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

65 Pages·03.105 MB·English

by  Jurney, Edward T

a m m or habxoactot BBXMORATiaii
M OF ABSORPTION AND 00INCIDENCE COUNTING KOT0M
BY
Y. OtJENKY
Indiana .university
LIBRARIES
BLOOMINCTON
SUBHXTTlSD TO THE FACULTY OF W GRADUATE SCHOOL III PARTIAL
FmilUMT OF THE EEviUffiEMSITS FOE TtE DSORES,
DOCTOR OF PHILOSOPHY, III THK DEFAHTIENT
OF PHYSICS* INDIANA UmMSITT
0-uly, iS*fS
,p f(/J *
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Z« Intonation 1
II* The Xteter&in&tien of Beta and ta rn Eagr
Bnergiee bj Absorption 3
A* The tedlsjw ^norgjr of anE lection Spsctmm 4
Bo The Mmlmm ISmrgj of a Ckwa Speetnm IQ
HI* 08sa88$a*"^as@$a and Bata^wsa QvimMmom 14
A* mimM m ms 14
B* Beta-gaffim eoinaidsrnms 17
17* ttw Beeults of Studies of Particular Bisirv~
tegmilen
A* Scandium 46 ax
Bo Gallium 7a 23
G. Gold 198 25
D* Bubidiua 86 a?
B* totiiionj 124 31
F. Khodlm 106 3?
Go Antimony 125 40
Ho Praseodymium 24a 47
7* ApjHrndi*
A* the Coincidence Circuit 51
Bo Saw Bay Counting Sensitivity 36
Hftferencfcea 61
x. iMtmimmm
Special counting techniques frequently prove miceesss-
ful m mi aid In solving the laqmrtant problem of determining
disintegration scheme® for the radioactive nuclei* In most
of such dislr&egrutiomi particles, either positive or negative
electrons, are emitted by Mm dtsintegrating nucleus almost
simultaneously with one or no re quanta of radiation* Much
information cun be obtained from tm counters armigod %dth
suitable sir cults m that rates of counting these "coincidental1*
events can be determined. Such an arrangement pm video a
powerful tool for obtaining direct information about modes of
dieintegration*
The first experiments of this kind, called coincidence
counting, were done by Bayer and v» Both© (HI). Much oub©e~
quant work was done by otter groups, notably Marling (Ml),
Mitchell, Lsmger, and McDaniel (Ml), Deutsoh and l&ilot (mainly
in direct conjunction with a magnetic lens spectrometer) (01),
Handevlllo and Soterb (M2), and #tedm\’mok md Chu (wl)«
Mitchell (K3) has published a survey of the results obtained
through coincidence counting prior to 1948*
The construction of a disintegration scheme naturally
requires m accurate measurement of the energy of tte particles
and radiation which are emitted* Often precise energy nie&sure-
meats alone are sufficient to yield the scheme by testing the
possible combinatlona of the energies involved. In marly all
instances it is asost practical to make energy measurement*
with mxm typo of magnetic analyser. Beta my energies are
measured directly in a magnetic spectrometer | gmm ray ener­
gies are inferred from a measurement of the energy of the
secondary electrons produced by them in s&m target material
or by the process of interrial conversion. In general the
measurement of gamma my energies of the magnitude of those
associated with nuclear disintegrations by crystal spectres*-
star techniques (02) is not practical because of the excep­
tionally strong sources which are required,
A less accurate, although mom convenient means of
energy Measurement i® that of detemining the minimi amount
of absorbing isaterial which Is required to prevent electrons
(sitter beta rays or secondary electrons produced by gamma
rays) from entering: a counter * L&pirlcol relalleniihips exist
which connect particle energy with particle range in a given
material. A detailed discussion of energy date rain* tion by
absorption vdll te given in Chapter XI,
In the present scries of liwestimations epectroastric
energy sseasurcesnte were aval labia for most of the nuclei
which were studied. Thus the absorption measurements of energy
were nearly always treated as confirmatory and reliance placed
on their accuracy is to be secondary to that placed on the
epeobrone trie deternin&ti om •
n . 'tm nwmmnhmoM of beta mb um h my
ehkroxbs by ad3orptxqk
Whan accuracy of measurement Is the primary oonaid*
©ration, Magnetic analysis I® to be greatly preferred aver
other method a for th© determination of parti ela and quantum
energies* A serious drawback is encountered, however: The
resolving power of a magnetic spectre*; tor depends ultimately
on source strength. In such a spectrometer, electron (either
beta raye or secondary electrons produced by grata rays)
Monentan is the directly mmxirwd quantity and i» proportional
to the product M/> of a const mb magnetic field and the radius
of Mi® electron^ trajectory in that field* In taking a mag­
netic i$eetrott«»tria measurement one must then count the nwher
of electrons which lie in a JsMenium Interval W/» to \\(ptdp)
whore dp la roughly the am of ilia source width and the
width of a defining slit placed near the counting device.
High resolution of th& instrument is achieved, only if the
quantity M & p / H p 1® made astall, and in order to
obtain reasonably high eeuntiqg rates when-this condition Is
satisfied a source of high activity is required*
A further, although lee® frequently encountered dis­
advantage of the magnetic speotrcweter is the relatively large
amount of time required to obtain a precise energy measurement.
This place® a serious lower limit on the decay period; of iho
radioactive specie* which can be .ftiudicd without the necessity
of raking repeated bef&artiments to produce freeh 3«ress,
Absorption methods for making enow measurements also
have these two limitations placed on thorn, bat sources with
much lower specific activity end with much shorter decay
periods can be conveniently studied by .absorption methods than
by the mo of a magnetic spectrometer* tier© Mm specific ac­
tivity limitation is encountered only when the thickness of
the source reaches. a eiastale fraction of the range of the most
energetic electrons present in a beta ray source £ for investi­
gation of gamma ray energies by absorption method® the lower
limit cm. flie specific activity of a source Is governed by its
physical else. For rapidly decaying sources, a rsaeonebly
successful absorption measurement of energy can be made in a
few minutes9 time. In order to gain these advantages over
spcotrcmstric measurements, however, two sacrifices must be
made: 1. Loss of accuracy, and 2. Inability 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 plication of special methods of data analysis;
these methods will b© discussed in the section to follow).
A. m MAXIMUM ttWQY OF AM BUCTBQM SFafiTtHJH.
Th® followng procedure is employed to obtain an ab­
sorption curve for the electron distribution from a radio­
active source:
The source to bo studied 1® placed a short distance
from a Gelfer«Mulier counter, preferably om wldch present©
vdiy little absorbing material for tit© elect m is to traverse
in order to bo counted**, Different thicknesses of absorbing
ate©ts arc placed between the source ate the counter, and tbe
counting rate la measured for each thickness. A plot of the
aountlng rate m a faction of absorber thickness then show*
•that fraction of the initial electron distribution (I.e.,
the distribution for no absorber) which survives the pasture
through & particular absorber thickness.
The absorption of electron® in u medium lu accomplished
by two competing mechanism®: X» Ionisation of tte atoms in
tte absorbing material fey Coulomb collisions, and 2* Hadlativ©
collision® titfe nuclei, with the release of brenwatrablung.*
For tte electron energies involved in nuclear disintegration,
absorption fey radiative collision represents only a small
fraction of tte total energy loss, particularly in materials
which have .low atomic weight, Tte process of energy loss by
Ionisation is almost a continuous one a« an electron pauses
through m tter. Tte average energy 1®®® suffered by an electron
*I» practice on© can e&eily construct G-K Counters for teta
rays which have "window®" with a surface density of the order
of two to ten sag/cm through which the ©lactone must pass In
order to roach the sensitive regions of tte counter*
per ionls&ng collision is around thirty electron volts* The
total mmb*r of such collision© per unit length of path tra­
versed is not constant for electrons of ft given energy, however*
A group of electrons which is initially homogeneous in energy
will thus be transformed to a group with & diet rlbution of
energies on traversing a » U thickness of absorbing material*
fids distribution will poems© a maximum which appi&ra at a
lower energy then the initial energy of Urn group. Increased
absorber thicknesses produce a further distortion of the homo­
geneity of tbs electron® wtiioh are transmitted, and it la
this distortion which makes it impossible to differentiate
an absorption curve md thereby to arrive *t the steps of a
given electron distribution.
Jt Is, however, possible to Unci that particular ab­
sorber thickness tfelch will absorb essentially all the electrons
emitted by the source. On m absorption curve this thickness
Is the one which corresponds to the point at which the count­
ing rate first reaches the constant background rate, Feather
(FI) has constructed an empirical fomula which relates the
range of an elect ran in art absorbing with its energy:
ft s 0*5118 - 0,091.
Here II la the range in grams per square centimeter and d is
th© electron energy in Kev* The nature of tte absorption
processes tees riot permit such a ©I mpls linear rsnge-energy
relation for energies \®m than about ?0Q Kev,
On® ssight expect to bo ablo to arrive at th* maximum
energy of an electron spectrum quite accurately by locating
visually the and point of an absorption curve, Comparison
of result® so obtain®<1 with spootrcmetric measurements show*
however* that on® usually wnaercetimfttett the range* «D3»»ti«»o
by as much as twenty percant, the error a in determining the
range visually become especially large for low counting rates
or tdian the scums oontJlbutos a high background counting
rat© from gam mdlatlon* Eleuler and &untl (112) have pro­
posed. a method of data analysis for beta my absorption curves
which successfully avoids the necessity for locating th© end
point, their system'is briefly m .fellows s
If all beta spectra yloMod absorption curves of the
smm shape* it *ould b© easy to fins th© endpoint energy of
any given beta ray amittar by comparing; its absorption curve
with that of some standard beta ray eoltber** The shapes of
absorption curves arc net alike* to « w , but a**© found to be
influenced by three factor®* 1, In the Xm m®r.$y region
the miig©-©n®v$y dependence is not a linear one* but rather
Hie range increases relatively slowly with Increasing energyj
'“This is* in fact* the basic for Fo&th&r’a method of analysis
(F2), Unfortunately, Feather obese the beta ray spectrum of
Radium R* which is well known to have an anomalous shape (hi) *
to produce hie standard curve.