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