GAMMA-RAY SPECTROSCOPY FOLLOWING HIGH-FLUX 14-MEV NEUTRON ACTIVATION.

摘要

The Rotating Target Neutron Source (RTNS-I), a high-intensity source of 14-MeV neutrons at the Lawrence Livermore National Laboratory (LLNL), has been used for applications in activation analysis, inertial-confinement-fusion diagnostic development, and fission decay-heat studies. The fast-neutron flux from the RTNS-I is at least 50 times the maximum fluxes available from typical neutron generators, making these applications possible. Facilities and procedures necessary for gamma-ray spectroscopy of samples irradiated at the RTNS-I were developed.;In support of efforts by the Laser Fusion Program at LLNL to use 14-MeV neutron-activation as a fuel-density diagnostic of inertial-confinement-fusion (ICF) targets, various materials were irradiated to examine their suitability for advanced diagnostics. Enhanced positron-annihilation radiation was observed in gamma-ray spectra from trace amounts of Br mixed with target material, indicating Br is a viable candidate to be used as a seed gas added to the fuel to achieve a direct fuel-density measurement. The cross section for the ('79)Br(n,2n)('78)Br reaction was measured.;The final application of high-flux, fast-neutron irradiations was the production of ('148)Pr, a radioisotope of interest in fission decay-heat studies, from ('148)Nd. From gamma-ray spectra obtained from a single detector, the energies and intensities of gamma rays in the decay of ('148)Pr have been tabulated. The success of these irradiations has demonstrated the feasibility of production of shorter-lived radioisotopes, such as ('100)Nb and ('82)As, for similar decay scheme studies.;Standard reference materials (fly ash, orchard leaves, and bovine liver) from the National Bureau of Standards were analyzed for 29 elements using high-flux, multielement fast-neutron activation analysis (FNAA). From gamma-ray spectra collected after the 14-MeV neutron irradiations of these materials, over 80 radioisotopes were observed from 37 elements. Procedures were developed that avoided problems due to the complicated irradiation geometry and minimized interferences from multiple reactions. The results were the most complete analyses of any samples to date by 14-MeV neutron activation, and demonstrated these procedures to be reliable and accurate.