Measurement of fusion-evaporation cross sections for reactions of Ca44 with Gd154,156,157,160 targets

Abstract

Background: Previously reported fusion-evaporation cross sections of residues in $^{45}\mathrm{Sc}$-induced reactions with lanthanide targets are much smaller than $^{48}\mathrm{Ca}$-induced reactions on the same targets. $^{44}\mathrm{Ca}$ is one proton removed from $^{45}\mathrm{Sc}$ and could be used to produce nuclei with a relative neutron content between those produced in the $^{45}\text{Sc-}$ and $^{48}\mathrm{Ca}$-induced reactions.Purpose: Several experiments worldwide have attempted to discover elements beyond the currently heaviest known element, oganesson ($Z=118$). Due to a lack of appropriate targets, these efforts focused on projectiles other than $^{48}\mathrm{Ca}$, which has been widely used for a number of successful element discovery experiments. The present study continues our previous work to understand the influence of various projectiles on the compound nucleus in fusion-evaporation reactions, and addresses the influence of target neutron number on fusion-evaporation cross sections.Methods: In experiments performed at the Cyclotron Institute at Texas A University, a beam of $^{44}\mathrm{Ca}^{6+}$ with an energy of $\ensuremath{\approx}5\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}/u$ was delivered by the K500 superconducting cyclotron to the Momentum Achromat Recoil Spectrometer (MARS). The $^{44}\mathrm{Ca}$ projectiles bombarded various isotopically enriched Gd targets in the MARS target chamber to create evaporation residues, which were spatially separated from unreacted projectiles by MARS and identified via their characteristic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{decay}$ energies. Excitation functions for the reactions of $^{44}\mathrm{Ca}$ with $^{154,156,157,160}\mathrm{Gd}$ were measured at several projectile energies each.Results: The maximum $4n$ cross sections in the $^{44}\mathrm{Ca}{+}^{154,156,157,160}\mathrm{Gd}$ reactions were $0.038\ifmmode\pm\else\textpm\fi{}0.008, 0.83\ifmmode\pm\else\textpm\fi{}0.08, 3.8\ifmmode\pm\else\textpm\fi{}0.2$, and $3.0\ifmmode\pm\else\textpm\fi{}0.5$ mb, respectively. Production cross sections for the more neutron-rich targets were surprisingly constant even given the substantial changes in the difference in neutron binding energy and fission barrier of the compound nuclei.Conclusions: Collective enhancements to level density caused a reduction in compound nucleus survivability for all targets. While this effect was required to obtain good agreement between theoretical calculations and experimental data, it was not sufficient to explain the cross sections for the reaction with the most neutron-deficient target studied $(^{154}\mathrm{Gd})$. Instead, it appears that the difference in the fission barrier and neutron binding energy is the dominant factor affecting the survival of the compound nucleus in this case.