Illuminating photon production with GEANIE at LANSCE

GEANIE is a multidetector, high-resolution gamma-ray spectrometer that is operated as a collaborative effort between Lawrence Livermore National Laboratory and Los Alamos National Laboratory. The Weapons Neutron Research high-energy spallation neutron source at the Los Alamos Neutron Science Center provides GEANIE's beam. GEANIE enables new measurements of neutron-induced reaction cross sections and fission fragment studies with neutrons with incident energies from 1 to 200 MeV and excellent gamma-ray energy resolution. Results of first studies are designed to provide information for such diverse needs and calibration of high-energy neutron fluence monitors for applications in the accelerator production of tritium and accelerator transmutation of waste. Current efforts with GEANIE are focused on determining the 239Pu(n,2n)238Pu reaction cross section from threshold to 15 MeV from gamma-ray measurements. Because of the large fission-neutron background, this reaction is difficult to measure by neutron detection techniques. Only two direct measurements of this cross section as a function of neutron energy from threshold to higher energies have been reported, with large differences in the results. By measuring many partial gamma-ray cross sections in the decay cascade and modeling the reaction with a sophisticated preequilibrium-plus-Hauser-Feshbach computer program (GNASH), the author expects to achieve an absolute cross-section measurement of 10% accuracy or better. Key to current efforts has been the production of a very pure 239Pu sample with almost all 241Am removed. As a test of their methods, and for other interests, he is measuring the 235U(n,nprimeγ) and (n,2nγ) cross sections and 238U(n,xnγ) reactions. The author is starting a program of fission fragment studies with French collaborators to look at spontaneous fission and neutron-induced fission