We propose a new approach for a high-density free-neutron target, primarily aimed at nuclear astrophysics reaction studies in inverse kinematics with radioactive ions circulating in a storage ring. The target concept integrates four key subsystems: a neutron production source driven by a supercompact cyclotron utilizing 9Be(p; xn) reactions, an optimized moderator/reflector assembly using either heavy water or beryllium oxide with a graphite reflector shell to thermalize fast neutrons, a cryogenic liquid hydrogen moderator to maximize thermal neutron density in the interaction region, and beam pipe geometries that enable neutron-ion interactions while maintaining vacuum conditions for ion circulation. This integrated approach focuses on feasibility by incorporating readily available technologies. Using a commercial supercompact cyclotron delivering a proton beam of 130 mu A, the design achieves thermal neutron areal densities of similar to 3.4 & times; 106 n/cm2 for a proof-of-concept demonstrator at the CRYRING ionstorage ring at GSI Darmstadt. This autonomous accelerator-target assembly design enables deployment at both in-flight and ISOL facilities to exploit their complementary production mechanisms. Potential upgrades based on higher-energy and/or higher-current cyclotrons will enable an increase in areal density to similar to 109 n/cm2. In combination with a customized low-energy storage ring and a radioactive ion-beam facility, the proposed solution could deliver luminosities above 1023 cm-2 s-1, thereby enabling neutron capture measurements of similar to mb cross sections within a few days of experiment. The proposed system represents a significant milestone toward enabling large neutron-capture surveys on short-lived nuclei, thereby opening a new avenue for understanding the synthesis of heavy elements in our universe.

Neutron-induced reaction cross sections of short-lived nuclei are imperative to understand the origin of heavy elements in stellar nucleosynthesis and for societal applications, but their measurement is extremely complicated due to the radioactivity of the targets involved. One way of overcoming this issue is to combine surrogate reactions with the unique possibilities offered by heavy-ion storage rings. In this work, we describe the first surrogate-reaction experiment in inverse kinematics, which we successfully conducted at the Experimental Storage Ring (ESR) of the GSI/FAIR facility, using the ^{208}Pb(p,p^{'}) reaction as a surrogate for neutron capture on ^{207}Pb. Thanks to the outstanding detection efficiencies possible at the ESR, we were able to measure for the first time the neutron-emission probability as a function of the excitation energy of ^{208}Pb. We have used this probability to select different descriptions of the γ-ray strength function and nuclear level density, and provide reliable results for the neutron-induced radiative capture cross section of ^{207}Pb at energies for which no experimental data exist.

The probabilities for gamma-ray and particle emission as a function of the excitation energy of a decaying nucleus are valuable observables for constraining the ingredients of the models that describe the deexcitation of nuclei near the particle emission threshold. These models are essential in nuclear astrophysics and applications. In this paper, we have for the first time simultaneously measured the gamma-ray and neutron emission probabilities of 208Pb. The measurement was performed in inverse kinematics at the Experimental Storage Ring (ESR) of the GSI/FAIR facility, where a 208Pb beam interacted through the 208Pb(p, p') reaction with a hydrogen gas jet target. Instead of detecting the gamma rays and neutrons emitted by 208Pb, we detected the heavy beamlike residues produced after gamma and neutron emission. These heavy residues were fully separated by a dipole magnet of the ESR and were detected with outstanding efficiencies. The comparison of the measured probabilities with model calculations has allowed us to test and select different descriptions of the gamma-ray strength function and the nuclear level density available in the literature.

Radioactive nuclei with lifetimes on the order of millions of years can reveal the formation history of the Sun and active nucleosynthesis occurring at the time and place of its birth1,2. Among such nuclei whose decay signatures are found in the oldest meteorites, 205Pb is a powerful example, as it is produced exclusively by slow neutron captures (the s process), with most being synthesized in asymptotic giant branch (AGB) stars3-5. However, making accurate abundance predictions for 205Pb has so far been impossible because the weak decay rates of 205Pb and 205Tl are very uncertain at stellar temperatures6,7. To constrain these decay rates, we measured for the first time the bound-state β- decay of fully ionized 205Tl81+, an exotic decay mode that only occurs in highly charged ions. The measured half-life is 4.7 times longer than the previous theoretical estimate8 and our 10% experimental uncertainty has eliminated the main nuclear-physics limitation. With new, experimentally backed decay rates, we used AGB stellar models to calculate 205Pb yields. Propagating those yields with basic galactic chemical evolution (GCE) and comparing with the 205Pb/204Pb ratio from meteorites9-11, we determined the isolation time of solar material inside its parent molecular cloud. We find positive isolation times that are consistent with the other s-process short-lived radioactive nuclei found in the early Solar System. Our results reaffirm the site of the Sun's birth as a long-lived, giant molecular cloud and support the use of the 205Pb-205Tl decay system as a chronometer in the early Solar System.

Stable Tl205 ions have the lowest known energy threshold for capturing electron neutrinos (νe) of Eνe≥50.6 keV. The Lorandite Experiment (LOREX), proposed in the 1980s, aims at obtaining the longtime averaged solar neutrino flux by utilizing natural deposits of Tl-bearing lorandite ores. To determine the νe capture cross section, it is required to know the strength of the weak transition connecting the ground state of Tl205 and the 2.3 keV first excited state in Pb205. The only way to experimentally address this transition is to measure the bound-state beta decay (βb) of fully ionized Tl81+205 ions. After three decades of meticulous preparation, the half-life of the βb decay of Tl81+205 has been measured to be 291−27+33 days using the Experimental Storage Ring (ESR) at GSI, Darmstadt. The longer measured half-life compared to theoretical estimates reduces the expected signal-to-noise ratio in the LOREX, thus challenging its feasibility. Published by the American Physical Society 2024

Developments in the description of the masses of atomic nuclei have led to various nuclear mass models that provide predictions for masses across the whole chart of nuclides. These mass models play an important role in understanding the synthesis of heavy elements in the rapid neutron capture ($r$-) process. However, it is still a challenging task to estimate the size of uncertainty associated with the predictions of each mass model. In this work, a method to quantify the mass uncertainty using \textit{ensemble Bayesian model averaging} (EBMA) is introduced. This Bayesian method provides a natural way to perform model averaging, selection, calibration, and uncertainty quantification, by combining the mass models as a mixture of normal distributions, whose parameters are optimized against the experimental data, employing the Markov chain Monte Carlo (MCMC) method using the No-U-Turn sampler (NUTS). The average size of our best uncertainty estimates of neutron separation energies based on the AME2003 data is 0.48 MeV and covers 95% of new data in the AME2020. The uncertainty estimates can also be used to detect outliers with respect to the trend of experimental data and theoretical predictions.

We report the charge-changing cross sections (σcc) of 24 p-shell nuclides on both hydrogen and carbon at about 900A MeV, of which 8,9Li, 10-12Be, 10,14,15B, 14,15,17-22N and 16O on hydrogen and 8,9Li on carbon are for the first time. Benefiting from the data set, we found a new and robust relationship between the scaling factor of the Glauber model calculations and the separation energies of the nuclei of interest on both targets. This allows us to deduce proton radii (Rp) for the first time from the cross sections on hydrogen. Nearly identical Rp values are deduced from both target data for the neutron-rich carbon isotopes; however, the Rp from the hydrogen target is systematically smaller in the neutron-rich nitrogen isotopes. This calls for further experimental and theoretical investigations.

Stable ^{205}Tl ions have the lowest known energy threshold for capturing electron neutrinos (ν_{e}) of E_{ν_{e}}≥50.6  keV. The Lorandite Experiment (LOREX), proposed in the 1980s, aims at obtaining the longtime averaged solar neutrino flux by utilizing natural deposits of Tl-bearing lorandite ores. To determine the ν_{e} capture cross section, it is required to know the strength of the weak transition connecting the ground state of ^{205}Tl and the 2.3 keV first excited state in ^{205}Pb. The only way to experimentally address this transition is to measure the bound-state beta decay (β_{b}) of fully ionized ^{205}Tl^{81+} ions. After three decades of meticulous preparation, the half-life of the β_{b} decay of ^{205}Tl^{81+} has been measured to be 291_{-27}^{+33}  days using the Experimental Storage Ring (ESR) at GSI, Darmstadt. The longer measured half-life compared to theoretical estimates reduces the expected signal-to-noise ratio in the LOREX, thus challenging its feasibility.

Background: Experiments involving nuclear reactions in a storage ring offer exceptional possibilities for precise measurements in inverse kinematics. These experiments provide excellent angular and energy resolution by particle spectroscopy, in addition to high luminosities. However, the extremely low-pressure environment maintained in the storage rings poses significant difficulties for experiments employing detectors or any outgassing material in the ring. Purpose: We investigate nuclear reactions in inverse kinematics using the storage-ring technique. The reactions were induced by scattering of a 20 Ne beam off a hydrogen target at an energy of 50 MeV/u. Method: A beam of fully stripped 20 Ne ions was injected into the ESR storage ring at an energy of 50 MeV/u. The beam interacted with an internal hydrogen gas-jet target. An ultrahigh vacuum compatible detector setup was installed around the gas jet inside the ring to measure the recoiling particles generated by nuclear reactions. Results: Multiple reaction channels were observed during the experiment. In particular, we present the results from studies on elastic and inelastic scattering, as well as the neutron transfer reaction 20 Ne( p , d ) 19 Ne & lowast; . The experimental data were compared to calculations that took into account the most significant excited states, using a coupled-reaction channel approach. A very good agreement with the experimental data was achieved. Conclusions: The present results are the first demonstration of the investigation transfer reactions using detectors directly installed in the ring. This provides an important proof-of-principle for prospective studies with far-from- stability radioactive beams in the future.

Isomer spectroscopy of heavy neutron-rich nuclei beyond the N=126 closed shell has been performed for the first time at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. New millisecond isomers have been identified at low excitation energies, 985.3(19) keV in 213Tl and 874(5) keV in 215Tl. The measured half-lives of 1.34(5) ms in 213Tl and 3.0(3) ms in 215Tl suggest spins and parities 11/2- with the single proton-hole configuration h11/2 as leading component. They are populated via E1 transitions by the decay of higher-lying isomeric states with proposed spin and parity 17/2+, interpreted as arising from a single s1/2 proton hole coupled to the 8+ seniority isomer in the (A+1)Pb cores. The lowering of the 11/2- states is ascribed to an increase of the h11/2 proton effective single-particle energy as the second g9/2 orbital is filled by neutrons, owing to a significant reduction of the proton-neutron monopole interaction between the h11/2 and g9/2 orbitals. The new ms-isomers provide the first experimental observation of shell evolution in the almost unexplored N>126 nuclear region below doubly-magic 208Pb.

The nuclear two-photon or double-gamma (2γ) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the 2γ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to ∼100  keV and half-lives as short as ∼10  ms. The half-life for the 2γ decay of the first-excited 0^{+} state in bare ^{72}Ge ions was determined to be 23.9(6) ms, which strongly deviates from expectations.

The cross-shell excited states of 34Si have been investigated via beta decays of the 4- ground state and the 1+ isomeric state of 34Al. Since the valence protons and valence neutrons occupy different major shells in the ground state as well as the intruder 1+ isomeric state of 34Al, intruder levels of 34Si are populated via allowed beta decays. Spin assignments to such intruder levels of 34Si were established through gamma -gamma angular correlation analysis for the negative-parity states with dominant configurations (nu d3/2)-1 (R) (nu f7/2)1 as well as the positive-parity states with dominant configurations (nu sd )-2 (R) (nu f7/2p3/2)2. The configurations of such intruder states play crucial roles in our understanding of the N = 20 shell gap evolution. A configuration interaction model derived from the FSU Hamiltonian was utilized in order to interpret the intruder states in 34Si. Shell model interaction derived from a more fundamental theory with the valence space in medium similarity renormalization group method was also employed to interpret the structure of 34Si.

The excited states of N=44 ^{74}Zn were investigated via γ-ray spectroscopy following ^{74}Cu β decay. By exploiting γ-γ angular correlation analysis, the 2_{2}^{+}, 3_{1}^{+}, 0_{2}^{+}, and 2_{3}^{+} states in ^{74}Zn were firmly established. The γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 2_{2}^{+}, 3_{1}^{+}, and 2_{3}^{+} states were measured, allowing for the extraction of relative B(E2) values. In particular, the 2_{3}^{+}→0_{2}^{+} and 2_{3}^{+}→4_{1}^{+} transitions were observed for the first time. The results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the N=40 gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize ^{74}Zn in its ground state. Furthermore, an excited K=0 band with a significantly larger softness in its shape is identified. A shore of the N=40 "island of inversion" appears to manifest above Z=26, previously thought as its northern limit in the chart of the nuclides.

Determining the cross sections of neutron-induced reactions on short-lived nuclei is imperative to rate calculations in stellar nucleosynthesis and applications of nuclear physics. It is also an immense experimental challenge due to the radioactivity of the targets involved. Our goal is to circumvent this obstacle by using surrogate reactions in inverse kinematics at the heavy-ion storage rings of GSI/FAIR. We present here preliminary results from the first proof of principle experiment, where a beam of 208 Pb impinged on a H 2 gas jet target in the Experimental Storage Ring (ESR).

beta-delayed gamma-neutron spectroscopy has been performed on the decay of A=84 to 87 gallium isotopes at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of 3He neutron counters (BRIKEN). beta-2n-gamma events were measured in the decays of all of the four isotopes for the first time, which is direct evidence for populating the excited states of two-neutron daughter nuclei. Detailed decay schemes with the gamma branching ratios were obtained for these isotopes, and the neutron emission probabilities (P-xn) were updated from the previous study. Hauser-Feshbach statistical model calculations were performed to understand the experimental branching ratios. We found that the P-1n and P-2n values are sensitive to the nuclear level densities of 1n daughter nuclei and showed that the statistical model reproduced the P-2n/P-1n ratio better when experimental levels plus shell-model level densities fit by the Gilbert-Cameron formula were used as the level-density input. We also showed the neutron and gamma branching ratios are sensitive to the ground-state spin of the parent nucleus. Our statistical model analysis suggested J <= 3 for the unknown ground-state spin of the odd-odd nucleus Ga-86, from the I gamma(4(+)-> 2(+))/I-gamma(2(+)-> 0(+)) ratio of Ga-84 and the P-2n/P-1n ratio. These results show the necessity of detailed understanding of the decay scheme, including data from neutron spectroscopy, in addition to gamma measurements of the multineutron emitters.

The matter radius of the doubly magic ^56 Ni was extracted from a measurement of the differential cross section by employing, for the first time, elastic proton scattering in inverse kinematics with a radioactive beam at E_kin=390.2 MeV/nucleon circulating in a storage ring and passing an internal hydrogen gas-jet target with a revolution frequency of around 2 MHz. The novel experimental scheme is based on UHV-compatible Si detectors operated as active vacuum windows, which were implemented in the ESR storage ring at GSI. A matter radius ^1/2=3.74^+0.03_-0.06 fm was extracted for the doubly-magic self-conjugate nucleus ^56 Ni.

Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the r-process path and following freeze-out region impact the resulting r-process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei. Purpose: We aim to improve the understanding of the nuclear structure of 160Gd, specifically the K & pi; = 4+ bands, as well as study the & beta; decay of 160Eu into 160Gd. Methods: High-statistics decay spectroscopy of 160Gd resulting from the & beta;-decay of 160Eu was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility. Results: Two new excited states and ten new transitions were observed in 160Gd. The & beta;-decaying half-lives of the low-and high-spin isomers in 160Eu were determined, and the low-spin state's half-life was measured to be t1/2 = 26.0(8) s, & AP;16% shorter than previous measurements. Lifetimes of the two K & pi; = 4+ bandheads in 160Gd were measured for the first time, as well as & gamma; -& gamma; angular correlations and mixing ratios of intense transitions out of those bandheads. Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the K & pi; = 4+ bandheads in 160Gd is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in 160Gd heavily populated in & beta; decay is shown to have positive parity, which raises questions regarding the structure of the high-spin & beta;-decaying state in 160Eu.

This contribution is based on our input to the NuPECC LRP on per-spectives of precision experiments at heavy-ion storage rings in the realm of nuclear structure, atomic-and astrophysics. A focus here is on experi-ments with secondary beams of heavy ions, which can either be stable or long-lived nuclei in specific, high atomic charge states, or unstable nuclides.

Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the $r$-process path and following freeze-out region impact the resulting $r$-process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei.Purpose: We aim to improve the understanding of the nuclear structure of $^{160}\mathrm{Gd}$, specifically the ${K}^{\ensuremath{\pi}}={4}^{+}$ bands, as well as study the $\ensuremath{\beta}$ decay of $^{160}\mathrm{Eu}$ into $^{160}\mathrm{Gd}$.Methods: High-statistics decay spectroscopy of $^{160}\mathrm{Gd}$ resulting from the $\ensuremath{\beta}$-decay of $^{160}\mathrm{Eu}$ was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility.Results: Two new excited states and ten new transitions were observed in $^{160}\mathrm{Gd}$. The $\ensuremath{\beta}$-decaying half-lives of the low- and high-spin isomers in $^{160}\mathrm{Eu}$ were determined, and the low-spin state's half-life was measured to be ${t}_{1/2}=26.0(8)$ s, $\ensuremath{\approx}16%$ shorter than previous measurements. Lifetimes of the two ${K}^{\ensuremath{\pi}}={4}^{+}$ bandheads in $^{160}\mathrm{Gd}$ were measured for the first time, as well as $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ angular correlations and mixing ratios of intense transitions out of those bandheads.Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the ${K}^{\ensuremath{\pi}}={4}^{+}$ bandheads in $^{160}\mathrm{Gd}$ is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in $^{160}\mathrm{Gd}$ heavily populated in $\ensuremath{\beta}$ decay is shown to have positive parity, which raises questions regarding the structure of the high-spin $\ensuremath{\beta}$-decaying state in $^{160}\mathrm{Eu}$.

New mass values measured with the TITAN MR-TOF-MS are reported for the short-lived isotopes 24−26Ne, produced at TRIUMF's ISAC facility using a uranium carbide target and cold FEBIAD ion source. A least-squares adjustment within the framework of the Atomic Mass Evaluation was performed and the improved precision of the new mass values is highlighted. The improved mass accuracy in this N=14 mid-shell region can also point to the refinement in values of the charge radii. By reducing the mass uncertainty of isotopes, their contribution of the mass shift uncertainty in laser spectroscopy can be negated. The work is part of developments for reaching the N=20 Island of Inversion, where high molecular contamination hinders mass measurements of the neon isotopes.