Anomaly of sup(116mg)Sb and sup(118mg)Sb isomeric pair yield in the (α, n) reaction

Isomeric cross section ratios σsub(m)(E)/σsub(g)(E) are measured in nuclear reactions sup(113)In(α, n)sup(116mg)Sb and sup(115)In(α, n)sup(118mg)Sb up to E=31 MeV. Anomalous behaviour of those ratios for sup(116mg)Sb and sup(118mg)Sb was observed in the energy range Esub(α)=25-27 MeV. The experimental results were compared with the statistical model calculations, the spin dependence of the level density being treated in the Fermi-gas (FGM), in particle independent pairing (IPM) and superconductivity models (S/CM). With the α-particle energy varying from the reaction threshold up to 22 MeV, the theoretical calculations of the FGM for sup(116mg)Sb agree with the experiment at the minimum permissible value of the level density parameter a=1/8 A MeV-1. For the isomeric ratio of sup(118mg)Sb the best agreement of the theoretical calculations with the experiment is observed in SCM up to 20 MeV. At a further increase in the α-particle energy the isomeric ratios σsub(8m)/σsub(g) of the sup(116mg)Sb and sup(118mg)Sb isotopes increase sharply exceedthe valuesing predicted by statistical theory