A study of radioactive disintegration by means of absorption and coincidence counting methods

Citation preview

a

M

m m or habxoactot BBXMORATiaii

OF ABSORPTION AND 00INCIDENCE COUNTING KOT0M

BY

Y. OtJENKY

Indiana .university LIBRARIES BLOOMINCTON

SUBHXTTlSD TO THE FACULTY OF W GRADUATE SCHOOL III PARTIAL Fm ilU M T OF THE EEviUffiEMSITS FOE TtE DSORES, DOCTOR OF PHILOSOPHY, III THK DEFAHTIENT OF PHYSICS* INDIANA UmMSITT 0-u l y , iS*fS ,p f/J * (

ProQuest Number: 10295197

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J 6 - f > -■ **?

c m

its

Pag® Z« II*

In to n a tio n

1

The Xteter&in&tien o f Beta and t a r n Eagr Bnergiee bj Absorption

A* The tedlsjw ^norgjr of anElection Spsctmm Bo The Mmlmm ISmrgj of aCkwa H I*

14

m im M m m s

14

B* Beta-gaffim eoinaidsrnms

17

ttw Beeults of Studies of Particular Bisirv~ tegm ilen A* Scandium 46

ax

Bo

Gallium 7a

23

G.

Gold 198

25

D* Bubidiua 86

a?

B*

to tiiio n j 124

31

F.

Khodlm 106

3?

Go Antimony 125 Ho

7*

4

Speetnm IQ

08sa88$a*"^as@$a and Bata^wsa Q vim M m om A*

17*

3

Praseodymium 24a

40 47

ApjHrndi* A* the Coincidence Circuit

51

Bo

36

S a w Bay Counting S e n sitiv ity

Hftferencfcea

61

x.

iM tm im m m

Special counting techniques fre q u e n tly prove miceesssf u l m mi a id In solving th e laqm rtant problem of determ ining d is in te g ra tio n scheme® f o r th e ra d io a c tiv e nuclei*

In most

o f such dislr& egrutiom i p a r t i c l e s , e ith e r p o s itiv e o r negative e le c tro n s , a re em itted by Mm d ts in te g ra tin g nucleus alm ost sim ultaneously w ith one or no re quanta of ra d ia tio n *

Much

inform ation cun be obtained from t m counters arm ig o d %dth s u ita b le s i r c u lts m t h a t r a te s of counting th ese " c o in c id e n ta l1* events can be determ ined.

Such an arrangement pm video a

powerful to o l fo r obtaining d ir e c t inform ation about modes o f d ie in te g ra tio n * The f i r s t experiments o f th is kind, c a lle d coincidence counting, were done by Bayer and v» Both© (H I).

Much oub©e~

quant work was done by o t t e r groups, notably Marling (M l), M itc h e ll, Lsmger, and McDaniel (Ml), Deutsoh and l& ilo t (mainly in d i r e c t conjunction w ith a magnetic len s spectrom eter) (0 1 ), Handevlllo and Soterb (M2), and #tedm\’m ok m d Chu (w l)« M itch ell (K3) has published a survey of the r e s u lts obtained through coincidence counting p rio r to 1948* The c o n stru ctio n of a d is in te g r a tio n scheme n a tu ra lly re q u ire s m a ccu rate measurement of the energy of t t e p a r tic le s and ra d ia tio n which a re emitted*

Often p re c ise energy nie&sure-

meats alone are s u f f ic ie n t to y ie ld the scheme by te s tin g the

possible combinatlona o f the energies involved.

In m a rly a l l

in stan ces i t is asost p ra c tic a l to make energy measurement* with mxm typo of magnetic an aly se r.

Beta m y energies are

measured d ir e c tly i n a magnetic spectrometer | gm m ray ener­ g ies a re in fe rre d from a measurement of the energy of the secondary e le ctro n s produced by them in s&m ta rg e t m ateria l o r by th e process o f interrial conversion.

In general the

measurement of gamma m y energies of the magnitude o f those associated with nuclear d isin te g ra tio n s by c ry s ta l spectres*s t a r techniques (02) i s not p r a c tic a l because of the excep­ tio n a lly strong sources which are required, A le s s accu rate, although mom convenient means o f energy Measurement i® th a t of d etem ining the m in im i amount o f absorbing is a te ria l which Is required to prevent electro n s ( s i t t e r beta rays or secondary electro n s produced by gamma rays) from entering: a counter * L&pirlcol relalleniihips e x is t which connect p a rtic le energy with p a rtic le range in a given m a te ria l.

A d e ta ile d discussion of energy date rain* tio n by

absorption v d ll te given in Chapter XI, In th e p resent s c r ie s of liw estim ations e p e c tro a stric energy sseasurcesnte were aval lab ia fo r most o f the nuclei which were stu d ied .

Thus the absorption measurements of energy

were n early always tre a te d as confirm atory and re lia n c e placed on t h e i r accuracy i s to be secondary to th a t placed on the epeobrone t r i e deternin& ti om •

n .

't m nwmm nhmoM of beta

mb

um h

my

ehkroxbs by ad3orptxqk Whan accuracy o f measurement Is the primary oonaid* © ration, Magnetic an aly sis I® to be g re a tly p referred aver o th er method a for th© determ ination of p a r t i ela and quantum energies*

A serious drawback i s encountered, however:

The

resolving power o f a magnetic spectre*; to r depends u ltim a te ly on source stre n g th .

In such a spectrom eter, e le ctro n (e ith e r

beta raye or secondary electrons produced by grata rays) Monentan i s the d ir e c tly mmxirwd quantity and i» proportional to the product M/> of a const m b magnetic fie ld and the radius o f Mi® e le c tr o n ^ tra je c to ry i n th a t field*

In taking a mag­

n e tic i$eetrott«»tria measurement one must th en count th e nw her o f electrons which l i e in a JsMenium In te rv a l whore dp

W/» to \ \ ( p t d p )

la roughly the am o f ilia source width and the

width o f a defining s l i t placed near the counting device. High reso lu tio n of th& instrum ent i s achieved, only i f the q uantity

M &p / H p

1® made astall, and in order to

obtain reasonably high eeuntiqg ra te s when-this condition Is s a tis f ie d a source o f high a c tiv ity is required* A fu rth e r, although lee® frequently encountered d is ­ advantage o f the magnetic speotrcw eter is the r e la tiv e ly larg e amount of time required to obtain a precise energy measurement. This place® a serio u s lower lim it on the decay period; o f iho

rad io activ e specie* which can be .ftiudicd without th e n ecessity o f raking repeated bef&artiments to produce freeh 3 « r e s s , Absorption methods for making e n o w measurements a lso have these two lim ita tio n s placed on thorn, bat sources with much lower sp e cific a c tiv ity end with much sh o rte r decay periods can be conveniently studied by .absorption methods than by the m o o f a magnetic spectrometer*

tier© Mm sp e c ific ac­

t i v i t y lim ita tio n i s encountered only when the thickness o f th e source reach es. a eiastale fra c tio n of the range o f the most energetic electro n s present in a beta ray source £ fo r in v e s ti­ gation o f gamma ray energies by absorption method® the lower lim it cm. flie sp e cific a c tiv ity o f a source Is governed by i t s physical e ls e .

For rap id ly decaying sources, a rsaeonebly

successful absorption measurement of energy can be made in a few m inutes9 tim e.

In order to gain these advantages over

spcotrcm stric measurements, however, two s a c rific e s must be made:

1. Loss of accuracy, and 2. In a b ility to separate the

components of complex spectra (under fkvcrab.1® conditions the groups which comprise a complex beta ray spectrum can be re­ solved by the up p lic a tio n of sp ecial methods of data a n a ly sis; these methods w ill b© discussed in the section to follow ). A.

m

MAXIMUM ttWQY OF AM BUCTBQM SFafiTtHJH.

Th® fo llo w n g procedure i s employed to o b tain an ab­ sorption curve fo r the ele ctro n d is trib u tio n from a radio­

a ctiv e source: The source to bo studied 1® placed a short d istan ce from a Gelfer«Mulier counter, preferably om wldch present© vdiy l i t t l e absorbing m aterial fo r tit© e le c t m is to tra v e rse in order to bo counted**,

D ifferent thicknesses o f absorbing

ate©ts arc placed between th e source a te th e counter, and tbe counting ra te la measured fo r each th ick n ess.

A p lo t of the

aountlng ra te m a f a c t i o n o f absorber thickness then show* •that fra c tio n of the i n i t i a l ele ctro n d is trib u tio n ( I . e . , the d is trib u tio n fo r no absorber) which survives the p asture through & p a rtic u la r absorber th ic k n e ss. The absorption of electron® in u medium lu accomplished by two competing mechanism®:

X» Io n isatio n o f t te atoms in

t t e absorbing m aterial fey Coulomb c o llis io n s , and 2* Hadlativ© collision® titfe n u c le i, with the rele ase o f brenwatrablung.* For t t e electro n energies involved in nuclear d is in te g ra tio n , absorption fey ra d ia tiv e c o llis io n represents only a sm all fra c tio n of t t e t o ta l energy lo s s , p a rtic u la rly in m aterials which have .low atomic weight,

Tte process of energy lo s s by

Io n isatio n i s alm ost a continuous one a« an e le ctro n pauses through m t t e r .

Tte average energy 1®®® suffered by an e lectro n

*I» p ra c tic e on© can e&eily construct G-K Counters fo r t e t a rays which have "window®" with a surface density of the order o f two to ten sag/cm through which the © lactone must pass In order to roach the se n sitiv e regions of t t e counter*

per ionls&ng c o llis io n i s around t h ir t y e le ctro n volts*

The

to ta l mmb*r of such collision© per u n it length of path t r a ­ versed is not constant fo r electrons of ft given energy, however* A group o f electro n s which i s i n i t i a l l y homogeneous in energy w ill thus be transformed to a group with & d ie t rlb u tio n o f energies on trav ersin g a » U

thickness of absorbing m aterial*

fid s d is trib u tio n w ill poems© a maximum which appi&ra a t a lower energy then the i n i t i a l energy o f Urn group.

Increased

absorber thicknesses produce a fu rth e r d is to rtio n of the homo­ geneity o f tbs electron® wtiioh a re tran sm itte d , and i t la th is d is to rtio n which makes i t impossible to d iffe re n tia te an absorption curve m d thereby to a rriv e * t the ste p s of a given e lectro n d is trib u tio n . J t I s , however, possible to Unci th at p a rtic u la r ab­ sorber thickness tfelch w ill absorb e s s e n tia lly a l l the e lectro n s emitted by th e source.

On m absorption curve t h is thickness

I s the one which corresponds to the point a t which the count­ ing ra te f i r s t reaches the constant background ra te ,

Feather

(FI) has constructed an em pirical fom ula which re la te s the range o f an e le c t ran in art absorbing

with i t s energy:

ft s 0*5118 - 0,091. Here II la the range in grams per square centim eter and d is th© ele ctro n energy in Kev* The nature of t te absorption processes tees riot permit such a ©I mpls lin ea r rsnge-energy

relation for energies \®m than about ?0Q Kev, On® ssight expect to bo ablo to a rriv e a t th* maximum energy of an electro n spectrum q u ite accurately by lo ca tin g v isu a lly the and point o f an absorption curve,

Comparison

o f result® so obtain® 0 .?3 ,

Thom «©aaur®TOiits combined with a&m coincidence

eaperiassnta led poaooctk to propose the d isin te g ratio n mhmo shown In

f i g . 11,

Xn the axp^risienta hero, a heat ray absorption

curve,

taken with aluaiinuKs. Absorbers# and analysed by the Bleuler and Zurtbi ( 81) method, yielded, a mjduaiM energy of 3.5^ Kev, with indications

of &

lower energy group.

The energy o f tte

hardeet gauM ray' was deters&nad by the coinci dence aboorption of the Gosipten recoil electrons produced in an alwdrmm radiator mid was found to be 1.3 Hev.

To

observe

and betfe-guma coincidences, t ie source was situated between a Fb-lined gaumc ray counter and. a tliln*edndow beta ray counter, as was described in the la st section. The gama-gassm coincidence rat© per ganne my recorded by the pb-linod counter was (0,15 4 0.049) x 10"^, which in d icates th at at least a pair of g$mm rays io eedtted in cascade# in t& m o f tli® disintegrations,

The psim ray counting e ffic ­

iency fo r the beta ray counter combined with m altsajjnuffl radi­ ator le not known, however, and no further in fo im tio n can be

figure

II 4.0

Rh

106

Pd 106

3.0 2.30 3 .5 5

2.0

0.51

1.25 0.73

1.0

Mi v

inferred from the

coincidence rate*

?h© b®t&«

&mm coincidence rat® per recorded ie ia roy m a function of absorber thickness i s given in Pig, 12; the curve &howe clear evidence o f & complex beta my spectrum, with a low energy group ufeae m&xXmuat

Is around 2*3 Mev. Above 2,3 >J»v

oo bsta-gs^aaa coincidences are observed, from which fact on® -concludes that the higher energy group o f beta rays lead© d irectly to the ground state of Pdi0 &* The rem its presen.ted hero are in good ogrmmmfo with Peacock's proposed ciisiategr&tion schoaio.

Kern* H iic h e ll, and Z&it&rmo (t£2), using a m gnetic lens apectrosaetor, have found the following beta and gwm& riiy tsicrgics associated with the (lieintegration oi. Bets, rsy groups with end point energies of 0*621, 0*288 K«v; qp&tm m y line® of 0*646, 0*609, 0*431 (I* C .), 0*174 (X* C*), 0.123, and 0*110 (I# 0*) Hev, in which the symbol (X, C.) denotes internal conversion*

Friedlftitcler, Goldhaber mti

Bcharff-Goldhubcr (F3) report te th e r that a mstast&bl® lev el of nppr ox&nately two montha* hal£«Xl$» ex ists In T sr^ , and their absorption »o$ur«icnt® on tbs conversion, electrons from the delayed ga&ma radiation Indicates a quantum energy of 120 Kev.

Kern, et

above 1» delayed.

(K2), fltid that the 110 Kov line sieniiomai

BETA

RAY

ABSORBER

(G/CM

(VI

IO M O

Tbs absorption mid coincidence experiments to* bo dess™ cribed her# are for two cases s X. sixth a a©urea which contains the two month tellurium g«mm& ray a c tiv ity , and 2. with a source which has had cbomical separation from tellurium,,

Identical

counter geometries were umid both tla e s . In the crqiarimimt© with the f ir s t «source, m aluminum absorption curve of the beta rays appeared to law® an end point at approximately 0 .7 Kev, with considerable “ta ilin g off* for absorber t hicknesses out to about one centimeter. The activity in the Hfcaiiw proved to bo characteristic £ x radiation £rom tellurium, following Internal conversion of g&mm. rays, as shown by tern c r itic a l absorption curves of Pig. 13*

1b®s® curve® were taken with enough aluminum to

stop beta rsy® of energy leas th&a 0.300 Mev.

?b® customary

bet&~gamfcta coincidence plot displayed the saddle chape shown in Fig. 14«

I t seamed lik ely tfc&fc a considerable number of

coincidences between $mmm rays and x-rsys following internal conversion were being recorded (at a rate to be symbolised by *» „

«). fife ratio

/UK should be independent of th* absorber

thickness placed before the beta ray counter, which In the present expariaortts was th© only counter capable of recording the x-rays • By exti^polation of tbs w©d.®

bote

fte low energy

group endpoint appeared at Q.% fftrv am th e value of again 0.34 x 1 0 ^ for m absorber before t o

^ 0 f I ^ (i

b*te ray counter*

I t was not e x a c te d th a t enough

of

t o con~

version electron® from t o 0.110 Mev $&»»»& ray could be bran*** isi t tod by the coimter window to have m aterial e ffe c t on the ste p s of t o Ms# / Mf H.

v s. absorber thickness p lo t.

.Praeeodymjiim 142. Mandeviile (MXO) ha® reported a caries of coincidence

and absorption experiments on the r&dtetionis from the 19.3 ~ hour (D3) a c tiv ity of Pr^*%

His re m its indicate a complex

bote ray spectrum, with endpoint® of 2.21 and 0.215 Hev* and a t least two gamma rsys* t o o f 1.74 Mev*

tete-giisma

hardest of which tea an energy

He also reports gsam-*g&!Ki8a coincidence® and

coincidences which involve only t o

low energy group .

its a part of the present investigation, the beta my mid point energy was found to b® 2.52 Itov by absorption in aluminum, with t o

usual Eleuler and Zunti data analysis (BX),

mid the energy te* the hardest gamma ray was found to bo 1.53

M&v by coincidence absorption of Cmpian reco il electrons* The ra ta of counting beta raya muj muah higher than th a t o f count ihg g m

rays in the m xwl coinddesiee counting

an'&rige^sntj apparently « . t of Use traifc-itions fmm pr4^ In this »m»

Xe&d d irectly to the gxwum state of arrang assent the

coincidence rate waa nearly aero,

partly because of the low se n sitiv ity of the tete rsy counter for ccomtlng gamma naya* With the source placed between two Pb-cathode gaistt counters Use ^^naa^asiia coincidence rate was found to bo (0.10 4 0*075) x 10"*^per counted g m m ray*

The

fraction of 1.53 H»v gmim. r&ye i s known to be around1*2 x 10"3 for the geometiy e&iployodj i t is not reasonable to ouppoee the 1*53 Kov ga&m my to be simply in cascade with a single lower energy gemma my, then, for the counter aan« a itlv ity for a g&ma rey o f the difference energy (approxi~ m&tely 0*7 Mw) la about 0*7 x

and for the cascaded pair

one would expect a gamar-fpum* *, la the fraction of the 1*53 Hw gemma my a coiuitad arid

is the fractio n of the- 0.7 M©v g?msm rsgrs counted. The plot of

A>

as a function of beta ray ab­

sorber thickness is given in Fig a 16 • As was expected, no

-4 S -

* to

-j

RAY

10 d

CM

o

CM

o ABSORBER

(6/CM

* o

BETA