Spin distributions and cross sections of evaporation residues in the Si28+Yb176 reaction

Abstract

Background: Non-compound-nucleus fission in the preactinide region has been an active area of investigation in the recent past. Based on the measurements of fission-fragment mass distributions in the fission of $^{202}\mathrm{Po}$, populated by reactions with varying entrance channel mass asymmetry, the onset of non-compound-nucleus fission was proposed to be around ${Z}_{p}{Z}_{t}\ensuremath{\sim}1000$ [Phys. Rev. C 77, 024606 (2008)], where ${Z}_{p}$ and ${Z}_{t}$ are the projectile and target proton numbers, respectively.Purpose: The present paper is aimed at the measurement of cross sections and spin distributions of evaporation residues in the $^{28}\mathrm{Si}+^{176}\mathrm{Yb}$ reaction (${Z}_{p}{Z}_{t}=980$) to investigate the hindrance which, in turn, would give information about the contribution from non-compound-nucleus fission in this reaction.Method: Evaporation-residue cross sections were measured in the beam energy range of 129--166 MeV using the hybrid recoil mass analyzer (HYRA) operated in the gas-filled mode. Evaporation-residue cross sections were also measured by the recoil catcher technique followed by off-line $\ensuremath{\gamma}$-ray spectrometry at few intermediate energies. $\ensuremath{\gamma}$-ray multiplicities of evaporation residues were measured to infer about their spin distribution. The measurements were carried out using NaI(Tl) detector-based 4\ensuremath{\pi}-spin spectrometer from the Tata Institute of Fundamental Research, Mumbai, coupled to the HYRA.Results: Evaporation-residue cross sections were significantly lower compared to those calculated using the statistical code pace2 [Phys. Rev. C 21, 230 (1980)] with the coupled-channel code ccfus [Comput. Phys. Commun. 46, 187 (1987)] at beam energies close to the entrance channel Coulomb barrier. At higher beam energies, experimental cross sections were close to those predicted by the model. Average $\ensuremath{\gamma}$-ray multiplicities or angular momentum values of evaporation residues were in agreement with the calculations of the code ccfus + pace2 within the experimental uncertainties at all the beam energies.Conclusions: Deviation of evaporation-residue cross sections from the fusion + statistical model predictions at beam energies close to the entrance channel Coulomb barrier indicates hindrance at these beam energies which would lead to non-compound-nucleus fission. However, reasonable agreement of average angular momentum values of evaporation residues at these beam energies with those calculated using the coupled-channel with the statistical codes ccfus + pace2 suggests that suppression at beam energies close to the entrance channel Coulomb barrier where populated $l$ waves are low is not $l$ dependent.