Speaker
Description
Nucleosynthesis of elements beyond iron is an interesting area of research as it describes the evolution of our universe. It is governed mainly by slow ($s$-process), rapid ($r$-process) neutron capture, and $p$-process. On the neutron-deficient side of the valley of stability, there is a small fraction of the heavy elements known as $p$-nuclei. 35 stable isotopes in the mass region $^{74}$Se - $^{196}$Hg are called $p$-nuclei. These are formed by reactions of the $p$-process such as ($p$,$\gamma$), ($n$,$\gamma$), ($\alpha$,$\gamma$) and their reverse photo-disintegrations, commonly known as the $\gamma$-process [1].
Precise cross-sections and reaction rates for the $p$-process are essential for extracting the isotopic abundances of $p$-nuclei via network calculations. We report the experimentally measured cross-sections and $S$-factors for three different reactions, namely, $^{106}$Cd($p$,$\gamma$)$^{107}$In, $^{107}$Ag($p$,$\gamma$)$^{108}$Cd and $^{108}$Pd($p$,$\gamma$)$^{109}$Ag at multiple beam energies from 2.5 - 4.5 MeV. The $^{108}$Pd($p$,$\gamma$)$^{109}$Ag reaction has been studied for the first time in this energy range. These reactions are important for the partial knowledge of the $p$-process paths eventually leading to the formation of different $p$-nuclei.
The measurements were carried out at proton energies relevant for the $p$-process at the Folded Tandem Ion Accelerator (FOTIA) facility in BARC, Mumbai, India. Isotopically enriched $^{106}$Cd ($\approx 99.57\ \%$ enriched), $^{107}$Ag ($\approx 99.12\ \%$ enriched) and $^{108}$Pd ($\approx 99.50\ \%$ enriched) foils were prepared and used as targets inside a stainless steel chamber for this experiment. The characteristic $\gamma$-transitions feeding the ground states of $^{113}$In, $^{108}$Cd and $^{109}$Ag were detected using a HPGe detector. The detector was placed at a distance of 20 cm from the target position and at 90{$^\circ$} with respect to the beam direction. The energy and efficiency calibrations were carried out using $\gamma$-rays from $^{152}$Eu, $^{137}$Cs and $^{60}$Co radioactive sources. The most intense $\gamma$-transition was used for the calculation of the total radiative $p$-capture cross-sections for $^{107}$Ag and $^{108}$Pd targets, as other transitions were much less intense compared to them. The $\gamma$-transitions from the 1st and 3rd excited states to the ground states of $^{108}$Cd and $^{109}$Ag were considered in the calculations, respectively. In the case of the reaction $^{106}$Cd($p$,$\gamma$)$^{107}$In, six $\gamma$-transitions feeding the ground state of $^{107}$In were taken into account for the reaction cross-section measurements. The experimentally measured cross-sections for all these reactions have been compared with theoretical calculations using the TALYS code [2] based on the Hauser-Feshbach theory.
$\textbf{References}$
[1] M. Arnould and S. Goriely, Phys. Rep. $\textbf{384}$, 1 (2003).
[2] Arjan Koning, Stephane Hilaire and Stephane Goriely, $\textit{TALYS: modeling of nuclear reactions}$, European Journal of Physics A59 (6), 131 (2023).