%
2 An 8.15-gm/em2 brass absorber was inserted between the sample
and the tube. This absorber eliminated all beta particles with maximum energies up to 6 Meve The purpose of this plate was to estimate
the detected results of the radiation interaction in the aperture
plate used for geometry definition.
The absorber plate was identical
to this aperture plate except for the aperture.
The count thus
obtained was subtracted from the original count of the sample to obtain
the beta activity (Ap) of that sample .*
3. The count was corrected for geometry (G), defined as the
fraction of solid angle subtended by the sensitive volume of the G=i
tube. This factor was determined by using the first three terms of
the Blachman Series.6/ Succeeding terms of this series are insignifi-~
cant and were not used for this correction. The G values in Table A.l
appear to be low because the counting arrangement was designed in such
a manner to insure the correct absorber placement.
4e
Backscattering determinations
(F;,) were made by mounting a
tube in a hollow support of lead bricks approximately three feet from
the floor. This arrangement provided negligible backscattering from
the floor of the support.
The geometry defining aperture
tends to
minimize the effects of scattering from the walls of the supporte
Equal aliquots of dissolved fallout from the shot under analysis were
dissolved in nitric acid and pipetted into counting cupse One cup
with a bottom of a very thin rubber film (0.45 mg/cm?) was measured
in the arrangemnt,which provided negligible backscattering,and one
glass bottom cup was measured in the regular counting apparatus. The
backscattering correction factor, which was obtained by dividing the
count obtained in the regular apparatus by the count obtained in the
arrangement with negligible backscattering,was used to correct all
samples from that shot. Since the energy distribution of mixed fis-~sion products is known to be time dependent, this correction was made
for various times. However, it was found that the variation was in~
significant during the tims the measurements were made on the concerned
samples. For examples cf (Fi) for various times see Table A.2e_
5« A correction (F,) was made for absorption by the air between
the sample and the tube window, and absorption of the tube window ite
self.7/ To obtain this correction, precise absorption curves were run
on @ sample from each shot.
the equation
A correction factor was calculated from
Fa *
Mh
1
.=mt
No
=@
(361)
¥ It is now felt that the use of this absorber was not proper because
the geometry factor for the aperture outweighed the geometry factor
for the rest of the plate, resulting in an estimate that was too high.
Yowever, the fact that this estimate in all cases was very small (approximately 2 per cent) in comparison with the beta count indicates
that the radiation interaction with the aperture is of no importance.
The use of the plate has been discontinued. Regular absorption curves
made with aluminum absorbers indicate that the detected gamma back-~
ground is of the order of one per cent.
This would be expected because
of the low sensitivity of the tube to gamma radiatione
35