where ny = corrected counting rate observed with thickness t between the sample and the sensitive volume N, # true beta counting rate at zero thickness +% = thickness of material between the source and sensitive volume m = miss absorption coefficient expressed in om?/mg " = Qt * eA oo mt (3.2 MeL At = counting rate at thickness t 4A + The best straight line was drawn through the experimental points and the slope(m) was calculated accordingly. This mthod 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 semi-log plot. It can be seen from the examples given in Appendix A that this is the case ana, therefore,the aforementioned determination of Fa was usede 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/em2 and has an energy greater than 0.4 Mev requires no self< absorption correction._8/ Furthermore, according to Hunter and Ballou, the nuclides with maximum energies below this value which contribute more than 1 per cent each to the gross fission activity constitute approximately 10 per cent of the total activity of the sample at the time the measurements for this report were made, i.e. approximately at H plus 200 hr. Therefore, the errer entailed by the assumption of a negligible correction should be 10 per cent or lesse The practice of ignoring this correction has been further justified by comparison of the defined geometry method with four=pi counting techniquese_9/ In these comparisons the experimental error ranged from 3 to 7 per cent. 7. The sample beta activity (Ay) was treated by the above corrections to obtain the sample activity (Aq) in disintegrations per minute e . A, Aa = RY (3-3) 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 preducts sample is believed to result in measurements within 10 per cent of the actual rate. It is true that secondary particles (eege internal conversion electrons) will be detected as primary beta particles. However, the error in disintegration rate due to this 36