Observed differences between argon- and krypton-induced reactions leading to the same compound nuclei,Er158andEr156

Abstract

Cross-bombardement experiments were carried out to produce the same compound nucleus ${}_{}{}^{158}{}_{}{}^{}\mathrm{Er}$ either in (${}_{}{}^{84}{}_{}{}^{}\mathrm{Kr}$ + ${}_{}{}^{74}{}_{}{}^{}\mathrm{Ge}$) or (${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ + ${}_{}{}^{118}{}_{}{}^{}\mathrm{Sn}$) reactions. Also the compound nucleus ${}_{}{}^{156}{}_{}{}^{}\mathrm{Er}$ was formed by using ${}_{}{}^{72}{}_{}{}^{}\mathrm{Ge}$ and ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$ targets. Evaporation residues ${}_{}{}^{152}{}_{}{}^{}\mathrm{Er}$ and ${}_{}{}^{153}{}_{}{}^{}\mathrm{Er}$ were detected, and excitation functions were constructed for $\mathrm{Kr},xn$ and $\mathrm{Ar},xn$ reactions, where x varies from 3 to 6. Some measurements were also made of $\mathrm{Ar},pxn$ and $\mathrm{Kr},pxn$ reactions leading to ${}_{}{}^{152}{}_{}{}^{}\mathrm{Ho}$. A comparison of the absolute cross sections shows that complete fusion is more severly limited in the case of Kr ions than for Ar ions, at the same values of excitation energy and of maximum orbital angular momentum. It was deduced that the critical angular momentum is lower in the case of krypton bombardments, $52\hslash$ instead of $76\hslash$ for Ar at ${\mathrm{E}}^{*}$ = 80 MeV. Also, the measured excitation functions are narrower for $\mathrm{Kr},xn$ than for $\mathrm{Ar},xn$ reactions and the Kr thresholds are shifted to higher energies by some 15 MeV, as if more energy were necessary for emitting a given number of neutrons in the $\mathrm{Kr},xn$ reactions than in the $\mathrm{Ar},xn$ reactions. Such an effect cannot be explained in terms of the deexcitation of the compound nucleus. We conclude that in the case of krypton-induced reactions on germanium, some particular aspects of the formation stage of the compound system may dissipate energy prior to attainment of full equilibrium.