Mass distribution and mass resolved angular distribution of fission products in ${}^{28}\mathrm{Si}+{}^{232}\mathrm{Th}$

Background: Fission process with heavier projectiles and actinide targets has contributions from processes, such as compound nucleus fission, transfer-induced fission, and noncompound nucleus fission. Mass distribution and mass-dependent anisotropy can be used to identify and delineate the contributions due to these different processes.

Purpose: Mass distribution in ${}^{28}\mathrm{Si}+{}^{232}\mathrm{Th}$ has been studied at beam energies of 180 and 158 MeV to investigate the nature of mass distribution arising from complete and incomplete momentum-transfer fission events. Mass-dependent angular anisotropy has been measured at 166 MeV to investigate the dominant noncompound nucleus process contributing to the fission.

Method: Mass distribution and mass resolved angular distribution of fission products were measured by the recoil catcher method followed by off-line $\gamma$-ray spectrometry.

Results: Mass distributions for full momentum-transfer fission processes were found to be symmetric, and those for transfer-induced fission were found to be asymmetric at both beam energies. The relative contribution from transfer-induced fission was found to be higher at lower beam energy. The anisotropy of the fission product angular distribution was found to increase with decreasing mass asymmetry.

Conclusions: The mass distribution indicates that, apart from the full momentum-transfer fission process, there is a significant contribution due to transfer-induced fission. The mass dependence of angular anisotropy indicated that preequilibrium fission is the dominant noncompound nucleus process in the present reaction system at near barrier energy $(E_{\mathrm{c}.\mathrm{m}.}/V_{\mathrm{C}}=1.06)$.

Figure 1

Schematic of the target catcher assembly for the mass distribution measurement.

Figure 2

Plot of independent yields of Sb and I isotopes at $E_{\mathrm{lab}}=180\mathrm{MeV}$.

Figure 3

Fractional independent yield (FIY) of Sb and I isotopes as a function of their A/Z at $E_{\mathrm{lab}}=180\mathrm{MeV}$.

Figure 4

Fractional independent yield (FIY) of Sb and I isotopes as a function of corresponding $Z-Z_P$ values at $E_{\mathrm{lab}}=180\mathrm{MeV}$.

Figure 5

Mass yields of the fission products as a function of their mass number at $E_{\mathrm{lab}}=180\mathrm{MeV}$.

Figure 6

Mass yields of the fission products as a function of their $A/Z$ at $E_{\mathrm{lab}}=180\mathrm{MeV}$.

Figure 7

Mass distribution in the ${}^{28}\mathrm{Si}+{}^{232}\mathrm{Th}$ reaction at $E_{\mathrm{lab}}=180\mathrm{MeV}$. The experimental FMT fission mass yield ($■$) and the reflected data ($\square$) are fitted to a Gaussian distribution (solid line). The extracted TF mass yield data ($●$) are fitted to two Gaussians (dotted line). ($▲$) are FMT fission yields not used in the Gaussian fitting (see the text for details).

Figure 8

Mass distribution at $E_{\mathrm{lab}}=158\mathrm{MeV}$. The solid line is the FMT fission mass distribution obtained by normalizing the FMT fission mass distribution fit at higher beam energy (see text for details). The dotted lines are Gaussian fits to the extracted TF mass yields.

Figure 9

(a)–(m) Angular distributions of different fission products in ${}^{28}\mathrm{Si}+{}^{232}\mathrm{Th}$ at $E_{\mathrm{lab}}=166\mathrm{MeV}$. The products formed in FMT fission are shown as solid squares ($■$). (i)–(k) The products with $A/Z>2.47$, formed mainly in TF, are shown with open circles ($○$).

Figure 10

Anisotropy of different fission products as a function of the mass number of the products. The mass yield weighted average anisotropy is shown as a dotted line.

Figure 11

Comparison of $K_0^2$ values for various target-projectile combinations leading to fissioning systems in the actinide and trans-actinide regions [11, 24, 25, 26, 27]. The projectile energy, in each case, is chosen such that the $\langle l^2\rangle$ values and compound nucleus temperature are close to that of the present system. The present experimental data is represented by the open circle ($○$).