mentioned that the electron multiplication gain M of

the PMT, (which is approximately equal to the coefficient raised to the

power n (i.e. M n)) is strongly dependent on the biasing voltage V which is supplied to the PMT Anode or cathode. This voltage is divided

by a potential divider using a set of resistances to bias the cathode, all

dynodes and the anode with the nominal voltages. It is recommended to

supply the PMT with the nominal voltage, since the increase of V will

increase the factor M, but at the same time it will shorten, strongly, the

service life-time of the PMT.

- different types of radiations are detected using different

scintillators. Table (3-1) represents the most widely used scintillators for

different types of radiations. All these scintillators emit violet light with

wave length shown in table (3-1).

- Alpha particles and protons can be easily detected using a thin

layer (about 1mm thickness) zinc sulphide crystal doped with silver ZnS

(Ag), while electrons and positrons can be detected using organic

crystals or liquids.

- The Sodium Iodide crystal with Thallium NaI(Tl) is the best

scintillation crystal that can be used to detect gamma radiation with a

higher efficiency, due to its high density. Moreover, the addition of a

small ratio of Thallium to the Sodium iodide makes the crystal capable

for emission of light photons at room temperature. To meat the required

detection efficiency of gamma radiation, the NaI(Tl) crystal is grown

with a different thicknesses. These crystals are available in the market,

mainly, in a cylindrical form with dimensions ranging from 1/2 inch

diameter x 1/2 inch height, up to more than 10 "

x 10 ". Generally

speaking, the scintillation gamma detectors are much sensitive to detect

gamma radiation, in comparison with gas detectors, and the detector

with 3" x 3

" NaI(Tl) crystal is considered as a reference one, so that, the

relative efficiency of any other gas and solid detectors, is given referring

to this reference one.

50

- Fast neutrons can be easily detected by scintillation detectors

using secondary charged particles, which arise as a result of the neutron

elastic scattering or nuclear reaction. For example, these neutrons can be

detected by putting a very thin layer of polyethylene in front of the

ZnS(Ag) crystal, so that neutrons will collide with hydrogen atoms of

the polyethylene, yielding recoil protons, which are detected in this

crystal.