Production of long-lived hafnium isomers in reactor irradiations

Experiments on production of long-lived Hf178m2 isomer in reactor irradiations are described. Properties of this nuclide are promising for its potential application as a relatively safe power source characterized by high density of accumulated energy. Metal natHf samples were activated in the Dubna IBR-2 reactor at positions corresponding to different neutron fluxes. Samples were bare or shielded by Cd and B4C layers. The gamma activity of the samples was analyzed with Ge gamma spectrometers during a two-year period following their irradiation. In the presence of dominant activation products 175Hf and 181Hf, the high-spin isomers Hf178m2 and Hf179m2 were also detected despite relatively low levels. The isomer-to-ground state ratios and cross-sections were determined from the measured yields. For Hf178m2, the cross-section for burnup (destruction) by neutron capture after its production was also estimated, clarifying the results from earlier experiments. In the context of suggestions for use of Hf178m2 for applications, the results confirm that large-scale production of this isomer by reactor irradiations is not feasible. In contrast, the efficiency of production of Hf179m2 is much higher and an amount of about 1016 atoms may be produced in standard reactor irradiations. For Hf178m2, more productive methods are known, in particular fast neutron irradiations at En ≥ 14 MeV and spallation reactions at intermediate energies. Neutron cross-sections for isomers may also be significant in astrophysics.