Radiochemical studies of neutron induced fission in heavy nuclides

The presence of tin-129 isomers has been confirmed among fission products. Their half-lives were measured and found to be 7.5 + 0.1 min and 2 min. No evidence was found for a longer-lived isomer previously reported. 0.17 ± 0.01 and 0.39 + 0.03% were obtained for the cumulative yields of 7.5 min tin-129 and antimony-129 respectively in the thermal neutron induced fission of uranium-235.

The cumulative yields of 10 fission products (bromine-83, and -84, strontium-91, zirconium-97, molybdenum-99, ruthenium-105, silver-113, antimony-129, cerium-143 and praseodymium-145) from the fission of protactinium-231 induced by 3 MeV neutrons have been measured radio-chemically using a recoil method. The massyield distribution was found to be highly asymmetric with a peak to trough ratio of about 100 and a peak width at half-height of 14 mass units. The maxima were observed at masses 91 and 138 with yields around 7.2%.

Previously a correlation between the relative width ratio for symmetric fission and neutron re-emission and a term depending on the relative energies available for the two processes was derived and tested for 14 MeV neutron induced fission of heavy nuclides. In the present work this was tested for fission induced by 3 MeV neutrons. The fission cross-section of protactinium-231 for 14.8 MeV neutrons was measured by a radiochemical method and found to be 1.43 + 0.12 barns. This result was used in a discussion of the correlation mentioned above.

In a search for irregularities in yields in the mass region 131-135 for 14.8 MeV neutron induced fission of protactinium-231, the independent yield of xenon-135 and the cumulative yields of iodine-131, -133 and -134 and xenon-133 and -135 were measured radio-chemically. Fine structure was observed at mass-134.. A comparison of the results with those obtained in 14 MeV neutron induced fission of thorium-232 and uranium-238 suggests that the prominence of the fine structure increases with decreasing atomic number.

In all measurements zirconium-97 or molybdenum-99 and where possible both of them were used as reference masses. Counting was carried out using an end-window gas-flow β-proportional counter for solid, and a gas Geiger-counter for gaseous samples.