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

TL;DR

This study probes incomplete fusion in the $^{12}$C+$^{193}$Ir system at $E_{lab}\approx 64$–$84$ MeV by measuring channel-by-channel evaporation-residue cross sections using stacked-foil activation and offline $\gamma$-spectroscopy, then interpreting the data with PACE4 across varying level-density parameters. It finds that $xn$ and $pxn$ channels are consistent with complete fusion (CF) when analyzed with $a=A/13\ \text{MeV}^{-1}$, while $\alpha$-emitting channels show strong enhancements attributable to ICF, quantified by $F_{ICF}$ rising from ~12% to ~18% with energy; $F_{ICF}$ also correlates with entrance-channel parameters like mass asymmetry, Coulomb factor, and neutron skin thickness, and ICF onset is observed below $\ell_{crit}$. The study demonstrates that projectile structure and $Q_\alpha$ values significantly influence ICF at 5–7 AMeV, with breakup contributing to fusion suppression; applying an Improved Fusion Function (IFF) reduces the inferred CF suppression to ~6%, underscoring the importance of $\ell$-dependent barrier effects in near-barrier reaction modeling. Overall, the results provide quantitative constraints on low-energy breakup-fusion dynamics for an $\alpha$-cluster projectile system and inform high-spin spectroscopy and reaction modeling needs.

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 $γ$-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 $α$-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.

Figures (16)

• Figure 1: $(a)$ A portion of $\gamma$-ray spectrum obtained at 84 MeV for $^{12}$C+$^{193}$Ir system, $(b)$ the decay curve of $^{201}$Bi nuclei reproducing a half-life of 103 min by following $E_\gamma$ = 629.1 keV.
• Figure 2: Experimental EF of $^{201}$Bi($4n$) compared with PACE4 calculations using different values of $K$ = 9, 11, 13 and 15.
• Figure 3: The EF of $^{200}$Bi($5n$) with PACE4 for $K$ = 13.
• Figure 4: $(a)$ The experimental EF of $^{201}$Pb (p3n) residue, $(b)$$\sigma_{exp}^{cum}$, $\sigma_{exp}^{pre}$ and $\sigma_{exp}^{ind}$ contributions are compared with PACE4 calculations for $K$ = 13.
• Figure 5: Experimental EF of $^{200}$Pb(p4n) residue. The contributions of $\sigma_{exp}^{pre}$ and $\sigma_{exp}^{ind}$ are plotted with $\sigma_{exp}^{cum}$ and PACE4 calculations for $K$ = 13.