Spin distributions and cross sections of evaporation residues in the ${}^{28}\mathrm{Si}+{}^{176}\mathrm{Yb}$ reaction

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 $Z_p{Z}_t\sim 1000$ [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 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 $\gamma$-ray spectrometry at few intermediate energies. $\gamma$-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 $\gamma$-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 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.

Figure 1

Singles and evaporation-residue-gated $\gamma$-ray fold distribution spectra in the ${}^{28}\mathrm{Si}+{}^{176}\mathrm{Yb}$ reaction at $E_{\mathrm{lab}}$ of 165.8 MeV. The inset shows the counts in each fold.

Figure 2

Plot of relative transmission efficiency of the HYRA as a function of the fraction of ERs emitted in the acceptance angle of the HYRA. The relative transmission efficiency of the ERs in the ${}^{28}\mathrm{Si}+{}^{176}\mathrm{Yb}$ reaction is deduced from earlier measurements in the ${}^{31}\mathrm{P}+{}^{170}\mathrm{Er}$ system [22].

Figure 3

Cross sections of evaporation residues in the ${}^{28}\mathrm{Si}+{}^{176}\mathrm{Yb}$ reaction. The parameters for the pace2 calculations were fixed by reproducing the fission cross section data for the ${}^{16}\mathrm{O}+{}^{188}\mathrm{Os}$ reaction [12].

Figure 4

(a) Typical fitting of the experimental fold distribution ($x$-axis scale on the top) data at $E_{\mathrm{lab}}=144\mathrm{MeV}$. (b) Multiplicity distributions obtained using the $\gamma$-ray detection efficiency as measured at 662 keV and by varying the efficiency by $\pm 10\%$ compared to that measured at 662 keV. (c) Average multiplicity distribution.

Figure 5

$\gamma$-ray multiplicity distributions extracted from the measured fold distributions in coincidence with evaporation residues in the reaction of ${}^{28}\mathrm{Si}+{}^{176}\mathrm{Yb}$.

Figure 6

The average $\gamma$-ray multiplicities of the evaporation residues in the ${}^{28}\mathrm{Si}+{}^{176}\mathrm{Yb}$ reaction as a function of beam energy. The average $\gamma$-ray multiplicities predicted from the statistical model code pace2 [16] with fusion $l$ distribution calculated using the code ccfus [17] also are given for comparison.