Isomer production studied with simultaneous decay curve analysis for alpha-particle induced reactions on natural platinum up to 29 MeV

The isomeric ratios of \(^{198}\)Au, \(^{197}\)Hg and \(^{195}\)Hg produced by \(\alpha \)-particle induced reactions on natural platinum were investigated experimentally up to 29 MeV by using the standard stacked foil activation technique and \(\gamma \)-ray spectrometry. The isomeric ratios of \(^{197}\)Hg and \(^{195}\)Hg determined by the conventional activation cross section formula showed strong cooling time dependence. The time dependence was resolved by adjusting the isomeric transition branching ratios for the two isotopes within a simultaneous decay curve analysis framework. Our analysis suggests 94.5±0.7% and 48.9±1.8% as the isomeric transition branching ratios of \(^{197m}\)Hg (24 h) and \(^{195m}\)Hg (42 h), respectively. The isomeric ratios and independent production cross sections of \(^{198}\)Au, \(^{197}\)Hg, \(^{195}\)Hg and some other Hg, Au and Pt isotopes were also measured down to 6 MeV with these corrected isomeric transition branching ratios, and compared with predictions of statistical and pre-equilibrium models by TALYS-2.0 to discuss spin cutoff parameter dependence. We found the measured isomeric ratios are better predicted if we reduce the spin cutoff parameter to half or less from that estimated with the rigid body moment of inertia.