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 ²⁰²Po, populated by reactions with varying entrance channel mass asymmetry, the onset of non-compound-nucleus fission was proposed to be around Z_pZ_t ∼ 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 ²⁸Si+¹⁷⁶Yb reaction (Z_pZ_t = 980) to investigate the fusion 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 γ-ray spectrometry at few intermediate energies. γ -ray multiplicities of evaporation residues were measured to infer about their spin distribution. The measurements were carried out using NaI(Tl) detector-based 4π-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 model code PACE2 [Phys. Rev. C 21, 230 (1980)] with the coupled-channel fusion model 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 γ-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 fusion 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 fusion model with the statistical model codes CCFUS + PACE2 suggests that fusion suppression at beam energies close to the entrance channel Coulomb barrier where populated l waves are low is not l dependent.