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The general features of the mass yield curve in the pileneutron fission of thorium-232 have been determined earlier, particularly by Turkevich and Niday using radiochemical methods; their yield data were accurate to about plus or minus 20 percent. Work was done to determine more accurately the yields of some mass numbers in thorium fission by radiochemical methods. It was hoped that the accuracy would be sufficient to determine radiochemically any fine structure in the yield curve similar to the mass spectrometrically determined fine structure in the thermalneutron fission of uranium-235. In order to obtain greater accuracy with radiochemical methods, a four-pi beta proportional counter was used rather than a GeigerMuller counter. Small counting samples with high specific activities were used to minimize self-absorption corrections. Thin aluminum- coated plastic films were used as sample supports to minimize absorption in the sample holder. The thorium samples, in the form of metal foils, were irradiated in the fast-neutron faculities or the Argonne National Laboratory, Lemont, Illinois. The fastneutron flux was of the order of 10/sup 13/ neutrons per second per square centimeter. The relative mass yields of mass numbers 89, 91, 99, 141, 143, 144, and 147 were determined by separating and countingmore » strontium- 89 yttrium91, molybdenum-99, cerium-141, praseodymium -143, cerium-144, and neodymium-147. The relative yields of mass numbers 90 and 140 were determined by separating strontium-90 and barium-140 and then milling and counting their daughters, yttrium-90 and lanthanum-140. Published fission product separation procedures were modified for use with one milligram of appropriate carriers. Additional steps were used to ensure removal of the large amount of protactinium- 233 produced in the neutron irradiation of thorium-232. The recoveries of the strontium and barium carriers were found gravimetically; the recoveries of the other carriers were found spectrophotometrically. The mass numbers and their yields relative to mass number 140 were found to be: 89, 1.04 plus or minus 0.06; 90, 1.09 plus or minus 0.06; 91, 0.64 plus or minus 0.04; 99, 0.40 plus or minus 0.03; 140, 1.00; 141, 0.95 plus or minus 0.06; The 91 mass yield was unaccountably low, but the others, including the yields of mass numbers 143 and 147 fit on the smooth yield curve of Turkevich and Niday. These yields are believed accurate to about plus or minus 6 per cent. Within this accuracy, no fine structure was found in these mass regions. (auth)« less

Radiochemical studies of some pile-neutron fission yields of thorium-232