Measurements of 14 MeV neutron cross-sections for the production of isomeric states in Hafnium isotopes

A package of 15 mm diameter foils of various materials was irradiated using an intense (d,t) neutron generator, which produced a maximum neutron energy of 15.6 MeV and a mean neutron energy of 14.8 MeV. The package was subjected to a neutron fluence of 1018 neutrons per square cm over a period of 11 days. The materials consisted of one foil each of hafnium, tantalum, tungsten, titanium and low-activation stainless steel, together with monitor foils. The activity of each foil was determined using a germanium (lithium) detector connected to a 4096-channel pulse height analyzer system. The stored gamma spectra were analyzed using the SABRE software. Isotope identification was carried out with reference to a computerized catalogue of gamma-ray isotope lines based on the GAMDAT-78 library. The neutron fluence was determined from measurements on the monitor foils of the activity of specific reaction products with known cross sections. A similar method was then used to calculate the cross sections for the reactions producing the isotopes seen in the sample foils. In the hafnium foil measurements the gamma rays from the 31-year isomeric state of hafnium 178 began to show clearly after the first year which was dominated by the decay of the metastable state of hafnium 179 and of hafnium 175. Tungsten, tantalum, titanium, and stainless steel foil measurements are also briefly discussed. The measured cross sections agree within a factor of about two with theoretical predictions based on pre-equilibrium and compound nucleus theories. 4 refs, 4 figs, 8 tabs