Fission fragment angular distributions in pre-actinide nuclei

Abstract

Background: Complete fusion of two nuclei leading to formation of a heavy compound nucleus (CN) is known to be hindered by various fission-like processes, in which the composite system reseparates after capture of the target and the projectile inside the potential barrier. As a consequence of these non-CN fission (NCNF) processes, fusion probability $({P}_{\mathrm{CN}})$ starts deviating from unity. Despite substantial progress in understanding, the onset and the experimental signatures of NCNF and the degree of its influence on fusion have not yet been unambiguously identified.Purpose: This work aims to investigate the presence of NCNF, if any, in pre-actinide nuclei by systematic study of fission angular anisotropies and fission cross sections $({\ensuremath{\sigma}}_{\mathrm{fis}})$ in a number of nuclear reactions carried out at and above the Coulomb barrier $({V}_{\mathrm{B}})$.Method: Fission fragment angular distributions were measured for six $^{28}\mathrm{Si}\text{-induced}$ reactions involving isotopically enriched targets of $^{169}\mathrm{Tm},\phantom{\rule{0.16em}{0ex}}^{176}\mathrm{Yb},\phantom{\rule{0.16em}{0ex}}^{175}\mathrm{Lu},\phantom{\rule{0.16em}{0ex}}^{180}\mathrm{Hf},\phantom{\rule{0.16em}{0ex}}^{181}\mathrm{Ta}$, and $^{182}\mathrm{W}$ leading to probable formation of CN in the pre-actinide region, at a laboratory energy $({E}_{\mathrm{lab}})$ range of 129--146 MeV. Measurements were performed with large angular coverage $({\ensuremath{\theta}}_{\mathrm{lab}}={41}^{\ensuremath{\circ}}$--${170}^{\ensuremath{\circ}})$ in which fission fragments (FFs) were detected by nine hybrid telescope $(E\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}E)$ detectors. Extracted fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ were compared with statistical model (SM) predictions.Results: Barring two reactions involving targets with large non-zero ground state spin $(\mathcal{J})$, viz., $^{175}\mathrm{Lu}\left({\frac{7}{2}}^{+}\right)$ and $^{181}\mathrm{Ta}\phantom{\rule{4pt}{0ex}}\left({\frac{7}{2}}^{+}\right)$, experimental fission angular anisotropies were found to be higher in comparison with predictions of the statistical saddle point model (SSPM), at ${E}_{\mathrm{c}.\mathrm{m}.}$ near ${V}_{\mathrm{B}}$. Comparison of present results with those from neighboring systems revealed that experimental anisotropies increasingly deviated from SSPM predictions as one moved from pre-actinide to actinide nuclei. For reactions involving targets with large nonzero $\mathcal{J}$, this deviation was subdued. Comparison between measured ${\ensuremath{\sigma}}_{\mathrm{fis}}$ and predictions of SM indicated the presence of NCNF in at least four systems, when shell effects, both in the level density and the fission barrier, were included in the calculation.Conclusions: Systematic SM analysis of measured fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ confirmed the onset of NCNF in pre-actinide nuclei. Discrepancies between results about the degree of its influence on complete fusion, as deduced from various experimental probes, remain challenges to be solved. Complete measurement of all signatures of NCNF for many systems and preferably a dynamical description of the collisions between projectile and target nuclei are warranted for a deeper understanding.