Measurement of low energy beta-spectra

67 Pages·03.051 MB·English

by  Motz, Joseph W

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FUiFIUtfEH? OF THE JUftVSUSfSNr; FOii m DKffUSK,
Doom OF PHILGSOHflf, IK 1ES DEPARTMENT
of m usics, Indiana w a rn cm
August, 1%9
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^ The writer wishes to express hi® tippreaiatlon to
c-v
^ Prof. L« K. Langer tor the v&lmbl® assistance rendered m
**r \ ,
well a® far the original suggestion of the problem.
He is also grateful to Prof. A«C*G. Mitchell for
the friendly eneoiaragesient. and for tho generous use of the
laboratory •
tm m or oomwrs
Chapter
1 Introduction
A. Xntoroat in low Energy Meaaureaianta 1
B. Sxpoilacntal Problems 1
C* Purpose of Present gxparlsont 2
XI ^Kperimontal Procedure
A* Prelimin&iy gaqparlmnt 4
B« Betermiiwition of 8 Effective* frintiow 7
0# Design of Spectra* ter 15
D. Keeoiution and f rssnsmis sion of Spec­
trometer 16
£« Kelmhoitss Coils for the Production of
Hie Magnetic Field 19
F. Deaigu of Counter 25
G» Electrical & Vacuum Apparatus 27
B« ibcpurliaAntal ArrangoRniit 28
1. Calibration of Spectrometer 30
J* Ur*© of Alcohol Filled Countar 32
K. Use of Thin Counter hindowa 36
L, Uaa of window!!eee Counter 37
M* Preparation of Sources 39
111 F^crimental Results 45
tmm of mmmn (conu)
I? Disc usaion of Results
A* B«ta^p*otrum of 3^5 52
&« of Souro® Thicknasa on th®
l'^X47 Spectrum Sliape 53
C* M fmt of Counter Window Thiskrtosa on
the Mpocfcrm Rhap® 55
&• 'Wiiuhwlaaa* Counter U^mnrmmntn 56
V Geneluaions 5@
VI References ^0
VII Uat of Illustration® 62
Introduction
Interest in Low Knarigy U&mummnU
Helatlvely little exjmliaenfc&X data is available in
the extremely low energy region of Beta spectra. As a result,
the validity of the feral Theory (PI) has not yet been defi­
nitely established in this region. In fact, recent experimental
investigations (H , L2) of th© Beta spectra of Cu*&, Cu^,
and £$$ have suggested deviations from Hie theory at low energies.
Further studies are needed to help clarify H i a question, as well
aa to determine the extent to which ins trusssntal effect® influ­
ence the results.
Such studies would provide other Interesting Information.
An accurate description of the ©haps of the spectra would reveal
whether there are certain forbidden transition* whose shapes differ
tnm that of an allowed shape (13# 14# Ml) in the low energy region
any differences would throw light on the particular interaction
between the nucleon and ©lootron-neutrino field which should bo
assumed (2a). Further, trm He shape of curtain low energy Beta
spectra such a® K?, It is possible to evaluate the upper limit
of the neutrino rent mass (Cl, 12),
Experimental Problems
It is no accident Hat little information is available
at low energies, since exporissantal difficulties are greatest in
this region. Certain factor®, which wight rave little or no effect
on to spectrum tope at the higher energies, eon haw a pronounced
effect at the lower energies. These factors bo stated as Allows:
(a) Thickness of m wtm to scores tasking (12, Ml,
C2)
(b) Absorption of Counter Window
(o) Scattering of electron© from walla, $m ants,
baffles of speotro&teier
(d) iieeoluiion of electrometer (FI)
Along with these teeters, it 1® important to not® that there ere
relatively few counts at to low energies, and therefore it 1®
necessary In order to obtain good statistic a, that the spectrom­
eter have m high a tonamission as poasibl® ffer a glvaa resolu­
tion.
The irbhlau then, of ©fetatniaf reliable msiuiursmsnts
of to low energy Beta spectra, requires to t adequate account
be taken of to effect of each of the above factor® and that m
attempt be made to aditisd.ee or completely eliminate these effects.
Purpose of present tey to itl
The object of the present experinent is to-folds
(a) to design and build a epectometer especially
adapted for low energy aeasuroments,
(b) to measure to low energy Beta spectra of m
allowed transition 3?$, to of a forbidden transition, ggpmW,
In particular, It is intended 'to raduco the above
awntioned eftpsrlasntftl effects, and in to mm of § to
study the magnitude of the offsets of source thickness ana
~2-
window thickness on tee top® of the spectrum. apetol attention
is to be given to the problem of absorption in the counter window
t o to a method designed to considerably minimise this effect.
ftKpey&iMmfraX Procedure
In ardor to obtain any Bata Measurements at all In the
low energy rang**, it is first of all necessary to have a counter
window tbln enough to transmit such electrons* Therefor®, the
question of how to make ’thin enou#* counter windows boeoitt&s the
first problem to b® settled*
Ons possible approach to the solution of thie problem
Is to uso a detector which doe s not deperti on the ionisation of
gases for its operation* and which can bo connected to tbs spec­
trometer vacuum e lumber without the intsreeption of a window*
liuch a detector might bo found for example* in the si®©iron mul­
tiplier*' (Al), or the eolntilXation counter (C3).
On the other band* to attain the Ideal condition of
no Window with & M l counter, it is logical to inquire what offset
is produced by actually removing the window from the counter and
allowing tbs $a>a to diffuse through the slit Into th* v m m
chamber of tbs spectrometer. Such a step would mean that for a
small enough slit opening* a pressure gradient would exist out-
^The writer initially construe ted Mid ra$&stfjBmtecit with an
electron multiplier tuba In the hops of vtaing it for the detec­
tion of beta rays. Tits results of this* investigation revealed
that the nature of the alec trod© surfaces was very critical for
Beta ray detection. Further* the preparation of such aurfaeee
could not bo easily controlled or kept constant on exposure to
air. Hough coaparieone between the counting miss of the mul­
tiplier tube and a G-M mmh&t indicated that wMla the fbrmor
could aetoct tatavy particles ouch as alpha particles with 100M
officioncy* it could only detect & emeu fraction of incident
Beta particles. This agrees with Allen*a results (AX) which
report a decrease of 301 in efficiency for counting electrons
abcrvc 6 lev. #
-4*"
side the counter silt* whtoh would present an 'effective* window
having a certain density md thickness to my incoming particles*
Further* it would be necessary to maintain a constant pressure
in the counter in order mb to ©hang© its Derating characteristics *
as wall m to quickly r»w e toe gas glowing into to vacuus
ehsator*
It is at onoe apparent that to maintain a constant
pressure in the counter by to use of a ballast tank filled with
the usual counter Rilscfcure of 2 am alcohol and 7 m Argon* would
be quite Impractical due to the large slue of to tank that would
be required* This i* roughly demonstrated by the following cal­
culations
To simplify m iters, assume the s lit opening 1®
small compared to to mm free path and that there
is only 7 oi! of Argon in the counter* These condi­
tions would certainly ©all .tor a smaller tank than
to actual case, then*
(Eats of effusion In ec/scc/m^)s /—ft- .._ \ ® (1)
( a rrs)
to r e p « 7 x 13*6 x 980 tiynm/m**'
(pressure of Argon gee)
f z 1»53 x 10*^ gtas/cc (Density
of Argon Ga® at 20° C and 7 dm
pressure)
Hence,
a # f £ » f f r ) ^ = *985 utw^