Mass and isotopic yield distributions of fission-like events in the ${}^{19}\mathrm{F}+{}^{169}\mathrm{Tm}$ system at low energies

In the present work, 22 fission-like residues with mass number $67\le A\le 156$ produced via complete fusion-fission and/or incomplete fusion-fission have been identified and their production cross sections measured at three projectile energies, viz., $92.0\pm 1.8$, $102.5\pm 1.5$, and $105.4\pm 1.6$ MeV in the ${}^{19}\mathrm{F}+{}^{169}\mathrm{Tm}$ system. The absolute cross section of each fission-like residue is measured by recoil-catcher activation technique using off-line $\gamma$-ray spectroscopy. The data have been analyzed to deduce the parameters for isotopic yield and isobaric charge distributions. The deduced isotopic yields for indium (${}^{107,108,109,110m}\mathrm{In}$) and neodymium (${}^{134,135,137,139}\mathrm{Nd}$) isotopes are reproduced by Gaussian distribution and their variance is in good agreement with that reported in literature. The mass distributions of fission-like residues at three energies are also found to be Gaussian peaked, which confirms their production via deexcitation of a compound nucleus through complete fusion-fission and/or incomplete fusion-fission processes. It has been observed that, the mass variance (${\sigma }_M^2$) values of fission fragments for two different systems, ${}^{12}\mathrm{C}+{}^{169}\mathrm{Tm}$ and ${}^{19}\mathrm{F}+{}^{169}\mathrm{Tm}$, at nearly the same excitation energy and angular momentum are distinctly different, indicating the importance of the entrance channel effect in fission reaction dynamics. The theoretically calculated isobaric charge dispersion parameters for these residues are found to be in good agreement with those obtained from the experimental data.

Figure 1

A typical $\gamma$-ray spectrum of ${}^{19}\mathrm{F}+{}^{169}\mathrm{Tm}$ interaction at 105.4 $\pm$ 1.6 MeV, where identified $\gamma$ lines assigned to different reaction products expected to be populated via cff and/or iff processes are indicated.

Figure 2

Isotopic yield distribution of indium isotopes (${}^{107,108,109,110m}\mathrm{In}$) measured at three incident energies. The blue line is the Gaussian fit (see text for details).

Figure 3

Isotopic yield distribution of neodymium isotopes (${}^{134,135,137,139}\mathrm{Nd}$) measured at three incident energies. The blue line is the Gaussian fit (see text for details).

Figure 4

Fractional independent yield (${FY}^{\prime }$) distribution for indium isotopes (${}^{107,108,109,110m}\mathrm{In}$) corresponding to corrected charge distribution. The blue line is the Gaussian fit (see text for details).

Figure 5

Fractional independent yield (${FY}^{\prime }$) distribution for neodymium isotopes (${}^{134,135,137,139}\mathrm{Nd}$) corresponding to corrected charge distribution. The blue line is the Gaussian fit (see text for details).

Figure 6

Mass distribution of fission-like events populated via complete fusion-fission and/or incomplete fusion-fission processes in the ${}^{19}\mathrm{F}+{}^{169}\mathrm{Tm}$ system at studied energies. The solid blue line is the Gaussian fitting (see text for details).

Figure 7

The observed mass variance (${\sigma }_M^2$) of fission fragment distribution plotted as a function of excitation energy ($E^{*}$) for the ${}^{19}\mathrm{F}+{}^{169}\mathrm{Tm}$ system. The solid line shows the linear increase in ${\sigma }_M^2$ with $E^{*}$. The vertical arrow indicates the value of excitation energy corresponding to the Coulomb barrier of the system.

Figure 8

A comparison of mass variance (${\sigma }_M^2$) with mass asymmetry at constant normalized energy ($E/V_b=1.12$) for three different systems, viz., ${}^{19}\mathrm{F}+{}^{169}\mathrm{Tm}$ (present work), ${}^{19}\mathrm{F}+{}^{209}\mathrm{Bi}$ [41], and ${}^{16}\mathrm{O}+{}^{232}\mathrm{Th}$ [42] (see text for details).

Figure 9

A plot of grazing angular momentum (${\ell }_{\mathrm{graz}}$) imparted to the system as a function of excitation energy ($E^{*}$) for the same deformed nucleus ${}^{169}\mathrm{Tm}$ for two different projectiles, ${}^{12}\mathrm{C}$ and ${}^{19}\mathrm{F}$. At the crossing point indicated by arrow, the two systems have nearly the same excitation energy and grazing angular momentum.

Figure 10

Mass distribution of identified fission fragments and heavy residues populated via complete fusion and/or incomplete fusion processes at $E_{\mathrm{lab}}=105.4\mathrm{MeV}$. The solid lines are the Gaussian fitting (see text for details).