Cross sections for the formation ofHg195m,g,Hg197m,g, andAu196m,gin α andHe3-particle induced reactions on Pt: Effect of level density parameters on the calculated isomeric cross-section ratio

Abstract

Excitation functions were measured for the reactions ${}^{\mathrm{nat}}$Pt($^{3}\mathrm{He}$$,\mathit{xn}$)$^{195}\mathrm{Hg}$${}^{m,g},{}^{\mathrm{nat}}$Pt($^{3}\mathrm{He}$$,\mathit{xn}$)$^{197}\mathrm{Hg}$${}^{m,g},{}^{\mathrm{nat}}$Pt($^{3}\mathrm{He}$$,x$)$^{196}\mathrm{Au}$${}^{m,g}$, and ${}^{\mathrm{nat}}$Pt$(\ensuremath{\alpha},\mathit{xn}$)$^{197}\mathrm{Hg}$${}^{m,g}$ over the energy range of 18--35 MeV for $^{3}\mathrm{He}$ particles and 17--26 MeV for \ensuremath{\alpha} particles. The reactions $^{197}\mathrm{Au}$$(p,n$)$^{197}\mathrm{Hg}$${}^{m,g}$ were also investigated over the proton energy range of 6--20 MeV. The three projectiles were produced at the J\"ulich variable-energy compact cyclotron (CV 28). Use was made of the activation technique in combination with conventional high-resolution as well as low-energy HPGe-detector \ensuremath{\gamma}-ray spectroscopy. For most of the reactions, the present measurements provide the first consistent sets of data. From the available experimental data, isomeric cross-section ratios were determined for the above-mentioned reactions. Nuclear model calculations using the code STAPRE, which employs the Hauser-Feshbach (statistical model) and exciton model (precompound effects) formalisms, were undertaken to describe the formation of both the isomeric and the ground states of the products. The calculations were compared with the results of the EMPIRE-II code. The excitation functions of the ($^{3}\mathrm{He}$$,\mathit{xn})$ and $(\ensuremath{\alpha},\mathit{xn})$ processes are described well by the theory. In the case of ($^{3}\mathrm{He}$$,\mathrm{pxn})$ reactions, however, considerable deviations were observed between the experiment and the theory, presumably due to strong contributions from direct interactions. A description of the isomeric cross-section ratio by the model was possible only with a very low value of $\ensuremath{\eta}$, i.e., the ${\ensuremath{\Theta}}_{\mathrm{eff}}/{\ensuremath{\Theta}}_{\mathrm{rig}}$ ratio. A mass dependence of $\ensuremath{\eta}$ is proposed.