AN IIM:3~GA'ltON aF :mE JU\AGG-GBAY PRINCIPLE WI~ P'LlJORESCEN';J.* X-RAYS

by

JIAROU) VINCmJ' .I.A.RJ()lf

A~IS subnitted to

ill. partf.al tultillment ot tbe requ:L~ts tor tbe degree or

MAS~ OF SCIENCE

June 1957 lillWt Redacted for Privacy

Redacted for Privacy IllrrcL farfrmr eil.$elrm Io ercilr of hrl Redacted for Privacy &.t d lnrrtxt of W.or Redacted for Privacy

&im of fahf {&re*t; fFililtttr Redacted for Privacy Srm d iBratr*r l&a[

ffr thai,r i,r lmntri T ',nut,A{r,,,(,1,*. nn* tr lu{rl.ry frilfir !lbe. author desires: to ~retts hie gratitude to Dr. w. c. Boesch tor e:u.ggestilla this problem e:nc1 to I. ~. M1ers 'for advice on ~bnt..

ques. AN DM$~00 OF 1HE BRAG<.J-GRAX PRINCJ:'PLE tttZ!..FLtJORESC!E:Nlr x...RAYS s.-~. ~: !!4t Introcluct1on l Ob,leetive 3 Method. ·5

~tion Cllam.'bers 9

Val.i\U:ty of the ~...G~ Pz1n.ciple 20

B1bl1~ 26 t?able No. 'atle !!I! l. !rheorettw Values of JAJ/Jair 10

2 ~rt:tical VaJ:.ue of Jcu/Ja.tr 10

3 ~~ ot ~tical tm4 Ex.Perl.Diental. Ve.lues o~ JAliJ'IJ.'Lr · leu/~a.J:r 23 4 Com.p.arison ot 'l!heo:retical. 8lld B:l(perimental Value ot J au/JAl 23 &1;.tle

1 ~son v.tt.b Attix's lata 4 2 Extre.:polation CUrves 2l 3 Extrapolation CUrves 22

a'=J:tle

~ IOllizat:ton Chamber AssemblY

2 ~ Vaeuurn Chamber .and tbe Iomzat:LQ!l ~ber :1u Relation to the Fluol'f:sc:ent X·Rau Source 15

1.f.tle

1 13 2 17 AN DM.S~GA1!0lf OF !l'fiE BRAGG-GRAY PRINCIPLE WI~ FLUORESCEN7,' X-RAYS

IN):'RODlTC').'ION

'-'he primary standard tor low energy X-ray dosimetry measurements is the tree a.1r ionization chamber . For some measurements, such as that o~ surface dOse,. the free air ionization cbam.ber is difficult or iDg?ossible to use. For these measurements the Bragg-Gray chamber

(12, pp.6oo-612), (13, PP·721-742)., and (14, pp.578-596) is a valu... able instrument. A compa.r:Lscn o:t tree air ionization chamber measurements with Bragg-Gray chamber data using lov energy x..ra.ys is needed to verti'y the Bragg•Gra.y principle and establis the lim:it• ations on a Bragg-Gray cbamber in this energy region.

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

1nvest1&f.t1ons (22, pp.l94·195). 1~ derivations of this principle are tound in the literature (ll, P.P·259-295), (19, pp.68-74), and (24, pp.581-589). A rigorous der·ivation ot the principle was made by Cormack and Johns (6, PP·7-9) and (7, pp.l34-l39) · For the mediUirl soft x..ray energy region, !artnelli (20, p.251) expresses the

Bragg-Gray principle in tbe tolloWing torm:

l) =S W ;r ::: h lrP.;11 q (l) D • 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 • energy in ergs necessa.'1.'7 to form an ion pair in the cas 2

S : ratio ot the mass stopping power of tbe va.U materla.l to that of the eas for the ionizine ;particles associated W1 th tb.e inci·dent radiation

11 : number of incident photons of energy h-v · per em?- per seconcl

hv• eJ.>ergy of the photon in ergs

,11•=mass energy abso~on eoeftie:f.ent Of the wall materi-al in cm2 p~r gam

:the mass energ;r absorption coetncient :ts det'ined by: ~<£>=/"It-s/ (2) .tN • mass absorption coefficient in em2 per gram

At low photon energies the mass energy absorption coefflc.:t.ent. ba.$ two cam,ponen~s .

.MIt = £G:.. .,. (1- ~,.,.. )Jll /-· . r... lt"V tl (3)

7; c photoelectric a..bsorptiou coefficl.ent per elect'l"'n in wall of atooti..c number Z in cm2

f- • fluorescent yield tor the K level.

~ =btM:In& energy ot tbe K level

~=¥= nUDber of electrons per gram of wall materieJ.

~ · Avogadro's number A • atomic weight

Wilson and his co-workers (1, pp .243-254), (16, PP·509-510), a.nd

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

Al . e.nd CU walled ionization chambers end f'or X..ra.ys with '' e~ective 11 3

energies fran 25 to 125 Kev by ustns an extrapolation chamber

{8, pp.2Q2...2i5) technique. 1'bis work is sul)jeet to crtt1~sm since heterogeneous X-rays were used ,as the source ot radiation. Siuce

these X-~s were J;l.Ot monoenergetic, the average mass energy absor,p..

t.ian eoettioient used in the eal..eula.tions does not nece:ss:arily con-espond to the average elierSf in the X-ra.r spectrum. !lbe extra..

. polatton teehnique used 1n ·tbelr e:lq)erinlents vas to vary the electrode spaeUJg betwetm o. 5 and 5 an ot air. As Will be shoVtl below, :for a go.od extrapole.tioli, eJ(l)erlmen'\;al data should. have been obtained for

eleetr

Gray pnDc1ple ill the· energr region h'otll 38 to 670 Kev using C, Al,

cu, sn, aDd Pb wa.Ued extrapolation Clmmbers. His method 'lila$ to

vaey tbe dlam~r spacing between o •.5 and l2 •· A curve ot current

per .arum ot Ur ill B~·Gl"&Y ebamber (J.z) relative to the current

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

chamber spae1ll8. 'W'aS extr&pole;te4 to zero clumiber .spaetug.. ·J:be Oasbe4 line in Figure 1 represents Attix•s Clata., For a sood extra•

pola.tiou the chamber spacing shoUld have been reduced another order

of JDqllituae. Also~ the, radiation W'!ed in tlUs ~~t w.a

fllterecl x...~. De.epite the heavy fUtrat1ou, .there rema:tne a.

oonsiderable spread 1n energy ot these x...~s.

~e present 'W'OJ:'k was undertaken to establish tl1e W:U.di.ty or l.aek 4

cu

0 LARSON, 34.3 KEV ATTIX, 38 KEV

AL -----...... ­..... ---­ --.... -- ......

FIGURE I

COMPARISON WITH ATTIX'S OA TA 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~ princ;1ple in the energy- r&r~&e f'1"0IIl

8.16 to 34.3 Kev tor copper 8.Dd allllliDUDl val.lecl ionization chambers.

MEtl'.HOD •

For low eDerQ' work the above method ot extrapola.tin& is

~tiw. NO't anJ.y would the plate sepe.n:tion measurements be

:lDAOcmr&te1 but also a small vriDkl.e 1n the vall material voul.d eauae electrical ditf'iculties 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 a.ir in a. l3ra.gg-Grq cbanlber rel.e.tive to the current per sram of air as· measured by a. tree air ion cbember versus pressure could. be extra,... polated to z~o pressure. ~s latter e~ra.polation technique all.ows 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 relationslU.p betveen the current (in amperes) per unit pressure (J') and the ion current per gram of the detecting ,gas. (4) 8 V =~..2"/;2 X/0/ J / (~) ( .:z:.) ~-'it ?.: J; = 76o mm-bs Itt • volume of the Braeg-.G~ chamber in cm3 .r • 273 .20Jc ~ •

'r• absolute tell;pera.ture of the air in 01( 6

Equation (4) is a conversion of ox.perlmenta:J. data according to the pertect gas la.vs. For a tree air ion chamber: .. (5) J;;,. = ~. 24.1 ~ ID ISf:.Z:•'~) ( ~';.) ( p':-- ) ~~~ 70 pd,,..

~,. .. ion current per gram of gas in the collecting volume of the tree air 1oni2:at1on chamber r.,.,. =current in amperes as read by tbe vibrating :reed electro­ meter

V.ir .. eollectiDg volume of the tree a.i.r ion chamber in cm3

7J;, ~; absolute temperature of the a.:l.r in the free air ion chamber in OK

P.;,. • atmospheric pressure in Dlll-hg

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

I .r;7: - J" ( ,tl,;. >( 7' ) (6) /" .,.,. -.r.,j../"'' Vc, "/ r.;: Also, for chambers With different vall materials, but the ssme detecting gas: (7) ~- /JAt =;c.'k', Kt1I J;/'1A1 vc~ One ot the purposes ot tbis work is to compare equation (6) to its tbetretica.l counterpart, equation (10},~ in the energ,y region. from 8.16 to 34·3 Kev. A nearly monoenergetic photon source (18, pp.l00-102} proViding energi.es han 8.16 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~ unif'orm six-inch beam with max:1.mum intensity, depending on ~ energ;J", from 0.055 to 14.1 r/hr.

~s instrument eHmna.ted the need for usina heterogenous X-rqs. 7

~e radiators used tor tbe experiment were copper, zirconium~ ca.dmium, and lanthanum which correspond to energies of 8.16, 16.1, 23.7 a.n;d 34·3

Kev respect!vely. ~ above energies wre verified b1 absorption measurements in an aluminum medi'um.

VaJ.ues tor the factor nlrv (equatton 1) ea.n 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 cm'2. per gram

~ above method. of obta.initl8 hli'V is impractical for high energy gamma. rays Since the range of the ionizing electrons would exceed a praoti­ cal. cheimber electrode spacing. · A substitution of equation (8) into e uation (1) recults in:

(9) or (10) .T/J;;,. =~ ·;~.:.,. s

W cancel.s and does not eppear :tn equation {10) beea.use 1t de~ ow.yl on· the gas used which ic as.cumed to be air tor both the tree o.ir ion cb:lmber and the ~-GI'S¥ eho.mb -r. ~ J ' s on the lett

1 ~re r:;ay bo a dependence of W on the e~rgy of the secondary elec­ trons. In that case1 W is a slightly different sort ~ average value in (1) and (8). ~e e:ttect o'f this difference can be esti­ mated and shown to be less than O.ltf, 1n the value of W. To the a.coura.cy ot the J_n."esent experiments W ca.n be ce.ucelled in goil'lg to equation (10)• 8

:1n equation (10) a.re the quantities measured in the present eJC;peri­ ment. ~ quantities on tbe right ere reasonably well. known. ~ right side ot tbe equation V1ll be called the 'theoretical. value ot J/JeJ.r• It the theoretical and e~rimental value are found to agree1 1t would 1mpl.y the valicU.'tf o:t the Braas-Grq prtnciple.

Cc:mpton bsorpt1on coetticients per electron fG;} were obtaiDed f'rall a gt"aph by Nelms (21, p .88} 8Dd photoel ctric absorption co­ efi'icients per tan (~ z) were interpolated trail tables canpiled by White (26, pp.48-58). ~ avera.ge K shell bind1og energies

(Ett) vere ~ by weigbt1Jl8 tbe e~es of the K ser1es emission lines (10, pp.36-3'7) by values ot theU' relative intena1t1es o'bte.1ned fn;n Canpto:n .8Jl4 Allison (5, pp,6J7-642). :t'be values used for tbe fluorescent yields (4, p.722) of aluminum 8Dd copper were 0.035 8lld o.46 respective~. 1.!lree correctiollS vere made to the absorption coetfic1ents. 7!he first was a correction for tbe sma.ll amount ot high energy scattered radiation in the fluorescent X·nq beam end tbe second vas a correction tor tbe ~ti sin the wa.U ~J~aterials. ~ 1m;purities in t.M aluminum were determined by a quaz.rt1te.tive spectro. chemical anal,ysis 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 photo­ electric effect to the mass energy bsorption coeftieients.

stowi..Jlg power ratios were cazwuted tran the non-relativ.Lstic

Bet.he•Blocb ~ory (15, p.220) Vith tbe aasUJZQ;>tion tbat the co: ot (11) <.;%: >w s--~- <;!>,-

(£) ~o'.tw

N

lt Q.

:t ( • )f

h'c >> ~ (1.2) { t1 • JIIIEUdr... 4Uienaion ot 10

Energy SAl e:: (c:m2jg) JAJfJair

Kev Al Air

8.16 0.835 43.3 7-84 6.61£ 0.26

16.1 0.851 5~75 0.942 7·17 l 0.30 23.7 o.86o 1.81 0.283 7.41t- t. 0.32 34-3 o.864 0-556 o.o9lf,4 6.82f.. 0.29

ergy seu ea (em2fg) Jeu/Ja:J..r ' cv Cu r 1 .1 o.683 46. .942 72. l .l 23.7 0 .700 rr .o 0 .283 85 .8f.. 3.6 34-3 0.709 6.21 .0944 2.8 l 3·9 11

However in determining the amount o.£ energy per gram tbat 1s absorbed. trail the X-~ beam (tJ=hlrzl~f) , the assumption must be maa.e tbat the energy })er gram. absorbed trcn the X-ra.y beam is tbe same every­ where in tbe absorbing medium. For this to be true the xponentia.l

X-rs.y absorption must be amaU and the medium ourroundU:Ig tbe caVity must 'be at least nne electron range thick. (13)

(l.4)

.# • linear absorption coetficient of the "'I8J.l material.

"t • extrapolation chamber wall thickneGs and (15)

R..,: range of the secondary electrons in the wall material.

'-'he .assembly of the ionization cll.embers that vere used 1n this . , f: \ . experi.ment is shown in Plate ·,1- and Photo~ph 1. !n oriler to lltlni•· m1ze absorption of the x..ray$ in the chamber \mlls, thin foils. Of

Cu and Al vere used. !l'he Al 'Wall~;~ were 5 •74 mgJc:m.2. ~ Cu valls were formed b'J evaporating 1.3 tNr)~ o't Cu on a 5.74 mg/cm2 Al

se. ~ Cu. tlttckncss w.a deterurtned by peeling the Cu. off the Al foil and we1.gbi.DS a known area of CU foil. Duco cc:ment was used to bond .the wall materials to the lu.cite rings (6" O.D. • 5·3/4" I.D. ­ l/8" long). Side walls were crewed onto the MFP...lo insulating rings (6" o.D ... 5-1/2" I.D. - 1/4" lonz) so tbat the · r electrode vas a:J.most canpl.etely Sllrro\U:\

~,.,,1::.~·:";~~·.:~1 t•>!.. - LUCITE RINGS '-----'1 -MFP-10 RINGS

PLATE I IONIZATION CHAMBER ASSEMBLY I:-' /\) 13

0: 1&.1 CD 2 :%:~ ~:X: ~0 0: C)Zoo ..... ­ 0~ :Z:N ~z 0

,UC•Gl .ICHLUID, WH . 14

1nsulat~ rings aud the el~ctrieal. Shield:f.ns (4 m:tl Al) ·~ the cunent leak.a&e to a. neeligible va.l:ue . A ~ el.eetl"Ode ebamber vas used &O that the coUectina electrode vou.ld. be eleetro­ stat.i~ shielded. Another adva.ntaae o~ the three electrode chamber was that 1t helped to elbdnate transfer current. By transfer ctlZ't'el)t is meant the current due to the seeond.aey ·electron$ Which cross the cbamber. 1-'lle etteet ot t:t:e.llSfer current vas complete~ elJ.mllnated by rf>'YeQ1:ttS the voltage on the outer eleetro4es ad avera.atns the two r~s. 1-!le cbamber lengths were measured w1th a micraneter caliper and the diameters of the lucite a.nd MFP-10

~s were determined. by a. metal ruler. Hw~r1 tJle largest error in ~ e.h$m'ber vol,_ detel'lltf.nat1on WW> due to Wrinkles 1n the thin metal f'oUs. At ~ssures above two m•hs the cond:f.tion o~ voltage satUt'ati® 'WaS obte.i.Ued :for both the QJ.Utninum. and. the eopper ion!· za.tton cllambers •

:rho ve.cuwn ebsmber &hGW ;tn, &te 2 was ·constructed ot 16" O.D. standard steel pipe. A steel plate l/4'' thick was welded over the tront of tM: p:l.pe. In the center ot the steel. plate a hole Sf.Jt.. inches 1n dieme~ was cut and a O.O.l" ~ellulose e:cetate 'Vitldow was mounted ~ the ~rture. :Pbe rear windov on tile pipe eonsisted o:£ a l/4n 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 t.he' 'Umbra ot the beam from tlle fluo:res¢ent X-ray souro• we.s all $bsolt"bed in the Al 'Window. Scattering trom the va.cuum cbambel' vas Qheeked by measurinS the current collected by the 15

•.. .. ~. ~ ~ •: ,..... ~ , ,, , . · ... . . ·.. " ...... II ,o a !---RADIATOR ~ . ·- ....__ • • ~ .. , . " .

tT'TI tT'TI ~ ~ ~ ~ ~ ~ 1- SHIEI.0£0 ~ · STEEL CA81.E ~ ~~~_,/ I I ~ ~ - LEAD ./ I I :11J'' ~-BRASS ELECT..._,~ I! ~ 1!!!!3 -ALUMIMJM FOil£ PUIII ~ • -PLASTIC IONIZATION_/ ~ CHAMBER ~ t%~ I ~ IIIANOIII:Jj' ~ ~In -~ ~ ~

0 1 I I 4

ICAL.l UICMI S

PLATE 2

THE VACUUM CHAMBER AND THE IONIZATION CHAMBER IN RELATION TO THE FLUORESCENT X-RAY SOURCE 16

extra.polatton chambers 1ll.th t.b.e vacuum chamber present and WithOut

the vacuum chamber vrcaent. !L'he scatter from the vacuum ehatn.'ber was

:tound to be :negllgibl.e. A l/4" lu.cite rod se~ted tbe g:rounded

vacuum ·chamber tran the extrapolation chambers. ,:he exper:tmenta.l

system 1G picture4 in .Pbotogra_ph 2.

~ ion current was measlll!ed wtth a Vibrating reed eJ.eotrcmeter.

~ Ballle glasa•see.led resistor was used in the head or the Vibre.t:ine

reed eleetraueter tor both the tree a.tr ion chamber current ~­ ments and tbe extrapolation cbamber current measurements. A value

tor tbe resistance (R} need not be known since the factor R SJ?pears

on both sides o:f equation (9) as a tactor in ea.eb of the J terms.

~ teur;perature . coei'fici~t (25, p.2} oi' the resietor ,;wed was l O.l5cfo/oc and tlle volta&e ~moient (25, p.2) was -o.o'!f,fv. ~se low coett1e1ents pemitted neglect ot cbanSes in resistance due to

temperature and vtil.tase .

A 11-tube oU manometer was usect as tbe pressure mea.aurtna deVice.

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

of the same 1nstl"Ultt8Dt f:i:Ued With mereur:r. One end ot the U•tube vas connected ~ugh a Pira.ni gauge to a rorepump. 'lhe P1ran1 suaee vas calibrated Vith a. ~Leod. ga.~. ~s end of the manometer

'Vas kept at e. pressure below 0.OC7 ~~m-hg. Ord.1na.ey f'orepurru> oil was used ·as the working fluid in the manometer. ~ density ot tbe

oU ws de1ierm.tned by we1shin8 an empty ,pipette of kn

the weight and the volume of tbe oi~, the density o£ the oil w.s PHOTOGRAPH 2 THE EXPERIMENTAL SYSTEM 18

C011I.P"41ted. !!'he va.:por presG\We of the f orepui:II!? oil vas less tb::m five microns. W;tth a.n oil rnaua:neter in COllju.nction with a. ca:thetometer1 e. pressure of one mm.-hg could be dete cd to ttGr than one ];)er eent aocuracy.

In equation (5) the factor la.ir/Va:J.r was determined by the use of a tree air ionization c:ballber in eonjunct1on Yith a v.t'brating reed electrometer. ~e tree atr ionization chamber consisted ot two ;par­ allel. o.lumin'um plates that were separated by a distance o~ nine inches·

~ proper plate separation vas detem.tned by plotttns a curve ot current ve:-sun plate s:r;ncins and ttnding that spacing beyond Yhich the current no longer depeoded on the plate spacing. !rhe dime,n...

11 sions or the plates vero 21" X 16.5 • A 0-3000 volt power su;ppl.y vas

OOlllleCted to ooo of the electrodes. ';t'he proper voltaee tor saturation was dete.nninad by plotting a curve ot ionization current versus volt­ age. !fbe other eJ.ect.rode was cut tnto three sections. ~ center section (1" x 16 .5n) 'WaS used a.s the collecting electrode aJld the two outer sections (1011 x 16.5" ) were grounded and. used as suard rings . !)lle :rluoreseent be8:4l ~ntez-ed the tree e.1r ion1zation chamber through a ttro~inch diameter hole that was dr:Uled in a l/'4" thick lead sbeGt. The etfective volume at the chamber was determined fran tbe area o£ this aperture1 the Width o-r the collecting cleetrode, alld. the average length of ~P between the BUSrd plates and the col­

11 le~tor. ~s was not the "eollectins vol'Ullle , but tll:ls volume is equivalent to the "coUeet1ng vol'Ullle'' for any point source or element o'f a. finite source,, ~e ionization current vas 11\eS.sured. by a 19

Vibra.titJg ~ electrometer.

!L'he x,..ray tube was alloved to wa.rm up for a period of e.pproX!.... mate}U twenty minutes before •~ents -were· taken. A radiator

·vas placed in tbe fluorescent source • 1'be free a.tr ionization ehtml... ber was centered on the ~ter line ot the side part ot the f~uores...

een.t source at a fiXed d1$talt\~e han the cenwr ot the ~~tor. ~ voltase acrosa tbe Vietoreen reSistor in the v:i.bratillg :reed ·electro.. 111eter vas recordecl on a chart reeo:rder tor appro.xtmately ten Jllinutes. ln this time interval tbe ·t"QQtn ~ture and p~ss\ll"e were also recorded. !1be free air 1oniut1on cluwber , was removed and. the al.uminun lWagg-Gr~ chamber was located so t.'h.a.t tbe center ot the coUeot~ volume was iu t.be :position that had pl'eviously been occupied by the free air ionization chamber. J'he press~ inaide the aluminum

'Bragg•Grtq ¢hamber was lowered to s~ pressure PJ... !rbe voltage, pre&sure, temperature, and proper saturation voltage vere recorded by

\be same procedure .as~ U$ed tor the tree air ionization cbam.ber.

· ~ voltase on the ou.ter el.e

:L'be al'UIIltnum ~...qray chatnber 'W'Q.S ~ved and the b-ee a.ir ioniza-­ tion cbamber was moved into ;position. Another set of tree atr ionization chamber measurements vas· ·recorded. ~us the tree air ionization dlamber rea.dtngs vere: not oncy used to detem:tne r;~~ vaJ.ue or Ja1r but also as a measu:rement of the stability ot the ~luoJ."esc.ent x-~ "*•· ~above expertmental steps were repeated tor the ¢0ppelr ~~ cbainbeT• n"ie X•Z'q beam W& ·~~ by keeping the values of' the X-ray tube voltege :atJl cun-eat the same tor all tbe above aeta ot readtneiJ. ~ ex.pertmentaJ. eu:nent l'$t10s were plotted wrsus press~ (ng. 2 and 3). ~ au.rrent ratio · w~ couide-l'ed to be the ~ data l"&ther tb8n the tndiv.td®. eunoent detel'!ltnations1 since the ratios 41<1 not conta!n ·the veJ;ue ot the electrometer 1J:ltpu.t reaiete.nee Ylth its assoei.ated t1 per cent uncerta:tnty. !lbe above e:x;pe~tal. steps were repeated tor aU ot the tl~sce.nt ~ators ..

~ ~tion curves tor the. Al aDA ·CU B:raag...Grat chambers are s~ in Fignres 2 and 3. ~ ordirlate is the ratio ot the curren1; per pam ot ai.r tn the chamber beiJlS tested to the CUl"J!'ent per snm of e.tr as measured b;r a bee atr lf.oaizat1on ehamber. ·'-be

X'trapolatecl values ~ theft .C\l:t"fes a.re· ~ ~ues desn-ed. 1l'hese val.'Uea ere llsted in ~bl.e 3 along Vl"th the theoretical ve.lues A ratio ot J'cu/JAJ. to'¥! .a, p8.1."ticul.ar ener&r can be ob~d trao. the ratio ot JfJrJJ~ to JAJ}J&ir 2llese -.lues are listed 1n ~ble 4.

shoa:l4 be.ve a ze.ro slope., However 1- at prese\lt'ea below two -h6 voltage saturation vas not attainable with tbe lb:'agg·G~ cbambers uae

0 0 0 0 0 0 ---~~ ~ ts 6­ cu A­ ~

c 8.16 KEV 6 16.1 KEV 0 23.7 KEV

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

I I I I I I I I

cu 100 ------f.. - f.. - .. - .. - .. - 0 34.3 KEV - -

~ - ~ '..,N

10 - 1­­ AL - ~----- 0 o----o 0 0 0 0 0 - f.. - f.. - 1­ - f.. - - - -

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 JAJ}Ja;J.r Jeu/Ja:tr

Kev :¥3r£ri.mEJlltal ts;oreticaJ. tWiffirimental (!!£0Nt1cal 8.16 6.41 l o.zr 6.61 i o.2S 16.1 7.o8 l 0.19 1·11 i 0.30 66 .8iL9 72.0 l 3·~

23 ·7 7.69£ 0.20 7 .44J. 0 . 32 85 .0 1:. 2.3 85.8 i 3.6 3!t·3 1 ·19 l. 0.21 6.82 l 0.29 lo8 l3 92·8 i 3·9

COMPARISON OF ~RmCAL AND KXPERnmtm.L VALl.Jm OF Jcu/JAJ.,

, Jou/JAJ..

.Kov 1.heoretical. r , 6.1 9·43l .34 10.0 l 0.42 ·1 ll. l. l .4 ll.5 t 0 .5 .6 34·3 .. 13·9 i .. ·5 13.6 1.. 24

A~ absorption correction wa-s a;P.Plied to the Cu and AJ.. ex:tra!)Olation chamber data. !L\10 air e.bsor.ption eorrections were made.· ~ :ttrst -was an a.bsorpti<>n ¢orreetion tor the air betw'een the front apertu-re end ·eoUeottng ele~s 11:1 the tree a.1r 1oni· te.tion Qhamber and. the seeol!ld -.a tor the lack ot air absorption in tbe f':1Ye inches ot vaeuum in *'

A ~rreetton was ma4e tor the non-Ut:l.ito.rm.tty ~the tluore&CQnt

X-~ beam. ~e correction was dete.tmined from the density on .exposed photographic plates. Both the free air :toni za.t1on ehamber and the vacuum cban\be:r: bad lO m:tl cellulose ~ete.te 'W'1n4ows. lfo1t­ ever, the w:!.tldow on the vacuum chatnber bQwed in. At 8.16 Rev a 1 •.51a· correction was_~ to the JAJ. val.W! to cam:pensate tor this t;Uttra :-' ebsorbing le;yer ot cellulose acetate . :the e1"l:''r on tbe current ana. prese'Ut"e me&suretne.l1ts' vas. esti.mated to be lcf, and the error on A , the ~vtt:r volume of \be ~-Greu ~bel"s 'We.$ ~. ·!l'he error on the mass etlerQ' abs~ion eoetti~nts ~ e.bout ~. ~ e.rrot­ on the wall tbiclaless ~term:l.na.tiQDS was 1'/J for the aluminU.la wl.ls anA 1/J ~or the cu vallE*. a'be data 1n ~bles 3 and 4 sub$tan'tiate the va.ll4ity of the lSrasg..G;ray pli.nciple :tn tbe · ~rQ' ~ t'ran 8.16 to 34.3 Kev .1or ehamberu ua1ns wall. materiels W1th atomic mwbers between alnudm.lm and copper. At 3)&...3 Kev the Scu/Js.ir and JAlfJair theoreti-cal and e~:tiJrlentel values (U.s~ by approx1mateJ¥ l4 to lff!,. aince the.

Jeu/JAl ex.perimelltsl .aai theo~ttcaJ. 'Values ~ee to 2fo1 the lna$S enerQ absorption ·CC>ett'ieient tor air at tbls enerty 1(0U].d seem to

• 25 be 1n error.. lbe fluorescent ~eld. .factors are ext:remeq tnqx>rta.nt in this energy rall8.e. For .~le, the Ct1 mass energy- e."bsorptton

coefticienio for 16.1 Kev re.diation that ts g!:ven as !Ki.3 ct.rt2/g tn

~bl.e 2 wou.l.a. be 29.9 em2/e it the fluoreseent neld 'factor were lett out of' e

1. Aly, S. M. and C. w. WUson. Ob~ervations on the ionizati.on produced by' high voltage radiatiOn. tn moulded ionization chambers with walls of va.riows · f.tectiTe atCIIIic numbera. British journal of radiologr 22a243-254. 1949.

2. Attix, n-ank H. Icmization chamber respans.e as a tunction ot wall matel"ial. Wa.shin(ton, llational bureau ot et&D:tards; 1953. 9p. (HatiODIJ. bureau ot etaD:iards. Report no.2491) .3. Attix,· Frank H. Ian.tzation chamber response as a functiCJD of wall illaterial. Wa$hingtoa, National bureau of st&D:Iards, 1953. 27p. (Nat10Dal bureau ot stml.ros. Report no.2771)

4. Broyles, C. D., D. A. 'lhau.s, ~ S. K. Haynes. The meame­ lllent arri i~~etaticm of the K Auger intensities of Sn.J.l3, cel-37, .m eul9U. Physical rmev 89•715-724 . 1953. $. Can.pton, Arthur H. arid Samuel K. All.is011. X..ra.ys in theory and experiment. 2d. ed. Rev YCB'k, D. Van Nostrand, 193$. 828p . 6. Carmack, D. V., et al.. Irradiatlcm or ferrous ammoa1:um sulphate solutieiit-eneru absat*ption am 10ftizat1cm. calcu.. latiQIIlS tar cobalt 60 aDd betatron radiation. Journal ot chemical physics .22a6.-l2. 19$'4.

7., Cor.II&Ck, D~ V. a.,d H. E. Johns. The MeaBllr...nt of b1gh-energ radiatieo. bit.uaity. Radiation research lt1))...1$7. 1954.

8. Failla, G. 'the DI8U\1raents ot tissue dose in tel"'lS ot the same cit :tOJ! aU S.Old.ziQS radiation. Radiology 29;202•21S. 1937.

9. Fano, U. lote 011 the Bragg.Gny cavity pr1Dciple for measuring eDer8f disa1patiefll. Radiatioa research 1a:?37·240. 1954.

10. Pine, S~ ad C. F. Hemee. X....ra7 critical...absarption aM emiseiarl energies in KeT. lucleonics 13s36-37. 195$.

11. ll'orler, \f. A., C. C. Lauritsen, am 't. Lauritsen. Quma.. radiatica .tr• excit~ states ot light nuclei. Reviews ot modern peyncs 20•2'36-277. 1948. 27

12. Gray, L. H. Radiation dosimetry. I. British journal of radiology 10:600-612. 1937.

13 . Gray, L. H. Radiation dosimetry. n. British jourD&l of radiology 10t721...742. 1~.37.

14. Gray, L. H. .An il.on1zation methOd tor the absolute measurement a£ ganna ray energy. Proceed1ngs Of the Royal: Society ot LomQil Al56:578-S96. 1936. .

15 . Heitlm-, r. the q\Wltum theory or radiat101\. 2d ed. Oxford, Oxf'ai-d University Press, 1944. 272p.

16. Ibrahim, Ali A. K. A ocrrection factcr to Gray• s theOJ."f of ionization. :fh7sical Sootety ot tomont proceedings A6luS09· 510. l9;:t.

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 at

19. Laurence, G. C. The lfteasurt$8nt of extra hard x-rqs aDi ga.nna rays in ~oentgens. Canadian journal or research 15167­ 7~ . 1937 .

20. Marinelli, Lf;onldaa n. Radiation dcJsimetry ani protectie~n. .Armual review ot nuclear acience 3:249·270. 19.53.

21. Helms, A. T. Graphs Of the Ccnpton energy.,.angle relationship ani the IO.ein.JRsbiDa formula from 10 Kev to 500 HeY. Wash­ ington, U.s. Govermnent printing oftiee, 1953. 89p. (Natioaal. bareau or stamards. Circular no.5h2) 22 . Sievert, R. M. Deter2rd..nation of ionization 1n biological objects. Acta radiology 2lal89-205. 1940. 23 . Spencer, L. V. The017' ot electron peD.etration. Hlys1.cal review 98 f 1597-1615. 1955. 28

24. Spie-er, B. r1. The absolute. maaSU!'ement Of high.-.energy radiati<;ltl. Australian journal of scienti£1c research 5t ~81...$91. 1952.

25 . Vlctareen 1nt9\r-.ent ¢cmpaq. Vtctare.n gl&iUs-sealed ~esistors . Cleveland, 1954. 6p. (Its form no. • )025) 26 . t-Jh1:bo, G. R. f.-ray attenutiOD coett'iotenta rrom 10 Kev to 100 Mev. Washin(lta, Matt~ bureau or tttandaros, 19S2 . 9.3p. (National bureau of st.tm.ards. Report no.l003)