UC Berkeley

The level density and $\gamma$-ray strength function of $^{243}$Pu have been determined using the Oslo method.\

A 12~MeV deuteron beam from the University of Oslo cyclotron was used to populate excited states in the quasi-continuum of $^{243}$Pu using the $^{242}$Pu$(d,p)$ reaction. The distribution of primary $\gamma$-rays as function of the excitation energy has been extracted from particle-$\gamma$ coincidence data. Based on the Brink-Axel asumption that the primary $\gamma$-ray spectrum is proportional to the product of the $\gamma$-ray transmission coefficient (which only depends on the transition energy) times the level density at the excitation energy of the final state. The $\gamma$-ray strength function is calculated from the $\gamma$-ray transmission coefficient assuming pure dipole radiation. Both, the level density and $\gamma$-ray strength function, are normalized using available experimental data from libraries. \

The level density of $^{243}$Pu follows closely the constant-temperature level density formula. An enhancement of the $\gamma$-ray strength function is seen at low energies that is similar to that previously measured in other actinides. This structure is interpreted as the M1 scissors resonance. Its centroid $\omega=2.42(5)\:\mathrm{MeV}$ and its total strength $B=10.1(15)\:\mu_N^2$ are in excellent agreement with sum-rule estimates.

The measured level density and $\gamma$-ray strength function were then used to calculate the $^{242}$Pu$(n,\gamma)$ cross section within the Hauser-Feshbach formalism.