Restricted spin-range correction in the Oslo Method: The example of nuclear level density and ${\hat{I}}^3$-ray strength function from ${}^{239}\mathrm{Pu}(\mathrm{d},\mathrm{p}{\hat{I}}^3{)}^{240}\mathrm{Pu}$

Abstract

The Oslo Method has been applied to particle-${\hat{I}}^3$ coincidences following the ${}^{239}\mathrm{Pu}$(d,p) reaction to obtain the nuclear level density (NLD) and ${\hat{I}}^3$-ray strength function (${\hat{I}}^3$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 ${\hat{I}}^3$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 ${\hat{I}}^3$SF shows a pronounced enhancement between 2-4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.