Incomplete fusion in $^{193}$Ir($^{12}$C, x)$^{205}$Bi reaction at $E_{lab}$ $\approx$ 5-7 AMeV

TL;DR

This study investigates incomplete fusion phenomena in $^{12}$C+$^{193}$Ir reactions at 5-7 AMeV, revealing how incomplete fusion fraction depends on energy and entrance-channel parameters, with implications for reaction modeling.

Contribution

It provides the first detailed measurement of incomplete fusion fractions and their dependence on entrance-channel parameters at these energies, using experimental data and statistical model analysis.

Findings

Incomplete fusion fraction increases linearly from 12% to 18% with energy.

Alpha-emitting channels show enhancement over model predictions, indicating incomplete fusion.

Projectile breakup suppresses complete fusion by approximately 12%.

Abstract

Low-energy heavy-ion induced reactions often involve incomplete fusion, but the dependence of ICF on various entrance-channel parameters remains unclear. In this work, we measure channel-by-channel production cross-sections of different evaporation residues populated via complete and/or incomplete fusion in $^{12}$C+$^{193}$Ir system at $E_{lab}$ $\approx$ 64--84 MeV ($\approx$ 5--7 AMeV) using the stacked-foil activation technique followed by offline $\gamma$-spectroscopy. Experimentally measured excitation functions have been analyzed in the framework of the statistical model code PACE4 using different values of the level-density parameter ($a$ = A/9-A/15 MeV${^{-1}}$). In the analysis of excitation functions, the $xn$ and $pxn$ channels (after correcting with their precursor contributions) have been explained fairly well with $a$ = A/13 MeV${^{-1}}$; however, almost all $\alpha$-emitting channels showed substantial enhancement over PACE4 predictions, which has been attributed to incomplete fusion. The incomplete fusion fraction ($F_{ICF}$) increases linearly with energy from 12\% to 18\% at 64 and 84 MeV, respectively. For better insights into the onset and strength of ICF, the variations of $F_{ICF}$ have been studied as a function of different entrance-channel parameters, which are found to increase with mass asymmetry, Coulomb factor, and neutron skin thickness. Further analysis of the data suggests the onset of ICF below the critical angular momentum ($\ell<\ell_{crit}$). Projectile breakup-driven incomplete fusion is found to suppress complete fusion by $\approx12\%$ and $\approx6\%$ w.r.t. the universal fusion function and the improved fusion function, respectively. These findings highlight the critical role of projectile structure at 5--7 AMeV energies, with implications for high-spin spectroscopy and reaction modeling.