Gamma-Ray Spectra of $R$-Process Nuclei

TL;DR

The paper tackles how gamma-ray spectra from r-process decays encode information about the site and strength of the r-process, addressing the challenge of linking observed lines to specific nuclei. It combines nucleus-by-nucleus gamma emission data with a PRISM-based nucleosynthesis network over a GRB cocoon trajectory to produce time-dependent spectra for four representative r-process strengths. A detailed spectral decomposition identifies the nuclei and lines that dominate the emission across eight timescales, highlighting strong lines such as from light to mid-mass isotopes early on and long-lived species like 60Co and Sb isotopes at late times, as well as heavy actinide–fission backgrounds in extended scenarios. The study provides reference tables and qualitative guidance to interpret future gamma-ray detections from r-process sites, underscoring the substantial modeling and data uncertainties that must be overcome.

Abstract

The radioactive decay of unstable nuclei created in the rapid neutron capture process release a large amount of $γ$-rays. When the ejecta is optically thick, these $γ$-rays may contribute to an associated kilonova. Once transparent, prominent spectral features will be directly observable in current and future $γ$-ray detectors. In this work, we study and compare the $γ$-ray spectra of a limited, weak, strong, and extended $r$-process across a broad timescale, identifying the nuclei which significantly contribute. We discuss these findings in the context of observability, noting that there are several practical challenges to connecting observed signatures to specific nuclei. However, if these challenges can be overcome, direct observation of $γ$-rays from $r$-process sites can provide insight into the fundamental physics underpinning the $r$-process.

Figures (7)

• Figure 1: Prompt fission spectra of $^{252}$Cf from the CGMF code as compared to the experimental results of 2013PhRvC..87b4601B and 2018PhRvC..98a4612Q. In each case, the experimental spectra shown are the ones obtained from the LaBr$_{3}$ detector.
• Figure 2: Final abundance pattern (at 100,000 years) of each of our $r$-process scenarios: a limited $r$-process (Simulation A), a weak $r$-process, (Simulation B), a strong $r$-process (Simulation C), and an extended $r$-process (Simulation D). All four patterns are normalized to the abundance per nucleon.
• Figure 3: Each panel gives the spectral decomposition for one of the four simulations at one of the eight timescales in the region 0.1 $<$ E $<$ 1 MeV. The solid black line gives the total spectra, while the colored crosses denote the contributions to this spectra from an individual nuclei. Colors for nuclei are consistent across simulations and timescales, though some colors represent multiple nuclei (on different panels). The complete figure set (12 images) is available in the online journal, but is also displayed in Appendix \ref{['sec:fullfig']} for this pre-print version.
• Figure 4: Each panel gives the spectral decomposition for one of the four simulations at one of the eight timescales in the region E $>1$ MeV. The solid black line gives the total spectra, while the colored crosses denote the contributions to this spectra from an individual nuclei. Colors for nuclei are consistent across simulations and timescales, though some colors represent multiple nuclei (on different panels). The complete figure set (10 images) is available in the online journal, but is also displayed in Appendix \ref{['sec:fullfig']} for this preprint version.
• Figure 5: Each panel gives the comparison of the spectra simulations at one of the eight representative timescales for the energy region $E >$ 1 MeV.