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The synthesis of heavy elements in supernovae is affected by low-energy $(n,p)$ and $(p,n)$ reactions on unstable nuclei, yet experimental data on such reaction rates are scarce. The SECAR (SEparator for CApture Reactions) recoil separator at FRIB (Facility for Rare Isotope Beams) was originally designed to measure astrophysical reactions that change the mass of a nucleus significantly. We used a novel approach that integrates machine learning with ion-optical simulations to find an ion-optical solution for the separator that enables the measurement of $(p,n)$ reactions, despite the reaction leaving the mass of the nucleus nearly unchanged. A new measurement of the ${}^{58}\mathrm{Fe}(p,n){}^{58}\mathrm{Co}$ reaction in inverse kinematics with a $3.66\pm 0.12$ MeV/nucleon ${}^{58}\mathrm{Fe}$ beam (corresponding to $3.69\pm 0.12$ MeV proton energy in normal kinematics) yielded a cross-section of $20.3\pm 6.3$  mb and served as a proof of principle experiment for the new technique demonstrating its effectiveness in achieving the required performance criteria. This novel approach paves the way for studying astrophysically important $(p,n)$ reactions on unstable nuclei produced at FRIB. Published by the American Physical Society2025 

The LEGEND Collaboration is searching for neutrinoless double-beta ( $0\nu \beta \beta$ ) decay by operating high-purity germanium detectors enriched in ${}^{76}\mathrm{Ge}$ in a low-background liquid argon environment. Building on key technological innovations from the GERmanium Detector Array (GERDA) experiment and the MAJORANA DEMONSTRATOR experiment, LEGEND-200 has performed a first $0\nu \beta \beta$ decay search based on 61.0 kg yr of data. Over half of this exposure comes from our highest performing detectors, including newly developed inverted-coaxial detectors, and is characterized by an estimated background level of ${0.5}_{-0.2}^{+0.3}\mathrm{cts}/\left(\mathrm{keV}\mathrm{ton}\mathrm{yr}\right)$ in the $0\nu \beta \beta$ decay signal region. A combined analysis of data from GERDA, the MAJORANA DEMONSTRATOR, and LEGEND-200, characterized by a 90% confidence level exclusion sensitivity of $2.8\times {10}^{26}\mathrm{yr}$ on the half-life of $0\nu \beta \beta$ decay, reveals no evidence for a signal and sets a new observed lower limit at $T_{1/2}^{0\nu }>1.9\times {10}^{26}\mathrm{yr}$ (90% confidence level). Assuming the decay is mediated by Majorana neutrinos, this corresponds to an upper limit on the effective Majorana mass in the range $m_{\beta \beta }<75–200\mathrm{meV}$ , depending on the adopted nuclear matrix element.