Level density andγ-ray strength function in the odd-oddNp238nucleus

Abstract

The level density and $\ensuremath{\gamma}$-ray strength function in the quasicontinuum of $^{238}\mathrm{Np}$ have been measured using the Oslo method. The level-density function follows closely the constant-temperature level-density formula and reaches $43\ifmmode\times\else\texttimes\fi{}{10}^{6}$ levels per MeV at ${S}_{n}=5.488$ MeV of excitation energy. The $\ensuremath{\gamma}$-ray strength function displays a two-humped resonance at low-energy as also seen in previous investigations of Th, Pa, and U isotopes. The structure is interpreted as the scissors resonance and has an average centroid of ${\ensuremath{\omega}}_{\mathrm{SR}}=2.26(5)$ MeV and a total strength of ${B}_{\mathrm{SR}}=10.8(12){\ensuremath{\mu}}_{N}^{2}$, which is in excellent agreement with sum-rule estimates. The scissors resonance is shown to have an impact on the $^{237}\mathrm{Np}$${(n,\ensuremath{\gamma})}^{238}$Np cross section.