Experimental determination of the isomeric yield ratios of 117mgIn and 120mgSb in photonuclear reactions

Experiments have been carried out using a Bremsstrahlung beam of maximal energy 43 MeV or 18 MeV on small amounts of material. The targets consisted of 80-90% enriched 118Sn, 121Sb and natSb. γ-spectra of the induced activity were measured and the isomeric yield ratios (IR) calculated from the photonuclear reactions. It is observed that IRs rise rapidly above the reaction threshold and became nearly constant after reaching the photon energy corresponding to the maximum of the giant dipole resonance cross section. The determination of the IRs is carried out by two approaches: 1/ using the photo peaks areas generated by two independent γ-transitions in the decays of the metastable and ground state nucleus of the isomeric pair; 2/ following the time evolution of the photopeak area generated by a single γ-transition. A comparison with IRs values reported by other authors is made. In the case of natural Sb target it is concluded that the contribution of 123Sb(γ,3n) to the reaction 121Sb(γ,n) 120m,gSb can be neglected. It is suggested to use weighting coefficients for the reactions observed after irradiation of natural targets. 2 tabs., 12 refs