8
ny
ch
where
corrected
t between
true beta
thickness
counting rate observed with thickness
the sample and the sensitive volume
counting rate at zero thickness
of material between the source
it
3
and sensitive volume
mass absorption coefficient expressed in
em* /mg
-o
At
ln.
nt
Me A t
(32)
Ned a + counting rate at thickness t #A +
The best straight line was drawn through the experimental
points and the slope(m) was calculated accordingly. This method is
applicable for any energy or group of energies as long as the first
part of the absorption curve is a straight line on a seml-log plot.
Jt can be seen from the examples given in Appendix A that this is the
case ana, therefore,the aforementioned determination of FP, was used.
6. Self absorption corrections for the samples in question
were considered negligible, since the weight per unit area was kept
in general between 5 and 10 mg/cm®. According to Coryell and
Sugarman,a radioactive sample which has a weight per unit area of 5
to 10 mg/em? and has an energy greater than 0.4 Mev requires no self=
absorption correction._3/ Furthermore, according to Hunter and Ballou,
the nuclides with maximum
more than 1 per cent each
approximately 10 per cent
time the measurements for
at H plus 200 hr.
energies below this value which contribute
to the gross fission activity constitute
of the total activity of the sample at the
this report were made, 1.8. approximtely
Therefore, the error entailad by the assumption
of a negligible correction should be 10 per cent or less.
The
practice of ignoring this correction has been further justified by
comparison of the defined geometry mthod with fow=pi counting
techniques.9/ In these comparisons the experimental error ranged from
3 to 7 per cento
7.
The sample beta activity (A,) was treated by the above
corrections to obtain the sample activity (Aq) in disintegrations per
arinute «
Ay =
*p
FE pe
(33)
A table of correction factors as well as examples of
various correction determinations and the activities A, of the samples
at the time of counting are given in Appendix A.
The above method has been used to determine the disintegra-
tion rate of known mixtures of nuclides with excellent results.10/
Its use in the determination of the disintegration rate for a mixed
fission prcducts sample is believed to result in measurements within
10 per cent of the actual rate. It is true that secondary particles
(e.g. internal conversion electrons) will be detected as primary beta
perticles.
However, the error in disintegration rate due to this
36