Helium-ion-induced fission excitation functions of terbium and ytterbium.

Abstract

${\mathrm{He}}^{4}$-ion-induced fission excitation functions of terbium (Z=65) and ytterbium (Z=70) were obtained by measuring fission cross sections in the energy range of 40--65 MeV using lexan polycarbonate plastic as the fission fragment track detector. The present measurements extend the range of low-Z elements in the deformed region for which ${\mathrm{He}}^{4}$-ion-induced fission excitation functions were obtained at moderate energies. The analysis of the ratio of ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{f}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{n}}$\ensuremath{\approx}${\mathrm{\ensuremath{\sigma}}}_{\mathrm{f}}$/${\mathrm{\ensuremath{\sigma}}}_{\mathrm{R}}$, which is a measure of the competition between fission and neutron emission widths, in terms of the statistical model expression, indicates the fission thresholds for ${\mathrm{Ho}}^{163}$ and ${\mathrm{Hf}}^{177.1}$ to be 31.5\ifmmode\pm\else\textpm\fi{}3.5 and 26.7\ifmmode\pm\else\textpm\fi{}3.0 MeV, respectively. The corresponding value for the fission level density parameter ${\mathrm{a}}_{\mathrm{f}}$ was found to be A/12 ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$ for both the compound nuclei. ${\mathrm{Ho}}_{67}^{163}$ represents the lightest compound nucleus for which fission barrier has been determined experimentally. The measured fission barriers compare very well with the theoretical fission barriers obtained by liquid-drop-based models. The present data have been used along with similar data available in the literature to bring out some systematics in the fission properties of low-Z elements. A systematic trend was observed in the ${\mathrm{a}}_{\mathrm{f}}$/${\mathrm{a}}_{\mathrm{n}}$ ratio of preactinide elements ranging from ${\mathrm{Ho}}^{163}$ to ${\mathrm{At}}^{213}$ (16 nuclei). A linear dependence of ${\mathrm{log}}_{10}$${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{f}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{n}}$ with ${\mathrm{Z}}^{2}$/A at constant excitation energies was observed extending over 8 orders of magnitude for low Z-compound nuclear systems from ${\mathrm{Ho}}^{163}$ to ${\mathrm{At}}^{213}$ all of which are characterized by a predominance of symmetric fission. A linear dependence was also observed in the variation of ${\mathrm{log}}_{10}$${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{f}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{n}}$ with ${\mathrm{E}}_{\mathrm{f}}^{\ensuremath{'}}$--${\mathrm{B}}_{\mathrm{n}}^{\ensuremath{'}}$ where ${\mathrm{E}}_{\mathrm{f}}^{\ensuremath{'}}$ is the effective fission barrier and ${\mathrm{B}}_{\mathrm{n}}^{\ensuremath{'}}$, neutron binding energy including 50% shell correction and pairing energy term, at constant excitation energies (40 MeV) for these 16 low-Z compound nuclear systems. The results suggest that shell effects tend to persist even at excitation energies of 40 MeV. An analysis was also done to separate the symmetric and asymmetric fission barriers. It is observed that asymmetric fission barrier ${\mathrm{E}}_{\mathrm{f}}$(asym) is much higher than symmetric fission barrier ${\mathrm{E}}_{\mathrm{f}}$(sym) for A\ensuremath{\leqslant}200 clearly suggesting that symmetric fission is the only observable mode below compound nuclear mass A=200.