Mass resolved angular distribution of fission products in 12C+232Th reaction at sub-barrier energy

This article reports the study of mass resolved angular distribution of the fission products and the angular anisotropy in ²³²Th(¹²C,f) reaction at sub-barrier energy to investigate the possible role of shell effects and non-compound nucleus fission. In literature, average anisotropy values reported earlier for this system at sub-barrier energy widely vary. The present study was carried out using the recoil catcher technique followed by off-line γ-ray spectrometry at 62.5 MeV beam energy. Angular anisotropies of fission products showed slight enhancement in the symmetric mass region. Significantly higher value of the experimental average anisotropy (1.46 ± 0.06) compared to that obtained by Statistical Saddle Point Model (SSPM) calculation indicated the possible role of shell effect and non-compound nucleus fission at the present beam energy. Consideration of shell effects resulted in an enhancement of the calculated anisotropy, though it was still slightly lower compared to the experimental value. The contribution from the non-compound nucleus fission was confirmed based of the deviation of the SSPM calculation in the symmetric mass region, where shell effects are not expected to be significant. In order to reproduce the error weighted average anisotropy (1.73 ± 0.18) for the symmetric mass region, the required value for the variance of K-distribution was much lower compared to the value obtained from the SSPM calculations. Presence of insignificant mass dependence of anisotropy suggests quasifission to be the dominant non-compound nucleus fission mechanism for the present system at the sub-barrier energy. This is consistent with the earlier studies [Mein et al., Phys. Rev. C55 (1997) R995; Williams et al. Phys. Rev. C 88 (2013) 034611] attributing anomaly in the overall fission fragment angular distribution to the contribution from quasifission.