Workshop on Radiation Protection

8/14/2019 Workshop on Radiation Protection

1/135

1

WORKSHOP ON RADIATION PROTECTIONWORKSHOP ON RADIATION PROTECTIONAN INTRODUCTIONAN INTRODUCTION

TOTO

RADIATION PROTECTIONRADIATION PROTECTIONRasel NirjhonRasel Nirjhon

Mawlana Bhashani Science & Technology UniversityMawlana Bhashani Science & Technology University,,Tangail, BangladeshTangail, Bangladesh


2/135

2

1.THE ATOM1.THE ATOMa. The emission of high-energy ionisingradiations and their interaction with matter arethe processes that occur on the microscopic

scale of atoms and their nuclei.Therefore, a brief over view on the atomic andnuclear structure is being presented for

recollecting the properties of ionising radiationand hence, understanding the guiding principlesof Radiation Protection.


3/135

3

b. A typical atom has diameter~10-7mm

and consists of a +(ve)ly chargednucleus, with a diameter~5 X 10-12 mm,

surrounded by a cloud of (ve)lycharged moving electrons (e-). Thenucleus consists of two types of

particles: the +(ve)ly charged proton,and uncharged neutron.


4/135

4

c. Proton and neutron have ~ equalc. Proton and neutron have ~ equal

mass and are ~ 2000 times the mass ofmass and are ~ 2000 times the mass ofelectron.electron.

d. Proton has 1 unit +ve charge, andd. Proton has 1 unit +ve charge, and

electron has 1 unit -(ve) chargeelectron has 1 unit -(ve) charge(( 1.6021.602 1010-19-19 coulombs, thecoulombs, the

elementary charge).elementary charge).

e. Some physical properties of thee. Some physical properties of theatomic particles are summarised inatomic particles are summarised in

Table.Table.


5/135

5

Atomic diameterAtomic diameter 1010-7-7 mmmm

Nuclear diameterNuclear diameter 1010-12-12 mmmmSo, Nuclear size isSo, Nuclear size is 101055 timestimes

smaller than an atom.smaller than an atom.


6/135

6

Table: Physical Properties of Atomic ParticlesTable: Physical Properties of Atomic Particles

Particle Symbol Mass(gm) Mass(u) Charge(e)

Proton p 1.6726 10-24 1 +1

Neutron n 1.6749 10-24 1 0

Electron - 9.11 10-28 0.00055 -1= atomic mass unit=1.66 10-24 g (1.66 10-21 mg)= elementary charge=1.602 10-19 coulomb


7/135

7

f. The nucleus is extremely small and compact comparedto the electron cloud. Thus, while the electron cloud takes-

up almost the whole of the atomic volume, 99.98% ofatomic mass is concentrated in the tiny nucleus.

g. In normal states, the +ve and -ve charges within an atom

are equal and therefore, the atom as a whole is electricallyneutral. But sometimes, due to chemical or physicalprocesses (changes), an atom may lose one or moreelectrons, or may gain (acquire) extra electrons. In such

case, the atom will have a net +(ve) or -(ve) charge and iscalled cation or anion, respectively, as they are attractedtowards cathode or anode.


8/135

8

1.1. Energy Levels in Atom1.1. Energy Levels in Atom

a. Electrons( in the electron cloud) in an atom are only permitted to reside in specific atomic orbitals around thenucleus.

b.Orbitals determine the energies of the electrons (i.e;anelectron in an outer orbital has higher energy than one ininner orbital).

c. Energy-Level for each orbital and the max no. ofelectrons permitted to reside in each orbital are governed

by the laws of Quantum Mechanics.


9/135

9

d. Electrons may move from one orbital to another only byemitting or absorbing a quantum of electro-magneticradiation (a photon) with energy equal to the difference

between the energies of the two orbitals, shown(fig:-)Emission of a photon when an electron moves fromthe 2nd orbital (high energy level) to the 1st orbital (lowenergy level) of a Lithium atom, and in doing so, it releases

excess energy.

e.g., 73Li

(photon)

Fig.


10/135

10

Depending on the atom, the nucleus and the energy levelsinvolved, the photon (emitted or absorbed) may be one of

infra-red, visible or ultra-violet light, or for heavy atomnuclei and inner orbitals where energy levels are widelyseparated, the photon may be of x-ray.

The specific energy of photons (emitted from atoms) isevident in the sp. colour of light emitted by lasers in thecharacteristic x-rays produced by X-ray machine.


11/135

11

1.2. The Nucleus1.2. The Nucleusa. It is comprised of protons & neutrons (collectively callednucleons) which are held together by a strong, short-ranged nuclearforce( an example of a Lithium nucleus, comprising 3 protons & 4neutrons, is shown in previous figure).

b. A nucleus (denoted by AZX), can be identified by its atomic

number, Z, and its mass number, A, where X takes symbol of anelement (e.g., H for Hydrogen, He for helium, etc.), Z is the no. of

protons and A is the total no. of nucleons in the nucleus.

However, the atomic no. is often not written with the symbol of anelement, as the proton no. is implied by the element symbol. e.g.,1H, 2H, 4He, 7Li, 12 C, 13 C, 13N , 14N, 58 Co, 60 Co, 88 Sr, 90 Sr,134 Cs, 137 Cs,etc


12/135

12

Nucleus Contd., .

c. Nuclei having (i) same atomic no (Z) but differentmass no(A) are called isotopes.

(ii) Same mass no. (A), different atomic no. (Z) are

Isobars;(iii) Same atomic no (Z) and mass no (A), but differentonly in energy state of their nuclei are called isomers;and

(iv) Same neutron nos. but different atomic no. (Z) arecalled Isotones, as shown in the following Table:


13/135

13

Nuclei Types ExamplesSame Z, but different A (Isotopes) 12 6C,

136C ,

146C ;

5928Ni ,

6028Ni ;

Same A, but different Z(Isobars) 14 6C,14

7N;58

26 Fe,58

28Ni ,58

27 Co

Same Z and A , different energy(Isomers)

60m28Ni,

6028Ni ;

99m43 Tc;

9943 Tc

137m56 Ba,

13756 Ba

Same neutron nos.(Isotones)13

6C,14

7N ;

Nucleus Contd., .

Table: Different Types of nuclei


14/135

14

Nucleus Contd., .

d. Only the element with Z=1 (I.e., Hydrogen) toZ=92 (Uranium) are stable and, therefore, existsin nature. Those with Z=93 (Nepturium) to Z=103(Lawrencium) are unstable (i.e.radioactive) andhave been synthesized by nuclear reactions.

e. Mentionable that, many of the stable elements

have also their unstable (i,e., radioactive) isotopes.


15/135

15

1.3. STABLE AND UNSTABLE Nuclei1.3. STABLE AND UNSTABLE Nucleia. Light nuclei have roughly equal nos. of protons and neutrons.

b. But, the heavier nuclei have increased nos. of neutrons comparedto protons. These excess neutrons provide the additional nuclearforce required to hold the nucleus together (against the repulsiveforce of electrostatic +(ve) charge of the protons).

c. Nuclei having the appropriate nos. of protons (z) and neutrons(n) such that the nucleus will hold together indefinitely are Stable.

A line of best fit, drawn by plotting N Vs P of all the stablenuclides is called the line of stability.

d. A stable nucleus holds together indefinitely;

e. All stable nuclides lie close to the line of stability.


16/135

16

Fig. Nuclei line of stability

N=A-Z

140

Z


17/135

17

f. Nuclei with Z > 82 and many nuclei produced artificially have N/p ratios that make the nucleus unstable. These nuclei lie somewhat away from the line of stability. These unstable nuclei canundergo one or more spontaneous transformations which result in achange in the N/P ratio and thus bring the nucleus closer to the lineof stability. During these transformations, the nucleus & hence theatom changes from one element (parent) to a different element

(daughter). Such spontaneous nuclear transformations alwaysrelease internal energy and most of this energy is carried awayfrom the nucleus in the from of fast moving particles and photonscollectively called nuclear radiation. The unstable nuclei are,

therefore, called radioactive and the spontaneous process is calledradioactive decay.


18/135

18

g. An unstable nucleus becomes more stable byundergoing a spontaneous transformation during which a

particle is ejected from the nucleus.The modes of the radioactive decay are called:

alpha ( )decay, beta( ) decay, and gamma( )decay.


19/135

19

h. The energy released in a single radioactive decay is small inabsolute terms, but is very large compared with the energies

involved (released or absorbed) in chemical reactions. Theenergy unit used is the eV.

(1eV=1 electron charge accelerated between 1 volt

= 1.602 X 10-19 coulombs X 1V=1.602 X 10-19 joule)

i. Chemical reactions usually involve energy changes 1eV peratom, while radioactive decay energies are typically 1 MeV

per nucleus.

ii. Electric charge and the total number of nucleons isconserved in all form of radioactive decay.


20/135

20

2. RADIOACTIVE DECAY MODES2. RADIOACTIVE DECAY MODES

2.1. Alpha decay: The alpha particle is a di-positivehelium ion, 42He

2+, and comprised of 2 protons & 2

neutrons. This stable configuration of nucleons(42He2+ ) is commonly ejected from heavier nuclei than

lead (Z=82), and not from light nuclei. The changes inZ &A that take place can be written as:

AZX

(A-4)(Z-2) Y +

42(

42He)


21/135

21

The -particles are mono energetic, i.e; they are not emitted withbroad range of energies, rather each - particle is emitted with aspecific energy.

Most -emitters give a group of discrete particle energies, because the parent nucleus may decay to one of many energystates of the daughter nucleus as shown in the figure(e.g., for212

83 Bi). A decay of an excited state of the daughter nucleus is thenhappned by one or more emissions of gamma rays ( ) as will beshown later. About 160 emitters have been identified, havingenergies from ~4MeV to 10 MeV. as shown in the example (212 83 Bi

transformation in the figure):


22/135

22

21283Bi

20881Tl+

42

The decay schemes of12 83 Bi shows decay paths that

produce the discrete - particle energies:1283 Bi

Parent nucleus

Energy-states of the daughter nucleus 208 81Tl

1 0.495 MeV

-particleenergies

1 5.601 MeV

2 5.621 MeV3 5.764 MeV4 6.050 MeV5 6.090 MeV

2 0.472 MeV

3 0.372 MeVEnergy levels of the daughternucleus

4 0.040 MeV

5

20881Tl

0MeV (Stable daughter20881Tl )


23/135

23

a. The particles also carry away energy from the

nucleus.b. The particles energies are very specific and canbe used to identify the radionuclide.

c. Energy unit- the electron-volt is written as:1eV= 1.602 X 10-19 J


24/135

24

2.2. Beta Decay(an isobaric transition )2.2. Beta Decay(an isobaric transition )

(i.e. A remains un-changed)(i.e. A remains un-changed)

A beta particle is an electron ( -) or a

position (+

) originating fromradioactive decay(a position is the antiparticle of an electron, it is identical in

every way except that its charge is oneunit+(ve)


25/135

25

a. ( -)decay. If the conversion of a neutron (10n) into a

proton1

1p) results in a more stable nucleus, then( -) decay will occur.A ( -) particle alone cannotbe emitted because such a change would violate thelaw of conservation of momentum. An antineutrino

-, is also produced with the ( -) particle. The( -) decay process is written as:

n p+ + - +- or, n p+ + - +-

Where `n is one of the neutrons with in the nucleus(free neutron also decay by this process with t1/2 : 12minutes).


26/135

26

The energy released in this decay process is

shared by electron (-

) and the nutrino.Therefore, unlike the discrete energies of particles, the - particles are emitted with arange of energies (from 0 upwards to a fixedmax).


27/135

27

A typical - energy spectrum is shown (figure--)


28/135

28

b.b. + decay.+ decay.

i. If the conversion of a proton into a neutron will result in a

move stable nucleus the + decay will occur. Analogously tothe - decay, the + decay will involve one of the protons inthe nucleus trans forming into a neutron. The process is writtenas:

p+ n + + +or, p+ n+ e+ +.

As in the - decay, the positron may have any energy over arange from zero to a fixed max. The e+ has a short-life,

annihilating with an electron(e-) soon after it is created throughformation & emission of two 0.511 MeV rays.


29/135

29

2. Rather than emitting a e+, a proton may

become a neutron by capturing an e-, this iscalled electron capture. Again, a neutrino isrequired to satisfy conservation laws. The

electron capture process is written as:p+ + e- n +


30/135

30

Some examples of beta decays are:

(a) Conversion of n p+ by emission, e,g.,

6037 Co 60 28Ni+ + - +- (produces - &- )

Conversion of n p+ by emission, e,g.,

137

55 Cs137

56 Ba + -

+

-

(produces -

&

-

)(b1) Conversion of p+ n by

+emission, e,g.,

127N 12 6C +

+ + (produces e+ & )

(b2) Conversion of p+ n electron capture , e,g.,

74Be + e- 73Li +( capture e- & produces).


31/135

31

The decay scheme of137 55 Cs, which decays by - decay, is

shown (fig.)

13755 Cs (parent)

- 0.512 MeV (94.6%)

0.662 MeV 137m 56 Ba (isomer)

excited state

- 1.173 MeV 0.662 MeV

(5.4%) (85.1%)

0 MeV 137 56 Ba (stable)

Fig. Decay of137 55 Cs to 137 56 Ba


32/135

32

Beta (Beta ( )decay ISOBARIC TRANSITIONS)decay ISOBARIC TRANSITIONS

-

decay: n p+

+ e-

+ (negative emission)A- remains same

Z increases by 1

+ decay: p+ n + e++(positive emission)

A- remains same 2 of 0.511 MeV (annihilation)

Z decreases by 1Or , P+ + e- n + (electron capture)

(Note: can use e- or - , e+or + )


33/135

33

Examples of ( -, + ) decay

- decay : 60 27Co 60 28Ni + - +

13755Cs

13756Ba +

- +

+decay : 12 7N 12 6C+ + +

74Be + e- 73Li+


34/135

34

2.3. Gamma Decay (an isomeric transition)2.3. Gamma Decay (an isomeric transition)

i. A daughter nucleus that exists in an excited stste (following an or decay process), can when decay further by the emission of

photons( rays). The Z and A of the nucleus remain unchanged in emission and, therefore, decay is an isomeric transition.

The life-time of the excited daughter nucleus is usually of the order10-14 sec., however there are a few exceptions with some- whatlonger half lives, e.g.

the transition 91m 41Nb 91 41Nb + (with t1/2 :60 days) is anextreme case.


35/135

35

ii. An example of decay is 60 27 Co 60 28Ni

(stable) where the parent nucleus (60 27 Co) firstundergoes - decay to an excited daughternucleus(60m 28Ni) which then undergoes decay as

follows:


36/135

36

6027 Co (Parent)

By - (0.318 MeV)decay (100%)

2.505 MeV (excited daughter60m 28Ni )By (1.173MeV) decay (100%)

1.332 MeV

By (1.332MeV) decay (100%)0 MeV 60 28Ni (stable state)

.. . E (total): (1.173+1.332)MeV=2.505MeV

Figure: Decay scheme of60

27 Co showing gamma decay following beta decay.


37/135

37

a. Can only occur if preceded by or decay (which

gives excited daughter prod.)b. The ray is a photon of electromagnetic radiation

(I.e. light). It carries away excess energy from the

nucleus and has no mass and no charge.Example 1

13755 Cs decays by - decay to 137 56 Ba (excited daughter).

The excited daughter nucleus then emits a - ray to getrid of its excess energy.


38/135

38

137

55Cs

By - (0.512 MeV)decay

(94.6%)

0.662 MeV 137 56 Ba (excited daughter)

by decay - by - (0.662 MeV)

(1.173 MeV) decay (5.4 %) decay (85.1%)

0 MeV (Stable daughter)

137

56 Ba... Total energy E (+ ): (0.512 +0.662)MeV=1.174MeV

Fig. 137 55 Cs Decay scheme showing gamma decay following beta

decay.


39/135

39

Example 2

192

77Ir decays to 192

78Pt. The decay process may take one of

many possible paths. Only the major transitions are shown:


40/135

40

19277 Ir (unstable) -0.536 MeV41.0%

- 0.672 MeV 0.921 MeV excited daughter

48.2%

2 5 0.784 MeV (192 78 Pt)

energies, intensities 0.612 MeV1 0.296 MeV 29.0%

2 0.308 MeV 29.7% 6 1 4

3 0.316 MeV 82.8% 0.316 MeV

4 0.468 MeV 48.1%

5 0.604 MeV 8.2%

6 0.612 MeV 5.3% 0 MeV

19278 Pt

Fig. 192 77 Irdecay scheme to 192 78 Pt


41/135

41

c. ray energies (like particle energies) arevery specific and can be used to identify theradionuclide.


42/135

42

2.4. Relative Intensity(% Probability) of Emission.2.4. Relative Intensity(% Probability) of Emission.

a. Not every decay/transformation of a particularradioisotope will produced in exactly. the samemanner. e.g. the 192 77 Ir decay scheme(fig. above)shows that, the unstable parent nucleus (192 77 Ir) may

decay by emitting - particles with max. energy ofeither 0.536MeV or o.672 MeV. Any one 192 77 Irnucleus can emit only of these - particles. The(%) probability that each - particle will be

emitted (expressed in %) is called relative intensity.


43/135

43

b. The decay of the excited daughternucleus can also processed by a no. ofdifferent paths with each path having adifferent (%) probability of occurring,

however each decay path can involve theemission of one or more rays (fig---).


44/135

44

2.5. Neutron Emission2.5. Neutron Emission

The most common source of abundant neutrons inthe nuclear reactor. The fission (spitting) of auranium or plutonium nucleus in a reactor is a

accompanied by the emission of several neutronswith a wide range of energies with a mean energy ~2MeV.

23592 U +

10n (

23692 U)

*

Fission products +Several 10n per fission.


45/135

45

Expect some fission fragments with very short half

lives, there are no radioisotopes that emit neutrons,however, 252 98 Cf undergoes spontaneous fission ataverage rate of 10 fissions per 313 alphatransformations 252 98 Cf thus stimulates a neutron

emitting radioisotope. The emitted neutrons energyrange is wide, average being ~ 2.3 MeV


46/135

46

All other sources depend on nuclear reactions e.g. commonlyused sources depending on the bombardment of 94Be or 11 5B

with alpha particles are:94Be + 42He (13 6C)* 12 6C + 10n .

115B + 42He (15 7N)* 14 7N + 10n .

(com. Nucleus excited)

The neutrons produced are of high energy (AV.range ~ 1 MeV 5 MeV).

Alpha sources used are : 210 84 P0, 226 88 Ra, 239 94 Pu, 241 95 Am

The target(Be or B) and alpha source are mixed together asfine powders and the mixture is sealed in a capsule.


47/135

47

By photodisintegration, the absorber nucleus

captures a ray, and in most instances emits aneutron,

e.g., 94Be + (94Be)* 84Be + 10n

is a useful source of mono-energetic neutrons. Thisreaction has a threshold energy and, therefore, theneutrons emitted with energy < the captured

gamma.e.g. the 2.75 MeV rays from 24Na decaygive mono-energetic neutrons with 0.83 MeV.


48/135

48

Neutron emissionNeutron emission contd.contd.

1) Nuclear reactor

Fission of235 U, average neutron energy ~ 2 MeV.

2) Spontaneous fission

Approx. 3% of252 98 Cf nuclei undergospontaneous fission instead of decay. Averageneutron energy is ~ 2.3 MeV.


49/135

49

3) Nuclear reactions

94B + 42 (15 7N)* 14 7N+ 10n

94B + 42 (13 6C)* 12 6C+ 10n

Average neutron energy is in the range 1 MeV to 5 MeV.4) Photodisintegration

e.g., 94B + (94Be)* 84Be + 10nNeutrons are mono energetic, in this case 0.83 MeV.

2 6 ACTIVITY2 6 ACTIVITY


50/135

50

2.6.ACTIVITY2.6.ACTIVITY

With radioactive materials, the most important quantity is the rateat which it is decaying/transforming in unit time and is calledactivity. Activity units are (i) curie(Ci)- the activity of a quantityof a material in which 3.7 X 1010 atoms (nuclei) aretransformed/sec-01cl unit;

(ii) The SI unit is the bequerel (Bq)-

the quantity of the material in which one atom (nucleus) istransformed or decayed per sec, i.e.

1 Bq= 1 transformation or decay or disintegration/sec.


51/135

51

Note: The bequerel does not mean the no. of particlesemitted by a radioactive source (Rs)/sec. In case of a

pure emitter, 1 Bq results in on particle/sec., butfor a complex transformations (e.g. decay of 60 27 Co,137

55 Cs etc.), each transformation releases one - particle and two rays; therefore total no. of

radiations per sec. per Bq=(1+2)=3The two units (Ci & Bq) are related by:

1 Ci = 3.7 X 1010 Bq = 37 GBq

Similarly 1mCi = 37 MBq and 1Ci = 37 kBq


52/135

52

2.6.1. RADIOACTIVE DECAY LAWS.

Radioactive decay/transformation is a random process. It isimpossible to say exactly, when any individual (particular)nucleus will decay. All that can be said is that, the individualnucleus has a probability at which it will decay in a giventime, is called decay constant (symbolized ) and is defined

as = the probability that a particular nucleus will decay inone sec., unit is per sec.(s-1). The decay constant () isdifferent for each isotope, but for any given isotope allnuclei have the same , regardless of its age, history or

environment. Any radioactive source will contain a huge no.of nuclei (of the order:1020 ) and statistics calculate accuratecalculation.


53/135


54/135

54

As time progresses, more & more nuclei will decay and,therefore, the no. of the unstable nuclei will decrease with time.

The no. of unstable nuclei left any time (t) can be calculated bythe mathematical relationship between N & t (called exponentialdecay law) as follows:

N = N0. e-t or

= N0.exp(- t)

Where, N0 = initial no. of unstable nuclei,

t = the elapsed time (sec), and

= the probability of decay of a nucleus/sec (decay constant)


55/135

55

Since the activity of the source is to the no. of unstable nuclei (i.e.,A N, or, A = N ), the activity (A) will also decrease with time inthe same manner ; i.e. A = A

0

.

e

-t or

= A0.exp(- t)

Where, A0 = initial activity and t = exposure time.

A commonly used term half-life, t1/2 , is related with as :

t1/2 = 0.693/ or, = 0.693/ t1/2

The t1/2 is defined as the time required for 1/2 of the unstable nucleito decay, or equivalently, the time for the activity to reduce to 1/2 ofthe initial activity.

i /


56/135

56

Putting = 0.693/ t1/2 in the above activity equation, we get A

= A0. e-t = A0.e( 0.693/ t1/2) Xt = A0 . 2-t/t1/2

A typical exponential decay curve of activity (MBq) Vs time(--) isshown (figure):

Time (yrs)

Figure: Decay curve of a radioactive source

Activity(MBq)


57/135

57

Activity, Decay Law Contd.

a. SI unit

1 becqerel= 1 Bq = 1 disintegration per second

b. Old unit

1 curie= 1 Ci = 37 GBq

c. The activity of a radioactive source is given by

A=N

Where, N is the number of nuclei present, and is the decay

constant (i.e. the probability that a particular nucleus will decay in 1second).


58/135

58

Activity, Decay Law Contd.

d. Exponential decay law

A = A0.e-t or, A = A0.2 t T1/2

where T1/2 is the half-life and is related to by T1/2 = 0.693/ .


59/135

59

Example.

A radiation gauge is labeled: 60 co, 250 mCi, 1st july 1972. What willits activity be on 1st November 1993, in Bq? (The half0life of60 Cois 5.27 years.)

Solution

Firstly, convert the activity to Bq.

A = 250 X 10-3 Ci X 3.7 X 1010 Bq/Ci = 9.25 GBq.

Secondly, use the equation for radioactive decay to find the activityon the required date. Elapsed time from 1/7/72 to 1/11/93 is 21.33

years.therefore, A = A0 2-t/T1/2 gives

A = 9.25 X 2-21.33/5.27 GBq

=0.56 GBq.

2 6 22 6 2 N t l R di ti itNatural Radioactivity


60/135

60

2.6.22.6.2 Natural RadioactivityNatural Radioactivity

Radioactive materials are divided into 2 groups:

a. Those that are found in nature & exhibit naturalradioactivity, have half-lives comparable to age of theuniverse, their unstable daughters, and thosecontinuously created by cosmic ray bombardment(e.g., 14 C). Some are listed (Table).

Table Some naturally occurring radionuclides


61/135

61

Table. Some naturally occurring radionuclides

Nuclide Decay mode Daughter radionuclide

Half-life(s)

4019 K - 40 20 Ca 4.10 1016

8737 Rb - 87 38 Sr 1.48 1018

11549 Tn - 115 50 Sn 1.58 1022

142

58

Ce 13856

Ba 1.58 1023

19078 Pt 186 76 Os 1.21 1019

20482 Pb 209 80 Hg 4.42 1024

23290 Th 228 88 Ra 4.45 1017

235 92 U 231 90 Th 2.25 1016238

92 U 234 90 Th 1.42 1017


62/135

62

The last 3 nuclides (232Th, 235U, 238U) are very

massive and produce a series of radioactive daughtersfor a no. of generations (radioisotopes of: Ra andRn). All these decay series ends with stable 207 82 Pb.

b. Those made by synthesis, and exhibit artificialradioactivity (many examples).

X-rayX-ray


63/135

63

X rayX ray

2.7. X- RAYS- Production and properties

X-rays are photons of electromagnetic radiation (EMR)and have the same physical properties as of gamma rays( ).

Production

i. Can be produced by radioactive decay e.g., decay byelectron capture create a vacancy in inner orbital (atom).

Then electrons from outer orbital (i.e, of higher energy)may drop down into the vacancy, and in doing so, releasetheir excess energy as a photon. If this excess energy issufficiently large, then the photon is an x-ray.


64/135

64

ii. In an X-ray tube.Electrons emitted from a

hot filament are accelerated to high velocitiesby applying high voltage and focused into abeam, impinge on a metal (e.g., 74W) anode.Instriking the anode, the high velocity electronsare rapidly decelerated and hence, emit thefollowing radiations:

a. breaking radiation(bremsstrahlung) andb. Characteristic radiation.


65/135

65

The breaking radiation has a wide energy distribution (0 eV to max.depending on the applied high voltage).

e.g., if the H.V.=50KV, then the max. X-ray energy is 50KeV.Characteristic radiation arises from the vacancies that are createdwhen electrons are knocked out of the inner orbitals by the highvelocity electrons (incident beam). Characteristic X-rays are mono

energetic(depends on the energies of the orbitals, hence, the nature ofatom i.e.anode material e.g., 74W).

iii.a. AV X-ray energy is controlled by changing the applied H.V.

b. Qty of the radiation produced in changeable by varying thecurrent flowing through the tube and the time of H.V. applied.


66/135

66

X-raysX-rays contd.contd.

X-rays


67/135

67

Salient Points

a. X-rays have the same physical properties as -rays, i.e. they are

photons of electromagnetic radiation(emr). b. The maximum x-rayenergy (in eV) is equal to the value of the high voltage(in V). c.The energy of the characteristic x-rays depends on the anodematerial.

d. Advantages of X-ray tubes: can be turned off very high photon flux photon energycan be changed

e. Disadvantages (- do - ):

bulky require power supplies


68/135

68

[b] The maximum x-ray energy (in-eV)is equal to the value of the high voltage(in V).

[c] The energy of the characteristics x-rays depends on the anode material.

33 I t ti f R di ti ith M ttI t ti f R di ti ith M tt


69/135

69

3.3. Interaction of Radiation with MatterInteraction of Radiation with Matter

( alpha, beta, gamma, x-ray, neutron)( alpha, beta, gamma, x-ray, neutron)

The common property of ionizing radiations is toionize matter. The mechanisms of interaction differmarkedly with type and energy of the radiation.

An understanding of these mechanisms is importantfor:

designing & selection of a detector,

understanding associated biological effects andidentifying the action needed.

The matter is of course composed of molecules (atoms) The


70/135

70

The matter is, of course composed of molecules (atoms). Theincident radiation ( alpha, beta, gamma, x-ray, neutron) mayinteract with either the nucleus or the orbital electrons. For

convenience, this interaction with matter is briefly discussed underthe following headings:

3-1. Interaction of Charged particles

(Section-A) a. heavy particles e.g., alpha

b. light particles e.g., betas (electron, positron)

3-2. Interaction of Un-charged particles

(Section-B) a. gamma and x radiations(photons)b. neutrons.

I t ti f Ch d ti l ( ) td


71/135

71

Interaction of Charged particles( ), contd.

3.1(Section -A) (a) Heavy Charged Particles (e.g.. ).

a. The distance to which the particles penetrate the matter(air, tissue, etc) is called Range (R) of the radiation in themedium.

b. Particular heavy charged particles of given energy:

have almost same range in matter (with in a very

narrow limits); and follow almost straight track.


72/135

72

Both these regularities are a result of the fact that, forenergies upto a few 100 MeV, the main process of losing

energy in passing through matter is the transfer for kineticenergy (K.E.) via coulomb forces to the electrons. Thisenergy transfer (K.E) results in the direct ionization (orexcitation), the heavy charged particle slowing down

(Slightly) at each interaction. However, because of thelight electrons (e-) compared with the incident particlethey are swept out of the way and the heavy particlecontinues almost undeflected, but with slightly diminished

energy. After a large no. of collisions, the particle K.E.will be used up & it will stop.


73/135

73

This simple picture must be modified by consideringthat, the collisions of heavy charged particle in its

passage through matter are random. therefore we canonly discuss the collision probability and if Ncollisions are expected to stop a particular energy

particle, then 34% of such particles will experiencecollisions outside of (N N) and, hence, the rangealthough quite uniform exhibits a straggling

phenomenon.

b Li h h d i l (b )


74/135

74

b. Light charged particles (betas: -, +):

The interaction of light charged particles, although still a result of

coulomb interactions differs in several ways from the heavy( ).Differences are a result of the lower mass which each deflection, thusundergo many large angle scatterings in collisions with electrons orwith the coulomb field of the nuclei. Further there is a probabilitythat an electron will radiate while being slowed down. (X-rays or

breaking radiation). (On the other hand, heavy charged particles do not radiate appreciable in this way, because there a accelerationsare generally much smaller). These radiative losses of beta becomemore important as E and Z increase. As a result of large angle

scatterings & radiative losses, the electrons initially of the sameenergy will traverse quite different thickness of a particular materialbefore coming to rest.


75/135

75

Beta particles ( -, +) interact in much the same waywith matter except that a + (being an anti matter) finally

annihilates with a -

of the target atom(material).The Emax (x-rays) emitted (in the breams strahlung process)

is just equal to E - .However, most of the EMR emitted

has only a small fraction of this Emax (x-rays). Breams strauhlug radiation is an important problem inshielding - sources because although - particles may

be easily absorbed, the more penetrating breams strauhlug produced in the shielding material must be absorbed orneutralized(but how?).


76/135

76

A further problem due to - interaction is that, -sources (distinct from conversion electron sources)emit a spectrum of a broad energy(not monoenergetic).

Despite the complications (large angle scattering,

radiative losses, poly energetic - sources), particles from a particular source may be consideredto have a particular range in considerably less than the

particle track length and the no. of - particles penetrating a particular thickness continuouslydecreases. this is in contras to heavy( ) track length.

Interaction of Heavy charged particle(Interaction of Heavy charged particle( ))


77/135

77

Interaction of Heavy charged particle(Interaction of Heavy charged particle( ))

The main range of particles (from a typical source,

E 5MeV) is 4 cm in air (Rair) and


78/135

78

On average--34eV energy must be deposited in air per ion pair produced, some energy deposited merely exciting

electrons to higher orbital, not producing ionization. Thus, a5MeV produces1.5X105 ion-pairs as it is brought to rest inair.

No.of ion-pairs produced/unit path length is Sp.

ionization's.A related term is linear energy transfer(LET),which is the energy deposited (KeV) --m of path, causes bothionization & excitation.


79/135

79

Sp. ionization & LET are not coast, but depend on

the particle 7 its energy. Commonly, the slower the particle the greater are the Sp. ionization & LET,because the particle has a longer time to interact withelectrons.

Thus, for a particular charged particle the no. of ionsproduced is a max. near the end of its range.(A plotof Sp. ionization along the track of a charged particle

is called Bragg Ionization Curve).

Interaction of ( - +) contd


80/135

80

Interaction of( , ) contd.

Salient Features heavy charged particles:

Alpha interaction with :

the nuclei are possible but rare;

the electrons ( -) occurs and all interactions via electrostaticattractions resulting excitation of ionization of the atom/molecule.

Emax acquired by an orbital electron 1/500 of E.

Some ejected electrons are of sufficient K.E. to cause furtherionization (producing delta rays), range of delta rays is alwayssmall compared to range of the but characteristic the high conc.Ionization along the alpha track (path).


81/135

81

Alpha LET is also knows as linear stopping power(s)

of the absorber. Rate of energy loss s = - E/x

(correction term is : sp. Energy loss).

Alpha particle emission is mono-energetic, I.e., for agiven radioisotope, all alpha particles (emitted) are ofthe same energy.


82/135

82

Range (R)

the paths of particles are quite straight,

No attenuation no. of -particles occurs until near the endof journey.

Range in air (with in 10%): for E < 4 MeV , R = 0.56E

(When Rcm, E MeV) for 4 < E < 8 MeV , R =1.24E-2.62

Range in other media (Rx) = (Pair/Px) X Rair (cm)

e.g., ,, ,, tissue Rt = (1.11 X 10-3) . R(air) cm.

I i f l h b C d


83/135

83

Interaction of alpha , betas, Contd.

Betas

size orbital electrons. Change in direction (by collisions): very much greater. Path traced (track):random

Ionizing capacity : < by alpha. Hence, they lose E moreslowly.

Energy loss mechanisms:

Same as alpha (e.g.excitation & ionization of atomic electronsdelta rays, etc.)

Strong electrostatic interactions with the nuclei


84/135

84

(rapid acceleration of the -, EMR losses (X-rays/bremsstrauhlung)

For typical - energies in low Z absorbers, the E photon(bream) is small and usually reabsorbed close to its oforigin.

Bremg. losses are:

E beta and Z2 (absorber material)

E emission from a particular nuclide is broad and varies up to

E (max) (characteristic of the nuclide).

Mean energy loss (in air) per ion pair, W = 33.7eV

Range (R)


85/135

85

R is quite indistinct compared to R for 3 main reasons:

betas do not travel in straight paths; ,, are not mono-energetic;

,, produce bremssg.

Plot log activity Vs absorber thickness provides two sections:

the steep section nearly liner, and the shallower ,, - linear anddue to attenuation of bremssg. Rmax is related to E (max) and the

absorber material. Thickness of absorber are oftenexpressed in a real density (g/cm2).I.e., distance (in g.cm-2)= distance (cm) X density (g/cm3). Using this unit, thecombined effected of different absorbers can be calculated

(e g )


86/135

3.2(Section-B): Unchanged Particles3.2(Section-B): Unchanged Particles


87/135

87

3.2(Section-B): Unchanged Particles

a. Gamma and X-radiations

The important parameters in interaction are the energy (E

, Ex) and

the Z of target material. Gamma and x-radiations are indirectlyionizing particles, because the primary interaction leads to partial or

complete transfer of the photon energy(E photon ) to E e(-). This ejectedfast moving electron then ionizes the material(as described).

Gamma and x-radiations do not have a definite range in matter, but

are attenuated according to the exponential law: N = N0. e-l.x

Where, N0 = no. of photons incident on the matter,

N = no. of photons remaining after penetrating

l = linear attenuation co-eff.


88/135

88

This exponential attenuation results from the one-shotnature of the interactions in which a photon is mainly

lost. The important mechanisms are : Photo electriceffect (P.E), Compton effect(CE), and Pair

production(PP).

The relative importance of the interaction ofUncharged Particles ( , x-ray)

The relative importance of the three process

(mechanisms) are a function of energy(Ephoton ) and Z(matter).

PEE


89/135

89

Ejection of an atomic (orbital) electron from an atom,

the incoming photon disappears. Because energy andmomentum are to be conserved, a free electron cannotwholly absorb a photon. The more tightly bound theelectron (e.g., K>L>M>N,etc. shell), the more probable is

this mode (PEE) of interaction. thus, for a sufficientenergy photon, the most probable interaction is with a K-shell electron or if the E photon < K absorption energy then L

shell electrons are more probably involved. For the same

reasons PEE is more probable in high Z material(innershell electrons are tightly bound).


90/135

90

This process (PEE) in most important at low E photon-

and the probability ((PEF) ) depends on E and Z(material) as per the equation:

(PEF) (proportional to) Zn .E-mWhere, n = 4 to 5 , m ~ 7/2

Th h t l t ill h K E (E ) t


91/135

91

The photon electron will have a K.E.(E-e- ) acc.to:

E(e-)

=

E

- B.E.(Where B.E. is binding energy of the

electron in its original shell)

Note: B.E. for even K-electrons of Pb = 88KeV and,hence, for E

>few 100KeV.

We have, Ee- E (for all materials)

The photo electrons behave as would any fast moving


92/135

92

The photo electrons behave as would any fast movingelectrons and ionize the medium, by losing energy in

collision & bremssg. processes. Now there is a vacancyin an inner shell (orbital) which will be filled up quickly by capture of free electrons and rearrangement,characteristic X-radiation (or perhaps Auger electrons)

will be emitted in the process.These variations are very important in the selection ofdetector or shielding materials. Particularly, gammaradiation shields are often Pb (high-Z).

Compton Effect


93/135

93

Compton Effect

CE

When a interacts with an outer-shell electron (looselybound), only a portion of the E photon is transferred tothis electron and a new photon (of lower energy scattered

photon, E photon and in different direction from theoriginal) is created. Because, all angles of scattering are

possible, the energy transferred to the electron (recoilelectron, Ee- ) can very betn. 0 to a large fraction of the E

(photon). energy transfer for any scattering angle can bederived through conservation of energy and momentumas the following schetch and equation:

S tt d h t


94/135

94

Scattered photon energy,

E photon = E (photon)

1 + E

(photon) m0c2(1-cos)Where, m0c2 = rest mass energy of an electron (= 0.511MeV), and

the E (photon) , E (photon) energies expressed in MeV.Note: This relationship is independent of the target material, and,the energy transferred to the recoil electron (Ee- ) is,

Ee(-)

= E (ph) - E (ph)


95/135

95

For small scattering angle (~0) , the E

(ph)

approximates to E (ph) and therefore, the transferredenergy is small. Max energy is transferred in backscattering (i.e. when = 180)

The probability of Compton effect (scattering) isrelatively independent of Z but varies inversely withthe E

(ph) . Angular distribution of the scattered

photon in non-isotropic. Particularly, there is a strongtendency for forward scattering at high incident energies (E

(ph) > 500keV).

Pair production (PP)Pair production (PP)


96/135

96

Pair production (PP)Pair production (PP)

If E

exceeds 1.02 MeV, pair production is energetically possible.

This occurs in the electric field of a nucleus and results in creationof an electron-positron pair. The K.E. of the pair is represent by:

E(pair) = (Ee- +Ee+ ) = E (ph) - 1.02 MeV

(Where, Ee- = electron energy and Ee+ = positron energy)The formed particles (e- & e+) lose their energies as described, thee+ subsequently annihilates after itself slowing down in the

medium. The probability (pp) ~ Z2 increases sharply asenergy E (ph) (above 1.02 MeV) i.e. (pp) Z2 . E

Att ti C ff (Li M T t l) f


97/135

97

Attenuation Co-effs. (Linear, Mass, Total) of s, x-rays :

For a photon beam, the contribution of each process to the

attenuation depends (in a complicated way) on E and z. The total linear attn. Co-eff. is the sum of the probabilities foreach process (PEE +CE +PP) as follows:

Thus, total =PEF +CE +PPWhere, total has units of cm-1 , and is stated as:

N = N0. e- total . X

l i i f h f i f i


98/135

98

Relative importance of three process as a function of energy isshown (fig..)

Mass attn. co-efficient (m) are also used and related to the linearCo-effs. (l), by the density () as follows:

m(cm2.g-1) = (cm-1) (g/cm3)

The simple exponential law eqn. (as above) is strictly valid only fora narrow beam photons. In the more general broad beam situation,photons scattered in the material will contribute to the beamemerging from any particular thickness of material.

Th th tt b


99/135

99

Then, the attn. eqn. becomes:

N = N0. B . e- lWhere, B is build up factor and is >1, dependson energy, material type and geometry and isdifficult to calculate theoretically. Howeverempirical values of B for various photonenergies, materials and specific geometries can

be found in the literature.

The amount of attn (in terms of intensity) 0f s & x-rays varies


100/135

100

The amount of attn. (in terms of intensity) 0f s & x rays variesexponentially with absorber (material) thickness. Under conditionsof good geometry (i.e., well collimated narrow beam) and using a

mono-energetic beam, the following equation holds:

I / I0 = e- lx

Or, ln(I / I0

) = -x lne = -lx

Where, x = the thickness absorber, and

I0 = intensity at zero thickness,

I = ,, after the absorber, andl = linear attenuation co-efficient (cm-1)

Half-Value Thickness (HVT) is the absorber thickness required


101/135

101

( ) qto reduce the radiation intensity to 50% of its original value:

HVT = 0.693 /l

Similarly, Tenth Value Thickness (TVT) is the absorberthickness required to reduce the radiation intensity 1/10 of itsoriginal intensity (i.e. 10% of Io) and is represented by:

TVT = 2.303 /l

The l is a function of photon energy and absorber (material).Hence, the above equations do not hold for hetero energetic

photon beams.

Using mass attn.C0-eff.(m) = l / cm2/g ( = density), weget

I / I0 = e-mxp

or, ln(I / I0) = -

mxplne = -

mxp


102/135

Each particle of the pair loses energy by ionization as it moves


103/135

103

Each particle of the pair loses energy by ionization as it movesfrom the point of origin. When the positron energy becomesquite low, it combines with a nearby electron (by a processwhich is nearly the reverse of the PP); two gamma photons arecreated to conserve momentum:

e(+) + e- 2 E (photon)

These are annihilation photons and have energies 0.51 MeVeach. The annihilation radiation peak can be detected whenrecording the gamma spectrum of a radionuclide which emitsgammas with an energy >1.02 MeV.

MAJOR INTERACTIONS OF GAMMA, X-raysMAJOR INTERACTIONS OF GAMMA, X-rays


104/135

104

PE Absorption

Photon is absorbed by an electron ( at energies of usually inner shellelectron) and the K.E of the ejected photo electron is : Ee- (pe) =(E+Eb)

Where, Eb= binding energy of the atomic electron

The ejection of a K-shell electron creates a vacancy which is quicklyfilled-up, therefore one or more characteristic X-rays may begenerated. The peak in the absorption Cross section occurs at the

binding energy of the K-electron. Photon energies less than this, areinsufficient for a PEE with K-electrons but sufficient for L electrons,etc. Probability of this effect (PE) increase with:decreasing E

, increasing Z.

Compton Effect(CE)Compton Effect(CE)


105/135

105

It is a partial interaction between the incident photon and electronwith conservation of K.E & momentum (like billiard ball collision).

The probability of Compton scattering:

- depends on the no. of electrons available, hence, increases with Z;

- decrease with gamma energy but not rapidly as PEE.


106/135

106

Pair Production(PP)

A photon in the coulomb field of the nucleus may disappearby transferring its energy (E

) to the creation of a electron- positron pair. Virtually, all of the photon energy (E

) is

transferred to the pair:

E transferred into (e+ + e-) + K.E (of the two particles)

The energy of each electron mass = 0.51MeV, hence, thethreshold energy for this PP reaction is = 0.511MeV X 2

particles =1.02 MeV. Any excess energy (E

- threshold) =

K.E is almost equally divided between the two particles.

RADIATION PROTECTIONRADIATION PROTECTION


107/135

107

4.1. Introduction

Ionizing radiation has the potential to cause injury to an exposedindividual. This injury is a result of the energy deposited in thetissues by the radiation.

The deposited energy results in excitation and ionization of

molecules of biological importance (e.g. DNA, proteins, etc.) Ingeneral, the more is the energy deposited (ionization produced),the greater the injury; however, the effect is also determined by theradiation type (e.g. , , , x-ray, n, etc.), the site exposed(organ), the exposure rate and other more subtle factors.


108/135

108

Note that, there is relatively little data on the biologicaleffects of radiation at the human level. The limited data

are available from: radiation therapy experience, studieson Japanese Atom Bomb survivors and some accidents atnuclear installations in the world.

Experiments on animals have provided the bulk datashowing an understanding of the mechanisms of injury ofan exposed animal. Such data is often due to relativelyhigh-dose radiations and, therefore, extrapolation to the

low-dose range may be in accurate.

Bi l i l ff t f i i i di ti i ll


109/135

109

Biological effect of ionizing radiation on man is usuallydivided into two categories:

SOMATIC and GENETIC effects.

Somatic effects are those that appear in the exposedindividual further sub-divided into early (within days or

weeks) and late somatic effects whereas, Genetic effectsexhibit themselves in the subsequent generations. Detailswill be provided in the subsequent lecture(s).

In passing through the body, the radiation deposits energy byi i i th b d l l (DNA P t i t ) Th i i d


110/135

110

ionizing the body molecules (DNA, Proteins, etc.). The ionizedmolecules result in modified bio-chemical reactions which may lead

to modifications in the cell-structure or cause the cell death.Eventually, the effects may manifest as the radiation-inducedinjury.

Many factors determine the occurrence, nature and severity of

the effect. Some of these are: Radiation dose dose amount, dose-rate, time gaps betweenexposures;

Radiation type - e.g. , , , neutron, X-ray;

The body organ(s) exposed, and

Others.


111/135

4 2 RADIATION DOSES : Fundamentals Concepts4 2 RADIATION DOSES : Fundamentals Concepts


112/135

112

4.2. RADIATION DOSES : Fundamentals Concepts4.2. RADIATION DOSES : Fundamentals Concepts

4.2.1. Exposure X

When passing through air, a photon (gamma, or X-ray) may strike an air molecule and eject an electron,

leaving the molecule with a +(ve) charge. These +(ve) ion and the ejected electron (-ve charge) arecombindly called an ion pair. Then the electron willtravel through the air, ionizing the molecules as it

passes through, and producing numerous ion pairs.

The amount of charge produced per unit mass (air) is a measure


113/135

113

of the radiation exposure X , and is expressed by the eqn:

Exposure X = Q/ m

Where, Q = total electric charge (either +ve or VE)

m = mass of air.

Note: Exposure relates only to photons ( , X-rays) but notother radiations.

Units of X : The SI unit is coulomb per kg, Ckg-1 . The old unit(which is still widely used) is the roentgen, R, 1R = 2.58x10-4

Coulomb.kg-1

(air).


114/135

114

Exposure Rate - X is defined as: the exposure dividedby the time of exposure

X = (exposure X / time) = dx/dt

Units are: C kg-1S-1 , R S-1 , mRh-1 , etc.

Note: Many radiation monitors show readings in mR/h.From radiation protection point of view, the quantities,Absorbed Dose and Dose Equivalent are more

useful (as discussed as follows).

4.2.2. Absorbed Dose D4.2.2. Absorbed Dose D


115/135

115

The energy deposited by ionizing radiation per unit mass of matter

and is expressed by: D = E/ mUnite: SI units is Joule per Kg-1 , JKg-1 , and its special nameGray(Gy). 1 Gy = 1.0 JKg-1

The old unit, still widely used, is rad (radiation absorbed dose).

1 rad = 0.01 JKg-1 = 0.01 Gy, or 1 Gy = 100 rad.

Notes: This quantity (D) and unit apply to all ionizing radiations( , , , x, n, etc.).

When using this quantity, the medium (material) involved mustbe specified (e.g. air, tissue, etc.)

Absorbed Dose Rate D :


116/135

116

Absorbed Dose-Rate D :

Defined by D = Absorbed Dose/Time = dD/dtSI units are: Gy s-1 , Gyh-1 , etc.

Old units are: rad s-1

, lm radh-1

, etc.Useful conversion: 1m radh-1 = 10 Gyh-1 .

4.2.2.1. Conversion of Exposure Reading4.2.2.1. Conversion of Exposure Reading


117/135

117

to Absorbed Dose in Air (i.e., X to D Air)to Absorbed Dose in Air (i.e., X to D Air)

We know, Exposure readings, X = Q/ m .we alsoknow, to produce 1 electron charge, on average, 33.7 eV (or5.39x10-18 Joule ) energy is required. To produce 1 coulombcharge in air, 33.7 Joule energy is required.

So, it can be shown that:

Absorbed dose in air (Gy) = 33.7 x Exposure (in CKg-1) Gy

( - do - ) (rad) = 0.87 x Exposure (in R)

4.2.2.2. Conversion of Exposure4.2.2.2. Conversion of Exposure


118/135

118

Readings to Absorbed Dose inReadings to Absorbed Dose in

Tissue (i.e. X to D tissue)Tissue (i.e. X to D tissue)In radiation protection, we are interested in the energy deposited intissue (and bone) i.e. in the Absorbed Dose (tissue). This willdepend on the mass energy absorption co-eff. of tissue for gamma

rays. This is turn depends on the energy of the gamma.

Now, D tissue mass absorption co-eff. Of tissue

D air mass absorption cvo-eff. Of air

We know, for an exposure X roentgen, D air = (0.87X) rad


119/135

119

Thus, D tissue = (0.87X) mass absorption co-eff. of tissue

mass absorption cvo-eff. Of airOf ten the factor 0.87 and the mass absorption co-effs are groupedtogether, called f factor .

Thus, D tissue = f XFor the common range of energies encountered in radiation

protection. ( say, 100 KeV to 2 MeV for tissue), f = 0.96. (For 30KeV, f = 0.90, for 10 MeV, f = 0.93). So, for converting an

exposure X CKg-1) Gy, the f factor under this commonconditions = 37.4

4.2.3. Equivalent Dose H4.2.3. Equivalent Dose HTT


120/135

120

This quantity has been introduced to allow for the factthat, stochastic effects of radiation depend on the qualityof the radiation (i.e., on the type & energy). We candefine,

Eq. Dose, H = R WR. DTR

Where, WR = a radiation weighting factor for theradiation type,

DTR = absorbed dose averaged over a particular tissue (ororgan) for that radiation type.

Unit: Sievert, SV.


121/135

121

Note. HT Can be compared with the old term Dose

Equivalent H which was related to Quality Factor forthe radiation Q and the Absorbed Dose D by H = QD.

The old units for dose equiv. Where rem and

Sieverts .Note: WR applies for stochastic effects (defined later) atlow doses, has specified values (Ref. ICRP60).

4.2.4. Effective Dose E4.2.4. Effective Dose E


122/135

122

The most important quantity in assessing radiationhazards and rinks. E depends on HT (i.e. the absorbeddose, D and the radiation weightily factor) and on theradiation sensitivity of the particular tissue or (organ)

being irradiated. This tissue sensitivity is related to thetissue weighting factor WT . (listed in Table, Ret:ICRP60). The effective dose is defined as

E = T WT. HT

Putting the value ofHT (from previous eqn.) in this eqn.,


123/135

123

We get: E = T

WT.

RW

R. D

TR

Units: Sieverts, Sv.

This E actually represents conversion of a dose to part of the bodyto whole-body dose which has the same effect i.e., it carries thesame risk.

Note : Weighting factor for a whole-body close WT = 1.

E is for estimating the probability of stochastic effects only forabsorbed doses well below the threshold for deterministic (or non-stochastic) effects (defined later).


124/135

4.2.4.2. SUMMARY of RADIATION QUANTITIES, UNITS,Their CONVERSION RELATIONS, Etc.

QUANTITY DEFN & REPRESN i CONVERSION


125/135

125

QUANTITY DEFN & REPRESN(SYMBOL, EQN.) Units

Old New

CONVERSIONRELATIONS

Exposure Charge/unit mass, (ai), X = Q/ m

Roentgen, R Ckg-1 1R = 2.58x10-4Ckg -1

AbsorbedDose

Energy deposited/ Unitmass (tissue, organ),

D = E/ m

Radiation AbsorbedDose, rad=0.01Jkg-

1

Gray (Gy)1Gy=1jkg-1

1 Gy = 100 rad

Eqv. Dose Absd. Dosex RadnWeighting Factor,

HT = D X WR

- Sievert -

Dose Eqv.

Absd. Dosex radn QualityFactor H=D x Q rem Sv1 Sv = 100rem

Eff. Dose Eqv. Dosex Tissue weig.Factor, E = HTWT

- Sv -


126/135

126

Note: For or x-rays, a useful conversion

relation for monitors reading are:1mR/h = 1mrad/h =1mrem/h =10 Gy/h = 10 Sv/h

4. Effective- Dose-limits4. Effective- Dose-limits

4 1 Historical Background


127/135

127

4.1 Historical Background

Roentgen produced the first radiograph (of his wifeshand). Numerous radiography where taken long exposuretimes often as long as an hour or so were given. Radiationinjuries were soon evident. (e.g., skin damage, loss of

hair, loss of nails & fingers, in some cases death). The first attempt to set a tolerance or permissible doserate was in 1902, when the limit was set at 10R/day(an

alarming level). Over the years until 1958, dose limitswhere reduced to 5R/y for whole body and remainedessentially the same until 1990, when it has been set tothe 2/5 yrs.


128/135

Another well known and respected organization is


129/135

129

Another well known and respected organization isthe National Council on Radiation protection and

Measurement(NCRP) set up in the US in 1928. It is alsoa non Govt. voluntary body having a purely advisoryfunction. There are several publications of ICRP, ICRU,

NCRP, etc. The national Health and Medicine ResearchCouncil of Australia make recommendation sonradiation standards for individuals in Australiafollowing the ICRP60 (1990).

4.3 Radiation Workers and Members of4.3 Radiation Workers and Members of

th P bli ( A t li )th P bli ( A t li )


130/135

130

the Public(e.g., Australia)the Public(e.g., Australia)

A radiation worker (sometimes called designated employee) isdefined as a person who, in the course of his employment, may

be exposed to ionizing radiation arising form his directinvolvement with sources of such radiation (NHMRC 1981).

Such workers are also described as Occupationally exposedworkers.

A member of the public is defined as person who in thecourse of this employment, has no direct involvement with

sources of ionizing radiation(NHMRC 1981).


131/135


132/135

132

Note:-

Such a response curve implies that,a. no recovery process from the radiation damage sustained;

b. the effects of the damage are additive and

c. the effects are independent of dose rate.d. Stochastic effects are observed through a study of statisticsfor a population.

e.g., incidence of Leukemia, death following whole-bodyirradiation, genetic effects.

4.4.2 non-stochastic (Deterministic) Effects.4.4.2 non-stochastic (Deterministic) Effects.


133/135

133

( )( )

Defined as: The probability of such effects varies with dose,and a threshold-dose may occur e.g., Cataract of the eye, non-malignant damage to skin , etc. Are illustrated by the sigmoidgraph as follows:

The main features are:

I) Severity of the effect increase in a non-linear way with dose,

ii) there is a threshold dose below which the effect is not

observed.

Note. The shape of graph implies that:


134/135

134

a) recovery process operate and the severity of effect will depend

on the dose rate,b) successive doses are only partly additive.

c) these effects are observed in individuals who have receiveddoses in excess of the thresholds.

d) In particle, it is often very difficult to determine the shape ofthe dose response curve at low doses, unless very large nos. oftest animals are used.

I cases of doubt, a linear dose response curve(stochastic) is oftenassumed any error then which might ensue would be on the safeside.

4.4.2. General Principles of Radiological4.4.2. General Principles of Radiological

Protection(Ref ICRP 60)Protection(Ref: ICRP 60)


135/135

Protection(Ref: ICRP 60)Protection(Ref: ICRP 60)

1. Justification a Particle: No practice involving exposures toradiation should be adopted unless it produces sufficient benefit to theexposed individuals or to society to off set the radiation detriment itcauses.

2. The optimization of protection: The magnitude of individualradiation doses, the no. of people exposes and the likelihood ofincurring exposures, should be kept as low as reasonably achievable,economic and social factors being taken into account(ALARA

principle)

3. Individual dose and the risk limits: Radiation exposures of

Documents

Workshop on Radiation Protection