The quest for improved neutron capture cross sections for advanced reactor concepts, transmutation of radioactive wastes as well as for astrophysical scenarios of neutron capture nucleosynthesis has motivated new experimental efforts based on modern techniques. Recent measurements in the keV region have shown that a 4π BaF 2 detector represents an accurate and versatile instrument for such studies. The present work deals with the potential of such a 4π BaF 2 detector in combination with spallation neutron sources, which offer large neutron fluxes over a wide energy range. Detailed Monte Carlo simulations with the GEANT package have been performed to investigate the critical backgrounds at a spallation facility, to optimize the detector design, and to discuss alternative solutions.
The 7.6-h isotope ' Es was produced from a 2.5-pg/cm target of ' Es by the (t,p) reaction. The reaction products were separated radiochemically, and the decay properties of ' Es were determined via P-y, y-y, and P-fission correlation techniques. From these measurements we were able to assign 57 y rays to 26 levels in the daughterFm. An isomeric level was observed at 1425 keV and assigned a spin and parity of 7 . This level has a t,~i of (70+5) ns and we observed two Pdelayed fissions with delay times in the proper time range to be associated with fission from this level. This gives a P-delayed fission probability of 2X 10 for this level and a partial fission half-life of 0.8+o, ms at the 95% confidence level.
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