Ca48 -induced reaction on the lanthanide target Gd154 and its decay to ground and metastable states within the dynamical cluster-decay model

Abstract

The decay mechanism of compound nucleus (CN) $^{202}\mathrm{Po}^{*}$, formed in $^{48}\mathrm{Ca}+^{154}\mathrm{Gd}$ reaction, is studied within the dynamical cluster-decay model (DCM) at various excitation energies ${E}_{\mathrm{CN}}^{*}$, where neutron emission $xn$, $x=3\text{--}5$, are the predominant decay modes. The study is of interest since $^{202}\mathrm{Po}^{*}$ decays to the ground state (g.s.) of $^{198}\mathrm{Po}$ by emission of $4n$ and to metastable states $^{199m}\mathrm{Po}$ and $^{197m}\mathrm{Po}$ via ($3n,5n$) emission, respectively. The DCM is applied here for the first time to the decays of metastable states. Both types of decays are analyzed separately, using neck-length $\mathrm{\ensuremath{\Delta}}R$ (equivalently, barrier-lowering) parameter, the only parameter in the DCM, to best fit the evaporation residue or channel cross section (${\ensuremath{\sigma}}_{xn}$) data and predict the quasifissionlike (qf-like) noncompound (${\ensuremath{\sigma}}_{\mathrm{qf}}$) and fusion-fission (${\ensuremath{\sigma}}_{\mathrm{ff}}$) cross sections. For g.s. to g.s. decay of $^{202}\mathrm{Po}^{*}$, possibly due to involving the deformed rare-earth lanthanide target $^{154}\mathrm{Gd}$, the observed $4n$ decay channel requires the noncompound nucleus (nCN) contribution, treated as the qf-like process. On the other hand, the decay mechanism of $^{202}\mathrm{Po}^{*}$ to metastable states (m.s.) $^{199m}\mathrm{Po}$ and $^{197m}\mathrm{Po}$, is a pure CN decay, i.e., the ${\ensuremath{\sigma}}_{\mathrm{qf}}$ is zero. In this study, we have included the deformation effects up to quadrupole deformations ${\ensuremath{\beta}}_{2i}$ and optimum orientations ${\ensuremath{\theta}}_{i}^{\mathrm{opt}.}$ for coplanar ($\mathrm{\ensuremath{\Phi}}={0}^{0}$) nuclei, using hot-compact configurations, supporting asymmetric fission of CN $^{202}\mathrm{Po}^{*}$. The variation of CN formation probability ${P}_{\mathrm{CN}}$ and survival probability ${P}_{\mathrm{surv}}$ with excitation energy ${E}_{\mathrm{CN}}^{*}$ is in complete agreement with the known systematic of other radioactive CN studied so far, thereby giving credence to the predicted ${\ensuremath{\sigma}}_{\mathrm{qf}}$ in g.s. to g.s. decay and fusion-fission cross section ${\ensuremath{\sigma}}_{\mathrm{ff}}$ of $^{202}\mathrm{Po}^{*}$. Interestingly, both the observed g.s. to g.s. and g.s. to m.s. processes occur at a fixed $\mathrm{\ensuremath{\Delta}}R=2.45\ifmmode\pm\else\textpm\fi{}0.20\phantom{\rule{0.28em}{0ex}}\mathrm{fm}$, which is within the nuclear proximity limit of $\ensuremath{\sim}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{fm}$, and hence useful for making predictions.