First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments

Abstract

We report the first experimental determination of independent isomeric yield ratios using direct ion counting with a Penning trap, which offered such a high resolution in mass that isomeric states could be separated. The measurements were performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility at the University of Jyv\"askyl\"a. The isomer production ratios of $^{81}\mathrm{Ge}$, $^{96,97}\mathrm{Y}$, $^{128,130}\mathrm{Sn}$, and $^{129}\mathrm{Sb}$ in the 25-MeV proton-induced fission of $^{\mathrm{nat}}\mathrm{U}$ and $^{232}\mathrm{Th}$ were studied. Three isomeric pairs ($^{81}\mathrm{Ge}$, $^{96}\mathrm{Y}$, and $^{129}\mathrm{Sb}$) were measured for the first time for the $^{\mathrm{nat}}\mathrm{U}(p,f$) reaction, while all the reported yield ratios for the $^{232}\mathrm{Th}(p,f$) reaction were determined for the first time. The comparison of the experimentally determined isomeric yield ratios with data available in the literature shows a reasonable agreement, except for the case of $^{130}\mathrm{Sn}$ for unspecified reasons. The obtained results were also compared with the gef model, where good agreement can be noticed in most cases for both reactions. Serious discrepancies can only be observed for the cases of $^{96,97}\mathrm{Y}$ for both reactions. Moreover, based on the isomeric yield ratios, the root-mean-square angular momenta (${J}_{\text{rms}}$) of the fission fragments after scission were estimated using the talys code. The experimentally determined isomeric yield ratios, and consequently the deduced ${J}_{\text{rms}}$, for $^{130}\mathrm{Sn}$ are significantly lower compared to $^{128}\mathrm{Sn}$ for both fissioning systems. This can be attributed to the more spherical shape of the fragments that contribute to the formation of $^{130}\mathrm{Sn}$, due to their proximity to the $N=82$ shell closure. The values of ${J}_{\text{rms}}$ for $^{129}\mathrm{Sb}$ are higher than $^{128}\mathrm{Sn}$ for both reactions, despite the same neutron number of both nuclides ($N=78$), indicating the odd-$Z$ effect where fission fragments with odd-$Z$ number tend to bear larger angular momentum than even-$Z$ fragments. The isomer production ratio for the isotopes of Sn is more enhanced in the $^{\mathrm{nat}}\mathrm{U}(p,f$) reaction than in $^{232}\mathrm{Th}(p,f$). The opposite is observed for $^{96}\mathrm{Y}$ and $^{97}\mathrm{Y}$. These discrepancies might be associated to different scission shapes of the fragments for the two fission reactions, indicating the impact that the different fission modes can have on the isomeric yield ratios.