Fission and capture cross-sections of curium

Optimization of the production of 252Cf and heavier isotopes requires knowledge of the capture and fission cross-sections of various isotopes formed in the production chain. Most important are minimization of losses due to fission of 245Cm, 247Cm, and 251Cf target material by neutrons. Also of interest are the capture cross-sections of the isotopes which undergo sub-barrier fission for resonance energy neutrons. In a recent experiment (Physics-8), utilizing a nuclear explosion as the neutron source, both fission and capture cross-sections were measured for 244Cm and 246Cm, and fission cross-sections were measured for the other Cm isotopes in the production chain: 245Cm, 247Cm, and 248Cm, from 20 eV to several MeV. Fission cross-sections were also determined for a number of other heavy isotopes, including 252Cf. This technique appears to be reasonably well suited for such studies, since only limited quantities (20 to 100 μg) of separated target isotopes are required, and the sample activity (up to 108 fissions/s) is no problem. Analysis of the data in the resolved resonance region above 20 eV yields all the partial cross-sections for the isotopes 244Cm, 246Cm, and 247Cm, under the assumption that the capture width for the odd-A target is constant and given by nuclear systematics. These measurements are also of interest because they offer experimental evidence concerning nuclear- structure-model predictions by Strutinsky and Nilsson on sub-barrier fission. The even curium and californium isotopes measured all show fission thresholds and appreciable sub-barrier resonance fission. However, the characteristic enhancement of sub-barrier resonance fission attributed to eigenstates in the second shape minimum does not seem to be present for the even Cm and Cf targets studied. More detailed statistical tests are required to establish the existence of such effects. (author)