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The astrophysical rapid neutron-capture process (r-process) is responsible for producing approximately half of the heavy elements in the Universe; however, its quantitative modeling remains limited by the scarcity of experimental knowledge of the structure of neutron-rich nuclei far from stability. In particular, nuclei in the $A \approx 115$ region provide an important benchmark for nuclear models used in r-process calculations, as their decay properties and level structures directly influence $\beta$-decay strength distributions and feeding patterns. The study of $\beta$-decay in this mass region allows for detailed exploration of low-lying states, spin–parity assignments, and the interplay between ground and isomeric configurations, which are essential for reliable theoretical predictions.
The present work focuses on the $\beta$-decay of $^{115}$Rh populating excited states in $^{115}$Pd, investigated at the IGISOL-4 facility at the JYFL Accelerator Laboratory (University of Jyväskylä, Finland). A high-purity beam of $^{115}$Rh was obtained using the JYFLTRAP Penning trap. The detection setup consisted of a $\beta$-plastic scintillator, three HPGe detectors, and a tape station. Excited states in $^{115}$Pd were investigated using $\gamma$-$\gamma$ and $\beta$-$\gamma$-$\gamma$ coincidence techniques, resulting in a substantially expanded and revised decay scheme. More than 98 previously unobserved $\gamma$ transitions and 25 new excited states were identified, extending the level scheme up to 1605 keV and significantly improving its completeness. For the first time, reliable absolute $\gamma$-ray intensities and $\beta$-feeding distributions have been determined, providing new insight into the structure of low-lying states.
A new half-life value of $T_{1/2} = 1.072(16)$ s was extracted, representing a significant improvement in precision compared to the previously adopted value ($T_{1/2} = 0.99(5)$ s [1]). Additionally, internal conversion coefficients for selected low-energy transitions were estimated using a $\beta$-$\gamma$-$\gamma$ versus single-$\gamma$ comparison method, enabling tentative multipolarity assignments for several key transitions. These results, combined with $\gamma$-branching ratios and $\log(ft)$ calculations, have led to revised spin–parity assignments for several states in $^{115}$Pd.
The presented findings provide important constraints for nuclear-structure models, particularly concerning the predicted shape evolution in Pd isotopes around N = 69 [2], and offer new insight into the band structure of $^{115}$Pd [3]. Additional knowledge of lifetimes, obtained by fast-timing techniques of excited states in $^{115}$Pd, will greatly expand and complement the presented results.
[1] J.Äystö et.al. “Identification and decay of new neutron-rich isotopes 115Rh and 116Rh”. In: Physics Letters B 201.2 (1988), pp. 211–214. issn: 0370-2693. doi: 10.1016/0370-2693(88)90214-6.
[2] J. Kurpeta, W. Urban. et. al. “Excited states in 115Pd populated in the $\beta$−decay of 115Rh”. In: Phys. Rev. C 82 (2 Aug. 2010), p. 027306. doi: 10.1103/PhysRevC.82.027306.
[3] J.O. Rasmussen, Y.X. Luo. et. al. “New insights into the nuclear structure in neutron-rich 112,114,115,116,117,118Pd”. In: Nuclear Physics A 919 (2013), pp. 67–98. issn:0375-9474. doi: 10.1016/j.nuclphysa.2013.10.002.