Fission resonances observed in the 237 Np(d,pf) reaction and the fission barrier topology of 238 Np

Abstract

The fission probability of 238Np was measured as a function of the excitation energy in the energy range of E∗=5.4−6.2 MeV in order to search for transmission fission resonances. A radioactive 237Np target was bombarded with deuterons of Ed = 12 MeV, whereas the energy of the protons was analyzed with a superior resolution of ΔE = 8 keV. The experiment was performed at the Tandem accelerator of the Maier-Leibnitz Laboratory in Garching employing the 237Np(d,pf) reaction. A group of fission resonances has been observed at excitation energies between E∗=5.5−5.8 MeV, which could be ordered into three (superdeformed) rotational bands with a rotational parameter of ℏ2/2Θ=3.507 keV, and identified as the first direct observation of transition states composing rotational bands in an odd-odd nucleus as they appear above the top of the outer fission barrier. Nuclear reaction code (talys1.95) calculations were also performed to extract the multi-humped fission barrier parameters of 238Np by fitting them to the experimental data of the present (d,pf) and previous (n,f) experiments. The extracted barrier parameters also support the above interpretation of the observed resonances.