AN IIM3~GAltON aF mE JUAGG-GBAY PRINCIPLE WI~ PLlJORESCENJ X-RAYS

by

JIAROU) VINCmJ IARJ()lf

A~IS

subnitted to

ill partfal tultillment ot tbe requL~ts tor tbe

degree or

MAS~ OF SCIENCE

June 1957

lillWt

Redacted for Privacy

Redacted for PrivacyIllrrcL farfrmr eil$elrm

Io ercilr of hrl

ampt d lnrrtxt of Wor

Redacted for Privacy

Redacted for Privacy

ampim of fahf ampret fFililtttr

Redacted for PrivacySrm d iBratrr lampa[

ffr thair ir lmntri T nutAr(1

nn tr lurlry frilfir

lbe author desires to ~retts hie gratitude to Dr w c Boesch

tor euggestilla this problem enc1 to I ~ M1ers for advice on ~bnt

ques

AN DM$~00 OF 1HE BRAGltJ-GRAX PRINCJPLE tttZFLtJORESCENlr xRAYS

s-~ ~ 4t Introcluct1on l

Obleetive 3

Method middot5

~tion Cllambers 9

ValiUty of the ~G~ Pz1nciple 20

B1bl1~ 26

table No atle I l rheorettw Values of JAJJair 10

2 ~rttical VaJue of JcuJatr 10

~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23

4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23

AN DM$~00 OF 1HE BRAGltJ-GRAX PRINCJPLE tttZFLtJORESCENlr xRAYS

s-~ ~ 4t Introcluct1on l

Obleetive 3

Method middot5

~tion Cllambers 9

ValiUty of the ~G~ Pz1nciple 20

B1bl1~ 26

table No atle I l rheorettw Values of JAJJair 10

2 ~rttical VaJue of JcuJatr 10

~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23

4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23

table No atle I l rheorettw Values of JAJJair 10

2 ~rttical VaJue of JcuJatr 10

~~ ot ~tical tm4 ExPerlDiental3 Velues o~ JAliJIJLr middot leu~aJr 23

4 Comparison ot lheoretical 8lld Bl(perimental Value ot J auJAl 23

amp1tle

1

2

3

~son vttb Attix s

Extrepolation CUrves

Extrapolation CUrves

lata 4

2l

22

a=Jtle

~

2

IOllizatton Chamber AssemblY

~ Vaeuurn Chamber and tbe IomzatLQl ~ber 1u Relation to the Fluolfscent XmiddotRau Source 15

1ftle

1

2

13

17

AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS

IN)RODlTC)ION

-he primary standard tor low energy X-ray dosimetry measurements

is the tree a1r ionization chamber For some measurements such as

that o~ surface dOse the free air ionization cbamber is difficult or

iDgossible to use For these measurements the Bragg-Gray chamber

(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu

able instrument A comparLscn ot tree air ionization chamber

measurements with Bragg-Gray chamber data using lov energy xrays

is needed to vertiy the BraggbullGray principle and establis the limitbull

ations on a Bragg-Gray cbamber in this energy region

~e Bragg-Gray principle has been the subject of numerous

1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle

are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and

(24 pp581-589) A rigorous dermiddotivation ot the principle was made

by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the

mediUirl soft xray energy region artnelli (20 p251) expresses the

Bragg-Gray principle in tbe tolloWing torm

l) =S W r h lrP11 q (l)

D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second

3 a ion current per gram ot gas in the cavity

W bull energy in ergs necessa17 to form an ion pair in the cas

2

S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation

11 number of incident photons of energy h-v middot per em- per seconcl

hv bull eJgtergy of the photon in ergs

11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam

the mass energr absorption coetncient ts detined by

~ltpoundgt=It-s (2)

tN bull mass absorption coefficient in em2 per gram

ltE) = the average energy- ot the electrons at the tJme of production in erss

At low photon energies the mass energy absorption coefflctent ba$

two camponen~s

MIt = poundG r (1- ~ )Jll (3)-middot ltV tl

ltPibull Calppton absorption ooefficient per electron in em2

7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2

f- bull fluorescent yield tor the K level

~ =btMInamp energy ot tbe K level

~=yen= nUDber of electrons per gram of wall materieJ

~ middot Avogadros number

A bull atomic weight

Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and

(17 pp 57-68) tried to establish tbe validity of equation 1) tor C

Al end CU walled ionization chambers end for Xrays with e~ective11

3

energies fran 25 to 125 Kev by ustns an extrapolation chamber

8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since

heterogeneous X-rays were used as the source ot radiation Siuce

these X-~s were JlOt monoenergetic the average mass energy absorp

tian eoettioient used in the ealeulations does not necessarily

con-espond to the average elierSf in the X-rar spectrum lbe extra

polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode

spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a

good extrapoletioli eJ(l)erlmenal data should have been obtained for

eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was

ignored in their cowputations

Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot

Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al

cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to

vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current

per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current

per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au

chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe

Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull

polatiou the chamber spacing shoUld have been reduced another order

of JDqllituae Also~ the radiation Wed in tlUs ~~t wa

fllterecl x~ Deepite the heavy fUtrat1ou there rematne a

oonsiderable spread 1n energy ot these x~s

~e present WOJk was undertaken to establish tl1e WUdity or laek

AN DMS~GA10lf OF lfiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7 X-RAYS

IN)RODlTC)ION

-he primary standard tor low energy X-ray dosimetry measurements

is the tree a1r ionization chamber For some measurements such as

that o~ surface dOse the free air ionization cbamber is difficult or

iDgossible to use For these measurements the Bragg-Gray chamber

(12 pp6oo-612) (13 PPmiddot721-742) and (14 pp578-596) is a valu

able instrument A comparLscn ot tree air ionization chamber

measurements with Bragg-Gray chamber data using lov energy xrays

is needed to vertiy the BraggbullGray principle and establis the limitbull

ations on a Bragg-Gray cbamber in this energy region

~e Bragg-Gray principle has been the subject of numerous

1nvest1ampft1ons (22 ppl94middot195) 1~ derivations of this principle

are tound in the literature (ll PPmiddot259-295) (19 pp68-74) and

(24 pp581-589) A rigorous dermiddotivation ot the principle was made

by Cormack and Johns (6 PPmiddot7-9) and (7 ppl34-l39) middot For the

mediUirl soft xray energy region artnelli (20 p251) expresses the

Bragg-Gray principle in tbe tolloWing torm

l) =S W r h lrP11 q (l)

D bull dose rate 1n tbe walls ot the chamber in ergs per gram per second

3 a ion current per gram ot gas in the cavity

W bull energy in ergs necessa17 to form an ion pair in the cas

2

S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation

11 number of incident photons of energy h-v middot per em- per seconcl

hv bull eJgtergy of the photon in ergs

11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam

the mass energr absorption coetncient ts detined by

~ltpoundgt=It-s (2)

tN bull mass absorption coefficient in em2 per gram

ltE) = the average energy- ot the electrons at the tJme of production in erss

At low photon energies the mass energy absorption coefflctent ba$

two camponen~s

MIt = poundG r (1- ~ )Jll (3)-middot ltV tl

ltPibull Calppton absorption ooefficient per electron in em2

7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2

f- bull fluorescent yield tor the K level

~ =btMInamp energy ot tbe K level

~=yen= nUDber of electrons per gram of wall materieJ

~ middot Avogadros number

A bull atomic weight

Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and

(17 pp 57-68) tried to establish tbe validity of equation 1) tor C

Al end CU walled ionization chambers end for Xrays with e~ective11

3

energies fran 25 to 125 Kev by ustns an extrapolation chamber

8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since

heterogeneous X-rays were used as the source ot radiation Siuce

these X-~s were JlOt monoenergetic the average mass energy absorp

tian eoettioient used in the ealeulations does not necessarily

con-espond to the average elierSf in the X-rar spectrum lbe extra

polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode

spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a

good extrapoletioli eJ(l)erlmenal data should have been obtained for

eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was

ignored in their cowputations

Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot

Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al

cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to

vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current

per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current

per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au

chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe

Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull

polatiou the chamber spacing shoUld have been reduced another order

of JDqllituae Also~ the radiation Wed in tlUs ~~t wa

fllterecl x~ Deepite the heavy fUtrat1ou there rematne a

oonsiderable spread 1n energy ot these x~s

~e present WOJk was undertaken to establish tl1e WUdity or laek

2

S ratio ot the mass stopping power of tbe vaU materlal to that of the eas for the ionizine particles associated W1th tbe incimiddotdent radiation

11 number of incident photons of energy h-v middot per em- per seconcl

hv bull eJgtergy of the photon in ergs

11bull=mass energy abso~on eoeftiefent Of the wall materi-al in cm2 p~r gam

the mass energr absorption coetncient ts detined by

~ltpoundgt=It-s (2)

tN bull mass absorption coefficient in em2 per gram

ltE) = the average energy- ot the electrons at the tJme of production in erss

At low photon energies the mass energy absorption coefflctent ba$

two camponen~s

MIt = poundG r (1- ~ )Jll (3)-middot ltV tl

ltPibull Calppton absorption ooefficient per electron in em2

7 c photoelectric absorptiou coefficlent per electln in wall of atootic number Z in cm2

f- bull fluorescent yield tor the K level

~ =btMInamp energy ot tbe K level

~=yen= nUDber of electrons per gram of wall materieJ

~ middot Avogadros number

A bull atomic weight

Wilson and his co-workers (1 pp 243-254) (16 PPmiddot509-510) and

(17 pp 57-68) tried to establish tbe validity of equation 1) tor C

Al end CU walled ionization chambers end for Xrays with e~ective11

3

energies fran 25 to 125 Kev by ustns an extrapolation chamber

8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since

heterogeneous X-rays were used as the source ot radiation Siuce

these X-~s were JlOt monoenergetic the average mass energy absorp

tian eoettioient used in the ealeulations does not necessarily

con-espond to the average elierSf in the X-rar spectrum lbe extra

polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode

spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a

good extrapoletioli eJ(l)erlmenal data should have been obtained for

eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was

ignored in their cowputations

Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot

Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al

cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to

vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current

per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current

per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au

chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe

Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull

polatiou the chamber spacing shoUld have been reduced another order

of JDqllituae Also~ the radiation Wed in tlUs ~~t wa

fllterecl x~ Deepite the heavy fUtrat1ou there rematne a

oonsiderable spread 1n energy ot these x~s

~e present WOJk was undertaken to establish tl1e WUdity or laek

3

energies fran 25 to 125 Kev by ustns an extrapolation chamber

8 pp2Q22i5) technique 1bis work is sul)jeet to crtt1~sm since

heterogeneous X-rays were used as the source ot radiation Siuce

these X-~s were JlOt monoenergetic the average mass energy absorp

tian eoettioient used in the ealeulations does not necessarily

con-espond to the average elierSf in the X-rar spectrum lbe extra

polatton teehnique used 1n middottbelr elq)erinlents vas to vary the electrode

spaeUJg betwetm o5 and 5 an ot air As Will be shoVtl below for a

good extrapoletioli eJ(l)erlmenal data should have been obtained for

eleetrltxle spacillgs belov 05 mm Also the fluorescent yield t was

ignored in their cowputations

Atttx (2 pp19) aryen1 (3 pp127) recentlY testeG the Blagsmiddot

Gray pnDc1ple ill themiddot energr region hotll 38 to 670 Kev using C Al

cu sn aDd Pb waUed extrapolation Clmmbers His method lila$ to

vaey tbe dlam~r spacing between o bull5 and l2 bullmiddot A curve ot current

per arum ot Ur ill B~middotGlampY ebamber (Jz) relative to the current

per gram of air as meas~ w a free atr ton cbam~ (Jatrgt ve~au

chamber spae1ll8 WaS extramppolete4 to zero clumiber spaetug middotJbe

Oasbe4 line in Figure 1 represents Attixbulls Clata For a sood extrabull

polatiou the chamber spacing shoUld have been reduced another order

of JDqllituae Also~ the radiation Wed in tlUs ~~t wa

fllterecl x~ Deepite the heavy fUtrat1ou there rematne a

oonsiderable spread 1n energy ot these x~s

~e present WOJk was undertaken to establish tl1e WUdity or laek

4

---shy----

cu

0 LARSON 343 KEV

ATTIX 38 KEV

AL

-----shy

FIGURE I

COMPARISON WITH ATTIXS OATA

THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS

MEASURED BY A FREE AIR ION CHAMBER (JAIR)

5

ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl

816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers

MEtlHOD bull

For low eDerQ work the above method ot extrapolatinamp is

~tiw NOt anJy would the plate sepention measurements be

lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae

electrical ditficulties aDd talst17 volUme ~ts Another

method ot extrapolating is to hold the volume ot the chaaber constant

and vary the pressure ~ a curve ot the current per sram of air

in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot

measured by a tree air ion cbember versus pressure could be extra

polated to z~o pressure ~s latter e~rapolation technique

allows the pressure of the gas between tlle electrodes and thus the

distance between the electrodes to be reduced by at least another

order of m88Jlitude as shown in Figure 1 Equation (4) expresses the

relationslUp betveen the current (in amperes) per unit pressure (J)

and the ion current per gram of the detecting gas

(4) V =~22 X0

8J (~) ( z)~-it

J = 76o mm-bs

Itt bull volume of the Braeg-G~ chamber in cm3

r bull 273 20Jc

~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3

r bull absolute tellperature of the air in 01(

6

Equation (4) is a conversion of oxperlmentaJ data according to the

pertect gas lavs For a tree air ion chamber

J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)

~~~ 70 pd

~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber

r =current in amperes as read by tbe vibrating reed electroshymeter

Vir eollectiDg volume of the tree air ion chamber in cm3

7J ~ absolute temperature of the alr in the free air ion chamber in OK

P bull atmospheric pressure in Dlll-hg

D1vi41ng equation (4) by equation (5) results in I

r7 - J ( tl gt( 7 ) (6) -rj Vc r

Also for chambers With different vall materials but the ssme

detecting gas

(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its

tbetretical counterpart equation (10~ in the energy region from

816 to 34middot3 Kev

A nearly monoenergetic photon source (18 ppl00-102 proViding

energies han 816 to 109 Kev was developed by Larson et eJ wbich

uses the K fluorescence ra41ation trom targets of different atanic

numbers ~ deVice provides a near~ uniform six-inch beam with

max1mum intensity depending on ~ energJ from 0055 to 141 rhr

~s instrument eHmnated the need for usina heterogenous X-rqs

5

ot valiclity ot the Bragg-G~ princ1ple in the energy- rampr~ampe f10IIl

816 to 343 Kev tor copper 8Dd allllliDUDl vallecl ionization chambers

MEtlHOD bull

For low eDerQ work the above method ot extrapolatinamp is

~tiw NOt anJy would the plate sepention measurements be

lDAOcmrampte1 but also a small vriDkle 1n the vall material vould eauae

electrical ditficulties aDd talst17 volUme ~ts Another

method ot extrapolating is to hold the volume ot the chaaber constant

and vary the pressure ~ a curve ot the current per sram of air

in a l3ragg-Grq cbanlber reletive to the current per sram of air asmiddot

measured by a tree air ion cbember versus pressure could be extra

polated to z~o pressure ~s latter e~rapolation technique

allows the pressure of the gas between tlle electrodes and thus the

distance between the electrodes to be reduced by at least another

order of m88Jlitude as shown in Figure 1 Equation (4) expresses the

relationslUp betveen the current (in amperes) per unit pressure (J)

and the ion current per gram of the detecting gas

(4) V =~22 X0

8J (~) ( z)~-it

J = 76o mm-bs

Itt bull volume of the Braeg-G~ chamber in cm3

r bull 273 20Jc

~ bull ltlensity ot gas at 0degC end 76 Cillbullha in grams per cm3

r bull absolute tellperature of the air in 01(

6

Equation (4) is a conversion of oxperlmentaJ data according to the

pertect gas lavs For a tree air ion chamber

J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)

~~~ 70 pd

~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber

r =current in amperes as read by tbe vibrating reed electroshymeter

Vir eollectiDg volume of the tree air ion chamber in cm3

7J ~ absolute temperature of the alr in the free air ion chamber in OK

P bull atmospheric pressure in Dlll-hg

D1vi41ng equation (4) by equation (5) results in I

r7 - J ( tl gt( 7 ) (6) -rj Vc r

Also for chambers With different vall materials but the ssme

detecting gas

(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its

tbetretical counterpart equation (10~ in the energy region from

816 to 34middot3 Kev

A nearly monoenergetic photon source (18 ppl00-102 proViding

energies han 816 to 109 Kev was developed by Larson et eJ wbich

uses the K fluorescence ra41ation trom targets of different atanic

numbers ~ deVice provides a near~ uniform six-inch beam with

max1mum intensity depending on ~ energJ from 0055 to 141 rhr

~s instrument eHmnated the need for usina heterogenous X-rqs

6

Equation (4) is a conversion of oxperlmentaJ data according to the

pertect gas lavs For a tree air ion chamber

J = ~ 241 ~ ID ISfZbull~)( ~) ( p-- ) (5)

~~~ 70 pd

~ ion current per gram of gas in the collecting volume of the tree air 1oni2at1on chamber

r =current in amperes as read by tbe vibrating reed electroshymeter

Vir eollectiDg volume of the tree air ion chamber in cm3

7J ~ absolute temperature of the alr in the free air ion chamber in OK

P bull atmospheric pressure in Dlll-hg

D1vi41ng equation (4) by equation (5) results in I

r7 - J ( tl gt( 7 ) (6) -rj Vc r

Also for chambers With different vall materials but the ssme

detecting gas

(7)~-JAt =ck Kt1I J 1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its

tbetretical counterpart equation (10~ in the energy region from

816 to 34middot3 Kev

A nearly monoenergetic photon source (18 ppl00-102 proViding

energies han 816 to 109 Kev was developed by Larson et eJ wbich

uses the K fluorescence ra41ation trom targets of different atanic

numbers ~ deVice provides a near~ uniform six-inch beam with

max1mum intensity depending on ~ energJ from 0055 to 141 rhr

~s instrument eHmnated the need for usina heterogenous X-rqs

7

~e radiators used tor tbe experiment were copper zirconium~ cadmium

and lanthanum which correspond to energies of 816 161 237 and 34middot3

Kev respectvely ~ above energies wre verified b1 absorption

measurements in an aluminum medium

VaJues tor the factor nlrv (equatton 1) ean be found by tree air

ion chamber measurements ~e equation relating errergy flux and

measured current per Ullit mass 1st

(8)

~= maoc energy absorption coefficieut for air in cm2 per gram

~ above method of obtainitl8 hliV is impractical for high energy gamma

rays Since the range of the ionizing electrons would exceed a praotishy

cal cheimber electrode spacing middot A substitution of equation (8) into

e uation (1) recults in

(9)

or

(10)TJ =~ middot~ s

W cancels and does not eppear tn equation 10) beeause 1t de~

owyl onmiddot the gas used which ic ascumed to be air tor both the tree

oir ion cblmber and the ~-GISyen ehomb -r ~ J s on the lett

1 ~re ray bo a dependence of W on the e~rgy of the secondary elecshytrons In that case1 W is a slightly different sort ~ average value in (1) and (8) ~e ettect of this difference can be estishymated and shown to be less than Oltf 1n the value of W To the acouracy ot the J_nesent experiments W can be ceucelled in goillg to equation (10) bull

8

1n equation (10) are the quantities measured in the present eJCperishy

ment ~ quantities on tbe right ere reasonably well known ~

right side ot tbe equation V1ll be called the theoretical value

ot JJeJrbull It the theoretical and e~rimental value are found

to agree1 1t would 1mply the valicUtf ot the Braas-Grq prtnciple

Ccmpton bsorpt1on coetticients per electron fG were obtaiDed

frall a gtaph by Nelms (21 p 88 8Dd photoel ctric absorption coshy

efiicients per tan (~z) were interpolated trail tables canpiled

by White (26 pp48-58) ~ average K shell bind1og energies

(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission

lines (10 pp36-37) by values ot theU relative intena1t1es obte1ned

fnn Canpton 8Jl4 Allison (5 pp6J7-642) tbe values used for tbe

fluorescent yields (4 p722) of aluminum 8Dd copper were 0035 8lld

o46 respective~ 1lree correctiollS vere made to the absorption

coetfic1ents 7he first was a correction for tbe small amount ot

high energy scattered radiation in the fluorescent Xmiddotnq beam end tbe

second vas a correction tor tbe ~ti sin the waU ~J~aterials ~

1mpurities in tM aluminum were determined by a quazrt1tetive spectro

chemical analysis and the copper impurities were determined by tbe

matmtacturer to be less than 0 5~ Because ot tbe relatively b1gh

values ot tbe copper absorption coetficients tbe 1DJpurities in

copper were les important thau the impurities in aluminum lbe

third correction vas tor the contribution of tbe L level photoshy

electric effect to the mass energy bsorption coeftieients

stowiJlg power ratios were cazwuted tran the non-relativLstic

BethebullBlocb ~ory (15 p220) Vith tbe aasUJZQgttion tbat the

co ot

lt gtw (11)

s--~-- ltgt

(pound)~otw

lt2 ~~

N

lt Q

t ( bull )f

hc gtgt ~ (12)

t1 bull JIIIEUdr 4Uienaion ot

10

Energy SAl e (cm2jg) JAJfJair

Kev Al Air

816 0835 433 7-84 661pound 026

161 0851 5~75 0942 7middot17 l 030

237 o86o 181 0283 741t- t 032

34-3 o864 0-556 oo9lf4 682f 029

ergy seu ea (em2fg) JeuJaJr

cv Cu r

1 1

237

34-3

o683

0 700

0709

46

rr o

621

942

0 283

0944

72 l l

85 8f 36

28 l 3middot9

11

However in determining the amount opound energy per gram tbat 1s absorbed

trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat

the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy

where in tbe absorbing medium For this to be true the xponential

X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity

must be at least nne electron range thick

(13)

(l4)

bull linear absorption coetficient of the I8Jl material

t bull extrapolation chamber wall thickneGs

and

(15)

R range of the secondary electrons in the wall material

-he assembly of the ionization cllembers that vere used 1n this f

experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot

m1ze absorption of the xray$ in the chamber mlls thin foils Of

Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls

were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al

se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al

foil and we1gbiDS a known area of CU foil Duco ccment was used to

bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy

l8 long) Side walls were crewed onto the MFPlo insulating

rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode

vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10

10

Energy SAl e (cm2jg) JAJfJair

Kev Al Air

816 0835 433 7-84 661pound 026

161 0851 5~75 0942 7middot17 l 030

237 o86o 181 0283 741t- t 032

34-3 o864 0-556 oo9lf4 682f 029

ergy seu ea (em2fg) JeuJaJr

cv Cu r

1 1

237

34-3

o683

0 700

0709

46

rr o

621

942

0 283

0944

72 l l

85 8f 36

28 l 3middot9

11

However in determining the amount opound energy per gram tbat 1s absorbed

trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat

the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy

where in tbe absorbing medium For this to be true the xponential

X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity

must be at least nne electron range thick

(13)

(l4)

bull linear absorption coetficient of the I8Jl material

t bull extrapolation chamber wall thickneGs

and

(15)

R range of the secondary electrons in the wall material

-he assembly of the ionization cllembers that vere used 1n this f

experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot

m1ze absorption of the xray$ in the chamber mlls thin foils Of

Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls

were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al

se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al

foil and we1gbiDS a known area of CU foil Duco ccment was used to

bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy

l8 long) Side walls were crewed onto the MFPlo insulating

rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode

vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10

11

However in determining the amount opound energy per gram tbat 1s absorbed

trail the X-~ beam (tJ=hlrzl~f) the assumption must be maae tbat

the energy )er gram absorbed trcn the X-ray beam is tbe same everyshy

where in tbe absorbing medium For this to be true the xponential

X-rsy absorption must be amaU and the medium ourroundUIg tbe caVity

must be at least nne electron range thick

(13)

(l4)

bull linear absorption coetficient of the I8Jl material

t bull extrapolation chamber wall thickneGs

and

(15)

R range of the secondary electrons in the wall material

-he assembly of the ionization cllembers that vere used 1n this f

experiment is shown in Plate middot1- and Photo~ph 1 n oriler to lltlnibullmiddot

m1ze absorption of the xray$ in the chamber mlls thin foils Of

Cu and Al vere used lhe Al Wall~~ were 5 bull 74 mgJcm2 ~ Cu valls

were formed bJ evaporating 1 3 tNr)~ ot Cu on a 574 mgcm2 Al

se ~ Cu tlttckncss wa deterurtned by peeling the Cu off the Al

foil and we1gbiDS a known area of CU foil Duco ccment was used to

bond the wall materials to the lucite rings (6 OD bull 5middot34 ID shy

l8 long) Side walls were crewed onto the MFPlo insulating

rings (6 oD 5-12 ID - 14 lonz) so tbat the middot r electrode

vas aJmost canpletely SllrroUltled bs the wall tertal ~ lFP-10

r - WALL MATERIAL

tgggSI - AL ELECTRICAL SHIELDING

1~middot~~middot~1 - LUCITE RINGS~ tbullgt

-----1 -MFP-10 RINGS

PLATE I IONIZATION CHAMBER ASSEMBLY

I- )

13

0

1amp1 CD 2

~

~X

~0

0

C)Z

o

oshy0~

ZN

~z

0

UC

bullGl IC

HL

UID

WH

14

1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~

the cunent leakaampe to a neeligible value A ~ eleetlOde

ebamber vas used ampO that the coUectina electrode vould be eleetroshy

stati~ shielded Another advantaae o~ the three electrode

chamber was that 1t helped to elbdnate transfer current By transfer

ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which

cross the cbamber 1-lle etteet ot ttellSfer current vas complete~

elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad

averaatns the two r~s 1-le cbamber lengths were measured w1th

a micraneter caliper and the diameters of the lucite and MFP-10

~s were determined by a metal ruler Hw~r1 tJle largest error

in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin

metal foUs At ~ssures above two mbullhs the condftion o~ voltage

satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot

zatton cllambers bull

rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD

standard steel pipe A steel plate l4 thick was welded over the

tront of tM plpe In the center ot the steel plate a hole SfJt

inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was

mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound

a l4n JU Plate Eoth tbe front and the rev ~s were vacuum

sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere

chosen so that the Umbra ot the beam from tlle fluorescentent X-ray

sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum

cbambel vas Qheeked by measurinS the current collected by the

13

0

1amp1 CD 2

~

~X

~0

0

C)Z

o

oshy0~

ZN

~z

0

UC

bullGl IC

HL

UID

WH

14

1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~

the cunent leakaampe to a neeligible value A ~ eleetlOde

ebamber vas used ampO that the coUectina electrode vould be eleetroshy

stati~ shielded Another advantaae o~ the three electrode

chamber was that 1t helped to elbdnate transfer current By transfer

ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which

cross the cbamber 1-lle etteet ot ttellSfer current vas complete~

elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad

averaatns the two r~s 1-le cbamber lengths were measured w1th

a micraneter caliper and the diameters of the lucite and MFP-10

~s were determined by a metal ruler Hw~r1 tJle largest error

in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin

metal foUs At ~ssures above two mbullhs the condftion o~ voltage

satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot

zatton cllambers bull

rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD

standard steel pipe A steel plate l4 thick was welded over the

tront of tM plpe In the center ot the steel plate a hole SfJt

inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was

mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound

a l4n JU Plate Eoth tbe front and the rev ~s were vacuum

sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere

chosen so that the Umbra ot the beam from tlle fluorescentent X-ray

sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum

cbambel vas Qheeked by measurinS the current collected by the

14

1nsulat~ rings aud the el~ctrieal Shieldfns (4 mtl Al) middot~

the cunent leakaampe to a neeligible value A ~ eleetlOde

ebamber vas used ampO that the coUectina electrode vould be eleetroshy

stati~ shielded Another advantaae o~ the three electrode

chamber was that 1t helped to elbdnate transfer current By transfer

ctlZtel)t is meant the current due to the seeondaey middotelectron$ Which

cross the cbamber 1-lle etteet ot ttellSfer current vas complete~

elJmllnated by rfgtYeQ1ttS the voltage on the outer eleetro4es ad

averaatns the two r~s 1-le cbamber lengths were measured w1th

a micraneter caliper and the diameters of the lucite and MFP-10

~s were determined by a metal ruler Hw~r1 tJle largest error

in ~ eh$mber vol_ detellltfnat1on WWgt due to Wrinkles 1n the thin

metal foUs At ~ssures above two mbullhs the condftion o~ voltage

satUtatireg WaS obteiUed for both the QJUtninum and the eopper ionmiddot

zatton cllambers bull

rho vecuwn ebsmber amphGW tn ampte 2 was middotconstructed ot 16 OD

standard steel pipe A steel plate l4 thick was welded over the

tront of tM plpe In the center ot the steel plate a hole SfJt

inches 1n dieme~ was cut and a OOl ~ellulose ecetate Vitldow was

mounted ~ the ~rture Pbe rear windov on tile pipe eonsisted opound

a l4n JU Plate Eoth tbe front and the rev ~s were vacuum

sealed by Ott rings [he ciimerui~~ons ot the windows and pi_pe vere

chosen so that the Umbra ot the beam from tlle fluorescentent X-ray

sourobull wes all $bsoltbed in the Al Window Scattering trom the vacuum

cbambel vas Qheeked by measurinS the current collected by the

15

bull ~

tTTI

~ ~ ~ ~

11J~ ~ ~

middot -

~ middot STEEL

~ - LEAD

~-BRASS 13 -ALUMIMJM

middot ~ middot bull ~ ~ II o

a ---RADIATOR

__ ~ bull ~ bull

tTTI

~ ~ ~

1- SHIEI0pound0 CA81E

I_ I~~~ I I

ELECT_~ I

FOilpound bull -PLASTICPUIII

IONIZATION_~ ~ CHAMBER~ ~t IIIIANOIIIJj ~

-~~In ~ ~

0 1 I I 4

ICALl UICMI S

PLATE 2

THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE

16

extrapolatton chambers 1llth tbe vacuum chamber present and WithOut

the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was

tound to be negllgible A l4 lucite rod se~ted tbe grounded

vacuum middotchamber tran the extrapolation chambers he expertmental

system 1G picture4 in Pbotogra_ph 2

~ ion current was measllled wtth a Vibrating reed eJeotrcmeter

~ Ballle glasabullseeled resistor was used in the head or the Vibretine

reed eleetraueter tor both the tree atr ion chamber current ~shy

ments and tbe extrapolation cbamber current measurements A value

tor tbe resistance (R need not be known since the factor R SJpears

on both sides of equation (9) as a tactor in eaeb of the J terms

~ teurperature coeifici~t (25 p2 oi the resietor wed was

l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se

low coett1e1ents pemitted neglect ot cbanSes in resistance due to

temperature and vtiltase

A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice

~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat

of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube

vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1

suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer

Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil

was used middotas the working fluid in the manometer ~ density ot tbe

oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd

tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS

the weight and the volume of tbe oi~ the density opound the oil ws

16

extrapolatton chambers 1llth tbe vacuum chamber present and WithOut

the vacuum chamber vrcaent Lhe scatter from the vacuum ehatnber was

tound to be negllgible A l4 lucite rod se~ted tbe grounded

vacuum middotchamber tran the extrapolation chambers he expertmental

system 1G picture4 in Pbotogra_ph 2

~ ion current was measllled wtth a Vibrating reed eJeotrcmeter

~ Ballle glasabullseeled resistor was used in the head or the Vibretine

reed eleetraueter tor both the tree atr ion chamber current ~shy

ments and tbe extrapolation cbamber current measurements A value

tor tbe resistance (R need not be known since the factor R SJpears

on both sides of equation (9) as a tactor in eaeb of the J terms

~ teurperature coeifici~t (25 p2 oi the resietor wed was

l Ol5cfooc and tlle voltaampe ~moient (25 p2) was -ooffv ~se

low coett1e1ents pemitted neglect ot cbanSes in resistance due to

temperature and vtiltase

A 11-tube oU manometer was usect as tbe pressure meaaurtna deVice

~ sensitiVity o~middot an oU-filled instruanent is about 15 times tbat

of the same 1nstlUltt8Dt fiUed With mereurr One end ot the Ubulltube

vas connected ~ugh a Pirani gauge to a rorepump lhe P1ran1

suaee vas calibrated Vith a ~Leod ga~ ~s end of the manometer

Vas kept at e pressure below 0OC7 ~~m-hg Ord1naey forepurrugt oil

was used middotas the working fluid in the manometer ~ density ot tbe

oU ws de1iermtned by we1shin8 an empty pipette of knltNn vollllle aDd

tben ve1gblog the same pipette Y1tb torepump oil in it KnowlrIS

the weight and the volume of tbe oi~ the density opound the oil ws

PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM

18

C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five

microns Wtth an oil rnauaneter in COlljunction with a cathetometer1

e pressure of one mm-hg could be dete cd to ttGr than one ])er eent

aocuracy

In equation (5) the factor lairVaJr was determined by the use of

a tree air ionization cballber in eonjunct1on Yith a vtbrating reed

electrometer ~e tree atr ionization chamber consisted ot two parshy

allel oluminum plates that were separated by a distance o~ nine inchesmiddot

~ proper plate separation vas detemtned by plotttns a curve ot

current ve-sun plate srncins and ttnding that spacing beyond Yhich

the current no longer depeoded on the plate spacing rhe dimen

sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull

OOlllleCted to ooo of the electrodes the proper voltaee tor saturation

was detenninad by plotting a curve ot ionization current versus voltshy

age fbe other eJectrode was cut tnto three sections ~ center

section (1 x 16 5n) WaS used as the collecting electrode aJld the

two outer sections (1011 x 165 ) were grounded and used as suard

rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber

through a ttro~inch diameter hole that was drUled in a l4 thick

lead sbeGt The etfective volume at the chamber was determined fran

tbe area opound this aperture1 the Width o-r the collecting cleetrode

alld the average length of ~P between the BUSrd plates and the colshy

le~tor ~s was not the eollectins volUllle11 but tllls volume is

equivalent to the coUeet1ng volUllle for any point source or element

of a finite source ~e ionization current vas 11eSsured by a

19

VibratitJg ~ electrometer

Lhe xray tube was alloved to warm up for a period of epproX

mateU twenty minutes before bull~ents -weremiddot taken A radiator

middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml

ber was centered on the ~ter line ot the side part ot the f~uores

eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~

voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro

111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes

ln this time interval tbe middottQQtn ~ture and p~sslle were also

recorded 1be free air 1oniut1on cluwber was removed and the aluminun

lWagg-Gr~ chamber was located so that tbe center ot the coUeot~

volume was iu tbe position that had pleviously been occupied by the

free air ionization chamber Jhe press~ inaide the aluminum

BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage

preampsure temperature and proper saturation voltage vere recorded by

be same procedure as~ U$ed tor the tree air ionization cbamber

middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull

ber vas reversed end the voltllge etc were recorded ~se two

voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot

tDeasurements were repeated ~or a series of lower pressures P21 P3 ete

Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy

tion cbamber was moved into position Another set of tree atr

ionization chamber measurements vasmiddot middotrecorded ~us the tree air

ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue

or Ja1r but also as a measurement of the stability ot the ~luoJescent

18

C011IP41ted he vapor presGWe of the f orepuiII oil vas less tbm five

microns Wtth an oil rnauaneter in COlljunction with a cathetometer1

e pressure of one mm-hg could be dete cd to ttGr than one ])er eent

aocuracy

In equation (5) the factor lairVaJr was determined by the use of

a tree air ionization cballber in eonjunct1on Yith a vtbrating reed

electrometer ~e tree atr ionization chamber consisted ot two parshy

allel oluminum plates that were separated by a distance o~ nine inchesmiddot

~ proper plate separation vas detemtned by plotttns a curve ot

current ve-sun plate srncins and ttnding that spacing beyond Yhich

the current no longer depeoded on the plate spacing rhe dimen

sions or the plates vero 21 X 16511 A 0-3000 volt power supply vasbull

OOlllleCted to ooo of the electrodes the proper voltaee tor saturation

was detenninad by plotting a curve ot ionization current versus voltshy

age fbe other eJectrode was cut tnto three sections ~ center

section (1 x 16 5n) WaS used as the collecting electrode aJld the

two outer sections (1011 x 165 ) were grounded and used as suard

rings )lle rluoreseent be84l ~ntez-ed the tree e1r ion1zation chamber

through a ttro~inch diameter hole that was drUled in a l4 thick

lead sbeGt The etfective volume at the chamber was determined fran

tbe area opound this aperture1 the Width o-r the collecting cleetrode

alld the average length of ~P between the BUSrd plates and the colshy

le~tor ~s was not the eollectins volUllle11 but tllls volume is

equivalent to the coUeet1ng volUllle for any point source or element

of a finite source ~e ionization current vas 11eSsured by a

19

VibratitJg ~ electrometer

Lhe xray tube was alloved to warm up for a period of epproX

mateU twenty minutes before bull~ents -weremiddot taken A radiator

middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml

ber was centered on the ~ter line ot the side part ot the f~uores

eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~

voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro

111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes

ln this time interval tbe middottQQtn ~ture and p~sslle were also

recorded 1be free air 1oniut1on cluwber was removed and the aluminun

lWagg-Gr~ chamber was located so that tbe center ot the coUeot~

volume was iu tbe position that had pleviously been occupied by the

free air ionization chamber Jhe press~ inaide the aluminum

BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage

preampsure temperature and proper saturation voltage vere recorded by

be same procedure as~ U$ed tor the tree air ionization cbamber

middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull

ber vas reversed end the voltllge etc were recorded ~se two

voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot

tDeasurements were repeated ~or a series of lower pressures P21 P3 ete

Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy

tion cbamber was moved into position Another set of tree atr

ionization chamber measurements vasmiddot middotrecorded ~us the tree air

ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue

or Ja1r but also as a measurement of the stability ot the ~luoJescent

19

VibratitJg ~ electrometer

Lhe xray tube was alloved to warm up for a period of epproX

mateU twenty minutes before bull~ents -weremiddot taken A radiator

middotvas placed in tbe fluorescent source bull 1be free atr ionization ehtml

ber was centered on the ~ter line ot the side part ot the f~uores

eent source at a fiXed d1$talt~e han the cenwr ot the ~~tor ~

voltase acrosa tbe Vietoreen reSistor in the vibratillg reed middotelectro

111eter vas recordecl on a chart reeorder tor approxtmately ten Jllinutes

ln this time interval tbe middottQQtn ~ture and p~sslle were also

recorded 1be free air 1oniut1on cluwber was removed and the aluminun

lWagg-Gr~ chamber was located so that tbe center ot the coUeot~

volume was iu tbe position that had pleviously been occupied by the

free air ionization chamber Jhe press~ inaide the aluminum

BraggbullGrtq centhamber was lowered to s~ pressure PJ rbe voltage

preampsure temperature and proper saturation voltage vere recorded by

be same procedure as~ U$ed tor the tree air ionization cbamber

middot ~ voltase on the outer eleltrtrodes or the alumtnum Bragg-Gray ohembull

ber vas reversed end the voltllge etc were recorded ~se two

voltage rea81nss we~ a~~ to eltbull1nate transter current ~se bullmiddot

tDeasurements were repeated ~or a series of lower pressures P21 P3 ete

Lbe alUIIltnum ~qray chatnber WQS ~ved and the b-ee air ioniza-shy

tion cbamber was moved into position Another set of tree atr

ionization chamber measurements vasmiddot middotrecorded ~us the tree air

ionization dlamber readtngs vere not oncy used to detemtne r~~ vaJue

or Ja1r but also as a measurement of the stability ot the ~luoJescent

x-~ bullmiddot ~above expertmental steps were repeated tor the

cent0ppelr ~~ cbainbeTbull nie XbullZq beam Wamp middot~~ by keeping

the values of the X-ray tube voltege atJl cun-eat the same tor all

tbe above aeta ot readtneiJ ~ expertmentaJ eunent l$t10s were

plotted wrsus press~ (ng 2 and 3) ~ aurrent ratio middot w~

couide-led to be the ~ data lampther tb8n the tndivtdreg eunoent

detelltnations1 since the ratios 41lt1 not contan middotthe veJue ot the

electrometer 1Jltput reaietenee Ylth its assoeiated t1 per cent

uncertatnty lbe above expe~tal steps were repeated tor aU ot

the tl~scent ~ators

~ ~tion curves tor the Al aDA middotCU BraagGrat chambers

are s~ in Fignres 2 and 3 ~ ordirlate is the ratio ot the

curren1 per pam ot air tn the chamber beiJlS tested to the CUlJent

per snm of etr as measured br a bee atr lfoaizat1on ehamber middot-be

Xtrapolatecl values ~ theft Cltfes aremiddot ~ ~ues desn-ed 1lhese

valUea ere llsted in ~ble 3 along Vlth the theoretical velues A

ratio ot JcuJAJ toyen a p81ticular enerampr can be ob~d trao the

ratio ot JfJrJJ~ to JAJJampir 2llese -lues are listed 1n ~ble 4

shoal4 beve a zero slope However1- at preseltea below two -h6

voltage saturation vas not attainable with tbe lbaggmiddotG~ cbambers

uaeltl in tb1s eXPertment J]leretore1 this tact could not be

verified

21

---~~ ~ cu

c 816 KEV

6 161 KEV

0 237 KEV

0 0

ts

0 0

6shy

0 0

Ashy ~

PRESSURE IIIII - HGI

FIGURE 2

fXTRAPOLATION CURVES

THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jzl RELATIVE TO THE CURRENT PER GRAM OF AIR AS

MEASURED BY A FREE AIR ION CHAMBER (JAlA)

--------

22

100

I I

f f

0

10 shy1shy

I I I I I I

cu - -

343 KEV

-------

-

-AL -

~

~

N

~-----f

f

1shy

f

0 o----o 0 0 0 0 0 -----

- -

-

I I I I I I I I I I I I I

0 2 4 6 8 10 12

PRESSURE (liM - H6)

FIGURE 3

EXTRAPOLATION CURVES

THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS

MEASURED BY A FREE AIR ION CHAMBER (JAIR)

23

Energy JAJJaJr JeuJatr

Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal

816 641 l ozr 661 i o2S

161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36

3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9

COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ

JouJAJ

Kov 1heoretical r

61 9middot43l 34 100 l 042

middot1 ll l l 4 ll5 t 0 5

34middot3 13middot9 i middot5 136 1 6

24

A~ absorption correction wa-s aPPlied to the Cu and AJ

extra)Olation chamber data L10 air ebsorption eorrections were

mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between

the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot

tetion Qhamber and the seeolld -a tor the lack ot air absorption

in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs

A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt

X-~ beam ~e correction was detetmined from the density on

exposed photographic plates Both the free air toni zat1on ehamber

and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy

ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot

correction was_~ to the JAJ valW to campensate tor this tUttra -

ebsorbing leyer ot cellulose acetate the e1lr on tbe current

ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A

the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error

on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy

on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls

anA 1J ~or the cu vallE

abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the

lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or

ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm

and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and

e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the

JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S

enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to

bull

--------

22

100

I I

f f

0

10 shy1shy

I I I I I I

cu - -

343 KEV

-------

-

-AL -

~

~

N

~-----f

f

1shy

f

0 o----o 0 0 0 0 0 -----

- -

-

I I I I I I I I I I I I I

0 2 4 6 8 10 12

PRESSURE (liM - H6)

FIGURE 3

EXTRAPOLATION CURVES

THE CURRENT PER GRAM OF AIR IN A BRAGG-GRAY CHAMBER (Jz) RELATIVE TO THE CURRENT PER GRAM OF AIR AS

MEASURED BY A FREE AIR ION CHAMBER (JAIR)

23

Energy JAJJaJr JeuJatr

Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal

816 641 l ozr 661 i o2S

161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36

3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9

COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ

JouJAJ

Kov 1heoretical r

61 9middot43l 34 100 l 042

middot1 ll l l 4 ll5 t 0 5

34middot3 13middot9 i middot5 136 1 6

24

A~ absorption correction wa-s aPPlied to the Cu and AJ

extra)Olation chamber data L10 air ebsorption eorrections were

mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between

the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot

tetion Qhamber and the seeolld -a tor the lack ot air absorption

in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs

A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt

X-~ beam ~e correction was detetmined from the density on

exposed photographic plates Both the free air toni zat1on ehamber

and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy

ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot

correction was_~ to the JAJ valW to campensate tor this tUttra -

ebsorbing leyer ot cellulose acetate the e1lr on tbe current

ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A

the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error

on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy

on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls

anA 1J ~or the cu vallE

abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the

lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or

ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm

and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and

e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the

JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S

enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to

bull

23

Energy JAJJaJr JeuJatr

Kev yen3rpoundrimEJlltal tsoreticaJ tWiffirimental (pound0Nt1cal

816 641 l ozr 661 i o2S

161 7o8 l 019 1middot11 i 030 668iL9 720 l 3middot~ 23 middot7 769pound 020 7 44J 0 32 85 0 1 23 858 i 36

3tmiddot3 1 middot19 l 021 682 l 029 lo8 l3 92middot8 i 3middot9

COMPARISON OF ~RmCAL AND KXPERnmtmL VALlJm OF JcuJAJ

JouJAJ

Kov 1heoretical r

61 9middot43l 34 100 l 042

middot1 ll l l 4 ll5 t 0 5

34middot3 13middot9 i middot5 136 1 6

24

A~ absorption correction wa-s aPPlied to the Cu and AJ

extra)Olation chamber data L10 air ebsorption eorrections were

mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between

the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot

tetion Qhamber and the seeolld -a tor the lack ot air absorption

in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs

A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt

X-~ beam ~e correction was detetmined from the density on

exposed photographic plates Both the free air toni zat1on ehamber

and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy

ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot

correction was_~ to the JAJ valW to campensate tor this tUttra -

ebsorbing leyer ot cellulose acetate the e1lr on tbe current

ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A

the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error

on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy

on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls

anA 1J ~or the cu vallE

abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the

lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or

ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm

and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and

e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the

JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S

enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to

bull

24

A~ absorption correction wa-s aPPlied to the Cu and AJ

extra)Olation chamber data L10 air ebsorption eorrections were

mademiddot ~ ttrst -was an absorptiltgtn centorreetion tor the air between

the front apertu-re end middoteoUeottng ele~s 111 the tree a1r 1onimiddot

tetion Qhamber and the seeolld -a tor the lack ot air absorption

in tbe f1Ye inches ot vaeuum in ltmt Cft the extrapolation ~rs

A ~rreetton was ma4e tor the non-Utlitormtty ~the tluoreampCQnt

X-~ beam ~e correction was detetmined from the density on

exposed photographic plates Both the free air toni zat1on ehamber

and the vacuum cbanber bad lO mtl cellulose ~etete W1n4ows lfo1tshy

ever the wtldow on the vacuum chatnber bQwed in At 8 16 Rev a 1 bull51amiddot

correction was_~ to the JAJ valW to campensate tor this tUttra -

ebsorbing leyer ot cellulose acetate the e1lr on tbe current

ana preseUte meampsuretnel1ts vas estimated to be lcf and the error on A

the ~vttr volume of be ~-Greu ~bels We$ ~ middotlhe error

on the mass etlerQ abs~ion eoetti~nts ~ ebout ~ ~ errotshy

on the wall tbiclaless ~termlnatiQDS was 1J for the aluminUla wlls

anA 1J ~or the cu vallE

abe data 1n ~bles 3 and 4 sub$tantiate the vall4ity of the

lSrasgGray plinciple tn tbe middot ~rQ ~ tran 816 to 343 Kev 1or

ehamberu ua1ns wall materiels W1th atomic mwbers between alnudmlm

and copper At 3)amp3 Kev the ScuJsir and JAlfJair theoreti-cal and

e~tiJrlentel values (Us~ by approx1mateJyen l4 to lff aince the

JeuJAl experimelltsl aai theo~ttcaJ Values ~ee to 2fo1 the lna$S

enerQ absorption middotCCgtettieient tor air at tbls enerty 1(0U]d seem to

bull

25

be 1n error lbe fluorescent ~eld factors are extremeq tnqxgtrtant

in this energy rall8e For ~le the Ct1 mass energy- ebsorptton

coefticienio for 161 Kev rediation that ts gven as Ki3 ctrt2g tn

~ble 2 woula be 299 em2e it the fluoreseent neld factor were

lett out of eltlUtiou 3middot ~ ~ent W$ uot Smiddotensttive eneUgh

t-o test the Betbe-iloch tbeOry ~ stopping power f011mlla eoul-d

be in error by au appreciable Mlmlnt but the en-ors eould ean0$1

out in the stopping paVer ratio Jhe ourves in ~ l extend over

a lar e r~ of pressure ~ese data ~ihov that at high poessureamp

the DraeemiddotG~ chamber response epprooches that of an air reJ1ed

chamber (9~PPmiddot231middot240)~

BIBLIOGRAPHY 26

1 Aly S M and C w WUson Ob~ervations on the ionization produced by high voltage radiatiOn tn moulded ionization chambers with walls of variows middot ftectiTe atCIIIic numbera British journal of radiologr 22a243-254 1949

2 Attix n-ank H Icmization chamber respanse as a tunction ot wall matelial Washin(ton llational bureau ot etampDtards 1953 9p (HatiODIJ bureau ot etaDiards Report no2491)

3 Attix middot Frank H Iantzation chamber response as a functiCJD of wall illaterial Wa$hingtoa National bureau of stampDIards 1953 27p (Nat10Dal bureau ot stmlros Report no2771)

4 Broyles C D D A lhaus ~ S K Haynes The meameshylllent arri i~~etaticm of the K Auger intensities of SnJl3 cel-37 m eul9U Physical rmev 89bull715-724 1953

$ Canpton Arthur H arid Samuel K Allis011 X rays in theoryand experiment 2d ed Rev YCBk D Van Nostrand 193$ 828p

6 Carmack D V et al Irradiatlcm or ferrous ammoa1um sulphate solutieiit-eneru absatption am 10ftizat1cm calcu latiQIIlS tar cobalt 60 aDd betatron radiation Journal ot chemical physics 22a6-l2 19$4

7 CorIIampCk D~ V ad H E Johns The MeaBllrnt of b1gh-energ radiatieo bituaity Radiation research lt1))1$7 1954

8 Failla G the DI8U1raents ot tissue dose in tellS ot the same cit tOJ aU SOldziQS radiation Radiology 29202bull21S 1937

9 Fano U lote 011 the BraggGny cavity pr1Dciple for measuring eDer8f disa1patiefll Radiatioa research 1a37middot240 1954

10 Pine S~ ad C F Hemee Xra7 criticalabsarption aM emiseiarl energies in KeT lucleonics 13s36-37 195$

11 llorler f A C C Lauritsen am t Lauritsen Quma radiatica trbull excit~ states ot light nuclei Reviews ot modern peyncs 20bull236-277 1948

27

12 Gray L H Radiation dosimetry I British journal of radiology 10600-612 1937

13 Gray L H Radiation dosimetry n British jourDampl of radiology 10t721742 1~37

14 Gray L H An ilon1zation methOd tor the absolute measurement apound ganna ray energy Proceed1ngs Of the Royal Society ot LomQil Al56578-S96 1936

15 Heitlm- r the qWltum theory or radiat101 2d ed Oxford Oxfai-d University Press 1944 272p

16 Ibrahim Ali A K A ocrrection factcr to Graybull s theOJf of ionization fh7sical Sootety ot tomont proceedings A6luS09middot 510 l9t

17 Ibrahim Ali A K and C W Wilson The ionization produced by bigh voltage radiation in an extrapolat-ion chamber with walls of various atltllldc ttUJibers alld its relation to that to be expected theoretically British jOlnlal or radiology 2g t 5768 1952

18 Larsan Harold V1Dcent Ira Thomas Myers aDi William Carl Roeseh Widebeam fluorescent xray source N1Cleon1os 13100-102 1955

19 Laurence G C The lfteasurt$8nt of extra hard x-rqs aDi ganna rays in ~oentgens Canadian journal or research 15167shy7~ 1937

20 Marinelli Lfonldaa n Radiation dcJsimetry ani protectie~n Armual review ot nuclear acience 3249middot270 1953

21 Helms A T Graphs Of the Ccnpton energyangle relationship ani the IOeinJRsbiDa formula from 10 Kev to 500 HeY Washshyington Us Govermnent printing oftiee 1953 89p (Natioaal bareau or stamards Circular no5h2)

22 Sievert R M Deter2rdnation of ionization 1n biological objects Acta radiology 2lal89-205 1940

23 Spencer L V The017 ot electron peDetration Hlys1cal review 98 f 1597-1615 1955

28

24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952

25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)

26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)

28

24 Spie-er B r1 The absolute maaSUement Of high-energy radiatiltltl Australian journal of scientipound1c research 5t ~81$91 1952

25 Vlctareen 1nt9r-ent centcmpaq Vtctaren glampiUs-sealed ~esistors Cleveland 1954 6p (Its form no bull )025)

26 t-Jh1bo G R f-ray attenutiOD coettiotenta rrom 10 Kev to 100 Mev Washin(lta Matt~ bureau or tttandaros 19S2 93p (National bureau of sttmards Report nol003)