The Oslo method has been applied to particle-$\gamma$ coincidences following the ${}^{239}\mathrm{Pu}(d,p)$ reaction to obtain the nuclear level density (NLD) and $\gamma$-ray strength function ($\gamma \mathrm{SF}$) of ${}^{240}\mathrm{Pu}$. The experiment was conducted with a 12 MeV deuteron beam at the Oslo Cyclotron Laboratory. The low spin transfer of this reaction leads to a spin-parity mismatch between populated and intrinsic levels. This is a challenge for the Oslo method as it can have a significant impact on the extracted NLD and $\gamma \mathrm{SF}$. We have developed an iterative approach to ensure consistent results even for cases with a large spin-parity mismatch, in which we couple Green's function transfer calculations of the spin-parity dependent population cross section to the nuclear decay code rainier. The resulting $\gamma \mathrm{SF}$ shows a pronounced enhancement between 2–4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.

• Received 8 April 2019
• Revised 21 June 2019
• Corrected 19 February 2021

DOI:https://doi.org/10.1103/PhysRevC.100.024305