Scintillation counter studies and the branching ratio of osmium-185

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B vm m m to tm fa cu lty or tm graduate school m p a r tia l FULFILLHBIT OF THE li^Um m ETB FOE THE D O S , DOCTOR OF PHILOSOPHY, IE THE DSPAKTKEHT OF PHYSICS, INDIaHA U R im siT Y

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ProQuest Number: 10295200

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uest ProQuest 10295200 Published by ProQuest LLC (2016). Copyright of th e Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346

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ii,

BBS IKSTRiaEHT a.

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m m a te n m ii n

B.

CBtCBITS

C.

PHOSPHORS

B.

OPERATION

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ia siB ’M ia .A s A

A.

BIAS Cl&VKS OH BETA ABC GAMMA 3PECTBA

pr o k w io h a i,

sm ce

B. PBBSE-HSIOiff ASU^FHSKMCY STUDIES FOR IQU MffiROT

ayscm>as C.

ANALYSIS ABB CONCLUSIONS 08 PROPORTIONAL COUNTING

i n . STOB? 0T BRANCHING RATIOS A.

G30MBTRT AND ISPFICISiCISS

B.

STUDIES ON Be?

C.

BRAJJCHIHG RATIO (S' Ca18?

IV. .. AOCBOWUIXHatafT V,

APPi»DJX

VI.

a iB U O M

i* S c in tilla tio n counters were used for the d etectio n o f nuclear p a r tic le s fey Butherford and Crookes in some o f th e ir ea rly in v e stig a tio n s in to rad ioactivity*

Since th e ir use r e -

quirad ted iou s v isu a l observations o f s c in t illa t io n s , the counter was discarded as nuclear physics cane o f age and b etter methods o f d etectio n ware introduced*

gtau and Dreyfus*- appear to be

th e f i r s t to bave reintroduced the s c in tilla tio n counter in i t s modernised form.

By using a photoJHultipH© r tube feeding in to

& se n sitiv e galvanometer to d etect the s c in tilla tio n s they were ab le to elim in ate most o f the tedium o f the older method. method bhm gave a measure o f the integrated a c tiv ity .

This

The f ir s t

work m in d ivid u al pulse d etection by means o f the modernised s c in t illa t ie n counter seema to have hem done independently and concurrently by Coltmao and Marshall2 a t th e Westinghouse labora­ to r ie s and by Kall&s&n^at the Kaiser Wilhelm I n s titu te .

Because

o f the many apparent advantages of the counter i t became the sub­ je c t o f in v estig a tio n and development by many researchers.

The

stu d ies described below Were begun In th is laboratory In October 1 a.

fubk@& o f tm im m M k tz m The fundamental purpose o f these stu d ies was to develop

a s c in tilla tio n counter Mid t e s t i t s u t i l it y so that i t could be used advantageously fo r nuclear in v e stig a tio n s.

Among i t s

JMB& apparent advantages n a w i - X) the high d en sity o f tt im d etector feX attye b*:

nuclear d etecto rs, advantageous for th e measure-*

n est o f ih em ere penetrating* t e a r ionising. p a r tic le a; 2} i t s r j^ C response and abort dead-tim e, lim ited only Nr the phosphor ex cita tio n decay tim es and the- response o f the photom ultiplier •

*'

'*'

■ ‘'

»* ■

"■

' v

10“^ to I0~^ see* for th e most ir e fu l phosphors* tube response r is e times le e s than 10“^ seconds) | 3)

o f the output pulse to the energy lo s t by

the rad iation to the phosphor* 4) I t s vdndowleas featu re which allow s 14 to be used in a vacuum; and $) the dependability and a v a ila b il­ i t y o f the photom ultiplier tubes and th e ir reasonable c o s t. In the .course o f th is in v e stig a tio n i t was found th at ,t

the high dao&ty-makes tim s c in t illa t io n counter advantageous fo r gamma eoanting. but th a t i t has few advantages w a r the Geiger counter a s a beta p a r tic le d etector; that i t s rapid response makes . i t . id e a l fo r fa s t counting and fa s t coincidence workj th at i t has advaBtagee a s a proportional counter but that p recise and absolu te data are d if f ic u lt to a tta in ; that i t s windowless featu re y ie ld s no ad m d agei and that, great care mast he taken with the c* th e f ir s t th ree have the same phys­ i c a l dimensions and contain the same 9 stage electro d e structure* mute they have e s s e n tia lly the same m u ltip lica tio n fa cto r fo r electro n s em itted from the f i r s t dynode, although they d iffe r in th e ir sp ectra l response*

The 1F28 tubes are se lec ted 931-A tu b es,

the c r ite r ia being prim arily lig h t s e n s it iv it y and low n o ise lev e l*

for the 931-A, the 1P21, and the u lt r a v io le t tra n s*, which was designed fo r nee in s c in t illa t io n counting, Is larger and has an ad d ition al m u ltip lica tio n stage*

I t s meet a ttr a c tiv e featu re Is i t s large

lig h t eoiX eetiag photocahhode which Is deposited on the in sid e o f th e H a t top o f the tube* r o f lad square inches

fhe p h otosen sitive surface has m area to .3 square Inches for the sm aller

tubes end i s

a larger so lid angle sin ce i t

i s located on the g la ss

I t has a maximum sp ectra l

response a t approximately 4300 Angstmas * Because o f the d ifferen ce in sp ectral response, the tube type must be selected to match as nearly m p o ssib le o f the phosphor used*

Most of the work

was carried out with the 1F2& photom ultiplier

-3 -

the excitation spectra of the phosphors availsfele* ffce s e n s itiv ity and a m p lific a tio n o f th e se phototubes In crease ra p id ly w ith an in c re a se in the v o lta g e p e r stag® , b u t coneom saitantly th e n o ise p u lses a lso in c re a se in am plitude*

It

i s n ecessary , th e re fo re , to fin d an optimum com bination o f th e v o lta g e p e r stag® on tb s p h o to sau ltip lio r and o f th e a m p lifie r g ain f o r th e ra d ia tio n measured so th a t th e tru e p u lse s w ill n o t be

m m lead by th e n o ise background * Far fl?ost b e ta and gamma ra d ia ­ tio n s i t was found th a t between 60 and 70 v o lts p er s ta g e , w ith an a m p lifie r having a g ain o f 10^ to IG^ gave a maximum "sig n a l to n o ise " ra tio *

For alp h a p a r tic le s , which give r i s e to appre­

c ia b ly more lig h t due to th e ir high s p e c ific io n is a tio n , stoat tru e p u lse s a re ap p reciab ly g re a te r than th e n o ise background p u lse s so th a t th e se co n sid era tio n s are n o t so im portant* Since th e photocathode and th e anode have a d jacen t pins on th e tu b es and so ck ets o f the H -p in tu b e s, c a re must be taken to p reven t leakage between th e se p is s of tb s 600 to 700 v o lte p o te n tia l d iffe re n c e . th e r e s is tiv e o r 00 leakage i s n o t o b jec­ tio n a b le , o f co u rse, fo r p u lse measurement, b u t corona leakage

\

can give many p u lse s which a re a p t d istin g u ish a b le from tru e s ig ­ n a l o r n o ise pulses*

I t was found th a t m illin g a s lo t in th e

socket between th ese pine elim in ated th e sp u rio u s p u lse s in a l l b u t the m ost humid weather*

th e lig h t- tig h t chamber fo r the

tube* and t h e ir so ck ets was a ls o vacuum tig h t no th a t in humid w eather o r f o r s e n s itiv e measurements th e chamber could be evac-

Udted, thus elim in atin g the spurious p u lses com pletely.

Coltman^

describ es other tech n iq u e fo r elim in ating t h is leakage toy tre a t­ ment o f the socket or tub e,

the mu 5&L9 photom ultiplier has

elim inated t h is d iffic u lty toy placing the photocathodo connection ml th e a id s of the tuba in the g la ss envelope. B* GJBWXTS C ircu it requirements are sim ilar to those for oth er nuclear eculiters such as ttoe Oeiger counter and the proportional counter.

Since th e photom ultiplier p u lses have r is e tim es o f the

order o f 10**® seconds and most o f th e phosphors used giv e lig h t pulses whose duration may be from 1Qt 7 or 10“"® for anthracene to several microseconds fo r scaae o f the inorganic phosphors^ the time constants o f th e c ir c u it must toe short and ta ilo red somewhat to the phosphor being used.

Coltman^ has developed an a n a ly sis

fo r the d erivation o f optim al c ir c u it constants in tern s o f the input tim e constant to the f i r s t am p lifier tube and i t s output time con stan t.

The c ir c u its used in th is laboratory had time

constants o f about 1 microsecond to m ich the phosphors used and to allow for fa s t coincidence counting. Figure 1 inclu des a block diagram o f the c ir c u its used; the power su p p lies are not shown, fhe pu lses could toe fed e ith e r in to the lin e a r a m p lifier, then discrim inated and in to the sc a la r , or in to the coincidence a m p lifiers, which had b u ilt in discrim ­ in a to r s, to the discrim inator s e t for e ith e r sin g le s or coincidence output, and thence to the sc a la r,

the pr© -am plifiers and lin e a r

am p lifier are sim ila r to the to s Alamos model 100 c ir c u its , the

~5~

y that M ilt in to the Htginbotham ae&l&r,

w&& the ee®iaarel&lly a v a ila b le Unclear Shemieal C o lo ra tio n *s model ZBL 161 Scale o f 128.

?bo coincidence a m p lifier was developed In th is laboratory*

It

la q u ite conventional 9 embodying som a m p lification sta g es which lead in to m biassed discriM im tor tuba feeding a m u ltivib rator C ircuit*

fba constant M igh t constant width (about 1 microsecond)

a&mvibr&bor pulse fro® each channel la fed in to a double trio d e

W m th e d lsc r isto a to r , preeeding the sc a la r , can

isixer tu b e,

be s e t to accept sin g les eeuu&s or coincidence counts, ■-;;:

M ace, as was pointed out before, the s e n s itiv ity and.

gaiil b f the p h o to m ltip lier tabes in crea ses rapidly with an in ­ crease in th e Voltage per sta g e , and sin ce the gain o f the ampli­ f ie r Is- dependent m the supply v o lta ge, both tbs negative high v a lta g e for th e pfcetessaXtipliers and th e supply voltages fo r the a i^ llf ie r s ^ e*

be ‘w e ll filte r e d and regu lated,

F a^F m s

Phosphors su ita b le for nuclear counting vary g r e a tly n- .:, ■,. ‘ v ':: V, ill th e ir s e n s itiv ity to d iffe re n t ra d ia tio n s, in th e ir lig h t « u tput decay constants, and in th eir sp ectra l ■emission c h a r a c te r istic s, tV: '• •* th e organic phosphors a l l m m to give lig h t p u lses appreciably

lam than one microsecond in duration, whereas the inorganic phosphors g iv e lig h t p u lses whose duration i s one microsecond or greater5 .

From sim ple d en sity con sid eration s, the inorganic

phosphors respond b etter to gamma rays than an equal volume o f er^pnie phosphor sin ce the p rob ab ility of stepping the gamma ray

-6 -

As greater*

However, i t i s d if f ic u lt to g en era lise ©a the e f f i c ­

iency o f phosphors fo r d iffe r e n t rad iation s sin ce o ste n sib ly the

m m phosphors may g iv e w idely m iy in g responses to m y given rad­ ia tion *

t h is i s thought to he an impurity e ffe c t* Shim types o f phosphors have been used p r in c ip a lly in

th is laboratory* the choice dependent to a great exten t on th e ir a v a ila b ility *

th ey m vm

H&ptltalene obtained in about 1 m*

cubic c r y s ta ls , prepared by Marahaw Chemical Corp.! Calcium Fluor­ id e or "Fluorite* which warn obtained both m the natural c r y sta l and as sy n th e tic , from Marsha# Chemical Corp. in the form o f a prism about 3 cm* lm% and | on. on a s id e , and both natural and syn th etic oaloln a iungst& te or “a c h e e lite 15* 111 o f th ese phos­ phors have em ission spectra ranging from blue to u ltr a -v io le t so th at the 1F2S becomes the lo g ic a l tabs to u se.

For Caleiusa

bttngabaie, th® 93X~A or the 1F21 ph oioasu ltiplisrs have s lig h tly b etter response e h a r a c te r istic s, but sin ce no lF21*s m m a v a ila b le , comparison was made only be tween the 921-A and the 1F28* fb * AP2S prov even a t the optimum d is— crizainator se ttin g such th at only large n oise pu lses weie couted. However, th is represents data taken w ith a 93X~A phototube, the

mmm i

i

2 - CHANNEL T ) '

t ■

,8 1 i?

PR E-AMPLIFIER

jra

EXIT BAFFLE

X

'

X

r

LINEAR

COINCIDENCE

AMPLIFIER

AMPLIFIER

■^r \ BAFFLE

\>i X \\1

x

LEN S COIL

DISCRIM­ INATOR

n rrn iL :~ -Trm zr* SCALER

SOURCE

DlFFUStON PUM P

A

I P 28

B

phosphor

F ig . 1 .

p h o to m u ltip lie r tu b e s (fluorite)

Geometiy for spectrom eter stu d ies and block diagram o f the c ir c u it s .

HV — To i n t e g r a t e r

A B C D E

Photomult iplier t u b e s Crystal Probe Light b a f f l e s E l e c t r o n gun

El e c t r o n gun d e t a i l

A

K -HV Diffusion Pump

Fig* 2 .

Geometiy for the electro n gun stu d ie s.

A B C D E F

F ig , 3*

Photomultiplier tubes CaW03 crystals Bakelite light shield Absorbers Lucite source holder S o urce

Geometry for the branching r a tio stu d ies.

b e st Of 9 sam ples,

liitte b e tte r tu b es, such &s the 1P21 and a few

o f the Xp2@»s sim ple p u lse height discrim ination is u su ally su ff i d tan*, fee> the tru e counts,,

nfefcja asosi o f the n o ise p u lses ana s t i l l reta in when the electro n energy i s so 1m that most

o f the p u lses are sim ilar in height to n o ise p u lse s, c o in c itechniques, w hile they may increase the sig n a l to n oise r a tio by a fa cto r o f 500, out down the counting e ffic ie n c y so L, T h is i s dm to the fa ct are o f th e m m order o f magnitude as n o ise i, the t m

pu lses seen by one tube are the r e s u lt o f a sm all and the chance a for coincidence with photons

fro® the smm w e n t seen by the second tube are consequently sm aller.

For higher energy electro n s which create a large number

o f photons the p rob ab ility for recording a coincidence i s very geod, but with such a large number o f photons, & good tube with l i t t l e n o ise w ill give a pulse much larger than the average n oise pulse m th at discrim ination can be used to elim in ate moat o f the

Another method of reducing the number o f n oise p u lse s, and at the same time in creasin g the e ffic ie n c y o f most phosphors, thus increasing sig n a l to n o ise r a tio , is that of coolin g the tube to dry ic e temperatures or lom r$*

This ha® bean done by

various researchers and was tr ie d a lso in th is laboratory*

while

the method i s good i t i s unfortunately qu ite inconvenient, espec­ i a ll y I f the detector i® to be used in & vacuum, or i f long readings are to be taken*

Furthermore, except fo r low energy electron s and

-9 -

low energy gamma

o f a good tube w ith m. adequate

diserimtn&ber c ir c u it I s u su ally q u ite s a t is fa cto ry , so th a t the

m e o f co olin g i s rceGsmadsd on ly as & la s t r e so r t. attem pts a t using th e s c in t illa t io n counter as a de­ te c to r in a magnetic Iona spectrom eter were carried o u t.

I n it ia l

09Eperiaenbs m m u n sa tisfa cto ry due to inadequate magnetic sh ield ­ ing o f the p h otoaailtiplier tu b es.

la te r experiments with m

electro n gun ind icated th at the th in window Geiger counter is s h i l l the most sa tisfa c to r y sin g le p a r tic le detector fo r spectrom­ e te r work both a t high and low energies*

Therefore th is project

was p o stp o n e , although I t is hoped that la te r use o f th is spec­ trom eter can ho med to c a lib r a te the e ffic ie n c y o f the s c in t i­ lla tio n counter for the normal range o f beta spectra* Figures 1 , 2 , and 3 show the geom etries for th e sp ecteam ster stu d ie s, the electro n gun stu d ies and the branching r a tio stm U as,

— 10—

ir n v m m ir a s a pnoromoM AL pggxcs

H.

A. BIAS a s m s 08 H lti A® OUHA SPSCTKA One o f tb s advantages of s c in t illa t io n ecm ntsrs, as mentioned b efore, i s that the pulse output o f the photomulti­ p lie r tube i s dependent on the energy of the detected eradiation. Here s p e c ific a lly , th© photon y ie ld of the phosphor i s propor­ tio n a l to the energy lo s t by the radiation to the phosphor, so th at i f the p a r tic le lo se s a l l i t s energy to the phosphor, the phwtoiaultip li« r output pu lse w ill be proportional to the energy o f the in cid en t rad iation .

Proportional counters have th is

property fo r the heavier p a r tic le s but are not su ita b le for the lig h te r p a r tic le s such as beta and gam e rays sin ce th e ir spe­ c i f i c io n iza tio n i s low and they lo se l i t t l e o f th e ir energy to the counter*

Sim® the s c in t illa t io n counter u t ilis e s a so lid

phosphor c r y sta l as the detector I t can be used for d etectin g rad iation s o f lower sp e c ific ion isation *

A lso, for average d is­

in teg ra tio n energies th e phosphor m y be chosen as to d en sity and voluse so th a t m e t or e l l o f the energy o f the rad iation i s lo s t to th® phosphor.

In view o f th ese indicated advantages, t e s t s

were conducted to determine the s u ita b ility o f th® s c in tilla tio n counter as a ^proportional counter11 for beta and g a n a ra d ia tio n s. P ulse height d istrib u tio n data were taken for the known sp ectra o f # 6 * s3 5 , # 8 5 ,

qs 137,

R # 116, T&182, and ThG« .

The

in te g ra l pulse height d istr ib u tio n s are a l l p lotted in fig u res 5 and 6 ,

Is a pure beta em itter having an end point o f .156

Mev whose spectrum may be eith er allowed or second forbidden?,

542729

-n-

1 C14 .156 Mev.

f t

2 S3 5 .l68Mev./3 3 W185 .428 Mev. /3 4 Csi37.56 Mev. 5 R e 86 1.08 Mev. f t

.6 6 Mev y (Converted) 214 Mev.y.138 Mev.y (Converted)

6 To182 .53 Mev./Q, i.22,1.13 etc. to .15 Mev.y (some lower energies converted) 7 ThC" 1.72 Mev. f t 2 . 6 2 Mev. y

8

Bias Fig. 4.

Integral bias curves for beta and gamma-rays of various energies.

has a pure beta spectrum ©f end poin t .169 Mev, which i s has a beta sp ectrm o f end point .420 Mev which to be allowed but whose f t value in d ica tes th a t U>© tra n sitio n i s second forbidden^.

Gork^ has recen tly

reported the presence of a *134 Mev gamma ray associa ted with the M^^ tr a n sitio n whereas former in v estig a tio n s have d isclo sed no ganma ra d ia tio n . .€**37 has a complex spectrum ^:

a low energy

beta group o f f% abundance with end point of .521 Mev which appears to have a f i r s t forbidden shape and which has associated w ith i t a h igh ly converted .663 Mev gamma, and a high energy beta group o f 5$ abundance with end point a t 1.2 Mm, apparently second forbidden.

has a beta spectrum of end point energy 1*0?

Mev which has an allowed shape but whose f t value would in d ica te a forbidden tr a n sitio n , and gammas o f .130 (converted) and .214 Mev^*

has apparently a sim ple, allowed beta spectrum of

end p oin t energy .525 Mev but a very complicated |» & w ith ,a t le a s t 31 d iscr ee t gamm en erg ies^ *

spectrum

ranging from 1 .2 4 ,

1*22, 1*13 Mev to a dense group o f 28 d iscreet sta te s from 328 to 46 Kv, many of which are converted,

fhc« has a beta group o f end

point l*v?2 Mev^6 and a gamma o f 2.62 Mev^*. 411 sources were measured w ith no photo electron radia­ to r , and no sp e c ia l care was taken to make very th in sources, Since, the *663 Mev Gs, the *138 Mev h e, arid many o f the fa gamma rays are in te r n a lly converted, sharp lin e s e x is t in the beta sp ectr a :©f th ese elements*

I f d etected , th ese lin e s would serve to

in d tcate t^e reso lu tio n o f the proportional nature of the a ein -

t illa t ie t u The curves In fig u res 4 , 5 , and 6 war© a l l taken with

& syn th etic Oaf^ c r y sta l m the phosphor, obtained frc© the Barafe&w Qheeiieal Corp.

the d etectors were 1F28 photom u ltip lier tubes

operated a t 60 volt© per stage feeding in to the coincidence amp­ l i f i e r s operated a t a gain o f appmdlmately 10^.

Integral bias

curves were taken with one photom ultiplier d etector alone {tensed s in g le s ) , w ith two photom ultlpliers biassed equally and w ith out­ put fed to a coincidence c ir c u it (termed coin cidence), and with two photom ultipliers in coin cidence, one se t a t minimum b ias w hile th e other was varied (termed sem i-coincidence)» Tit© sin g le s and sem i-coincidence b ia s curves hfsd the same simp®, w ithin ex­ perim ental error*

Since the sem i-eoioeictenee bias curves were

store s e lf-c o n s is te n t, due to a much sm aller background, the sin g le s method was not used for a l l the spectra*

Figure 6 shows the

shapes o f the coincidence and sem i-coincidence curve© for I t i s rea d ily ^parent from the curves that the coincidence method discrim inates against the larger p u lse s. That i s , sin ce the min­ imum number o f photons correspond!ng to the discrim inator se ttin g s on th e am p lifiers must now reach each tube sim ultaneously, eith er the energy lo s t to the phosphor must be greater to provide the r e q u isite number o f photons, or th e operation must be farth er out on the probab ility curve for a given energy.

Thus, from these

curves i t can be seen more q u a n titativ ely that the coincidence technique m a method for reducing the noise background lo se s i t s advantage sin ce for ©mail pu lses which are d if f ic u lt to separate

-13-

.0 0 0

0 .5 0 0

0 .1 0 0

0 .0 5 0

5 &6 o .o i o

D .0 0 5

0 .0 0 1 20 Fig* 5. IQS

24

Semi-logarithmic plot of

the in teg ra l bias curves shown in figu re 4*

F ig. 6.

,185

s-c

185

.005

A comparison of coincidence and "sem i-coincidence” in te g ra l bias curves fo r Inr- *.

£rm the n oise pt&ms the d etectio n p rob ab ility i s decreased. aince th e sin&Xes and sia& i-eoineiderne curves have the mm> shape th e aaw&^olneideisiea method im advantageous sin ce I t does g iv e greater sig n a l to n oise ratios* Curves 5 and 4 present the in teg ra l b ia s curves as taken by the s©mi-coincid©fiCQ method on both lin ear and logar­ ithm ic p lo ts*

I t i s evid en t from the curves that q u a lita tiv e

energy determ inations may he deduced for pure beta spectra by scans o f the s c in t illa t io n counter.

However, r esu lts are Incon­

c lu siv e for combined beta and gaw a spectra, a s can be seen from the overlapping ghaniun and Tantalus curves, sin ce the gm m rays can be detected d ir e c tly as gaama rays by the phosphor, as e l l as by th e ir conversion electron spectra*

Absorption techniques

would be required to separate the pur® gsmaa spectra from th e beta plus conversion electron spectra*

The resolu tion of the

instrument was disappointing sin ce the conversion electrons from the Gssiimt, Rhenium, and Tantalum isotop es gave no p o sitiv e in d i­ cations*

A fter attem pts a t more p recise analyses o f the curves

had been made with l i t t l e su ccess, i t was concluded that more res­ o lu tio n would be required so th a t ca lib ra tio n s by means o f sharp in te rn a l conversion lin e s could be determined, as w ell as by monoen ergetic accelerated ele ctr o n s. Since th is tim e, B e ll and h is colleagues a t Oak Ridge have published some ©xeellezit work with remarkable resolu tion i^ > i71$ They employed large anthracene c ry sta ls with the new $819 photo-

-14-

o f tl& anthracene c r y sta ls and of the photom ultiplier d etecto r, coupled with the mm o f a channel discrim inator which allowed d iffe r e n tia l b ias curves to he taken accurately over sm all In ter v a ls, com bing to give a resolu tion in energy o f approxiasatoly $0$ fo r the 312 Kev Ijv^'* conversion lin e ; i . e . , the width a t faal f-maximum i s approximate ly 130 &ev. The Kurie p lo t o f th eir data on

in d ica tes a confirm ation of i t s forbidden shape, a l­

though in s u ffic ie n t data Um bean published on th e ir source thick­ ness and th e ir no na&lination and ca lib ra tio n procedures.

Indica­

tio n s are that the B c in tilla tia n counter may be a valuable to o l fo r the measurements o f radiations which are not su ffic ie n tly in ten se to measure in a h i$ i transm ission, low resolu tion magnetic

a.

w f i s m s i m n .rag sH m o t? m stio m m m studies

fo e

mm-

w m m M M M m m m m m 600 to $6gg v o lts Studies on low energy electrons were conducted to de­ termine the advantages and disadvantages o f the s c in tilla tio n cou n ter, as a windowlea s counter, for the simple d etection as w ell as the energy deieysainstion o f electro n s.

The Geiger counter,

which I s a very e ffic ie n t d etector of electro n s, has the disad­ vantage th at the windows, although made painstakingly th in , stop the very low energy beta rays before they can reach the counting region .

The proportional counter which cm give a measure o f the

energy of, the electron has th is mm disadvantage and in addition

i s much le e s e ff ic ie n t than the Geiger ©abater, S ince many o f th e phosphors may be operated in a vacuum, which can a lso con­ ta in tb® beta ray source, almost a l l o f th e energy o f the e le c ­ trons could be d issip a ted in the phosphor,

k proportionate number

o f photons would be en itted which could then be counted by the photom ultiplier tube. to t e s t the e ffic ie n c y of th is process and to find the dependence of p u lse-b sig h i on electron energy, the apparatus shown in figu re 2 was prepared.

The electron gun was a 5BF1

cathode ray tube whose cathode and filam ent were operated a t various negative p o te n tia ls from taps on the bleeder o f a nega­ tiv e high voltage supply» the beam was p a r tia lly fo ce^ d by means o f a battery biassed potentiom eter control on the f ir s t g rid . The second grid was operated from on® of tbs taps on the high voltage b leed er, and the p late was operated a t ground p o te n tia l, til© filam ent m s operated a t about h a lf it® rated v o lta g e,

right

b a ffle s were introduced between the electron gun and the photo­ m u ltip lier tube to prevent th e lig h t of the electron gun filam ent from reaching the photom ultiplier tubes. beam an electrom eter grid of about ^ust in front of the phosphor c r y sta l.

To monitor the electron

transm ission was placed

A guard rin g , operated

a t the -tGO v o lt p o ten tia l supplied to the phototubes, was placed to fro n t of the electrom eter grid to prevent secondary electron em ission from the g r id .

Since the electron beam was operated

between H T ^ and IQ-**2 amperes am fluctuated during given read­ in gs by fa cto rs ©f about 2 , an in tegratin g electrom eter was b u ilt

-16-

Counts

per

i nci dent

Electron

>

20

30

40

Pulse Height (volts x I0 10 53)) F ig . 7 .

The pulse height d istrib u tio n s from a calcium tungstate phosphor. Curves A, . B, C, D, E and F are for electron energies of 5600, 4300, 3200, 2000, 1200 and 700 v o lts resp ectiv ely .

50

150

250

350

microvolts F ig . Q.

The pulse height d istrib u tio n s from a calcium fluorid e phosphor.

per

Incident E lectron

r4

Counts

o/

2

i

I

i

i

3

4

5

6

7

Electron Energy - kev Fig* 9*

E fficien cy curves for pulses equal to or greater than 120 micro­ v o lts and 200 m icrovolts. Curves a , a 1 are for calcium tungstate and b , b 1 are for calcium flu o r id e.

t© cover th is cur ren t region*

I t was eoiw ention&l in d esign , em­

p loyin g a 959 acorn tub© as the electrom eter tub s, operated in the P » r recommended by » W

9 , which triggered a 2050 Thyratron.

A f a s t r ela y in th e p la te c ir c u it of the thyratron reset the input condenser and triggered a magnetic mechanical counter*

The in te ­

grator behaved qu ite lin e a r ly and c o n siste n tly , except in the most humid weather* P ulse height distribution® and e ffic ie n c y curves were taken on natural p olish ed calcium tungstate crystal® from Kern County, C a lifo rn ia , and on syn th etic calcium flu orid e c r y sta ls obtained from the Harshaw Chemical Go* These were the only good phosphors a v a ila b le to us idiieh could be operated in the required vacuum* Their photon output was detected by an EGA 1P28 photom u ltip lie r tube feeding in to the p re-am p lifier, lin ea r am p lifier combination shown in the block diagram in fig u re 1* are p lo tted in fig u res ? , 0 , and 9 .

The r e su lts

Figure 7 shows the pulse height

d istr ib u tio n s from the calcium tungstate phosphor for various en­ e rg ies o f accelerated electro n s s the ordinate i s in count® per in cid en t electron^ the ab eissa represent® the photom ultiplier out­ put p u lse h eig h t, operated at 60 v o lts per sta g e , in m icrovolts* Curves A, B, C, I), E and F are fo r electro n energies o f 5600, 4300, 3200, 2000, 1200, and 700 v o lt s , resp ectively*

Figure 8 i s a sim­

ila r curve fo r calcium flu o rid e * Figure 9 shows the v ariation in e ffic ie n c y w ith electro n energy for calcium tungsi& te and calcium flu orid e*

Two s e ts of curves are shown, one (a , b) representing

A ll p u lses equal to or greater than 133 m icrovolts, the oth er> (a* b 1)

-17-

sim ila r ly J fe r 200 m icro v o lts. d&uhed

fo r

The s o lid carves are fo r Ca&O^, the

Again, th e ordinate is p lo tted In counts

per in cid en t ele ctr o n ; th e abcisaa i s p lo tted in k ilo v o lts . Due to errors in th e absolute ca lib ra tio n o f the e le c ­ trom eter and tfee electrom eter g rid transm ission, in the s o lid ®dgl@ gecaaotry between the c r y sta l and the ph otom ultiplier, and ia the s e n s itiv ity o f the photom ultiplier for tfe© p a rticu la r em ission sp ectra o f th e c r y s ta ls , an absolute value o f the quantum e ffic ie n c y o f tfe® c r y sta ls i s d if f ic u lt to a ssig n .

I t i s tfe® com­

bined d etecto r e ffic ie n c y which i s p lotted as ord in ate.

This

e ffic ie n c y is probably accurate w ithin 50$; the r e la tiv e e f f ic ­ ie n c ie s and d istr ib u tio n data are probably accurate w ithin 2D$ or le ss* Assuming a s o lid angle o f 10$ and a production in the pfeotoM ultipli© r o f 1 electron fo r every 10 photons, we css make a rough ca lc u la tio n o f the e ffic ie n c y in transform ation o f electro n energy to photon energy.

Hie a m p lificatio n o f the 1P28, operated

a t 60 v o lts per stag© w ill be approximately 10^ according to the m anufacturer.

I f we asstwm a d istrib u ted input capacity to tfe®

pream p lifier ©f 10 M icm -m icrofarads we f in i that an electron em itted from the photo cathode w ill give r is e to a 160 m icrovolt p u lse .

Thus, 100 phoiona from the phosphor would g iv e r is e to a

160 m icrovolt p u lse .

The curves show that the p ro b a b ility for a

56$0 v o lt ele ctr o n givin g r is e to & 160 m icrovolt or greater pulse i s 2xicH *.

Thus the p ro b a b ility fo r e ffic ie n c ie s o f &% or b etter

-1 0 -

m th er markedly w ith the 10% average e ffic ie n c y quoted by B elli® in h ie summary o f the- recent s e in i 1 11s t i on counter confer once a t

%to* Oak Eidge n ation al laboratory.

However* in s u ffic ie n t in fo r­

m ation i® a va ila b le to determ ine i f the r e su lts should be compar­ a b le . The shapes o f the puls© height d istr ib u tio n curves seem t© b© b est approximated by str a ig h t lin e s on a logarithm ic p lo t. U t i l e sig n ific a n c e i s ascrib ed to th e d ev ia tio n from lin e a r ity in the sm all pulse, height region which may have been due to in stru ­ m ental errors* both in ca lib ra tio n and detection*

However, the

str a ig h t lin e logarithm ic in teg ra l p lo t in d ic a tes a stra ig h t lin e logarithm ic d iffe r e n tia l p lot a s w ell* so that the data in d ica tes th a t th e pulse h eigh t d istr ib u tio n must be exponential in th is low energy region*

S in ce a l l th e curves have alm ost id e n tic a l

slo p es i t appears d if f ic u lt , i f not im possible, to use the scin ­ t i ll a t i o n counter fo r low energy indication*

Thus, an in te n se ,

lew energy source might ex a ctly overlap a weak, high energy source, so th a t a knowledge o f source strength would, be required before any energy doterraination could b© made* C.

C0$G1BSIQHS the s c in t illa t io n counter i s a t b e st a low reso lu tion

proportional counter fo r beta and ganaa rays*. u t i l it y fo r very low energy b eta—ray work t

1

I t baa l i t t l e

i t i s inadequate for

energy datera& m tions and has a poor d etectio n e ffic ie n c y .

It

should be u sefu l prim arily fo r rough energy determ inations o f

m ek eo a rces, fo r the d etectio n o f is&Basa rays, and for fa s t c o s t in g and coin cidence techniques*

“*■20—

H i*

f i i sm n : o f nmMamm k atios

S ince th© s c i n t i l l a t i o n counter is the most e f f ic ie n t , convenimtJLy o p erated gamma counter a v a ila b le , and sin ce i t has rapid response and l i t t l e dead time

thus i s em inently s u it­

a b le fo r coincidence work, i t should prove advantageous fo r branch­ ing r a ti# stu d ies* By branching r a tio i® meant the rati© of the probabil­ i t i e s o f the various modes o f decay open to a rad ioactive isotope* S p e c if ic a lly , i f the rad ioactive iso to p e I has open to i t the two modes o f decay k and B, the rati© o f the p ro b ab ility o f decay fey made A to th e t o t a l decay p ro b a b ility is c a lle a the branching r a t i o fo r A; sim ila r ly fo r B* As an example,

i s known t©

decay fey I capture t© R©^-* but in the decay the Rhenium can be l e f t c i t h e r in th e ground sta te o r in an ex cited sta te with the JA 51 subsequent em ission o f a .75 Mav gamma ray* 5 Thus the nusibar o f .75 H«v gamsa ray s p e r K cap tu re occurrence w ill fee th e branch­ in g r a t i o fo r th e e x c ite d sta te *

Mow & capture cannot be d e te c te d

p e r ge sin c e i t may fe® considered a® the capture o f one o f th e K e le c tro n s fey the nucleus Z and th e concurrent em ission o f a neu­ trin o *

Subsequently th e vacancy In th© K shell, of what i s now th e

g-1 iso to p e i s f i l l e d , giving r i s e to & K x -ra y c h a r a c te r is tic o f th e elem ent Z~l*

The occurrence i s d e te cted fey means o f measure-

m eats o f the Z ~l, K x-ray*

The theory fo r K capture processes

■f i r s t form ulated fey Yukawa and Sak&ts*^ and i s summarised in K oaopinski1© b e ta decay review a r t i c l e . 2^

-21—

f©.-measure the femnchiog rail© o f such & capture e le ­ ments in which the product nucleus may be l e f t e ith e r in the •KCiVtfat ■gmsmwl ©tat® ©fee*, one must be *.&L» to d e te c t both the % x-ray «a* th e :p n & ray, hut i t i s necessary to know the e ffic ie n c y © sly o f the gamma ray d e te c tio n , as m s h a ll se e .

I f we assume

H d isin teg r a tio n s per second, the x-ray counter w ill d etect

/V (uj£.) k

x-rad ia tio n g per second, where cj* i s the so lid angle subtended to th© counter by th® source and tio n fo r the x-ray* »A/ 0> £) r o f the p m

i s th© e ffic ie n c y o f detec­

S im ilarly th® gamma ray countar w ill d etect

gamma rays per second where oo-^is the so lid angle o untor and £-3- I t s e ffic ie n c y fo r the gamma ray*

S u ita b le c o r r e c tio n must fee made, o f course, for x-rays counted fey the gamma counter and g&iassa rays counted by the x-ray counter, as w ill fee seen in the a n a ly sis o f the d ata.

Mow i f n i s the

branching r a tio , the number o f tru e x-gamm coincidences observed I s th® ©ousters w ill fee

fl/n C ^ O x C ^ ^ j r

® Thus i f we measure

th e number o f tru e coincidences A and the number o f x-rays S in equal tim e in te r v a ls , and i f we know th© so lid -a n g le -e ffic ie n c y fa c to r dd£ fo r the gamma m y we can find the branching r a tio n, aims®

JL 3

=

^ ,m y

/V *

/V (u ;e .)x

A * s

~ B & t\

1fcUt axperiwwat w ill therefore require a calib ra ted gaaaaa ray counter*

- 2a-

&

m m m m m > w e w ic sm m Figure 3 shews the geometry used in th e branching r a tio The separation 'dotween source and c r y sta ls was kept to

ft M niw sa, lea v in g only s u ffic ie n t spas® fo r the in terp o sitio n o f absorbers.

The so lib -a n g lea between the source and phosphors are

sfamit p i sto ra d ia n s, but du© to the high index o f refra ctio n o f calcium tu n gstate c r y sta ls ( c r it ic a l angle equals 31°) the a ctu al s o lid angle between tbs c r y sta l and the p h otosen sitive surface o f about p i siera& tans y ie ld s an e ffe c tiv e s o lid angle o f only about i pi*

Thus the t o ta l so lid angle for d etectio n i s about

l/B p i sfeer&di&m or 1/35 o f u n it so lid angle*

Fortunately i t

i s not necessary to know the exact so lid angles or e ffic ie n c ie s in d iv id u a lly ; i t i s necessary only to know th e ir product for the gamma counter, as we sew before * This can be measured experiment­ a lly fey means o f known energy gm m rays whose coincidence proba­ b ilit y per beta or gmmm from the same nuclear event i s known. I f a cascade mode o f decay i s th e only p o s s ib ility , the coincidence p ro b a b ility w ill be u n ity . Two such iso to p es which are w ell su ited for ca lib ra tio n work are

mid C©^®«

Au*^^ i s known to decay by m issio n o f

ft 0*37 m r beta group to Mg 138, which omits a 0*41 Mev gamma ray w ith a h a lf - lif e o f 2*3xKT^ secon d s.2^

Some in v e stig a -

I s r s have reported a d d itio n a l gamma- ra y s, but accurate work and ^ in v e s tig a tio n s by others have su bstantiated the shove decay scheme,

Since the i*@so lvin g tim e o f our coincidence c ir c u it i s

-2 3 -

■31 m icroseconds , tfe© c ir c u it w ill see the beta—and gamma—r&y as e©ineid«m t, which i s necessary fo r our experim ent„ In th is ca se, sin ce we know the branching r a tio to be u n ity , by an a n a ly sis «&atX&r to th a t d iscu ssed above, we can fin d

(u> &) r

, the

s o lid angle e ffic ie n c y fa cto r fo r the gamma ray, by measuring the r a tio o f th e tru e coincidence counts to the true beta~ray counts« The method I s as fo llo w s; For sim p lic ity o f d iscu ssio n w© sh a ll c a ll the gamma counter which we wish to c a lib r a te , channel I I , or sim ply II; the oth er beta or x-ray counter we s h a ll c a ll channel I , or simply I . In channel I we measure the background and two sin g les counting r a te s, the f ir s t with no absorber between the source ami phosphor, th e second w ith an aluminum absorber o f thickness 0*164 cm, or 443 ®g/ r , t & f f i r f e r f '? v 1 K f< i &)/S

.M r . . ,7.7 . 5.24 x 00-3 N/0 6 .9 6 x 1530 ' p

Thus tb s so lid -a a g le , e ffic ie n c y fa cto r fo r the gamma counter for a 0*41 Mer g m a ray i s 5»24xX0“3* Ocr** decays by Isom eric tr a n sitio n , one isomer with a h a lf - lif e o f 10,7 m inutes, th e other with a h a lf - lif e of 5*3 years* W9

concern ou rselves only with the la tte r isomer*

Its

decay

scheme i s w e ll known and corroborated by many workers, c o n sist­ in g o f a 0*31 Hev beta group and cascade gamma o f 1*3 and 1*1 Mev, a i l th ree in co in cid en ce* ^

As w ith A u*^, by firs&iag the r a tio

o f tru e bets-gasama coincidence counts to beta counts we can find tw ice the so lid -e n g le , e ffic ie n c y fa cto r fo r the average gamma ray energy o f 1 .2 Mev, sin c e in th is

case the branching r a tio for the

1*2 Mev gamtaa must be considered to be 2* Again, in channel 1 we measure the background and two sin g le s counting r a te s , t t e one w ith no absorber and the o tte r w ith & 0.049 cm* or 132 mg*/cm^ aluminum absorber*

This absorber

i s s u ffic ie n t to atop a l l the 0*31 Hev beta rays, but a t the same

-25-

tim e a tten u a tes the 1*2 Mm gamma by 0*04*

A 540 J»g/W2 lead ab­

sorber ia ia tr o d u e e d between the source and channel H which is s u f f ic ie n t to step a l l the beta ray® w hile atten u atin g the 1*2 Mev gemma ray by 4$*

The single® counting ra te in charnel 11 la meas­

ured again for the purpose at p red ictin g the number o f random ©r chance co in cid en ces.

Coincidence counting rates w ith and without

the absorber in channel I then g ive us s u ffic ie n t inform ation to c a lc u la te the so lld -a n g le , e ffic ie n c y factor for the 1*2 Mev average gasman ray* Sample data and ca lcu la tio n s A ll data in count® per minute Channel 1

Channel IX

SbkgS I 55%

* • XT

B0 j

31,552

B* i

24,107

Mow; Channel 1

M/3 $ l y

8752

♦ %&gd X -

0*992 Coincidence

Coincidence

I

OP

0.96H ^r =

^&c

91*4

which g iv es:

7,302

4- %fcgd i $eb = $©c

0.992 X 0 , 9 i r r f Mbkgd * 0*% Mp-r «

133*1

i s% c

0 . 9 ^ ^ f 0.9&H y

so

^©c

~ %a *"

***

x 0*96!^ r ~ A

Moe_ Noa ~ *°°8 x Q .96B -rv 2 t (NoI - SBi)S ai i

B 133.1 - 91.4 - 0 .6 - 4 .4 a 36.7

36.7 so (w£) y S 2x0.96x7300 * 2.61 x 10-3

“26»

f in a lly th e so lid -a n g le , e ffic ie n c y fa cto r becomes 2,61x10”!

ta t 1*2 Her gamma rays*

Summarising, the e ffic ie n c y fo r 0 .4 1

Her gamma rays i s 5*24x10-3j th® e ffic ie n c y fo r th e 1 .2 Kev gaisma r^y i s 2*61x10“! » gy a lin e a r in terp o la tio n wo find the e ffic ie n c y fo r a 0*75 Mev gamma ray to be 4»12x10“! .

This e ffic ie n c y w ill

bs required f or the measurement o f the bran chii'^ ra tio in Os^^e I t m y be in te r e stin g to compare the ra tio o f th ese e f f ic ie n c ie s w ith th e p re d ic te d rati© for energy. lo s s o f th e g e n rays in the eals& as bungsi&te c r y s t a l'as determined by the to ta l absorption cro ss sectio n s*

~m find from the Handbook of Chemistry

and P o s ie s th at the ss&as absorption c o e ffic ie n ts for calcium o q tu p gstate a t 0*03 A or 0*41 Mm i s approximately 0 .3 and a t 0 .0 1 A or 1 .2 Hoy, approxim ately 0 .0 6 4 .

Thus sin ce the d en sity o f c a l­

cium tu a g sta te i s 6*06 gm/ea*!, the lin ea r absorption c o e ffic ie n ts are approxim ately 1*02 and 0*330 tiv e ly *

fo r 0*41 and 1 .2 Hev respec­

The th ick n ess of our c r y sta l was about 0*75 cm so that

we might expect th e r a tio o f e ffic ie n c ie s to be (l-e ^ * !^ )/(l-e ^ * ^ 9 ) or 0 . 743/O .252 which i s approximately 3 . e ff ic ie n c ie s was 5*24/2,61 or 2 .

The measured r a tio o f the

Mow for the 0*41

gamma rsy ,

the p h o to electric cro ss se c tio n is somewhat larger than the Compton sca tterin g crass se c tio n 5 fo r the 1*2 Mev gamma ray the Compton cro ss se c tio n i s the la r g e r , the pair production cross sectio n S t i l l being n eg lig ib le*

The Compton electro n s w ill have maximum

en erg ies o f about 0.25 and 1*0 Hev for the 0.41 and 1*2 Kev gammas r e sp e c tiv e ly , w hile th e ir photo electron s w ill have energies o f 0*34

sad 1*1 Metf. Thus i t appears th at th® higher energy electro n s

tomm b e tte r photon soaversism energy e ffic ie n c ie s than the low er, m

tafcal energy lo s e in the phosphors cannot fee the c r it e r ­

io n fa r d etermi n ing the r e la tiv e e ffic ie n c ie s fo r d iffe r e n t energy gamma rays*

B ather, the e ffic ie n c y in creases id th the energy a t

which t&e energy i s absorbed* fu

M ttK g iM ? The branching r a tio o f the decay by £ capture o f Be? i s

of in te r e s t b sessso i t may g iv e im fom ation o s the sp in sta te o f tb s e x c ite d Xdt? nu cleus. 8^ £»,< ^ )L i? rea c tio n .

th is i s o f in te r e s t because o f the A lso, assuming & sp in s t a t e , the branch­

in g r a tio »cy g iv e iM ons& tion as to the proper beta-decay se le c ­ tio n rules*

Busing tbe course o f our in v e stig a tio n , Williamson

and Eiefeards2^ have deianaiBed the branching r a tio o f Be? to be 0*107 £ 0.02*

measured the masher o f Be? atoms produced from

th e ti?(p*»)B «? rea ctio n by cour&ing neutrons and subsequently measured th e mm&m o f 0.476 Mev gammas from these atoms* A d ie -

ew m im of the sp in s ta te s of the excited H ? nucleus i s included in the paper* Xt mas hoped to measure the branching ra tio d ir e c tly by th e e sin c id sfies method* th e d if f ic u lt y being in the d etection o f

ttw U 7 ehiuractw lsU e X -ray.

It* energy Is « l « l 70 v o lts , since

ttM io n isa tio n p o te n tia l o f 1A**I» 75 W its .

In »iew of « * *»»»*

a ffic iw a y stu d ies i t eppMtred impossible to d etect the x-ray by

-*8-

lower tha» 3000 angstroms but l i t t l e additional in fo im tio n i s given for th e ir response a t very short y®m length u ltra -v io le t r a d ia tio n s. was prepared by th e Li^(4,»}Be^ rea ctio n , using accelerated dsuterom* * The Be? from the Lithium ta rg et iy Mias bmt&nmn according; to the method outlined in th e armeadiit-

T ests were than conducted to detomln© i f th

be d etected .

I f th is 17§ ^3 th e astray absorption law / ^ = c A which i s dou btfu l, the lin ea r f*1! 110

foj* a ir a t atmospheric pressure would be

Thus w ith a source-phosphor separation o f 5 a®* the

o f 1 micron o f mercury should give the ra te o f d etection

ae&eurud m& again bo detectable «n*©P*

,W vv«.,

To laaheih th e r e fle c tiv ity o f tfe® interposed paper, the source had bean atotmted on a sh eet

of eisiilar carbon paper. coincidence rate was measured*

It

than the calcu lated chance r a te , that it

e a s ily have been due to tta& tosiag*

ijsilu m fey

two methods to d e test the !£. could n et he per~ * If the expertiseat cm he

#m s It appear® that a

would he required with a resonance region*

o f attack weald fee to search ffcr

haps the meet fluorescent Lithium salt 0* m

Umm mch sure kaoiei* Per-

such a resonance*

tAfio or 0 #**^ 05 was prepared by means of the

reaction

>g cyclotron deuterone» The Qmixm fey M ae Lantenaan m described In o f measuring the branching r a tio was s im ila r to

deeaya to e ith e r m excited, sta te or the ground s ta te a t fio^® by X capture*

fhe radiation s f i t t e d w ill fee the

0*75 Hev g&ssaa from the excited BsM5 nucleus arid the K x~r®ye I t a s Bheniuau The p r in c ip a l XHmty w ill he the 60 le v

ray

but higher energy L* K* ®t«. lin e s w ill fee em itted ranging in energy up fee th e c r it ic a l absorption energy fo r the X electro n o f 71 Xev, toicfe corresponds t© © free electro n dropping in to the X s h a ll vacancy*

Me s h a ll cheese 62 Xev a s the average x-*ray

energy, noting th a t ©nr r e su lt w ill net fee very se n sitiv e to an error in t h is asslgnsent*

Since we use a %0 m g/crf Xsad absorber

fa r the d ifferen ce measurement the attenuation of the 62 Km x-ray w ill fee 99$* The range o f energies which hh© I x-rays nay have i s s t i l l le s s than S i Xev c r it ic a l X io n isa tio n p o ten tia l of lead and g rea ter than th e 16 le v maximum L io n isa tio n p o te n tia l, so th a t we expect no sharp d isc o n tin u itie s in the absorption curve* The atten u a tio n fo r the maximum 71 Ifev x -ra y , o f which there w ill fee few , I s % $, and as w ill fee seen from the ca lcu la tio n s below p fttsfe am erro r weubi fee & n e g lig ib le contribution to the fin a l

th e procedure fo r find in g toe branching r a tio i s as follow s; The background and two sin g le s counting rates in channel X are d etem iu ed , w ith and without absorber*

fh e ©ingles count

lea d absorber peraanently mounted in

attenuates toe 0.75 Mev are taken with aM th e absorber in

I.

t a o l 1 % II

X

9*2

2 I » 6* 0/< 4* H-r 4* 0fekgEl I —

Channel 1

which g iv es UK = 23,832

IMXL 1 K f 0*i 0 * i m rK f 0 * 0 1 ^ 4 %kgd 4 «^n » ^ 0 * 0 1 ^ .8 7 7 1 ^ 4 0 .0 X ^ .8 7 7 i^ 4 %kgs£ 4 hf,eh * &©« but I VK -

i*«* sine© toe geometries sad e ff ic ­

ie n c ie s are alm ost equal the number o f qgItjc idenees by a gamma in cimmml. I and a I x-ray In channel I I should equal the number o f coincidences w ith a gaiaiM in H and m x-ray in I* Thus;

0*887^v/< 4 ^bkgd 4 @*02i0S*OTr ^

or

# ^gg

%kgd 4 ^*eh - % g

Ot0l0Hri