Photonuclear yields of thePu237fission isomers

The half-lives and yield ratios $\frac{Y_{\mathrm{iso}}}{Y_{\mathrm{prompt}}}$ for the two ${}_{}{}^{237}{}_{}{}^{}\mathrm{Pu}$ fission isomers have been investigated in the photonuclear reaction ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$($\gamma$,$2n$). The observed half-lives ($t_{\frac{1}{2}}=77\mathrm{}\pm 16 \mathrm{ns}, 1050\pm 400 \mathrm{ns}$) agree well with the data from particle induced fission. The measured yield ratios were ${\left(\frac{Y_{\mathrm{iso}}}{Y_{\mathrm{prompt}}}\right)}_f^{\mathrm{short}}=(6.4\mathrm{}\pm 1.7)\times {10}^{-6\mathrm{}}$ and ${\left(\frac{Y_{\mathrm{iso}}}{Y_{\mathrm{prompt}}}\right)}_f^{\mathrm{long}}=(0.83\mathrm{}\pm 0.22)\times {10}^{-6\mathrm{}}$. From a statistical model analysis of the isomeric fission yield ratio ($\frac{Y_{\mathrm{short}}}{Y_{\mathrm{long}}}=7.7\mathrm{}\pm 2.9$) a spin assignment of the two shape isomers was attempted. The analysis provided, in the context of the statistical model, the most probable spin values $I=\frac{11}{2}$ for the exicted, long lived state and $I=\frac{5}{2}$ for the short lived ground state in the second minimum of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Pu}$. Comparing these spin values with single particle calculations, we tend to classify the excited state as the ${\mathrm{}\left[615\right]\mathrm{}}_{\frac{11}{2+}}$ Nilsson orbit and the short lived state as the ${\mathrm{}\left[862\right]\mathrm{}}_{\frac{5}{2+}}$ state.