The level density and $\gamma$-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 $43\times {10}^6$ levels per MeV at $S_n=5.488$ MeV of excitation energy. The $\gamma$-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 ${\omega }_{\mathrm{SR}}=2.26\left(5\right)$ MeV and a total strength of $B_{\mathrm{SR}}=10.8\left(12\right){\mu }_N^2$, which is in excellent agreement with sum-rule estimates. The scissors resonance is shown to have an impact on the ${}^{237}\mathrm{Np}$${(n,\gamma )}^{238}$Np cross section.

• Received 24 February 2014

DOI:https://doi.org/10.1103/PhysRevC.89.044323