notes for theoretical health physics

8/9/2019 Notes for Theoretical Health Physics

1/92

Notes for Theoretical Health Physics

Tae Young Kong

1. Stochastic processesa. Independent events

Reference: James E. Turner, Darryl J. Downing, and James S. Bogard, Statistical

Methods in Radiation Physics, pp.2!"#

$n some cases, gi%en addi&ional informa&ion will cause no c'ange in &'e

pro(a(ili&y of &'e e%en& occurring. T'en, in sym(ols, )r*+B-)r*+-, and so &'e

occurrence of B 'as no e/ec& on &'e pro(a(ili&y of &'e occurrence of +. 0e

&'en say &'a& e%en& + is independen& of e%en& B.

Two e%en&s + and B are independen& if and only if )r*+B-)r*+-

and)r*B+-)r*B-.

$ndependence T'eorem Two e%en&s, + and B, in a sample space are independen& if and only if

)r*+ 1B-)r*+-)r*B-.

Eample Two p'o&ons of a gi%en energy are normally inciden& on a me&al foil. T'e

pro(a(ili&y &'a& a gi%en p'o&on will 'a%e an in&erac&ion in &'e foil is 3.2.

4&'erwise, i& passes &'roug' wi&'ou& in&erac&ing. 0'a& are &'e pro(a(ili&ies

&'a& nei&'er p'o&on, only one p'o&on, or (o&' p'o&ons will in&erac& in &'e foil5Solu&ion

T'e num(er of p'o&ons &'a& in&erac& in &'e foil is a random %aria(le 6, w'ic'

can &a7e on &'e possi(le %alues 3, 8, or 2. T'ere are four simple e%en&s for &'e

sample space for &'e &wo p'o&ons: *n, n-, *n, y-, *y, n-, *y, y-.9ere y means

yes, &'ere is an in&erac&ion; and n means no, &'ere is no&,; &'e pair of

sym(ols in paren&'eses deno&ing &'e respec&i%e fa&es of &'e &wo p'o&ons. T'e

pro(a(ili&y of in&erac&ion for eac' p'o&on is gi%en as 3.2, and so &'e

pro(a(ili&y for i&s 'a%ing no in&erac&ion is 3.


2/92

Since &'e pro(a(ili&y of yes; for a p'o&on is 3.2 and &'a& for no; is 3.


3/92

T'e resolu&ion of &'e &o&al!energy pea7,

09 3.3or anormal cur%e, wi&' s&andard de%ia&ion P, i&

can (e s'own &'a& >09 2.#P. T'e

resolu&ion of a spec&rome&er depends on

se%eral fac&ors. T'ese include noise in &'e

de&ec&or and associa&ed elec&ronic sys&ems

as well as Quc&ua&ions in &'e p'ysical

processes &'a& con%er& radia&ion energy

in&o a measured signal. +pplying )oisson

s&a&is&ics, we can epress &'e resolu&ion in&erms of &'e a%erage num(er of

p'o&oelec&rons *wi&' s&andard de%ia&ion P

-:

µ µ

σ

µ

35.235.2===

FWHM R

wi&' >09 now referring &o &'e num(er, ra&'er &'an energy, dis&ri(u&ion. 0i&'

R 3.3< for &'e scin&illa&or, i& follows &'a& &'e a%erage num(er of

p'o&oelec&rons collec&ed per pulse is or di/eren& &ypes of de&ec&ors, &'e p'ysical limi&a&ion on resolu&ion imposed

(y &'e in'eren& s&a&is&ical spread in &'e num(er of en&i&ies collec&ed can (e

compared in &erms of &'e a%erage energy needed &o produce a single en&i&y.

>or &'e al de&ec&or Uus& gi%en, since an e%en& is regis&ered wi&' &'e

ependi&ure of 2 7eO, &'is a%erage energy is 0; *2333 eO-V*


4/92

Eample>or &'e scin&illa&or analyed in &'e eample gi%en af&er >ig. 83.3, i& was found

&'a& &'e a%erage energy needed &o produce a p'o&oelec&ron was 8## eO.*a- 0'a& is &'e resolu&ion for &'e &o&al!energy pea7 for "#3!7eO p'o&ons5*(- 0'a& is &'e wid&' of &'e &o&al!energy pea7 *>09- in 7eO5*c- 0'a& is &'e resolu&ion for 8.2!eO p'o&ons5

Solu&ion*a- T'e a%erage num(er of p'o&oelec&rons produced (y a(sorp&ion of a "#3!

7eO p'o&on is "#3,333V8## 233. T'e resolu&ion is &'erefore, R 2.#V

*233-8V2 3.3".*(- >or "#3!7eO p'o&ons, i& follows &'a& >09 3.3"A"#3 8. 7eO.*c- T'e resolu&ion decreases as &'e sHuare roo& of &'e p'o&on energy. T'us,

&'e resolu&ion for 8.2!eO p'o&ons is 3.3" *3."#3V8.2-8V2 3.32.

d. Deviation from Poisson statistics – ano factor

Reference: James E. Turner. Atoms, Radiation, and Radiation Protection,

pp.

T'e >ano fac&or 'as (een in&roduced as a measure of &'e depar&ure of

Quc&ua&ions from pure )oisson s&a&is&ics. $& is de=ned as &'e ra&io of &'e

o(ser%ed %ariance and &'e %ariance predic&ed (y &'e la&&er:

iance Poisson

ianceObserved F

var

var =

.Repor&ed %alues of >ano fac&ors for gas propor&ional coun&ers are in &'e range

from a(ou& 3.8 &o 3.2 and, for semiconduc&ors, from 3.3 &o 3.8#. >or

scin&illa&ion de&ec&ors, > is near uni&y, indica&ing a )oisson!limi&ed resolu&ion.

!. Nuclear physics basicsa. ield descriptions

Reference: Radia&ion )ro&ec&ion Lompe&ency 8.8 p.22!2#

Point source T'e in&ensi&y of &'e radia&ion =eld decreases as &'e dis&ance from &'e source

increases. T'erefore, increasing &'e dis&ance will reduce &'e amoun& of

eposure recei%ed. $n many cases, especially w'en wor7ing wi&' poin&

sources, increasing &'e dis&ance from &'e source is more e/ec&i%e &'an

decreasing &'e &ime spen& in &'e radia&ion =eld. T'eore&ically, a poin& source is an imaginary poin& in space from w'ic' all &'e

radia&ion is assumed &o (e emana&ing. 0'ile &'is 7ind of source is no& real *all

real sources 'a%e dimensions-, any geome&rically small source of radia&ion

(e'a%es as a poin& source w'en one is wi&'in &'ree &imes &'e larges&

dimension of &'e source. Radia&ion from a poin& source is emi&&ed eHually in all


5/92

direc&ions. T'us, &'e p'o&ons spread ou& &o co%er a grea&er area as &'e

dis&ance from &'e poin& source increases. T'e e/ec& is analogous &o &'e way

lig'& spreads ou& as we mo%e away from a single source of lig'& suc' as a lig'&

(ul(. T'e radia&ion in&ensi&y for a poin& source decreases according &o &'e $n%erse

SHuare Zaw w'ic' s&a&es &'a& as &'e dis&ance from a poin& source decreases or

increases &'e dose ra&e increases or decreases (y &'e sHuare of &'e ra&io of

&'e dis&ances from &'e source. T'e in%erse sHuare law (ecomes inaccura&e

close &o &'e source *i.e., wi&'in &'ree &imes &'e larges& dimension of &'e

source-.+s pre%iously men&ioned, &'e eposure ra&e is in%ersely propor&ional &o &'e

sHuare of &'e dis&ance from &'e source. T'e ma&'ema&ical eHua&ion is:

T'is eHua&ion is assuming &'e a&&enua&ion of &'e radia&ion in &'e in&er%ening

space is negligi(le and &'e dimensions of &'e source and &'e de&ec&or are

small compared wi&' &'e dis&ance (e&ween &'em. T'e in%erse sHuare law 'olds &rue only for poin& sourcesX 'owe%er, i& gi%es a

good approima&ion w'en &'e source dimensions are smaller &'an &'e

dis&ance from &'e source &o &'e eposure poin&. Due &o dis&ance cons&rain&s,

eposures a& cer&ain dis&ances from some sources, suc' as for a pipe or &an7,

canno& (e &rea&ed as a poin& source. $n &'ese si&ua&ions, &'ese sources mus&

(e &rea&ed as line sources or large surface sources.

x p er

S µ

π φ −=

24

for calcula&ion of Qu from &'e poin&

source

"ine Sources+n eample of a line source would (e a pipe carrying con&amina&ed cooling

wa&er or liHuid was&e, a con&rol rod, a series of poin& sources w'ic' are close

&oge&'er, or a needle inUec&ing a radioiso&ope in&o &issue. 0i&' line sources, an

assump&ion mus& (e made &'a& &'e dis&ri(u&ion of radioac&i%i&y is uniform

&'roug'ou& &'e source. 0'en no a&&enua&or is presen&, &'e rela&ions'ip


6/92

(e&ween &'e line source emission ra&e and &'e Qu a& &'e

recep&or *)- depends on &'e loca&ion of &'e recep&or wi&'

respec& &o &'e line source. 9owe%er, &'is rela&ions'ip is

more comple ma&'ema&ically &'an in &'e case of &'e

poin& source, and &'e use of calculus is reHuired. T'efollowing =gure and formula applies &o line sources.

x

p eh

Sl µ θ θ π

φ −

−= )(4

0

Plane Sources+n eample of a plane source would (e a spill of liHuid con&aining radioac&i%i&y

on &'e Qoor. +gain, w'en es&ima&ing &'e amoun& of

radioac&i%i&y emana&ing from an area source, an

assump&ion mus& (e made &'a& &'e dis&ri(u&ion of

radioac&i%i&y is uniform &'roug'ou& &'e source. >or an

area source wi&' an a&&enua&or presen&, &'e calcula&ions

(ecome %ery complica&ed. >or illus&ra&i%e purposes, an

eample of a circular area source wi&'ou& an a&&enua&or presen& is gi%en.

)1ln(4 2

2

h

aS a p +=φ

b. Interaction of radiation #ith matter and interaction ratesi. Production of annihilation radiation$ %remsstrahlung$ and

&uger electrons


pp., #


7/92

producing &'e radia&ion (ecause &'e deQec&ions are

s&ronger. + single elec&ron can emi& an 6!ray p'o&on

'a%ing any energy up &o i&s own 7ine&ic energy. +s a resul&,

a monoenerge&ic (eam of elec&rons produces a con&inuous

spec&rum of 6 rays wi&' p'o&on energies up &o &'e %alue of &'e (eam energy. T'ese con&inuous 6 rays are called

(remss&ra'lung, or (ra7ing radia&ion.;

&uger electrons>ollowing eUec&ion of &'e p'o&oelec&ron, &'e inner!s'ell %acancy in &'e

a&om is immedia&ely =lled (y an elec&ron from an upper le%el resul&ing

in a release of energy. +l&'oug' mos& of &'e &ime &'is energy is

released in &'e form of an emi&&ed p'o&on, &'e energy can also (e

&ransferred &o ano&'er elec&ron, w'ic' is eUec&ed from &'e a&om. T'is

second eUec&ed elec&ron is called an +uger elec&ron.

c. 'adioactive decayi. Half(life$ mean life$ decay constant$ activity

Reference: 9erman Lem(er. Health Physics. 4th edition, p.<

Half(life T'e &ime reHuired for any gi%en radionuclide &o decrease &o one!'alf of

i&s original Huan&i&y is a measure of &'e speed wi&' w'ic' i& undergoes

radioac&i%e &ransforma&ion. T'is period of &ime is called &'e 'alf!life and

is c'arac&eris&ic of &'e par&icular radionuclide. Eac' radionuclide 'as i&s

own uniHue ra&e of &ransforma&ion, and no opera&ion, ei&'er c'emical

or p'ysical, is 7nown &'a& will c'ange &'e &ransforma&ion ra&eX &'e

decay ra&e of a radionuclide is an unal&era(le proper&y of &'a& nuclide.

E6+)ZE ".8Lo(al&!3, a gamma!emi&&ing iso&ope of co(al& w'ose 'alf!life is #.

years, is used as a radia&ion source for radiograp'ing pipe welds.

Because of &'e decrease in radioac&i%i&y wi&' increasing &ime, &'e

eposure &ime for a radiograp' will (e increased annually. Lalcula&e &'e

correc&ion fac&or &o (e applied &o &'e eposure &ime in order &o accoun&for &'e decrease in &'e s&reng&' of &'e source.

Solu&ionEHua&ion can (e wri&&en as

n

A

A20 = *since n A

A

2

1

0

=-


8/92

By &a7ing &'e logari&'m of eac' side of &'e eHua&ion, we 'a%e

2loglog 0 n A

A=

w'ere n, &'e num(er of 3Lo 'alf!li%es in 8 year, is 8V*#.- 3.8


9/92

T'e uni& of ac&i%i&y is &'e BecHuerel *BH-, de=ned as one disin&egra&ion

per second: 8 BH 8 s\8. T'e &radi&ional uni& of ac&i%i&y is &'e curie *Li-,

w'ic' was originally &'e ac&i%i&y ascri(ed &o 8 g of 22Ra. T'e curie is

de=ned as 8 Li .A8383 BH, eac&ly.

ii. Simple decay


pp.


10/92

111 N

dt

dN λ −=

^

t

oe N N 1

11

λ −=

221122112 1 N e N N N

dt

dN t o λ λ λ λ

λ −=−= −

ul&iply (y

t e 2λ

for (o&' sides

dt e N dt N edN e t t t )(

1012221222 λ λ λ λ λ λ

−=+

dt e N e N d t t )(

1012122 ][

λ λ λ λ −=

$n&egra&e (o&' sides

t d e N dt e N t

t t

t

∫ ∫ −= 0)(

1010

2122 λ λ λ λ

t

t t e

N e N

0

)(

12

1012

122

−

= −λ λ λ λ λ

λ

]1[ )(

12

1012

122 −−

= − t t e N

e N λ λ λ

λ λ

λ

][ 21

12

1012

t t ee

N N λ λ

λ λ

λ −− −

−

=

iv. Serial decay


pp.


11/92

of daug'&er a&oms 2 per uni& &ime is eHual &o &'e ra&e a& w'ic' &'ey

are produced, +8, minus &'eir ra&e of decay, ]22:

2212 N A

dt

dN λ −=

dt N A

dN =− 2212

λ

w'ere +8 can (e regarded as cons&an&. $n&roducing &'e %aria(le u +8

\ ]22, we 'a%e du \]2d2 and,

dt u

du2λ −=

$n&egra&ion gi%es

ct N A +−=− 2221 )ln( λ λ

w'ere c is an ar(i&rary cons&an&. $f 23 represen&s &'e num(er of a&omsof nuclide *2- presen& a& & 3, &'en we 'a%e c ln*+8 \ ]223-.

t N A

N A2

2021

221ln λ λ

λ −=

−−

or

t e N A N A 2)( 2021221

λ λ λ −−=−

Since ]22 +2, &'e ac&i%i&y of nuclide *2-, and ]223 +23 is i&s ini&ial

ac&i%i&y,t t

e Ae A A 22 2012 )1( λ λ −− +−=

$n many prac&ical

ins&ances one s&ar&s wi&' a

pure sample of nuclide *8-

a& & 3, so &'a& +23 3,

w'ic' we now assume.

T'e ac&i%i&y +2 &'en (uilds

up as s'own in >ig. ".".

+f&er a(ou& se%en

daug'&er 'alf!li%es *&_

T2-, e\]2& G8 and EH.

reduces &o &'e condi&ion +8

+2, a& w'ic' &ime &'e

daug'&er ac&i%i&y is eHual

&o &'a& of &'e paren&. T'is condi&ion is called secular eHuili(rium. T'e

&o&al ac&i%i&y is 2+8. $n &erms of &'e num(ers of a&oms, 8 and 2, of &'e

paren& and daug'&er, secular eHuili(rium can (e also epressed (y

wri&ing


12/92

2211 N N λ λ =

Transient ,-uilibrium T1 T!0

w'en 23 3 and &'e 'alf!life of &'e paren& is grea&er &'an &'a& of &'e

daug'&er, (u& no& grea&ly so

)( 21

12

101222

t t ee N

N λ λ

λ λ

λ λ λ −− −

−=

0i&' &'e con&inued passage of &ime, e \]2& e%en&ually (ecomes negligi(le

wi&' respec& &o e\]8&, since ]2 ` ]8. $n addi&ion since +8 ]88 ]883e\

]8& is &'e ac&i%i&y of &'e paren& as a func&ion of &ime, &'is rela&ion says

&'a&

12

122

λ λ

λ

−=

A A

T'e &ime a& w'ic' &'e daug'&er ac&i%i&y is larges&

1

2

12

ln1

λ

λ

λ λ −=t

for maimum +2 T'e &o&al ac&i%i&y is larges& a& &'e earlier &ime

2

121

2

2

12 2ln

1

λ λ λ

λ

λ λ −−=t

for maimum +8 +2


13/92

v. &ctivation 2decay relations


p.22

Reference: Llass 3", Zec&ure 28

delayirr t t ee N A 22 )1(112

λ λ φ σ −−−=

d. Nuclear decay schemes

Reference: James E. Turner. Atoms, Radiation, and Radiation Protection, pp.2!

Refer &o +ppendi D and deduce &'e decay sc'eme of 28+l.28+l b: \82.288 eO,


14/92

e. Shielding and radiation attenuation

Reference: Radia&ion )ro&ec&ion Lompe&ency 8.8 p.8

+lp'a par&icles are rela&i%ely massi%e, slow mo%ing par&icles &'a& in&erac& (y

ionia&ion and eci&a&ion. T'erefore, alp'a radia&ion is no& %ery pene&ra&ing.

+lp'a radia&ion is no& an e&ernal 'aard and can (e s'ielded (y:+ few inc'es of air.

+ s'ee& of paper.+ dead layer of s7in.

Be&a par&icles are rela&i%ely lig'&, fas& mo%ing par&icles &'a& in&erac& (y

ionia&ion and eci&a&ion. Bea& radia&ion is modera&ely pene&ra&ing dependen&

on &'e energy or %eloci&y of &'e (e&a par&icle, and can (e an e&ernal 'aard if

i& can pene&ra&e &'e dead layer of s7in. Be&a radia&ion s'ould (e s'ielded (y


15/92

low a&omic num(er ma&erials *i.e., - &o pre%en& &'e produc&ion of

(remss&ra'lung radia&ion. T'ese ma&erials include:)las&ic.0ood.+luminum.

eu&ron s'ielding in%ol%es slowing down fas& neu&rons and a(sor(ing &'ermal

neu&rons. >or eample, con&rol rods in nuclear reac&ors can (e fa(rica&ed from

(oron, w'ic' is a good ma&erial &o a(sor( &'ermal neu&rons. eu&ron s'ielding

is 'ig'ly dependen& on &'e energy of &'e neu&ron. T'e goal in neu&ron

s'ielding is &o genera&e a c'arged par&icle %ia an in&erac&ion. T'e (es&

in&erac&ion for s'ielding neu&rons would (e an elas&ic collision wi&' a lig'&

nucleus suc' as a 'ydrogen a&om. + 'ydrogen nucleus consis&s of a single

pro&on and allows a signi=can& &ransfer of energy &o a pro&on (ecause &'e

masses of &'e pro&on and neu&ron are almos& &'e same. T'e neu&ron collides

wi&' &'e pro&on, &ransferring energy and recoils &'e pro&on away from i&s

elec&ron cloud. T'e li(era&ed pro&ons range is &'en %ery s'or&, causing

ionia&ions and eci&a&ions along &'e recoiled pro&ons pa&'. eu&rons can (e

s'ielded (y ma&erials wi&' a 'ig' 'ydrogen con&en& suc' as:0a&er.Loncre&e.)las&ic.>uel 4il.)araMn.

)'o&on s'ielding is also 'ig'ly dependen& on &'e energy of &'e p'o&on and &'e

a&omic num(er of &'e s'ielding ma&erial. +s in neu&ron s'ielding, &'e goal is

&o produce a c'arged par&icle %ia an in&erac&ion, prefera(ly &'e p'o&oelec&rice/ec&, in w'ic' all of &'e p'o&on energy is &ransferred &o &'e elec&ron. T'e

p'o&oelec&rons range in ma&&er is %ery s'or&, causing ionia&ions and

eci&a&ions in &'e s'ielding ma&erial. T'e energy of &'e p'o&on is &'en

&ransferred &o &'e s'ield (y p'o&oelec&rons. Since p'o&ons in&erac& wi&'

elec&rons, p'o&ons can (e s'ielded (y any ma&erial w'ic' pro%ides an

adeHua&e num(er of elec&rons. T'is can (e done (y use of 'ig' a&omic

num(er *'ig' -, suc' as lead or uranium. $f space is no& limi&ed, wa&er or

concre&e may (e a prac&ical s'ielding ma&erial.


pp.83!88

0'a& &'ic7ness of concre&e and of lead is needed &o reduce &'e num(er of

#33! 7eO p'o&ons in a narrow (eam &o one!four&' &'e inciden& num(er5

Lompare &'e &'ic7nesses in cm and in g cm\2. Repea& for 8.#!eO p'o&ons.Sol"tion


16/92

0e use EH. *or lead,3.2# e\3.3#8$# ,

and so $# 28.2 g cm\2 and # 2. cm. +& &'is energy &'e Lomp&on e/ec& is

&'e principal in&erac&ion &'a& a&&enua&es &'e (eam, and &'erefore all ma&erials

*ecep& 'ydrogen- gi%e compara(le a&&enua&ion per g cm \2. Zead is almos&

uni%ersally used w'en low!energy p'o&on s'ielding is reHuired.

3. Ioni4ing radiationa. Types and sources

Reference: >ran7 9. +&&i. $n&roduc&ion &o Radiological )'ysics and Radia&ion

Dosime&ry, pp.2!

T'e impor&an& &ypes of ioniing radia&ions &o (e considered are:i. r!rays5 Elec&romagne&ic radia&ion emi&&ed from a nucleus or in

anni'ila&ion reac&ions (e&ween ma&&er and an&ima&&er.ii. 6!rays: Elec&romagne&ic radia&ion emi&&ed (y c'arged par&icles *usually

elec&rons- in c'anging a&omic energy le%els *called c'arac&eris&ic orQuorescence !rays- or in slowing down in a Loulom( force =eld

*con&inuous or (remss&ra'lung !rays-. o&e &'a& an !ray and a y!ray

p'o&on of a gi%en Huan&um energy 'a%e iden&ical proper&ies, di/ering

only in mode of origin. Iamma rays origina&e in &'e nucleus. 6!rays

origina&e in &'e elec&ron =elds surrounding &'e nucleus or are mac'ine!

produced.


17/92

iii. >as& elec&rons: $f posi&i%e in c'arge, &'ey are called posi&rons. $f &'ey

are emi&&ed from a nucleus &'ey are usually referred &o as !rays

*posi&i%e or nega&i%e-. $f &'ey resul& from a c'arged!par&icle collision

&'ey are referred &o as h rays.i%. 9ea%y c'arged par&icles: Csually o(&ained from accelera&ion (y a

Loulom( force =eld in a Oan de Iraa/, cyclo&ron, or 'ea%y!par&icle

linear accelera&or. +lp'a par&icles are also emi&&ed (y some radioac&i%e

nuclei. Types include: )ro&on ! &'e 'ydrogen nucleus. Deu&eron ! &'e deu&erium nucleus, consis&ing of a pro&on and

neu&ron (ound &oge&'er (y nuclear force. Tri&on ! a pro&on and &wo neu&rons similarly (ound. +lp'a par&icle ! &'e 'elium nucleus, i.e., &wo pro&ons and &wo

neu&rons. 4&'er 'ea%y c'arged par&icles consis&ing of &'e nuclei of

'ea%ier a&oms, ei&'er fully s&ripped of elec&rons or in any case

'a%ing a di/eren& num(er of elec&rons &'an necessary &o

produce a neu&ral a&om. )ions ! nega&i%e !mesons produced (y in&erac&ion of fas&

elec&rons or pro&ons wi&' &arge& nuclei.%. eu&rons: eu&ral par&icles o(&ained from nuclear reac&ions ?e.g., *p, n-

or =ssion@, since &'ey canno& &'emsel%es (e accelera&ed

elec&ros&a&ically.

T'e $LRC *$n&erna&ional Lommission on Radia&ion Cni&s and easuremen&s,

88- 'as recommended cer&ain &erminology in referring &o ioniing radia&ions

w'ic' emp'asies &'e gross di/erences (e&ween &'e in&erac&ions of c'arged

and unc'arged radia&ions wi&' ma&&er:i. Direc&ly $oniing Radia&ion. >as& c'arged par&icles, w'ic' deli%er &'eir

energy &o ma&&er direc&ly, &'roug' many small Loulom(!force

in&erac&ions along &'e par&icles &rac7.ii. $ndirec&ly $oniing Radia&ion. 6! or r!ray p'o&ons or neu&rons *i.e.,

unc'arged par&icles-, w'ic' =rs& &ransfer &'eir energy &o c'arged

par&icles in &'e ma&&er &'roug' w'ic' &'ey pass in a rela&i%ely few large

in&erac&ions. T'e resul&ing fas& c'arged par&icles &'en in &urn deli%er&'e energy &o &'e ma&&er as a(o%e.

b. *haracteristics

S&a&ed a(o%e Types and sources6

c. ield -uantities


18/92

Reference: >ran7 9. +&&i. $n&roduc&ion &o Radiological )'ysics and Radia&ion

Dosime&ry, pp.

i. >ZCELEReferring &o >ig. 8.8, Ze& , (e &'e epec&a&ion %alue of &'e num(er of

rays s&ri7ing a =ni&e sp'ere surrounding poin& ) during a &ime in&er%ale&ending from an ar(i&rary s&ar&ing &ime t o &o a la&er &ime t . $f &'e

sp'ere is reduced &o an in=ni&esimal a& P wi&' a grea&!circle area of da,

we may de=ne a Huan&i&y called &'e Quence, j, as &'e Huo&ien& of &'e

di/eren&ial of Ne, (y da:

da

dN e=Φ

w'ic' is usually epressed in uni&s of m!2 or cm!2.ii. >ZC6 DES$TY *4R >ZCELE R+TE-

j may (e de=ned a(o%e for all %alues of t &'roug' &'e in&er%al from t =

t o *for w'ic' j jma-. T'en a& any &ime t wi&'in &'e in&er%al we may

de=ne &'e Qu densi&y or Quence ra&e a& ) as

=

Φ=

da

dN

dt

d

dt

d eϕ

w'ere dj is &'e incremen& of Quence during &'e in=ni&esimal &ime

in&er%al dt a& &ime t , and &'e usual uni&s of Qu densi&y are m!2 s!8 or cm!

2 s!8.iii. EERIY >ZCELE

T'e simples& =eld!descrip&i%e Huan&i&y w'ic' &a7es in&o accoun& &'e

energies of &'e indi%idual rays is &'e energy Quence k, for w'ic' &'e

energies of all &'e rays are summed. Ze& R (e &'e epec&a&ion %alue of

&'e &o&al energy *eclusi%e of res&!mass energy- carried (y all &'e ,

rays s&ri7ing a =ni&e sp'ere surrounding poin& ) during a &ime in&er%al

e&ending from an ar(i&rary s&ar&ing &ime t o &o a la&er &ime t . $f &'e

sp'ere is reduced &o an in=ni&esimal a& ) wi&' a grea&!circle area of da,

we may de=ne a Huan&i&y called &'e energy Quence, k, as &'e Huo&ien&

of &'e di/eren&ial of R (y da:

da

dR=Ψ

w'ic' is usually epressed in uni&s of J m!2 or erg cm!2.

>or &'e special case w'ere only a single energy E of rays is presen&, &'e

a(o%e eHua&ions are rela&ed (y

e EN R =


19/92

and

Φ=Ψ E

i%. EERIY >ZC6 DES$TY *4R EERIY >ZCELE R+TE- may (e de=ned a(o%e eHua&ion for all %alues of t &'roug'ou& &'e

in&er%al from & &o *for w'ic' k 3- &o & &ma *for w'ic' k kma-.

T'en a& any &ime & wi&'in &'e in&er%al we may de=ne &'e energy Qu

densi&y or energy Quence ra&e a& ) as:

=

Ψ=Ψ

da

dR

dt

d

dt

d

w'ere dk is &'e incremen& of energy Quence during &'e in=ni&esimal

&ime in&er%al d& a& &ime & , and &'e usual uni&s of energy Qu densi&y are J

m!2 s!8 or erg cm!2 s!8.

8 eO 8.32 A 83! erg 8.32 A 83!8 J

d. Interaction #ith matter

i. Ioni4ation$ e7citation$ 8(value

Reference: Radia&ion )ro&ec&ion Lompe&ency 8.8 p.#Reference: >ran7 9. +&&i. $n&roduc&ion &o Radiological )'ysics and

Radia&ion Dosime&ry, p.

L'arged par&icle radia&ions, suc' as alp'a par&icles or elec&rons, will

con&inuously in&erac& wi&' &'e elec&rons presen& in any medium

&'roug' w'ic' &'ey pass (ecause of &'eir elec&ric c'arge. T'ese

par&icles mus& undergo an in&erac&ion resul&ing in a full or par&ial

&ransfer of energy of &'e inciden& radia&ion &o &'e elec&ron or nuclei of

&'e cons&i&uen& a&om. $f &'e energy &ransferred &o &'e elec&ron is

grea&er &'an &'e energy 'olding &'e elec&ron &o &'e a&om, &'e elec&ron

will lea%e &'e a&om and crea&e ionia&ion. $onia&ion is &'e process of

&urning an elec&rically neu&ral a&om in&o an ion pair consis&ing of a

nega&i%ely c'arged elec&ron un(ound &o an a&om, and an a&om missing

one elec&ron crea&ing a ne& posi&i%e c'arge. $f insuMcien& energy is

&ransferred &o &'e elec&ron &o lea%e &'e a&om, &'e elec&ron is said &o (e

eci&ed. Eci&a&ion does no& crea&e ionia&ion or ion pairs, (u& does

impar& some energy &o &'e a&om. 0!%alue is &'e mean energy *in eO-

spen& (y a c'arged par&icle of ini&ial energy % o in producing eac' ion

pair:


20/92

N

T W o≅

w'ere N is &'e epec&a&ion %alue of &'e num(er of ion pairs produced

(y suc' a par&icle s&opping in &'e medium *usually a gas- &o w'ic' 0

refers. T'e %alue for elec&rons, & ". eO ip\8.

ii. 'ange$ *SD& range$ density thic9ness$ mean(free path

Reference: Radia&ion )ro&ec&ion Lompe&ency 8.8 p.8<

L'arged par&icles 'a%e a de=ni&e range in ma&&er. T'e range of a

c'arged par&icle in an a(sor(er is &'e a%erage dep&' of pene&ra&ion of

&'e c'arged par&icle in&o &'e a(sor(er (efore i& loses all of i&s 7ine&ic

energy and s&ops. T'e energy of &'e par&icle, w'ic' is a func&ion of &'e

mass of &'e par&icle and i&s %eloci&y, and &'e elec&rical c'arge of &'e

par&icle a/ec& &'e range of &'e c'arged par&icle in a ma&erial. T'e

a&omic densi&y *num(er of a&oms per cu(ic cen&ime&er- and &'e a&omic

num(er *- of &'e s'ielding ma&erial also a/ec& range.


p. 88,

*SD& is &'e contin"o"s'slo(in)'do(n appro#imation. $& ignores

Quc&ua&ions of energy loss in collisions and assumes &'a& a c'arged

par&icle loses energy con&inuously along i&s pa&' a& &'e linear ra&egi%en (y &'e ins&an&aneous s&opping power.


T'e fac&or &'a& a/ec&s &'e range of a c'arged par&icle in any ma&erial

is a uni& called densi&y!&'ic7ness. Density(thic9ness can (e

calcula&ed (y mul&iplying &'e densi&y of a ma&erial in grams per cu(ic

cen&ime&er *gVcm- (y &'e dis&ance &'e par&icle &ra%eled in &'a&

ma&erial in cen&ime&ers. T'e produc& is densi&y!&'ic7ness in uni&s of

grams per sHuare cen&ime&er *gVcm2

-. Densi&y!&'ic7ness can (econsidered a cross!sec&ional &arge& for a c'arged par&icle as i& &ra%els

&'roug' &'e ma&erial. T'e concep& of densi&y!&'ic7ness is impor&an& &o

discussions of (e&a radia&ion a&&enua&ion (y 'uman &issue, de&ec&or

s'ieldingVwindows, and dosime&ry =l&ers. +l&'oug' ma&erials may 'a%e

di/eren& densi&ies and &'ic7nesses, if &'eir densi&y!&'ic7ness %alues are

&'e same, &'ey will a&&enua&e (e&a radia&ion in a similar manner. >or


21/92

eample, a piece of ylar used as a de&ec&or window wi&' a densi&y of

mgVcm2 will a&&enua&e (e&a radia&ion similar &o &'e ou&er layer of

dead s7in of &'e 'uman (ody w'ic' 'as a densi&y!&'ic7ness of

mgVcm2.


p. 88

T'e mean free path is &'e mean dis&ance of &ra%el of a c'arged

par&icle (e&ween collisions. $& is &'e reciprocal of μ &'a& is &'e

macroscopic cross sec&ion, &'e pro(a(ili&y per uni& dis&ance of &ra%el

&'a& an elec&ronic collision &a7es place. *8V μ*

iii. Stopping po#er$ linear energy transfer$ linear energy transfer


pp. 88#, 82Reference:

'&&p:VVwww.med.'ar%ard.eduVUpnmVp'ysicsVnml&dVradprinVsec&V.2V2.

.'&ml

S&opping power is &'e a%erage linear ra&e of energy loss of a 'ea%y

c'arged par&icle in a medium, designa&ed \d+Vd # , in eO cm\8. T'e

linear energy &ransfer *ZET- is &'e ra&e of energy &ransfer per uni&

dis&ance along a c'arged!par&icle &rac7 as &'e Huo&ien& \d+ZVd # . T'e

ZET is similar &o &'e s&opping power ecep& &'a& i& does no& include &'e

e/ec&s of radia&i%e energy loss *i.e., Bremss&ra'lung- or del&a!rays. T'e

di/erence (e&ween ZET and s&opping power is &'a& ZET is local energy

deposi&ion only and s&opping power is concerned wi&' &'e &o&al energy

los& (y &'e par&icle. T'e s&opping power and ZET are nearly eHual for

'ea%y c'arged par&iclesX for (e&as ZET does no& include del&a!rays nor

Bremss&ra'lung. T'e ZET is rela&ed &o Biological Damage. T'e se%eri&y

and permanence of (iological c'anges are direc&ly rela&ed &o &'e local

ra&e of energy deposi&ion along &'e par&icle &rac7. T'e 'ig'er &'e ZET,

&'e 'ig'er &'e Huali&y fac&or in de&ermining dose eHui%alen&.

iv. *ompton e:ect$ photoelectric e:ect$ pair production


Photoelectric ,:ect$n &'e p'o&oelec&ric e/ec& &'e p'o&on impar&s all of i&s energy &o an

or(i&al elec&ron of some a&om. T'e p'o&on, since i& consis&ed only of


22/92

energy in &'e =rs& place,

simply %anis'es. T'e

energy is impar&ed &o

&'e or(i&al elec&ron in

&'e form of 7ine&icenergy of mo&ion,

o%ercoming &'e

a&&rac&i%e force of &'e

nucleus for &'e elec&ron

*&'e (inding energy- and

usually causing &'e

elec&ron &o Qy from i&s

or(i& wi&' considera(le %eloci&y. T'us, an ion pair resul&s. T'e

pro(a(ili&y of p'o&oelec&ric e/ec& a& i&s maimum, occurs w'en &'eenergy of &'e p'o&on is eHual &o &'e (inding energy of &'e elec&ron.

T'e &ig'&er an elec&ron is (ound &o &'e nucleus, &'e 'ig'er &'e

pro(a(ili&y of p'o&oelec&ric e/ec&, so mos& p'o&oelec&rons are inner

s'ell elec&rons. T'e p'o&oelec&ric e/ec& is seen primarily as an e/ec& of

low energy p'o&ons wi&' energies near &'e elec&ron (inding energies of

ma&erials and 'ig' ma&erials w'ose inner!s'ell elec&rons 'a%e 'ig'

(inding energies.

*ompton Scattering


23/92

$n Lomp&on

sca&&ering &'ere is a

par&ial energy loss

for &'e incoming

p'o&on. T'e p'o&onin&erac&s wi&' an

or(i&al elec&ron of

some a&om and only

par& of &'e p'o&on

energy is &ransferred

&o &'e elec&ron. +f&er

&'e collision, &'e

p'o&on is deQec&ed in a di/eren& direc&ion a& a reduced energy. T'e

recoil elec&ron, now referred &o as a Lomp&on elec&ron, producessecondary ionia&ion in &'e same manner as does &'e p'o&oelec&ron,

and &'e sca&&ered p'o&on con&inues on un&il i& loses more energy in

ano&'er p'o&on in&erac&ion. T'e pro(a(ili&y of a Lomp&on in&erac&ion

increases for loosely (ound elec&rons and, &'erefore, increases

propor&ionally &o &'e of &'e ma&erial. Lomp&on sca&&ering is primarily

seen as an e/ec& of medium energy p'o&ons and i&s pro(a(ili&y

decreases wi&' increasing energy.

)'o&on energy af&er Lomp&on sca&&ering

)cos1(12

0

'

θ −+=

cm E

E E

r

r r

$ncoming p'o&on energy

)cos1(12

0

'

'

θ −−=

cm

E

E E

r

r r

+ngle of &'e p'o&on sca&&er

ϕ θ

tan12

cot2

0

+=

cm

E

* recoiled elec&rons angle-

Pair Production)air produc&ion occurs w'en &'e p'o&on is con%er&ed &o mass. $n pair

produc&ion a p'o&on simply disappears in &'e %icini&y of a nucleus and

in i&s place appears a pair of elec&rons: one nega&i%ely and one

posi&i%ely c'arged *an&i!par&icles are also called elec&ron and posi&ron


24/92

respec&i%ely-. )air produc&ion is impossi(le unless &'e p'o&on

possesses grea&er &'an 8.322 eO of energy &o ma7e up &'e res& mass

of &'e par&icles. +ny ecess energy in &'e p'o&on a(o%e &'e 8.322 eO

reHuired &o crea&e &'e &wo elec&ron masses is simply s'ared (e&ween

&'e &wo elec&rons as 7ine&ic energy of mo&ion, and &'ey Qy ou& of &'ea&om wi&' grea& %eloci&y. T'e pro(a(ili&y increases for 'ig' ma&erials

and 'ig' energies. T'e pair produc&ion elec&ron &ra%els &'roug' ma&&er,

causing ionia&ions and eci&a&ions, un&il i& loses all of i&s 7ine&ic energy

and is Uoined wi&' an a&om. T'e posi&i%e elec&ron *7nown as a posi&ron-

also produces ionia&ions and eci&a&ions un&il i& comes &o res&. 0'ile a&

res&, &'e posi&ron a&&rac&s a free elec&ron, w'ic' &'en resul&s in

anni'ila&ion of &'e pair, con%er&ing (o&' in&o elec&romagne&ic energy.

T'us, &wo p'o&ons of #88 7eO eac' arise a& &'e si&e of &'e anni'ila&ion

*accoun&ing for &'e res& mass of &'e par&icles-. T'e ul&ima&e fa&e of &'eanni'ila&ion p'o&ons is ei&'er p'o&oelec&ric a(sorp&ion or Lomp&on

sca&&ering followed (y p'o&oelec&ric a(sorp&ion.


25/92

v. &ttenuation coe;cients

Reference: Radia&ion )ro&ec&ion Lompe&ency 8.8 p.23

0'en s'ielding agains& !rays and gamma rays, i& is impor&an& &o

realie &'a& p'o&ons are remo%ed from &'e incoming (eam on &'e (asis

of &'e pro(a(ili&y of an in&erac&ion *p'o&oelec&ric, Lomp&on, or pair

produc&ion-. T'is process is called a&&enua&ion and can (e descri(ed

using &'e linear a&&enua&ion coefficien&, , w'ic' is &'e pro(a(ili&y of

an in&erac&ion per pa&' leng&' &'roug' a ma&erial. T'e linear

a&&enua&ion coeMcien& %aries wi&' p'o&on energy and &ype of ma&erial.

a&'ema&ically, &'e a&&enua&ion of a narrow (eam of monoenerge&icp'o&ons is gi%en (y:

xe x

µ −= 0)(

w'ere:$*- Radia&ion in&ensi&y ei&ing a ma&erial of &'ic7ness $o Radia&ion in&ensi&y en&ering a ma&eriale Base of na&ural logari&'ms *2.8"......- Zinear a&&enua&ion coefficien& T'ic7ness of ma&erial.

T'is eHua&ion s'ows &'a& &'e in&ensi&y is reduced eponen&ially wi&'

&'ic7ness. $*- ne%er ac&ually eHuals ero (ecause !rays and gamma

rays in&erac& (ased on pro(a(ili&y and &'ere is a =ni&e *al(ei& small-

pro(a(ili&y &'a& a gamma could pene&ra&e &'roug' a &'ic7 s'ield

wi&'ou& in&erac&ing. S'ielding for !rays and gamma rays &'en

(ecomes an +Z+R+ issue and no& an issue of s'ielding &o ero

in&ensi&ies.


26/92

T'e formula a(o%e is used &o calcula&e &'e radia&ion in&ensi&y from a

narrow (eam (e'ind a s'ield of &'ic7ness , or &o calcula&e &'e

&'ic7ness of a(sor(er necessary &o reduce radia&ion in&ensi&y &o a

desired le%el. Ta(les and grap's are a%aila(le w'ic' gi%e %alues of

de&ermined eperimen&ally for di/eren& radia&ion energies and manya(sor(ing ma&erials. T'e larger &'e %alue of &'e grea&er &'e

reduc&ion in in&ensi&y for a gi%en &'ic7ness of ma&erial. T'e fac& &'a&

lead 'as a 'ig' for ! and gamma radia&ion is par&ially responsi(le for

i&s wide use as a s'ielding ma&erial.

vi. 'ayleigh scattering *oherent scattering0

Reference: >ran7 9. +&&i. $n&roduc&ion &o Radiological )'ysics and

Radia&ion Dosime&ry, p.8#

Rayleig' sca&&ering is called co'eren&; (ecause &'e p'o&on issca&&ered (y &'e com(ined ac&ion of &'e w'ole a&om. T'e e%en& is

elas&ic in &'e sense &'a& &'e p'o&on loses essen&ially none of i&s

energyX &'e a&om mo%es Uus& enoug' &o conser%e momen&um. T'e

p'o&on is usually redirec&ed &'roug' only a small angle. T'erefore &'e

e/ec& on a p'o&on (eam can only (e de&ec&ed in narrow!(eam

geome&ry. Rayleig' sca&&ering con&ri(u&es no&'ing &o 7erma or dose,

since no energy is gi%en &o any c'arged par&icle, nor is any ionia&ion

or eci&a&ion produced.


pp. 8


27/92

coeMcien&. $& follows &'a& e\ μ# is Uus& &'e pro(a(ili&y *i.e., NVN3- &'a& a

normally inciden& p'o&on will &ra%erse a sla( of &'ic7ness # wi&'ou&

in&erac&ing. T'e fac&or e\ μ# &'us generally descri(es &'e frac&ion of

uncollided p'o&ons; &'a& go &'roug' a s'ield.

+& low p'o&on energies &'e (inding of &'e a&omic elec&rons is impor&an&

and &'e p'o&oelec&ric e/ec& is &'e dominan& in&erac&ion. 9ig'!

ma&erials pro%ide grea&er a&&enua&ion and a(sorp&ion, w'ic' decrease

rapidly wi&' increasing p'o&on energy. T'e coeMcien&s for )( and C

rise a(rup&ly w'en &'e p'o&on energy is suMcien& &o eUec& a

p'o&oelec&ron from &'e K s'ell of &'e a&om. 0'en &'e p'o&on energy is

se%eral 'undred 7eO or grea&er, &'e (inding of &'e a&omic elec&rons

(ecomes rela&i%ely unimpor&an& and &'e dominan& in&erac&ion is

Lomp&on sca&&ering. Since &'e elemen&s *ecep& 'ydrogen- con&ain

a(ou& &'e same num(er of elec&rons per uni& mass, &'ere is no& a large

di/erence (e&ween &'e %alues of &'e mass a&&enua&ion coeMcien&s for

&'e di/eren& ma&erials.

T'ere are s'arp increases in &'e a&&enua&ion coeMcien& for &'e

p'o&oelec&ric e/ec& w'en &'e p'o&on energy Uus& eceeds &'e (inding

energy of &'e elec&ron s'ell *K s'ell- of a&om.

+#ample0'a& &'ic7ness of concre&e and of lead are needed &o reduce &'e

num(er of #33!


28/92

7eO p'o&ons in a narrow (eam &o one!four&' &'e inciden& num(er5

Lompare &'e&'ic7nesses in cm and in g cm\2.Sol"tion0e use EH. *


29/92

+n eample is gamma!ray cap&ure (y a 23)( nucleus wi&' emission of

a neu&ron: 23or &'ese reasons,

p'o&onuclear reac&ions can (e impor&an& around 'ig'!energy elec&ron

accelera&ors &'a& produce energe&ic p'o&ons.

T'e &'res'olds for *, p- reac&ions are of&en 'ig'er &'an &'ose for *, n-

reac&ions (ecause of &'e repulsi%e Loulom( (arrier &'a& a pro&on mus&

o%ercome &o escape from &'e nucleus. +l&'oug' &'e pro(a(ili&y for

ei&'er reac&ion is a(ou& &'e same in &'e lig'&es& elemen&s, &'e *, n-

reac&ion is many &imes more pro(a(le &'an *, p- in 'ea%y elemen&s.

+#ampleLompu&e &'e &'res'old energy for &'e *,n- p'o&odisin&egra&ion of

23

)(. 0'a& is &'e energy of a neu&ron produced (y a(sorp&ion of a 83!eO p'o&on5Sol"tion

T'e mass di/erences, -, from +ppendi D, are \2. eO for 23)(, \

2. eO for 23#)(, and


30/92

Kerma is de=ned as &'e &o&al ini&ial 7ine&ic energy of all c'arged

par&icles li(era&ed (y unc'arged radia&ion *or indirec&ly ioniing

radia&ion: p'o&ons and neu&rons- per uni& mass of mass. T'is Huan&i&y,

w'ic' 'as &'e dimensions of a(sor(ed dose, is called &'e erma

*Kine&ic Energy Released per uni& +ss-. By de=ni&ion, 7erma includes

energy &'a& may su(seHuen&ly appear as (remss&ra'lung and i& also

includes +uger!elec&ron energies. T'e 7erma decreases s&eadily

(ecause of &'e a&&enua&ion of &'e primary radia&ion wi&' increasing

dep&'. Speci=cally, 7erma and a(sor(ed dose a& a poin& in an irradia&ed

&arge& are eHual w'en c'arged!par&icle eHuili(rium eis&s &'ere and

(remss&ra'lung losses are negligi(le.

dm

dE " tr =

L'arged par&icle eHuili(rium *L)E- eis&s for &'e %olume O if eac'

c'arged par&icle of a gi%en &ype and energy lea%ing O is replaced (y an

iden&ical par&icle of &'e same energy en&ering in &erms of epec&a&ion

%alues.

ii. &bsorbed dose


pp. 2!

T'e primary p'ysical Huan&i&y used in dosime&ry is &'e a(sor(ed dose.

$& is de=ned as &'e energy a(sor(ed per uni& mass from any 7ind of

ioniing radia&ion in any &arge&. T'e uni& of a(sor(ed dose, J 7g\8, is

called &'e gray *Iy-. T'e older uni&, &'e rad, is de=ned as 833 erg g \8.

*8Iy833rad-.)'o&ons produce secondary elec&rons in air, for w'ic' &'e a%erage

energy needed &o ma7e an ion pair is & " eO ip\8 " JL\8.

#! $ %

$

#!

% R /1076.8

341058.21

34

−−

×=××

=

T'us, an eposure of 8 R gi%es a dose in air of


31/92

iii. ,7posure


p. 2

Eposure is de=ned for gamma and 6 rays in &erms of &'e amoun& of

ionia&ion &'ey produce in air. T'e uni& of eposure is called &'e

roen&gen *R-. T'e roen&gen is de=ned as &'e amoun& of energy

reHuired &o li(era&e 2.#< A 83!" L of c'arge from 8 7g of air a& s&andard

&empera&ure *2 K- and pressure *3 mm9g-. *8R 2.#


32/92

M

NaW

T N A

⋅==

2/1

693.0λ

s

%i

mole !

moleatoms !

s086.0

/107.3

1

/137

/1002.610

36002436517.30

693.010

233

=×⋅××⋅×××=

−

Eposure ra&e a& dis&ance r 8m from a poin& source of ac&i%i&y

3.3or eample, &'e de&ec&or ma&erial of


33/92

scin&illa&ion de&ec&or emi&s %isi(le lig'&. T'e lig'& s&ri7es &'e p'o&o ca&'ode

crea&ing elec&ron in &'e )*p'o&omul&iplier- &u(e.Some of &'e p'ysical and c'emical radia&ion e/ec&s &'a& apply &o radia&ion

de&ec&ion and measuremen& for 'eal&' p'ysics purposes are lis&ed in Ta(le

(elow.

b. +as(>lled detectors

Reference: a&ional uclear Securi&y +dminis&ra&ion. uali=ca&ion S&andard

Reference Iuide \ Radia&ion )ro&ec&ion, pp."

Methods in Radiation Physics, pp.2"8!38

Reference: 9erman Lem(er. Health Physics. 4th edition, p."2

Eac' &ype of radia&ion 'as a speci=c pro(a(ili&y of in&erac&ion wi&' &'e

de&ec&or media. T'is pro(a(ili&y %aries wi&' &'e energy of &'e inciden&

radia&ion and &'e c'arac&eris&ics of &'e de&ec&or gas. T'e pro(a(ili&y of

in&erac&ion is epressed in &erms of speci=c ionia&ion wi&' uni&s of ion pairs

per cen&ime&er. + radia&ion wi&' a 'ig' speci=c ionia&ion, suc' as alp'a, will

produce more ion pairs in eac' cen&ime&er &'a& i& &ra%els &'an will a radia&ion

wi&' a low speci=c ionia&ion suc' as gamma.

Ienerally, &'e pro(a(ili&y of in&erac&ion (e&ween &'e inciden& par&icle radia&ion

and &'e de&ec&or gas *and &'erefore &'e produc&ion of ions- decreases wi&'

increasing radia&ion energy. $n p'o&on in&erac&ions, &'e o%erall pro(a(ili&y of

in&erac&ion increases (ecause of &'e increasing con&ri(u&ion of &'e pair

produc&ion reac&ions. +s &'e energy of &'e par&icle radia&ion decreases, &'e

pro(a(ili&y of in&erac&ion increases, no& only in &'e gas, (u& also in &'e


34/92

ma&erials of cons&ruc&ion. Zow energy radia&ions may (e a&&enua&ed (y &'e

walls of &'e de&ec&or and no& reac' &'e gas %olume. +s &'e num(er of

radia&ion e%en&s s&ri7ing a de&ec&or increases, &'e o%erall pro(a(ili&y of an

in&erac&ion occurring wi&' &'e forma&ion of an ion pair increases. $n addi&ion,

&'e num(er of ion pairs crea&ed increases and &'erefore de&ec&or responseincreases.

T'e pro(a(ili&y of an in&erac&ion occurring (e&ween &'e inciden& radia&ion and

a gas a&om increases as &'e num(er of a&oms presen& increases. + larger

de&ec&or %olume o/ers more &arge&s; for &'e inciden& radia&ion, resul&ing in a

larger num(er of ion pairs. Since, eac' radia&ion 'as a speci=c ionia&ion in

&erms of ion pairs per cen&ime&er, increasing &'e de&ec&or sie also increases

&'e leng&' of &'e pa&' &'a& &'e radia&ion &ra%erses &'roug' &'e de&ec&or. T'e

longer &'e pa&', &'e larger &'e num(er of ion pairs.

onoenerge&ic (eam of par&icles s&opping in parallel!pla&e ionia&ion c'am(er

wi&' %aria(le po&en&ial di/erence / applied across pla&es )8 and )2

T'e amoun& of energy epended in &'e crea&ion of an ion pair is a func&ion of

&'e &ype of radia&ion, &'e energy of &'e radia&ion, and &'e c'arac&eris&ics of

&'e a(sor(er *in &'is case, &'e gas-. T'is energy is referred &o as &'e ionia&ion

po&en&ial, or 0!Oalue, and is epressed in uni&s of elec&ron %ol&s *eO- per ion

pair. Typical gases 'a%e 0!Oalues of 2#!#3 eO, wi&' an a%erage of a(ou& " eO

per ion pair.

$n &'e sec&ion on de&ec&or sie, i& was s'own &'e pro(a(ili&y of in&erac&ion

increases wi&' de&ec&or sie. $n many cases, &'ere is a prac&ical limi& &o

de&ec&or sie. $ns&ead of increasing de&ec&or sie &o increase &'e num(er of

&arge&; a&oms, increasing &'e pressure of &'e gas will accomplis' &'e same

goal. Ias under pressure 'as a 'ig'er densi&y *more a&oms per cm - &'an a

gas no& under pressure, and &'erefore o/ers more &arge&s, a 'ig'er pro(a(ili&y


35/92

of in&erac&ion, and grea&er ion pair produc&ion. >or eample, increasing &'e

pressure of a &ypical gas &o 833 psig increases &'e densi&y (y a(ou& &imes.

4nce &'e ion pair is crea&ed, i& mus& (e collec&ed in order &o produce an ou&pu&

pulse or curren& Qow from &'e de&ec&or. $f lef& undis&ur(ed, &'e ion pairs will

recom(ine, and no& (e collec&ed. $f a %ol&age po&en&ial is applied across &'e

elec&rodes, a =eld is crea&ed in &'e de&ec&ors, and &'e ion pairs will (e

accelera&ed &owards &'e elec&rodes. T'e s&ronger &'e =eld, &'e s&ronger &'e

accelera&ion. +s &'e %eloci&y of &'e elec&ron increases, &'e elec&ron may cause

one or more ionia&ions on i&s own. T'is process is 7nown as secondary

ionia&ion. T'e secondary ion pairs are accelera&ed &owards &'e elec&rode and

collec&ed, resul&ing in a s&ronger pulse &'an would 'a%e (een crea&ed (y &'e

ions from primary ionia&ion.


36/92

$f &'e applied %ol&age po&en&ial is %aried from 3 &o a 'ig' %alue, and &'e pulse

sie recorded, a response cur%e will (e o(ser%ed. >or &'e purposes of

discussion, &'is cur%e is (ro7en in&o si regions. T'e ion c'am(er region, &'e

propor&ional region, and &'e Ieiger!tller region are useful for de&ec&or

designs used in radiological con&rol. 4&'er regions are no& useful. $n &'erecom(ina&ion region, &'e applied %ol&age is insuMcien& &o collec& all of &'e ion

pairs (efore some of &'em recom(ine.

+s &'e %ol&age &o &'e de&ec&or is increased, a poin& is reac'ed a& w'ic'

essen&ially all of &'e ions are collec&ed (efore &'ey can recom(ine. o

secondary ionia&ion or gas ampli=ca&ion occurs. +& &'is poin&, &'e ou&pu&


37/92

curren& of &'e de&ec&or will (e a& a maimum for a gi%en radia&ion in&ensi&y

and will (e propor&ional &o &'a& inciden& radia&ion in&ensi&y. +lso, &'e ou&pu&

curren& will (e rela&i%ely independen& of small Quc&ua&ions in &'e power supply.

T'e ou&pu& of a gas!=lled de&ec&or w'en 833N of &'e primary ion pairs are

collec&ed is called &'e sa&ura&ion curren&.

$f &'e applied %ol&age po&en&ial is %aried from 3 &o a 'ig' %alue, and &'e pulse

sie recorded, a response cur%e will (e o(ser%ed. >or &'e purposes of

discussion, &'is cur%e is (ro7en in&o si regions.

%he %hree Re)ions 0se1"l 1or Radiation 2etection and Meas"rement T'e ion c'am(er region, &'e propor&ional region, and &'e Ieiger!tller region

are useful for de&ec&or designs used in radiological con&rol. 4&'er regions are

no& useful. $n &'e recom(ina&ion region, &'e applied %ol&age is insuMcien& &o

collec& all of &'e ion pairs (efore some of &'em recom(ine. $n &'e limi&ed

propor&ional region, nei&'er &'e ou&pu& curren& nor &'e num(er of ou&pu&

pulses are propor&ional &o &'e radia&ion le%el. Lali(ra&ion is impossi(le. $n &'e

con&inuous disc'arge region, &'e %ol&age is suMcien& &o cause arcing and

(rea7down of &'e de&ec&or gas.

%he Se"ence o1 +ents that 5cc"r 6ollo(in) an nitial oni8in) +ent in an

oni8ation 9ham:er, a Proportional 9o"nter and a ;ei)er'M


38/92

%o(nsend aalanche. 0'en &'is 'appens, &'e end of &'e propor&ional region is

reac'ed and &'e Ieiger region (egins. +& &'is poin&, &'e sie of all pulses !

regardless of &'e na&ure of &'e primary ioniing par&icle ! is &'e same. 0'en

opera&ed in &'e Ieiger region, &'erefore, a coun&er canno& dis&inguis' among

&'e se%eral &ypes of radia&ions. 9owe%er, &'e %ery large ou&pu& pulses *`3.2#O- &'a& resul& from &'e 'ig' gas ampli=ca&ion in a Ieiger!uller *I- coun&er

means ei&'er &'e comple&e elimina&ion of a pulse ampli=er or use of an

ampli=er &'a& does no& 'a%e &o mee& &'e eac&ing reHuiremen&s of 'ig' pulse

ampli=ca&ion. Since all &'e pulses in a I coun&er are a(ou& &'e same 'eig'&,

&'e pulse 'eig'& is independen& of energy deposi&ion in &'e gas.

c. Scintillation detectors


Reference Iuide \ Radia&ion )ro&ec&ion, p.#2

Reference: 9erman Lem(er. Health Physics. 4th edition, pp."!"<

Scin&illa&ion de&ec&ors measure radia&ion (y analying &'e e/ec&s of &'e

eci&a&ion of &'e de&ec&or ma&erial (y &'e inciden& radia&ion. Scin&illa&ion is &'e

process (y w'ic' a ma&erial emi&s lig'& w'en eci&ed. $n a scin&illa&ion

de&ec&or, &'is emi&&ed lig'& is collec&ed and measured &o pro%ide an indica&ion

of &'e amoun& of inciden& radia&ion. umerous ma&erials scin&illa&e ! liHuids,

solids, and gases. + common eample is a &ele%ision pic&ure ma&erial w'ic'

scin&illa&es is commonly called a p'osp'or or a Quor. T'e scin&illa&ions are

commonly de&ec&ed (y a p'o&omul&iplier &u(e.

+ scin&illa&ion de&ec&or is a &ransducer &'a& c'anges &'e 7ine&ic energy of an

ioniing par&icle in&o a Qas' of lig'&. Scin&illa&ion coun&ers are widely used &o

coun& gamma rays and low!energy (e&a par&icles.


39/92

d. Semiconduc&or de&ec&ors


Reference Iuide \ Radia&ion )ro&ec&ion, pp.#2!#

o&e: Solid!s&a&e de&ec&ors are more commonly referred &o as semiconduc&or

de&ec&ors *for eample, germanium \ a common semiconduc&or used in

radia&ion de&ec&ion-.$f a s&rong elec&ric =eld is applied &o &'e crys&al, &'e elec&ron in &'e conduc&ion

(and mo%es in accordance wi&' &'e applied =eld. Similarly, in &'e group of

=lled (ands, an elec&ron from a lower energy (and mo%es up &o =ll &'e 'ole

*%acancy- in &'e %alence (and. T'e 'ole i& lea%es (e'ind is =lled (y an

elec&ron from ye& a lower energy (and. T'is process con&inues, so &'e ne&

e/ec& is &'a& &'e 'ole appears &o mo%e down &'roug' &'e energy (ands in &'e

=lled group. T'us, &'e elec&ron mo%es in one direc&ion in &'e un=lled group of

(ands, w'ile &'e 'ole mo%es in &'e opposi&e direc&ion in &'e =lled group of (ands. T'is can (e li7ened &o a line of cars awai&ing a &oll (oo&', &'e &oll (oo&'

(eing &'e for(idden (and. +s a car lea%es &'e =lled %alence (and for &'e

un=lled conduc&ance (and, a 'ole is formed. T'e ne& car in line =lls &'is 'ole,

and crea&es a 'ole, and so on. LonseHuen&ly, &'e 'ole appears &o mo%e (ac7

&'roug' &'e line of cars.


40/92


41/92

+ny impuri&ies in &'e crys&alline s&ruc&ure can a/ec& &'e conduc&ing a(ili&y of

&'e crys&alline solid. T'ere are always some impuri&ies in a semiconduc&or, no

ma&&er 'ow pure; i& is. 9owe%er, in &'e fa(rica&ion of semiconduc&ors,

impuri&ies are in&en&ionally added under con&rolled condi&ions. $f &'e impuri&y

added 'as an ecess of ou&er elec&rons, i& is 7nown as a donor impuri&y,(ecause &'e e&ra; elec&ron can easily (e raided or dona&ed &o &'e

conduc&ion (and. $n e/ec& &'e presence of &'is donor impuri&y decreases &'e

gap; (e&ween &'e group of =lled (ands and &'e group of un=lled (ands. Since

conduc&ion occurs (y &'e mo%emen& of a nega&i%e c'arge, &'e su(s&ance is

7nown as an n!&ype ma&erial. Similarly, if &'e impuri&y does no& con&ain

enoug' ou&er elec&rons, a %acancy or 'ole eis&s. T'is 'ole can easily accep&

elec&rons from o&'er energy le%els in &'e group of =lled (ands, and is called an

accep&or su(s&ance. +l&'oug' elec&rons mo%e &o =ll 'oles, as descri(ed a(o%e,

&'e appearance is &'a& &'e 'oles mo%e in &'e opposi&e direc&ion. Since &'isimpuri&y gi%es &'e appearance of posi&i%e 'oles mo%ing, i& is 7nown as a p!

&ype ma&erial. Since any crys&alline ma&erial 'as some impuri&ies in i&, a gi%en semiconduc&or

will (e an n!&ype or a p!&ype depending on w'ic' concen&ra&ion of impuri&y is

'ig'er. $f &'e num(er of n!&ype impuri&ies is eac&ly eHual &o &'e num(er of p!

&ype impuri&ies, &'e crys&alline ma&erial is referred &o as an in&rinsic

semiconduc&or.;

+ semiconduc&or &'a& 'as (een doped; wi&' &'e proper amoun& of &'e correc&&ype of impuri&y &o ma7e &'e energy gap (e&ween &'e &wo groups of (ands

Uus& rig'& ma7es a good radia&ion de&ec&or. + c'arged par&icle loses energy (y

crea&ing elec&ron!'ole pairs.

$f &'e semiconduc&or is connec&ed &o an e&ernal elec&rical =eld, &'e collec&ion

of elec&ron!'ole pairs can lead &o an induced c'arge in &'e e&ernal circui&

muc' as &'e collec& of elec&ron!posi&i%e a&om pairs *ion pairs- is used &o

measure radia&ion in an ion c'am(er. T'erefore, &'e semi!conduc&or de&ec&or

relies on &'e collec&ion of elec&ron!'ole pairs &o produce a usa(le elec&rical

signal.

4ne disad%an&age of &'e semiconduc&or de&ec∨ is &'a& &'e impuri&ies, in

addi&ion &o con&rolling &'e sie of &'e energy gap also ac& as &raps. +s

elec&rons *or 'oles- mo%e &'roug' &'e crys&alline ma&erial, &'ey are a&&rac&ed

&o &'e impuri&y areas or cen&ers (ecause &'ese impuri&y cen&ers usually 'a%e

a ne& c'arge. T'e carrier *elec&ron or 'ole- may (e &rapped for a w'ile a& &'e


42/92

impuri&y cen&er and &'en released. +s i& (egins &o mo%e again, i& may (e

&rapped a& ano&'er impuri&y cen&er and &'en released again. $f &'e elec&ron or

'ole is delayed long enoug' during &ransi& &'roug' &'e crys&al, i& may no& add

&o &'e elec&rical ou&pu&.

T'us, al&'oug' &'e carrier is no& ac&ually los&, &'e ne& e/ec& on readou& is &'a&

i& is los&. +no&'er disad%an&age of &'e semiconduc&or de&ec&or is &'a& &'e

presence of impuri&ies in &'e crys&al is 'ard &o con&rol &o 7eep &'e energy gap

w'ere i& is desired. + newer &ec'niHue, &'e Uunc&ion coun&er, 'as (een

de%eloped &o o%ercome &'ese disad%an&ages.

$n a semiconduc&or Uunc&ion coun&er, an n!&ype su(s&ance is uni&ed wi&' a p!

&ype su(s&ance. 0'en &'e &wo are

di/used &oge&'er &o ma7e a

di/used Uunc&ion, a deple&ion layer

is crea&ed (e&ween &'e &wo

ma&erials. *T'is deple&ion layer is

formed (y &'e di/usion of elec&rons

from &'e n!&ype ma&erial in&o &'e p!

&ype ma&erial and &'e di/usion of

'oles from &'e p!&ype ma&erial in&o

&'e n!&ype ma&erial.-

T'is resul&s in a narrow region w'ic' is deple&ed of carriers and w'ic' (e'a%es

li7e an insula&or (ounded (y conduc&ing elec&rodes. T'a& is, a ne& c'arge on

eac' side of &'e deple&ion region impedes &'e fur&'er &ransfer of c'arge. T'is

c'arge is posi&i%e in &'e n!region and nega&i%e in &'e p!region. T'is (arrier can

(e (ro7en if we apply an e&ernal %ol&age &o &'e sys&em and apply i& wi&' &'e

proper (ias. + forward (ias; is applied w'en we connec& &'e posi&i%e

elec&rode &o &'e p!region. $n &'is case, &'e (arrier (rea7s down and elec&rons

Qow across &'e Uunc&ion. 9owe%er, if we apply a re%erse (ias; *nega&i%e

elec&rode connec&ed &o &'e p!region-, &'e (arrier 'eig'& is increased and &'e

deple&ed region is e&ended.

+ fur&'er ad%ancemen& in Uunc&ion coun&ers is &'e p!n &ype. T'is coun&er 'as

an in&rinsic region (e&ween &'e n and p surface layers. *+n in&rinsic

semiconduc&or was discussed earlier and is e/ec&i%ely a pure semiconduc&or.-

T'e presence of an in&rinsic region e/ec&i%ely crea&es a &'ic7er deple&ion area.

+ germanium!li&'ium Ie*Zi- de&ec&or is an eample of &'is &ype of de&ec&or.


43/92

Zi&'ium *an n!&ype ma&erial- is di/used in&o p!&ype germanium. T'e n!p

Uunc&ion &'a& resul&s is pu& under re%erse (ias, and &'e &empera&ure of &'e

ma&erial is raised. Cnder &'ese condi&ions, &'e li&'ium ions drif& &'roug' &'e

germanium, (alancing n and p ma&erial and forming an in&rinsic region.

T'e 'ea& and (ias are remo%ed and &'e crys&al cooled Huic7ly &o liHuid

ni&rogen &empera&ures. T'is in&rinsic region ser%es as &'e region in w'ic'

in&erac&ions can &a7e place. T'e in&rinsic region can (e &'oug'& of as a (uil&!in

deple&ion region.

Due &o &'e large sie of &'e deple&ion region and &'e reduced mo(ili&y of &'e

elec&rons and 'oles due &o &'e depressed &empera&ure, a 'ig' c'arge is

necessary &o cause conduc&ion. T'e c'arge is c'osen 'ig' enoug' &o collec&

ion pairs, (u& low enoug' &o pre%en& noise.

Due &o &'e increased s&opping power of germanium o%er air a& !28o> &'eenergy reHuired &o crea&e an ion pair is only 2. eO compared &o . eO for

air. T'is means &'a& (y &'eory, a germanium de&ec&or will respond &o any

radia&ion &'a& will crea&e ion pairs. $n ac&uali&y, 'owe%er, &'e response &o

radia&ions o&'er &'an gamma is limi&ed (y &'e ma&erials surrounding &'e

de&ec&or, ma&erial necessary &o main&ain &empera&ure. +no&'er considera&ion

limi&ing response is &'e geome&ry of &'e crys&al. T'e mos& eMcien& response

occurs w'en &'e in&erac&ion &a7es place in &'e cen&er of &'e in&rinsic region,

&'is can only occur for gamma.

Radia&ion in&erac&s wi&' a&oms in &'e in&rinsic region &o produce elec&ron 'olepairs. T'e presence of ion pairs in &'e deple&ion region causes curren& Qow.

T'is is similar &o a &ransis&or, in &'a& ins&ead of inducing c'arges in &'e cen&er

sec&ion *&'e (ase in a &ransis&or- (y a (a&&ery or ano&'er source, &'e c'arge is

induced (y &'e crea&ion of ion pairs. Since i& is no& necessary for &'e ion

produced &o reac' &'e p and n region &o (e collec&ed, as in a gas =lled

c'am(er, &'e response is fas&er.

Since &'e num(er of ion pairs produced is a func&ion of &'e inciden& energy,

and &'e resul&ing curren& is a func&ion of &'e amoun& of ion pairs, Ie*Zi-

response is in &erms of energy.

e. Special detectors


Reference Iuide \ Radia&ion )ro&ec&ion, pp.2!#

Thermoluminescent Dosimeter


44/92

T'ermoluminescence *TZ- is &'e a(ili&y of some ma&erials &o con%er& &'e

energy from radia&ion &o a radia&ion of a di/eren& wa%eleng&', normally in &'e

%isi(le lig'& range. T'ere are &wo ca&egories of &'ermoluminescence:

Quorescence and p'osp'orescence.>luorescence

T'is is emission of lig'& during or immedia&ely af&er irradia&ion of &'e

p'osp'or. T'is is no& a par&icularly useful reac&ion for &'ermoluminescen&

dosime&ry *TZD- use.)'osp'orescence

T'is is &'e emission of lig'& af&er &'e irradia&ion period. T'e delay &ime can (e

from a few seconds &o wee7s or mon&'s. T'is is &'e principle of opera&ion used

for TZD. T'e proper&y of &'ermoluminescence of some ma&erials is &'e main

me&'od used for personnel dosime&ers a& D4E facili&ies. TZDs use p'osp'orescence as &'eir means of de&ec&ion of radia&ion. Elec&rons

in some solids can eis& in &wo energy s&a&es, called &'e %alence (and and &'e

conduc&ion (and. T'e di/erence (e&ween &'e &wo (ands is called &'e (and

gap. Elec&rons in &'e conduc&ion (and or in &'e (and gap 'a%e more energy

&'an &'e %alence (and elec&rons. ormally in a solid, no elec&rons eis& in

energy s&a&es con&ained in &'e (and gap. T'is is a for(idden region.;$n some ma&erials, defec&s in &'e ma&erial eis& or impuri&ies are added &'a&

can &rap elec&rons in &'e (and gap and 'old &'em &'ere. T'ese &rapped

elec&rons represen& s&ored energy for &'e &ime &'a& &'e elec&rons are 'eld, as

s'own (elow in =gure


45/92

$n mos& ma&erials, &'is energy is gi%en up as 'ea& in &'e surrounding ma&erial,

'owe%er, in some ma&erials a por&ion of energy is emi&&ed as lig'& p'o&ons.

T'is proper&y is called luminescence. 9ea&ing of &'e TZ ma&erial causes &'e

&rapped elec&rons &o re&urn &o &'e %alence (and. 0'en &'is 'appens, energy is

emi&&ed in &'e form of %isi(le lig'&. T'e lig'& ou&pu& is de&ec&ed and measured

(y a p'o&omul&iplier &u(e and a dose eHui%alen& is &'en calcula&ed. + &ypical

(asic TZD reader con&ains &'e following componen&s:• 9ea&er ! raises &'e p'osp'or &empera&ure• )'o&omul&iplier &u(e ! measures &'e lig'& ou&pu&• e&erVRecorder ! display and record da&a

+ glow cur%e can (e o(&ained from &'e 'ea&ing process. T'e lig'& ou&pu& from

TZ ma&erial is no& easily in&erpre&ed. ul&iple pea7s resul& as &'e ma&erial is

'ea&ed and elec&rons &rapped in s'allow; &raps are released. T'is resul&s in a

pea7 as &'ese &raps are emp&ied. T'e lig'& ou&pu& drops o/ as &'ese &raps are

deple&ed. +s 'ea&ing con&inues, &'e elec&rons in deeper &raps are released.

T'is resul&s in addi&ional pea7s. Csually &'e 'ig'es& pea7 is used &o calcula&e

&'e dose eHui%alen&. T'e area under &'e cur%e represen&s &'e radia&ion energy

deposi&ed on &'e TZD.

Albedo DosimeterReference: Radia&ion )ro&ec&ion Lompe&ency 8.. p.R) 8.!8

TZDs used &o de&ec& neu&rons incorpora&e &wo iso&opes of li&'ium, Zi! and Zi!

, (o&' of w'ic' are eHually sensi&i%e &o gamma radia&ion. 9owe%er, Zi! 'as a

large cross sec&ion for &'e &'ermal neu&ron *n, - reac&ion. )roduc&ion of &'e

alp'a par&icle ini&ia&es &'e &'ermoluminescence process &'a& ul&ima&ely resul&sin a measure of &'e dose due &o &'ermal neu&ronsX w'ereas, Zi! is rela&i%ely

insensi&i%e &o &'ermal neu&rons. T'e Zi! p'osp'or will read (o&' neu&ron and

gamma radia&ion in&erac&ionsX w'ereas, &'e Zi! p'osp'or will read only

gamma in&erac&ions. eu&ron dose is de&ermined (y su(&rac&ing &'e Zi!

reading *r- from &'e Zi! reading *nr- and applying a con%ersion fac&or &o &'e

di/erence.


46/92

T'e &erm al(edo s&ands for reQec&ing. Some of &'e &'ermal neu&rons de&ec&ed

(y &'e Zi! are originally fas& neu&rons &'a& in&erac& wi&' 'ydrogen in &'e (ody,

are &'ermalied, reQec&ed or sca&&ered o/ &'e (ody and de&ec&ed. T'is ma7es

&'e al(edo dosime&er posi&ion sensi&i%eX &'erefore, i& mus& (e properly

orien&a&ed. Because &'e neu&rons can (e modera&ed &o &'ermal energies, &'ey

are reQec&ed from &'e (ody &'roug' &'e (ac7 of &'e (adge in&o &'e al(edo

dosime&er. T'erefore, i& is impor&an& &o wear &'e dosime&er e&remely close &o

&'e (ody *on &'e Qes'- &o o(&ain accura&e measuremen&s. T'e fron& of &'e

(adge is s'ielded wi&' cadmium &o reUec& e&ernal &'ermal neu&rons.

Pocket Dosimeter)oc7e& dosime&ers are compac&, easy!&o!carry de%ices &'a& indica&e an

indi%iduals accumula&ed dose &o radia&ion a& any&ime, &'us elimina&ing &'e

delay of =lm (adgeVTZD processing. 9owe%er, (ecause of &'e possi(ili&y of

faul&y readings due &o roug' &rea&men&, &'e dosime&er reading does no&

cons&i&u&e a permanen& legal record of dose recei%ed. + poc7e& dosime&er can

(e self!reading or no&. $n &'e self!reading &ype, a small compound microscope

is used &o o(ser%e &'e response. T'e &ype w'ic' is no& self!reading, called &'e

poc7e& c'am(er, is similar in cons&ruc&ion &o &'e self!reading &ype, (u& ano&'er

ins&rumen& called &'e c'arger reader mus& (e used &o read i&. T'e self!reading

&ype is normally preferred since i& can (e read anyw'ere and a& any &ime.+ self!reading poc7e& dosime&er consis&s of a small air!=lled c'am(er in w'ic'

a Huar& =(er elec&rome&er, a small microscope and a gradua&ed scope across

w'ic' &'e s'adow of &'e Huar& =(er mo%es &o indica&e &'e applied dose, is

suspended. T'e design and opera&ion of a self!reading poc7e& dosime&er u&ilies &'e

principle of disc'arging a pair of opposi&e c'arged surfaces w'en &'e air

(e&ween &'em is eposed &o ioniing radia&ion. T'e elec&ric c'arge reHuired &o

a&&rac& &'e ionied gas par&icles is impressed on &'e elec&rome&er and &'e

c'am(er wall (y means of a sui&a(le c'arging uni&. $oniing radia&ion

pene&ra&ing &'e c'am(er forms posi&i%ely and nega&i%ely c'arged gas

par&icles. T'ese c'arged par&icles are a&&rac&ed &o &'e opposi&ely c'arged

surfaceX i.e., &'e nega&i%e par&icles are a&&rac&ed &o &'e elec&rome&er and &'e

posi&i%e par&icles are a&&rac&ed &o &'e c'am(er wall. T'e migra&ion of &'e

nega&i%e par&icles &o &'e elec&rome&er permi&s &'e =(er &o mo%e closer &o &'e

frame, w'ic' in &urn causes &'e s'adow of &'e =(er &o mo%e across &'e

cali(ra&ed scale.

Film Badge


47/92

Reference: James E. Turner. Atoms, Radiation, and Radiation Protection, pp.

2#!2.

>ilm emulsions con&ain small crys&als of a sil%er 'alide *e.g., +gBr-, suspended

in a gela&ine layer spread o%er a plas&ic or glass surface, wrapped in lig'&!&ig'&

pac7aging. Cnder &'e ac&ion of ioniing radia&ion, some secondary elec&rons

released in &'e emulsion (ecome &rapped in &'e crys&alline la&&ice, reducing

sil%er ions &o a&omic sil%er. Lon&inued &rapping leads &o &'e forma&ion of

microscopic aggrega&es of sil%er a&oms, w'ic' comprise &'e la&en& image.

0'en de%eloped, &'e la&en& images are con%er&ed in&o me&allic sil%er, w'ic'

appears &o &'e eye as dar7ening of &'e =lm. T'e degree of dar7ening, called

&'e op&ical densi&y, increases wi&' &'e amoun& of radia&ion a(sor(ed. +n

op&ical densi&ome&er can (e used &o measure lig'& &ransmission &'roug' &'e

de%eloped =lm.

Doses from gamma and (e&a radia&ion can (e inferred (y comparingdensi&ome&er readings from eposed =lm (adges wi&' readings from a

cali(ra&ed se& of =lms gi%en di/eren&, 7nown doses under &'e same

condi&ions. T'e dar7ening response of =lm &o neu&rons, on &'e o&'er 'and, is

&oo wea7 &o (e used in &'is way for neu&ron personnel moni&oring.>ilm cali(ra&ion and &'e use of densi&ome&er readings &o o(&ain dose would

appear, in principle, &o (e s&raig'&forward. $n prac&ice, 'owe%er, &'e procedure

is complica&ed (y a num(er of fac&ors. >irs&, &'e densi&y produced in =lm from

a gi%en dose of radia&ion depends on &'e emulsion &ype and &'e par&icular lo&

of &'e manufac&urer. Second, =rm is a/ec&ed (y en%ironmen&al condi&ions,suc' as eposure &o mois&ure, and (y general aging. Ele%a&ed &empera&ures

con&ri(u&e &o (ase fog in an emulsion (efore de%elopmen&. T'ird, signi=can&

%aria&ions in densi&y are in&roduced (y &'e s&eps in'eren& in &'e =lm!

de%elopmen& process i&self. + serious pro(lem of a di/eren& na&ure for dose

de&ermina&ion is presen&ed (y &'e s&rong response of =lm &o low!energy

p'o&a

notes for theoretical health physics

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