Speaker
Description
It is well established fact that nearly $50\%$ of all heavy neutron-rich nuclei (viz. in the range Fe to U) are produced via the rapid neutron capture process or $r$-process. This process is proposed to occur in explosive astrophysical sites like Core-collapse Supernovae and Neutron Star Mergers (NSM) [1]. The latter is already established observationally, via multi-messenger astronomy [2]. Neutron star is one of the densest objects in the universe and several aspects of its physical properties are still unresolved and unexplored, even after decades of scientific pursuit. Hence, the complete understanding of NSM and understanding $r$-process is still an open challenge in the field of nuclear astrophysics. Recently [3], it has been shown via simulation using SkyNet nuclear reaction network code, that the $(n,\gamma)$ capture of the neutron-rich nuclei near neutron shell-closure has a significant effect on final isotopic abundance in the $r$-process at NSM. However, due to limited measurements of such neutron-rich nuclei (owing to practical difficulties), $r$-process models like SkyNet usually considers the results from the Hauser-Feshbach (HF) statistical models, as nuclear physics inputs. The main problem with such HF models (e.g. T. Rauscher [4] or TALYS code) is that their predictability near shell-closure might be limited for neutron-rich nuclei, compared to other methods, e.g. shell model calculations. Then again, HF considers relatively high nuclear level densities (NLDs), whereas loosely bound neutron-rich nuclei are expected to have lower density of states [5]. In fact, recent measurements of $(n,\gamma)$ reaction on neutron-rich nuclei have yielded results significantly different from HF predictions [6,7,8]. In present work, a large-scale shell model calculation (with modern n-n interaction) [9], combined with Direct break-up model [6,7], has been used to compute the neutron capture cross-sections and reaction rates, for some of the nuclei important in $r$-process nucleosynthesis [3]. Combining the reaction rate from this work with the help of TALYS code, an estimate of NLDs of the reaction products have been attempted, considering Fermi-gas model. As expected, it has been observed that NLDs obtained are always lower than the default consideration of TALYS and for a relatively tightly bound nucleus, density of states is higher compared to the loosely bound one. The authors would like to present the theoretical computations for a number of nuclei relevant to this scenario, along with the detailed method and results of NLD estimation. If time permits, some experimental data will be presented also.
References
[1] T. Kajino et al. Prog. Part. Nucl. Phys. 107 (2019) pp. 109-166.
[2] N. Domoto et al. Astrophys. J 913 (2021) 26
[3] D. Vescovi et al. Front. astron. space sci. 9 (2022).
[4] T. Rauscher et al. At. Data Nucl. Data Tables 75.1 (2000) pp. 1-351
[5] K. Sieja, $\&$ S. Goriely. Eur. Phys. J. A 57, (2021) 110
[6] U. D. Pramanik et al. Phys. Lett. B 551 (2003) pp. 63-70.
[7] U. D. Pramanik. Prog. Part. Nucl. Phys. 59 (2007) pp. 183-192.
[8] A. Bhattacharyya et al. Phys. Rev. C 104 (2021) p. 045801.
[9] T. Miyagi et al. Phys. Rev. C 102 (2020) p. 034320.