Fusion-fission events in the ${}^{11}\mathrm{B}+{}^{181}\mathrm{Ta}$ reaction up to 5.7 MeV/nucleon energy

Background: In heavy-ion fusion reactions, the compound nucleus' de-excitation process is observed via two competitive modes, evaporation of particles and fission, at relatively higher excitation energies. Although fusion-fission dynamics have been extensively studied in actinide nuclei, exploration of the same is yet to come for preactinide nuclei.

Purpose: The objective is to understand the fusion-fission dynamics involved in the ${}^{11}\mathrm{B}+{}^{181}\mathrm{Ta}$ reaction within 4.8–5.7 MeV/nucleon energy by measuring absolute cross sections of the residues, which are mainly populating through complete and incomplete fusion-evaporation (FE), pre-equilibrium (PEQ), and fusion-fission (FF) mechanisms, as well as to infer on the mass and charge distributions of fission fragments.

Method: The stacked-foil activation technique was employed, followed by off-line $\gamma$-ray spectroscopy, to measure the activity and production cross sections of radionuclides populated through FE and FF. Charge and mass distributions were studied to obtain dispersion parameters of fission fragments.

Results: The equilibrium and pre-equilibrium reaction models have been used to analyze residual cross-section data resulting from FE and PEQ processes in the framework of EMPIRE3.2.2 and PACE4 codes. PEQ reaction's strengths in $3n$-channel have been extracted and compared with other systems. The relative distribution of spins between the ground and isomeric state of ${}^{186}\mathrm{Ir}$ has been discussed in view of the measured isomeric cross-section ratios. Further, 22 fission-fragments have been identified within the mass range $24\le A\le 160$. The variance extracted from the measured isotopic yield distributions of Nd isotopes is in good agreement with the literature values. A well-consistent approach has been employed to determine the isobaric yield distribution of Nd and In isotopes.

Conclusion: The combination of Hauser-Feshbach formalism for compound evaporation and the Exciton model for PEQ reaction agrees with the measured data of $xn$-channel residues, which confirms their production from the CF mechanism. Indirect evidence of ICF was observed in the $\alpha$- and $p$-emitting channels. Further, fission fragments' mass distribution is broad and symmetric, indicating their production through the compound nuclear mechanism. The variance of fission fragments' mass distribution shows an increasing trend with raising excitation energy.

Figure 1

A pictorial representation of the formation of an excited compound nucleus that predominantly de-excites by particle evaporation or fission.

Figure 2

A typical $\gamma$-ray spectrum of the $E_{\text{lab}}=62.4$ MeV ${}^{11}\mathrm{B}$ irradiated ${}^{181}\mathrm{Ta}$ collected 30 min after the EOB. The energy of the $\gamma$-ray peaks are in keV.

Figure 3

Comparison between measured excitation function of (a) ${}^{189}\mathrm{Pt}$, (b) ${}^{188}\mathrm{Pt}$, (c) ${}^{187,186}\mathrm{Pt}$, and (d) ${}^{186+187+188+189}\mathrm{Pt}$ with the theoretical predictions of EMPIRE3.2.2 (denoted by EMP3.2) and PACE4.

Figure 4

Comparison between measured excitation function of (a) ${}^{188}\mathrm{Ir}$, (b) ${}^{187}\mathrm{Ir}$ (Cumulative), (c) ${}^{186}\mathrm{Ir}(m+g$)(Cumulative), and (d) ${}^{183m}\mathrm{Os}$ with the theoretical predictions.

Figure 5

(a) Variation of ${\mathrm{F}}_{\text{PEQ}}$ as a function of $E_{\text{c.m.}}$ for ${}^{11}\mathrm{B}+{}^{181}\mathrm{Ta}$ (Present work) and ${}^7\mathrm{Li}+{}^{93}\mathrm{Nb}$, ${}^{11}\mathrm{B}+{}^{89}\mathrm{Y}$, ${}^{11}\mathrm{B}+{}^{93}\mathrm{Nb}$, ${}^{12}\mathrm{C}+{}^{89}\mathrm{Y}$; Prajapat 2020 [31] systems, (b) Variation of ICR of ${}^{186}\mathrm{Ir}$ with $E_{\text{c.m.}}$.

Figure 6

Isotopic yield distribution for (${}^{137,139m,149}\mathrm{Nd}$) neodymium isotopes at (a) $E_{\text{c.m.}}=56.2$ MeV, (b) $E_{\text{c.m.}}=53.4$ MeV, and for (c) indium isotopes (${}^{108,109,110m}\mathrm{In}$) at $E_{\text{c.m.}}=56.2$ MeV. Red line represents the Gaussian fit.

Figure 7

Fractional yield (FY) distribution for neodymium isotopes (${}^{137,139m,149}\mathrm{Nd}$) as a function of corrected charge ($Z\text{−}{Z}_{\text{mp}}$) at (a) $E_{\text{c.m.}}=56.2$ MeV, (b) $E_{\text{c.m.}}=53.4$ MeV, and for (c) indium isotopes (${}^{108,109,110m}\mathrm{In}$) at $E_{\text{c.m.}}=56.2$ MeV. Red line represents the Gaussian fit.

Figure 8

Mass distribution of fission fragments (a), (b) produced via complete and incomplete fusion-fission processes in ${}^{11}\mathrm{B}+{}^{181}\mathrm{Ta}$ reaction at different incident energies. Red line represents the Gaussian fit.

Figure 9

The mass variance of fission fragments' mass distribution plotted as a function of excitation energy ($E^{*}$) for the ${}^{11}\mathrm{B}+{}^{181}\mathrm{Ta}$ reaction. Solid red line shows the linear fit of data.

Figure 10

Comparison of the reduced fusion excitation function of ${}^{11}\mathrm{B}+{}^{181}\mathrm{Ta}$ system with similar systems such as ${}^{11}\mathrm{B}+{}^{159}\mathrm{Tb}$; Mukherjee 2006 [23], ${}^{11}\mathrm{B}+{}^{209}\mathrm{Bi}$; Gasques 2009 [3], ${}^{12}\mathrm{C}+{}^{181}\mathrm{Ta}$; Babu 2003 [24], ${}^{12}\mathrm{C}+{}^{181}\mathrm{Ta}$; Crippa 1994 [25], ${}^{12}\mathrm{C}+{}^{159}\mathrm{Tb}$; Sufang 1989 [26], and ${}^{16}\mathrm{O}+{}^{174,176}\mathrm{Yb}$; Rajbongshi 2016 [27].