Energy dependence of the total reaction cross section of isomeric 18Fm on silicon below 400 MeV

A 25 MeV/nucleon primary 17O beam was used to produce an isomeric state 18Fm beam via the single-nucleon transfer reaction 17O (12C, 11B)18Fm. The total nuclear reaction cross section, σR, of 18Fm (metastable) on silicon was measured using a stack of silicon surface barrier detectors that included a position sensitive silicon detector (PSD). The total reaction cross sections were determined as a function of bombarding energy up to 400 MeV by measuring the energy loss in each detector. The reaction cross sections were measured both for a mixture of metastable- and ground-state ions and for a pure ground-state beam. The reaction cross sections for 14N and 16O beams were also measured simultaneously. The reaction cross sections for the metastable beam were deduced from the mixed- and the ground-state cross sections, corrected for the measured purity of the mixed-state beam. The observed reaction cross sections for 18Fm are larger at low energies than the cross sections measured for the ground state. The isomeric cross sections can be reproduced by an optical model potential having a diffuseness of 0.8 fm, compared to a more conventional diffuseness parameter of 0.5 fm, which is adequate to reproduce the 14N, 16O, and 18Fg.s. cross sections measured in this experiment