Speaker
Description
As per the Bohr’s independence hypothesis [1], the formation and decay of a compound nucleus (CN) are independent processes. As such, a nuclear reaction occurs in two steps: (i) the incident particle is absorbed by the target nucleus to form a quasi-stable CN with energy shared among all the nucleons, (ii) the equilibrated CN decays after a long time ($\approx 10^{-16}$ sec.). The CN theory was experimentally verified for the first time by Ghoshal in 1950 [2]. The CN reaction mechanism has been well studied for light-ion induced reactions. However, such studies for heavy ion (HI) reactions are scarce.
In the present work, two HI reaction systems, $^{16}\mathrm{O} + ^{172}\mathrm{Yb}$ and $^{19}\mathrm{F} + ^{169}\mathrm{Tm}$, forming the same compound nucleus $^{188}\mathrm{Pt}^*$, are studied. The experiments were carried out at the Inter University Accelerator Center, New Delhi. The stacked foil activation technique followed by offline γ-ray spectroscopy was used. The residues populated in the reactions were identified by their characteristic $\gamma$-lines and were confirmed by measuring their half-lives using decay curve analysis. The intensities of the identified $\gamma$-peaks were used to determine the fusion cross-sections for various reaction channels. The measured excitation functions for the $^{185}\mathrm{Pt} (3n)$, $^{184}\mathrm{Pt}(4n)$, $^{184}\mathrm{Ir}(p3n)$, and $^{183}\mathrm{Ir}(p4n)$ channels are also analyzed within the framework of statistical model code PACE4. At relatively higher excitation energies, the cross sections for the same exit channels are found to agree well for both the systems and with statistical model predictions. This supports the Bohr’s hypothesis. However, discrepancies in the measured cross-sections for the same exit channels populated via different entrance channels appear at lower excitation energies. In order to understand this discrepancy, the angular momentum distributions at several excitation energies were compared. The analysis shows that the mean angular momentum differs at lower excitation energies but becomes almost similar at relatively higher excitation energies.
These results suggest that Bohr’s hypothesis holds at relatively higher excitation energies where angular momentum distributions are nearly similar for both the systems. This highlights the role of angular momentum in CN decay in HI reactions. Further details of measurement and analysis will be presented.
References:
- N. Bohr; Nature 137, 344 (1936).
- S. N. Ghoshal; Phys. Rev. 80, 939 (1950).