2e An 8.15-gm/cem2 brass absorber was inserted between the sample and the tube. This absorber eliminated all beta particles with maxi- mum energies up to 6 Mev. 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 volumes of the G=-H tubee This factor was determined by using the first three terms of the Blachman Series.6/ Succeeding terms of this series are insignificant and were not used for this correction. The G values in Table A.1 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 fission 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 samplese For examples cf (Fy) for various times see Table A.2._ 5 A correction (F,) was made for absorption by the air between the sample and the tube window, and absorption of the tube window itself.7/ To obtain this correction, precise absorption curves were run on a sample from each shot. the equation A correction factor was calculated from Fa * Nt 1 No = = mt 6 (3.1) * 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 highe Yowever, the fact that this estimate in all cases was very small (ap- proximately 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 background is of the order of one per cent. This would be expected because of the low sensitivity of the tube to gamma radiatione 35