Search

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 birth. Among such nuclei whose decay signatures are found in the oldest meteorites, $^{205}$Pb 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) stars. However, making accurate abundance predictions for $^{205}$Pb has so far been impossible because the weak decay rates of $^{205}$Pb and $^{205}$Tl are very uncertain at stellar temperatures. To constrain these decay rates, we measured for the first time the bound-state $\beta^-$ decay of fully ionized $^{205}$Tl$^{81+}$, an exotic decay mode that only occurs in highly charged ions. The measured half-life is 4.7 times longer than the previous theoretical estimate 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 $^{205}$Pb yields. Propagating those yields with basic galactic chemical evolution (GCE) and comparing with the $^{205}$Pb/$^{204}$Pb ratio from meteorites, 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 $^{205}$Pb--$^{205}$Tl decay system as a chronometer in the early Solar System., Comment: 21 pages and 11 figures/tables. Published in Nature (2024)

Background: Globular clusters show strong correlations between different elements, such as the well-known sodium-oxygen anticorrelation. One of the main sources of uncertainty in this anticorrelation is the $^{22}$Ne($p,\gamma$)$^{23}$Na reaction rate, due to the possible influence of an unobserved resonance state at $E_\mathrm{x} = 8862$ keV ($E_\mathrm{r, c.m.} = 68$ keV). The influence of two higher-lying resonance states at $E_\mathrm{x} = 8894$ and $9000$ keV has already been ruled out by direct $^{22}$Ne($p,\gamma$)$^{23}$Na measurementsPurpose: To study excited states in $^{23}$Na above the proton threshold to determine if the unconfirmed resonance states in $^{23}$Na exist. Methods: The non-selective proton inelastic scattering reaction at low energies was used to search for excited states in $^{23}$Na above the proton threshold. Protons scattered from various targets were momentum-analysed in the Q3D magnetic spectrograph at the Maier-Leibnitz Laboratorium, Munich, Germany. Results: The resonance states previously reported at $E_\mathrm{x} = 8862$, $8894$ and $9000$ keV in other experiments were not observed in the present experiment at any angle. This result, combined with other non-observations of these resonance states in most other experiments, results in a strong presumption against the existence of these resonance states. Conclusions: The previously reported resonance states at $E_\mathrm{x} = 8862$, $8894$ and $9000$ keV are unlikely to exist and should be omitted from future evaluations of the $^{22}$Ne($p,\gamma$)$^{23}$Na reaction rates. Indirect studies using low-energy proton inelastic scattering are a simple and yet exceptionally powerful tool in helping to constrain astrophysical reaction rates by providing non-selective information of the excited states of nuclei., Comment: 1 single-panel figure and 5 double-panel figures. Updated version of paper with better organisation of parts

We used the $^{138}$Ba$(d,\alpha)$ reaction to carry out an in-depth study of states in $^{136}$Cs, up to around 2.5~MeV. In this work, we place emphasis on hitherto unobserved states below the first $1^+$ level, which are important in the context of solar neutrino and fermionic dark matter (FDM) detection in large-scale xenon experiments. We identify for the first time candidate metastable states in $^{136}$Cs, which would allow a real-time detection of solar neutrino and FDM events in xenon detectors, with high background suppression. Our results are also compared with shell-model calculations performed with three Hamiltonians that were previously used to evaluate the nuclear matrix element (NME) for $^{136}$Xe neutrinoless double beta decay. We find that one of these Hamiltonians, which also systematically underestimates the NME compared to the others, dramatically fails to describe the observed low-energy $^{136}$Cs spectrum, while the other two show reasonably good agreement.

Globular clusters contain multiple stellar populations, with some previous generation of stars polluting the current stars with heavier elements. Understanding the history of globular clusters is helpful in understanding how galaxies merged and evolved and therefore constraining the site or sites of this historic pollution is a priority. The acceptable temperature and density conditions of these polluting sites depend on critical reaction rates. In this paper, three experimental studies helping to constrain astrophysically important reaction rates are briefly discussed., Comment: Submission to conference proceedings of INPC2022 in Cape Town

We use a high resolution $^{32}{\rm S}(d,t)$ measurement to investigate the claimed existence of a 6401(3) keV state in $^{31}$S that may affect the $^{30}{\rm P}(p,\gamma)$ nuclear reaction rate in oxygen-neon (ONe) novae. Our data are shown to exclude the null hypothesis - that the state does not exist - with high significance. Additionally, the data also suggest the existence of a hitherto unreported state at 6648(4) keV. This state corresponds to a $^{30}{\rm P}(p,\gamma)$ resonance at $517(4)$ keV, located below the higher edge of the Gamow window for peak nova temperatures of about 0.4 GK.

[Background] Abundance anomalies in some globular clusters, such as the enhancement of potassium and the depletion of magnesium, can be explained in terms of an earlier generation of stars polluting the presently observed ones. It was shown that the potential range of temperatures and densities of the polluting sites depends on the strength of a few number of critical reaction rates. The reaction has been identified as one of these important reactions. [Purpose] The key ingredient for evaluating the thermonuclear reaction rate is the strength of the resonances which, at low energy, are proportional to their proton width. Therefore the goal of this work is to determine the proton widths of unbound 31P states. [Method] States in 31P were studied at the Maier-Leibnitz-Laboratorium using the one-proton transfer reaction. Deuterons were detected with the Q3D magnetic spectrometer. Angular distribution and spectroscopic factors were extracted for 27 states, and proton widths and resonance strengths were calculated for the unbound states. [Results] Several unbound states have been observed for the first time in a one-proton transfer reaction. Above 20 MK, the reaction rate is now entirely estimated from the observed properties of states. The reaction rate uncertainty from all resonances other than the resonance has been reduced down to less than a factor of two above that temperature. The unknown spin and parity of the resonance dominates the uncertainty in the rate in the relevant temperature range. [Conclusion] The remaining source of uncertainty on the reaction rate comes from the unknown spin and parity of the resonance which can change the reaction rate by a factor of ten in the temperature range of interest.

The isobaric multiplet mass equation (IMME) is known to break down in the first T = 2, A = 32 isospin quintet. In this work we combine high-resolution experimental data with state-of-the-art shell-model calculations to investigate isospin mixing as a possible cause for this violation. The experimental data are used to validate isospin-mixing matrix elements calculated with newly developed shell-model Hamiltonians. Our analysis shows that isospin mixing with nonanalog T = 1 states contributes to the IMME breakdown, making the requirement of an anomalous cubic term inevitable for the multiplet.

The occupancies and vacancies of the valence neutron orbitals across the stable tin isotopic chain from $112\leq A\leq 124$ have been determined. These were inferred from the cross sections of neutron-adding and -removing reactions. In each case, the reactions were chosen to have good angular-momentum matching for transfer to the low- and high-$\ell$ orbitals present in this valence space. These new data are compared to older systematic studies. The effective single-neutron energies are determined by combining information from energy centroids determined from the adding and removing reactions. Two of the five orbitals are nearly degenerate, below $N=64$, and approximately two MeV more bound than the other three, which are also degenerate., Comment: 11 pages, 10 figures

Background: The $^{136}$Ba isotope is the daughter nucleus in $^{136}$Xe $\beta\beta$ decay. It also lies in a shape transitional region of the nuclear chart, making it a suitable candidate to test a variety of nuclear models. Purpose: To obtain spectroscopic information on states in $^{136}$Ba, which will allow a better understanding of its low-lying structure. These data may prove useful to constrain future $^{136}$Xe $\to$ $^{136}$Ba neutrinoless $\beta\beta$ decay matrix element calculations. Methods: A $^{138}\mathrm{Ba}(p,t)$ reaction was used to populate states in $^{136}$Ba up to approximately 4.6 MeV in excitation energy. The tritons were detected using a high-resolution Q3D magnetic spectrograph. A distorted wave Born approximation (DWBA) analysis was performed for the measured triton angular distributions. Results: One hundred and two excited states in $^{136}$Ba were observed, out of which fifty two are reported for the first time. Definite spin-parity assignments are made for twenty six newly observed states, while previously ambiguous assignments for twelve other states are resolved., Comment: 16 pages, 15 figures, 1 table

We performed a high resolution study of $0^{+}$ states in $^{134}$Ba using the $^{136}$Ba($p,t$) two-neutron transfer reaction. Our experiment shows a significant portion of the $L = 0$ pair-transfer strength concentrated at excited $0^+$ levels in $^{134}$Ba. Potential implications in the context of $^{136}$Xe $\to$ $^{136}$Ba neutrinoless double beta decay matrix element calculations are briefly discussed.

Nucleon-transfer sum rules have been assessed via a consistent reanalysis of cross-section data from neutron-adding ($d$,$p$) and -removing ($d$,$t$) reactions on well-deformed isotopes of Gd, Dy, Er, Yb, and W, with $92\leq N\leq108$, studied at the Niels Bohr Institute in the 1960s and 1970s. These are complemented by new measurements of cross sections using the ($d$,$p$), ($d$,$t$), and ($p$,$d$) reactions on a subset of these nuclei. The sum rules, defined in a Nilsson-model framework, are remarkably consistent. A single overall normalization is used in the analysis, which appears to be sensitive to assumptions about the reaction mechanism, and in the case of sums using the ($d$,$t$) reaction, differs from values determined from reactions on spherical systems., Comment: 7 pages, 4 figures

New experimental data on the neutron single-particle character of the Pygmy Dipole Resonance (PDR) in $^{208}$Pb are presented. They were obtained from $(d,p)$ and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany. The new data are compared to the large suite of complementary, experimental data available for $^{208}$Pb and establish $(d,p)$ as an additional, valuable, experimental probe to study the PDR and its collectivity. Besides the single-particle character of the states, different features of the strength distributions are discussed and compared to Large-Scale-Shell-Model (LSSM) and energy-density functional (EDF) plus Quasiparticle-Phonon Model (QPM) theoretical approaches to elucidate the microscopic structure of the PDR in $^{208}$Pb., Comment: 9 pages, 4 figures

Spatially-correlated overabundances of $^{15}$N and $^{18}$O observed in some low-density graphite meteoritic grains have been connected to nucleosynthesis taking place in the helium-burning shell during core-collapse supernovae. Two of the reactions which have been identified as important to the final abundances of $^{15}$N and $^{18}$O are $^{18}$F($n,\alpha$)$^{15}$N and $^{18}$F($n,p$)$^{18}$O. The relative strengths of the $^{18}$F($n,\alpha$)$^{15}$N and $^{18}$F($n,p$)$^{18}$O reactions depend on the relative $\alpha_0$ and $p_0$ decays from states above the neutron threshold in $^{19}$F in addition to other properties. Experimental data on the charged-particle decays from these highly excited states are lacking or inconsistent. Two experiments were performed using proton inelastic scattering from LiF targets and magnetic spectrographs. The first experiment used the high-resolution Q3D spectrograph at Munich to constrain properties of levels in $^{19}$F. A second experiment using the Orsay Split-Pole spectrograph and an array of silicon detectors was performed in order to measure the charged-particle decays of neutron-unbound levels in $^{19}$F. A number of levels in $^{19}$F have been identified along with their corresponding charged-particle decays. The first state above the neutron threshold which has an observed proton-decay branch to the ground state of $^{18}$O lies 68 keV above the neutron threshold while the $\alpha$-particle decays from the neutron-unbound levels are generally observed to be much stronger than the proton decays. Neutron-unbound levels in $^{19}$F are observed to decay predominantly by $\alpha$-particle emission, supporting the role of $^{18}$F($n,\alpha$)$^{15}$N in the production of $^{15}$N in the helium-burning shell of supernovae. Improved resonant-scattering reaction data are required in order to be able to determine the reaction rates accurately., Comment: 13 pages, 5 figures, v3 more wrong affiliations

We used a high-resolution magnetic spectrograph to study neutron pair-correlated $0^+$ states in $^{136}$Ba, produced via the $^{138}{\rm Ba}(p,t)$ reaction. In conjunction with state-of-the-art shell model calculations, these data benchmark part of the dominant Gamow-Teller component of the nuclear matrix element (NME) for $^{136}$Xe neutrinoless double beta ($0\nu\beta\beta$) decay. We demonstrate for the first time an evaluation of part of a $0\nu\beta\beta$ decay NME by use of an experimental observable, presenting a new avenue of approach for more accurate calculations of $0\nu\beta\beta$ decay matrix elements.

The excitation spectra in the deformed nucleus 158Gd have been studied with high energy resolution by means of the (p,t) reaction using the Q3D spectrograph facility at the Munich Tandem accelerator. The angular distributions of tritons were measured for more than 200 excited states seen in the triton spectra up to 4.3 MeV. A number of 36 excited 0+ states (five tentative), have been assigned by comparison of experimental angular distributions with the calculated ones using the CHUCK code. Assignments for levels with higher spins are the following: 95 for 2+ states, 64 for 4+ states, 14 for 6+ states and about 20 for negative parity states. Sequences of states which can be treated as rotational bands are selected. The analysis of the moments of inertia defined for these bands is carried out. This high number of excited states in a deformed nucleus, close to a complete level scheme, constitutes a very good ground to check models of nuclear structure. The large ensembles of states with the same spin-parity offer unique opportunities for statistical analysis. Such an analysis for the 0+, 2+ and 4+ states sequences, for all K-values and for well-determined projections K of the angular momentum is performed. The obtained data may indicate on a K symmetry breaking. Experimental data are compared with interacting boson model (IBM) calculations using the spdf version of the model. The energies of the low-lying levels, the transition probabilities in the first bands and the distribution in transfer intensity of the 0+ states are calculated and compared with experiment., Comment: 25 pages, 15 figures

The pairing properties of the neutrinoless double-$\beta$ decay candidate $^{116}$Cd have been investigated. Measurements of the two-neutron removal reactions on isotopes of $^{114,116}$Cd have been made in order to identify 0$^+$ strength in the residual nuclei up to $\approx$3 MeV. No significant $L=0$ strength has been found in excited states indicating that the Bardeen-Cooper-Schrieffer (BCS) approximation is a reasonable basis to describe the neutrons in the ground state. This approximation avoids complications in calculations of double-$\beta$ decay matrix elements that use the quasiparticle random-phase approximation (QRPA) techniques. However this is not the case for the protons, where pair vibrations are prevalent and the BCS approximation is no longer valid, complicating the use of traditional QRPA techniques for this system as a whole., Comment: 5 pages, 3 figures, spreadsheet of supplemental material

Excited states in the 158Eu nucleus have been determined with the 160Gd(d, alpha)158Eu reaction, studied at an incident energy of 18.0 MeV with the Munich tandem and the Q3D spectrograph. More than 50 excited states have been determined up to 1.6 MeV excitation, some of them corresponding to states previously observed in the beta-decay of 158Sm. The number of levels found in this nucleus at low excitation energies follows the systematic trend of the level densities in the other isotopes with mass 152-156., Comment: 6 pages, 4 figures

An experiment addressing electron capture (EC) decay of hydrogen-like $^{142}$Pm$^{60+}$ ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7) s$^{-1}$ for automatic analysis and 0.0141(7) s$^{-1}$ for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4).

Excited states in the deformed nucleus $^{158}$Gd have been studied in the (p,t) reaction by using the Munich Tandem and Q3D spectrograph. 30 new excited 0$^+$ states (three tentative) have been assigned up to the 4.3 MeV excitation energy. The total number of 34 excited 0$^+$ states (four tentatively assigned) in a deformed nucleus, close to a complete level scheme, offers a new opportunity to test nuclear models and obtain more information on the structure of these special states., Comment: 10 pages, 7 figures

Detailed spectroscopic measurements of excited states in $^{50}$Cr and $^{62}$Zn were performed using 24~MeV (p,t) transfer reactions on $^{52}$Cr and $^{64}$Zn, respectively. In total, forty-five states in $^{50}$Cr and sixty-seven states in $^{62}$Zn were observed up to excitation energies of 5.5~MeV, including several previously unobserved states. These experimental results are compared to {\it ab-initio} shell-model calculations using chiral effective field theory ($\chi$-EFT) with the valence-space in-medium similarity renormalization group (VS-IMSRG) method. This comparison demonstrates good agreement in the level orderings with these new theoretical methods, albeit with a slight over binding in the calculations. This work is part of a continued push to benchmark {\it ab-initio} theoretical techniques to nuclear structure data in $0^+\rightarrow0^+$ superallowed Fermi $\beta$ decay systems., Comment: Submitted to Physical Review C as a Regular Article. 10 pages, 9 figures, 2 tables

A high-resolution $(p,t)$ experiment at $E_{p}$= 24 MeV was performed to populate low-spin states in the actinide nucleus ${}^{240}$Pu. The Q3D magnetic spectrograph of the Maier-Leibnitz Laboratory (MLL) in Munich (Germany) was used to identify the ejected tritons via $dE/E$ particle identification with its focal-plane detection system. Angular distributions were measured at nine different Q3D angles to assign spin and parity to the excited states based on a comparison with coupled-channels DWBA calculations. In total, 209 states have been excited in $^{240}$Pu up to an excitation energy of 3 MeV. Many previously known states have also been observed and their spin-parity assignments were confirmed. However, many of the populated states have been seen for the first time. The low-spin one-octupole phonon excitations, i.e. $K^{\pi} = 0^-,1^-,2^-,3^-$, could be observed and a new candidate for the $K = 3$ projection is proposed. Furthermore, the double-octupole or $\alpha$-cluster structure of the $0^+_2$ state in $^{240}$Pu has been studied in more detail. It is shown that the $0^+_2$ state in $^{230}$Th has a distinctly different structure. In addition, strongly excited $1^-$ states have been observed at 1.5\,MeV and 1.8\,MeV in $^{240}$Pu. The present study suggests that similar states might be observed in $^{230}$Th. At least two different and distinct structures for $J^{\pi} = 0^+$ states are present in the actinides. These are pairing states and states with enhanced octupole correlations. We have shown that it is crucial to consider negative-parity single-particle states being admixed to some $K^{\pi} = 0^+_2$ rotational bands to understand the $\alpha$-decay hindrance factors and enhanced $E1$-decay rates. Based on our analysis, we have identified the double-octupole or $\alpha$-cluster $K^{\pi} = 0^+$ candidates from $^{224}$Ra to $^{240}$Pu., Comment: This manuscript has been published in Phys. Rev. C 97, 064319 (2018). This is the preprint version

The $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction is an important source of neutrons for the s-process. Due to the difficulty in directly measuring the cross section of this reaction and the competing $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg reaction, indirect methods are frequently used to try to instead calculate the relevant reaction rates. Uncertainty, however, persists in the number and properties of the pertinent excited states of $^{26}$Mg. In this paper, the results of proton and deuteron scattering from $^{26}$Mg are presented, giving a list of levels in $^{26}$Mg. In addition, level assignments in $^{26}$Mg are discussed, including some possible changes to the previously accepted assignments in $^{26}$Mg., Comment: 12 pages, 3 figures, 2 tables. Fixed missing reference and some index labelling in figures, fixed page break problem under table

The rearrangements of protons and neutrons amongst the valence single-particle orbitals during double {\beta} decay of 100Mo have been determined by measuring cross sections in (d,p), (p,d), (3He,{\alpha}) and (3He,d) reactions on 98,100Mo and 100,102Ru targets. The deduced nucleon occupancies reveal significant discrepancies when compared with theoretical calculations; the same calculations have previously been used to determine the nuclear matrix element associated with the decay probability of double {\beta} decay of the 100Mo system., Comment: 18 pages, 13 figures, 37 pages of supplemental information

$Background:$ Previous measurements of $\beta$-delayed neutron emitters comprise around 230 nuclei, spanning from the $^{8}$He up to $^{150}$La. Apart from $^{210}$Tl, with a minuscule branching ratio of 0.007\%, no other neutron emitter is measured yet beyond $A=150$. Therefore new data are needed, particularly in the heavy mass region around N=126, in order to guide theoretical models and to understand the formation of the third r-process peak at $A\sim195$. $Purpose:$ To measure both, $\beta$-decay half-lives and neutron branching ratios of several neutron-rich Au, Hg, Tl, Pb and Bi isotopes beyond $N=126$. $Method:$ Ions of interest are produced by fragmentation of a $^{238}$U beam, selected and identified via the GSI-FRS fragment separator. A stack of segmented silicon detectors (SIMBA) is used to measure ion-implants and $\beta$-decays. An array of 30 $^3$He tubes embedded in a polyethylene matrix (BELEN) is used to detect neutrons with high efficiency and selectivity. A self-triggered digital system is employed to acquire data and to enable time-correlations. The latter are analyzed with an analytical model and results for the half-lives and neutron-branching ratios are derived using the binned Maximum-Likelihood method. $Results:$ Twenty new $\beta$-decay half-lives are reported for $^{204-206}$Au, $^{208-211}$Hg,$^{211-216}$Tl,$^{215-218}$Pb and $^{218-220}$Bi, nine of them for the first time. Neutron emission probabilities are reported for $^{210,211}$Hg and $^{211-216}$Tl. $Conclusions:$ The new $\beta$-decay half-lives are in good agreement with previous measurements in this region. The measured neutron emission probabilities are comparable or smaller than values predicted by global models like RHB+RQRPA., Comment: 17 pages, 21 figures, will be submitted at PRC

A $^{52}$Cr$(p,t)$$^{50}$Cr two-neutron pickup reaction was performed using the Q3D magnetic spectrograph at the Maier-Leibnitz-Laboratorium in Garching, Germany. Excited states in $^{50}$Cr were observed up to an excitation energy of 5.3 MeV. Despite significantly increased sensitivity and resolution over previous work, no evidence of the previously assigned first excited $0^+$ state was found. As a result, the $0^+_2$ state is reassigned at an excitation energy of $E_x=3895.0(5)$ keV in $^{50}$Cr. This reassignment directly impacts direct searches for a non-analogue Fermi $\beta^+$ decay branch in $^{50}$Mn. These results also show better systematic agreement with the theoretical predictions for the $0^+$ state spectrum in $^{50}$Cr using the same formalism as the isospin-symmetry-breaking correction calculations for superallowed nuclei. The experimental data are also compared to $ab$-$initio$ shell-model predictions using the IM-SRG formalism based on $NN$ and $3N$ forces from chiral-EFT in the $pf$-shell for the first time., Comment: Accepted for Publication in Physical Review C as a Rapid Communication

The $\beta$-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with $\beta$-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb and Bi in the mass region N$\gtrsim$126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the $\beta$-decay strength distribution. In doing so, it provides important constraints to global theoretical models currently used in $r$-process nucleosynthesis., Comment: Submitted to Phys. Rev. Lett

Uncertainties in the thermonuclear rates of the $^{15}$O($\alpha,\gamma$)$^{19}$Ne and $^{18}$F($p,\alpha$)$^{15}$O reactions affect model predictions of light curves from type I X-ray bursts and the amount of the observable radioisotope $^{18}$F produced in classical novae, respectively. To address these uncertainties, we have studied the nuclear structure of $^{19}$Ne over $E_{x} = 4.0 - 5.1$ MeV and $6.1 - 7.3$ MeV using the $^{19}$F($^{3}$He,t)$^{19}$Ne reaction. We find the $J^{\pi}$ values of the 4.14 and 4.20 MeV levels to be consistent with $9/2^{-}$ and $7/2^{-}$ respectively, in contrast to previous assumptions. We confirm the recently observed triplet of states around 6.4 MeV, and find evidence that the state at 6.29 MeV, just below the proton threshold, is either broad or a doublet. Our data also suggest that predicted but yet unobserved levels may exist near the 6.86 MeV state. Higher resolution experiments are urgently needed to further clarify the structure of $^{19}$Ne around the proton threshold before a reliable $^{18}$F($p,\alpha$)$^{15}$O rate for nova models can be determined., Comment: 5 pages, 3 figures, Phys. Rev. C (in press)

Background: The thermonuclear $^{34g,m}$Cl($p,\gamma$)$^{35}$Ar reaction rates are unknown due to a lack of experimental nuclear physics data. Uncertainties in these rates translate to uncertainties in $^{34}$S production in models of classical novae on oxygen-neon white dwarfs. $^{34}$S abundances have the potential to aid in the classification of presolar grains. Purpose: Determine resonance energies for the $^{34g,m}$Cl($p,\gamma$)$^{35}$Ar reactions within the region of astrophyical interest for classical novae to a precision of a few keV as an essential first step toward constraining their thermonuclear reaction rates. Method: $^{35}$Ar excited states were populated by the $^{36}$Ar($d,t$)$^{35}$Ar reaction at $E$(d)=22~MeV and reaction products were momentum analyzed by a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph. Results: Seventeen new $^{35}$Ar levels have been detected at a statistically significant level in the region $E_x\approx$~5.9-6.7~MeV ($E_r$ \textless~800~ keV) and their excitation energies have been determined to typical uncertainties of 3~keV. The uncertainties for five previously known levels have also been reduced substantially. The measured level density was compared to those calculated using the WBMB Hamiltonian within the $sd-pf$ model space. Conclusions: Most of the resonances in the region of astrophyical interest have likely been discovered and their energies have been determined, but the resonance strengths are still unknown, and experimentally constraining the $^{34g,m}$Cl($p,\gamma$)$^{35}$Ar reaction rates will require further experiments., Comment: 6 pages, 3 figures

Half of the heavy elements including all actinides are produced in r-process nucleosynthesis whose sites and history still remain a mystery. If continuously produced, the Interstellar Medium (ISM) is expected to build up a quasi-steady state of abundances of short-lived nuclides (with half-lives <100My), including actinides produced in r-process nucleosynthesis. Their existence in today's ISM would serve as a radioactive clock and would establish that their production was recent. In particular $^{244}$Pu, a radioactive actinide nuclide (81 My half-life), can place strong constraints on recent r-process frequency and production yield. Here we report on the detection of live interstellar $^{244}$Pu, archived in Earth's deep-sea floor during the last 25 My, at abundances lower by about two orders of magnitude than expected from continuous production in the Galaxy. This large discrepancy may signal a rarity of actinide r-process nucleosynthesis sites, compatible with neutron-star mergers or with a small subset of actinide-producing supernovae.

Measurements of sulphur isotopes in presolar grains can help to identify the astrophysical sites in which these grains were formed. A more precise thermonuclear rate of the 33S(p,gamma)34Cl reaction is required, however, to assess the diagnostic ability of sulphur isotopic ratios. We have studied the 33S(3He,d)34Cl proton-transfer reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. Deuteron spectra were measured at ten scattering angles between 10 and 55 degrees. Twenty-four levels in 34Cl over Ex = 4.6 - 5.9 MeV were observed, including three levels for the first time. Proton spectroscopic factors were extracted for the first time for levels above the 33S+p threshold, spanning the energy range required for calculations of the thermonuclear 33S(p,gamma)34Cl rate in classical nova explosions. We have determined a new 33S(p,gamma)34Cl rate using a Monte Carlo method and have performed new hydrodynamic nova simulations to determine the impact on nova nucleosynthesis of remaining nuclear physics uncertainties in the reaction rate. We find that these uncertainties lead to a factor of less than 5 variation in the 33S(p,gamma)34Cl rate over typical nova peak temperatures, and variation in the ejected nova yields of S--Ca isotopes by less than 20%. In particular, the predicted 32S/33S ratio is 110 - 130 for the nova model considered, compared to 110 - 440 with previous rate uncertainties. As recent type II supernova models predict ratios of 130 - 200, the 32S/33S ratio may be used to distinguish between grains of nova and supernova origin., Comment: 7 pages, 3 figures, 3 tables, Phys. Lett. B (in press)

$J^{\pi}$= 0$^+$ states have been investigated in the actinide nucleus ${}^{240}$Pu up to an excitation energy of 3 MeV with a high-resolution (p,t) experiment at $E_{p}$= 24 MeV. To test the recently proposed $J^{\pi}$= 0$^+_2$ double-octupole structure, the phenomenological approach of the spdf-interacting boson model has been chosen. In addition, the total 0$^+$ strength distribution and the $0^+$ strength fragmentation have been compared to the model predictions as well as to the previously studied (p,t) reactions in the actinides. The results suggest that the structure of the 0$^+_2$ states in the actinides might be more complex than the usually discussed pairing isomers. Instead, the octupole degree of freedom might contribute significantly. The signature of two close-lying 0$^+$ states below the 2-quasiparticle energy is presented as a possible manifestation of strong octupole correlations in the structure of the 0$^+_2$ states in the actinides., Comment: 6 pages, 5 figures, published in Phys. Rev. C 88, 041303(R) (2013)

The periodic time modulations, found recently in the two-body orbital electron-capture (EC) decay of both, hydrogen-like $^{140}$Pr$^{58+}$ and $^{142}$Pm$^{60+}$ ions, with periods near to 7s and amplitudes of about 20%, were re-investigated for the case of $^{142}$Pm$^{60+}$ by using a 245 MHz resonator cavity with a much improved sensitivity and time resolution. We observed that the exponential EC decay is modulated with a period $T = 7.11(11)$s, in accordance with a modulation period $T = 7.12(11)$ s as obtained from simultaneous observations with a capacitive pick-up, employed also in the previous experiments. The modulation amplitudes amount to $a_R = 0.107(24)$ and $a_P = 0.134(27)$ for the 245 MHz resonator and the capacitive pick-up, respectively. These new results corroborate for both detectors {\it exactly} our previous findings of modulation periods near to 7s, though with {\it distinctly smaller} amplitudes. Also the three-body $\beta^+$ decays have been analyzed. For a supposed modulation period near to 7s we found an amplitude $a = 0.027(27)$, compatible with $a = 0$ and in agreement with the preliminary result $a = 0.030(30)$ of our previous experiment. These observations could point at weak interaction as origin of the observed 7s-modulation of the EC decay. Furthermore, the data suggest that interference terms occur in the two-body EC decay, although the neutrinos are not directly observed., Comment: In memoriam of Prof. Paul Kienle, 9 pages, 1 table, 5 figures Phys. Lett. B (2013) online

The rapid neutron nucleosynthesis process involves an enormous amount of very exotic neutron-rich nuclei, which represent a theoretical and experimental challenge. Two of the main decay properties that affect the final abundance distribution the most are half-lives and neutron branching ratios. Using fragmentation of a primary $^{238}$U beam at GSI we were able to measure such properties for several neutron-rich nuclei from $^{208}$Hg to $^{218}$Pb. This contribution provides a short update on the status of the data analysis of this experiment, together with a compilation of the latest results published in this mass region, both experimental and theoretical. The impact of the uncertainties connected with the beta-decay rates and with beta-delayed neutron emission is illustrated on the basis of $r$-process network calculations. In order to obtain a reasonable reproduction of the third $r$-process peak, it is expected that both half-lives and neutron branching ratios are substantially smaller, than those based on FRDM+QRPA, commonly used in $r$-process model calculations. Further measurements around $N\sim126$ are required for a reliable modelling of the underlying nuclear structure, and for performing more realistic $r$-process abundance calculations., Comment: Nuclear Physics in Astrophysics VI, Lisbon 2013, Conference Proceedings

The 18F(p,\alpha)15O reaction rate is crucial for constraining model predictions of the \gamma-ray observable radioisotope 18F produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, 19Ne, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between J\pi=3/2+ states near the proton threshold (Sp = 6.411 MeV) and a broad J\pi=3/2+ state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical S-factor at nova temperatures. Here we report a new measurement of states in this energy region using the 19F(3He,t)19Ne reaction. In stark contrast with previous assumptions we find at least 3 resonances between the proton threshold and Ecm=50 keV, all with different angular distributions. None of these are consistent with J\pi= 3/2+ angular distributions. We find that the main uncertainty now arises from the unknown proton-width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects 18F production by at least a factor of two in the model considered., Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev. Lett

The pairing properties of the neutrinoless double beta decay $(0\nu2\beta)$ candidate $^{100}$Mo have been studied, along with its daughter $^{100}$Ru, to provide input for nuclear matrix element calculations relevant to the decay. The $(p,t)$ two-neutron transfer reaction was measured on nuclei of $^{102,100}$Ru and $^{100,98}$Mo. The experiment was designed to have particular sensitivity to $0^{+}$ states up to excitation energies of $\sim 3$ MeV with high energy resolution. Measurements were made at two angles and L=0 transitions identified by the ratio of yields between the two angles. For the reactions leading to and from $^{100}$Ru, greater than 95% of the L=0 $(p,t)$ strength was in the ground state, but in $^{100}$Mo about 20% was in excited $0^{+}$ states. The measured $(p,t)$ data, together with existing $(t,p)$ data, suggest that $^{100}$Mo is a shape-transitional nucleus while $^{100}$Ru is closer to the spherical side of that transition. Theoretical calculations of the $0\nu2\beta$ nuclear matrix element may be complicated by this difference in shape., Comment: 5 pages, 2 figures

The fission probability of 232Pa was measured as a function of the excitation energy in order to search for hyperdeformed (HD) transmission resonances using the (d,pf) transfer reaction on a radioactive 231Pa target. The experiment was performed at the Tandem accelerator of the Maier-Leibnitz Laboratory (MLL) at Garching using the 231Pa(d,pf) reaction at a bombarding energy of E=12 MeV and with an energy resolution of dE=5.5 keV. Two groups of transmission resonances have been observed at excitation energies of E=5.7 and 5.9 MeV. The fine structure of the resonance group at E=5.7 MeV could be interpreted as overlapping rotational bands with a rotational parameter characteristic to a HD nuclear shape. The fission barrier parameters of 232Pa have been determined by fitting TALYS 1.2 nuclear reaction code calculations to the overall structure of the fission probability. From the average level spacing of the J=4 states, the excitation energy of the ground state of the 3rd minimum has been deduced to be E(III)=5.05 MeV., Comment: 6 pages, 8 figures

Model predictions of the amount of the radioisotope 26Al produced in hydrogen-burning environments require reliable estimates of the thermonuclear rates for the 26gAl(p,{\gamma})27Si and 26mAl(p,{\gamma})27Si reactions. These rates depend upon the spectroscopic properties of states in 27Si within about 1 MeV of the 26gAl+p threshold (Sp = 7463 keV). We have studied the 28Si(3He,{\alpha})27Si reaction at 25 MeV using a high-resolution quadrupole-dipole-dipole-dipole magnetic spectrograph. For the first time with a transfer reaction, we have constrained J{\pi} values for states in 27Si over Ex = 7.0 - 8.1 MeV through angular distribution measurements. Aside from a few important cases, we generally confirm the energies and spin-parity assignments reported in a recent {\gamma}-ray spectroscopy study. The magnitudes of neutron spectroscopic factors determined from shell-model calculations are in reasonable agreement with our experimental values extracted using this reaction., Comment: accepted for publication in Phys. Rev. C

The discovery of naturally occurring long-lived isomeric states (t_1/2 > 10^8 yr) in the neutron-deficient isotopes 211,213,217,218Th [A. Marinov et al., Phys. Rev. C 76, 021303(R) (2007)] was reexamined using accelerator mass spectrometry (AMS). Because AMS does not suffer from molecular isobaric background in the detection system, it is an extremely sensitive technique. Despite our up to two orders of magnitude higher sensitivity we cannot confirm the discoveries of neutron-deficient thorium isotopes and provide upper limits for their abundances., Comment: 3 pages, 1 figure

An accurate knowledge of the neutron capture cross sections of 62,63Ni is crucial since both isotopes take key positions which affect the whole reaction flow in the weak s process up to A=90. No experimental value for the 63Ni(n,gamma) cross section exists so far, and until recently the experimental values for 62Ni(n,gamma) at stellar temperatures (kT=30 keV) ranged between 12 and 37 mb. This latter discrepancy could now be solved by two activations with following AMS using the GAMS setup at the Munich tandem accelerator which are also in perfect agreement with a recent time-of-flight measurement. The resulting (preliminary) Maxwellian cross section at kT=30 keV was determined to be 30keV = 23.4 +/- 4.6 mb. Additionally, we have measured the 64Ni(gamma,n)63Ni cross section close to threshold. Photoactivations at 13.5 MeV, 11.4 MeV and 10.3 MeV were carried out with the ELBE accelerator at Forschungszentrum Dresden-Rossendorf. A first AMS measurement of the sample activated at 13.5 MeV revealed a cross section smaller by more than a factor of 2 compared to NON-SMOKER predictions., Comment: Proceedings of the 11th International Conference on Accelerator Mass Spectrometry in Rome, Sept. 14-19, 2008; to be published in Nucl. Instr. Meth. A

Analysis of presolar grains in primitive meteorites has shown isotopic ratios largely characteristic of the conditions thought to prevail in various astrophysical environments. A possible indicator for a grain of ONe nova origin is a large 33S abundance: nucleosynthesis calculations predict as much as 150 times the solar abundance of 33S in the ejecta of nova explosions on massive ONe white dwarfs. This overproduction factor may, however, vary by factors of at least 0.01 - 3 because of uncertainties of several orders of magnitude in the 33S(p,gamma)34Cl reaction rate at nova peak temperatures (Tpeak ~ 0.1 - 0.4 GK). These uncertainties arise due to the lack of nuclear physics information for states within ~ 600 keV of the 33S+p threshold in 34Cl (Sp(34Cl) = 5143 keV). To better constrain this rate we have measured, for the first time, the 34S(3He,t)34Cl reaction over the region Ex(34Cl) = 4.9 - 6 MeV. We confirm previous states and find 15 new states in this energy region. New 33S(p,gamma)34Cl resonances at ER = 281(2), 301(2) and 342(2) keV may dominate this rate at relevant nova temperatures. Our results could affect predictions of sulphur isotopic ratios in nova ejecta (e.g., 32S/33S) that may be used as diagnostic tools for the nova paternity of grains., Comment: accepted for Phys. Rev. C, 2 tables, 2 figures

By comparing the Q-values for the 46Ti(3He,t)46V and 47Ti(3He,t)47}V reactions to the isobaric analog states the Q-value for the superallowed Fermi-decay of 46V has been determined as Q_{EC}(46V)=(7052.11+/-0.27) keV. The result is compatible with the values from two recent direct mass measurements but is at variance with the previously most precise reaction Q-value. As additional input quantity we have determined the neutron separation energy S_n(47Ti)=(8880.51+/-0.25) keV.

Results are presented from a one-neutron knockout reaction at relativistic energies on 56Ti using the GSI FRS as a two-stage magnetic spectrometer and the Miniball array for gamma-ray detection. Inclusive and exclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the orbital angular momentum of the populated states. First-time observation of the 955(6) keV nu p3/2-hole state in 55Ti is reported. The measured data for the first time proves that the ground state of 55Ti is a 1/2- state, in agreement with shell-model calculations using the GXPF1A interaction that predict a sizable N=34 gap in 54Ca., Comment: 12 pages, 3 figures, submitted to Physics Letters B 13 pages, 4 figures, resubmitted to Physics Letters B (major revision to meet the suggestions of the reviewers)

It is argued that orbital electron-capture decays of neutral $^{142}$Pm atoms implanted into the lattice of a solid (LBNL experiment) do not fulfil the constraints of true two-body beta decays, since momentum as well as energy of the final state are distributed among three objects, namely the electron neutrino, the recoiling daughter atom and the lattice phonons. To our understanding, this could be a reason for the non-observation of a periodic time modulation in the number of electron-capture decays of implanted neutral $^{142}$Pm atoms., Comment: 2 p. comment on arXiv:0807.0649v1

Although $^{79}$Se represents an important branching in the weak s process, the stellar neutron capture cross sections to this isotope have not yet been measured experimentally. In this case, experimental data is essential for evaluating the important branching in the s-process reaction path at $^{79}$Se. The total cross section of $^{78}$Se at a stellar energy of kT = 25 keV has been investigated with a combination of the activation technique and accelerator mass spectrometry (AMS), since offline decay counting is prohibitive due to the long terrestrial half life of $^{79}$Se (2.80$\pm$0.36 $\times10^5$ y) as well as the absence of suitable $\gamma$-ray transitions. The preliminary result for the total Maxwellian averaged cross section is $<\sigma>_{30 keV}$= 60.1$\pm$9.6 mbarn, significantly lower than the previous recommended value. In a second measurement, also the partial cross section to the 3.92 min-isomer was determined via $\gamma$-spectroscopy and yielded $<\sigma>_{30 keV}$(part.)= 42.0$\pm$2.0 mbarn., Comment: Proceedings "Nuclei in the Cosmos" NIC- IX, June 25-30 2006, CERN, Geneva/ Switzerland, published with Proccedings of Science, PoS(NIC-IX)089

We report on time-modulated two-body weak decays observed in the orbital electron capture of hydrogen-like $^{140}$Pr$^{59+}$ and $^{142}$Pm$^{60+}$ ions coasting in an ion storage ring. Using non-destructive single ion, time-resolved Schottky mass spectrometry we found that the expected exponential decay is modulated in time with a modulation period of about 7 seconds for both systems. Tentatively this observation is attributed to the coherent superposition of finite mass eigenstates of the electron neutrinos from the weak decay into a two-body final state., Comment: 12 pages, 5 figures

We report on the first measurement of the $\beta^+$- and orbital electron capture decay rates of $^{140}$Pr nuclei with the most simple electron configurations: bare nuclei, hydrogen-like and helium-like ions. The measured electron capture decay constant of hydrogen-like $^{140}$Pr$^{58+}$ ions is about 50% larger than that of helium-like $^{140}$Pr$^{57+}$ ions. Moreover, $^{140}$Pr ions with one bound electron decay faster than neutral $^{140}$Pr$^{0+}$ atoms with 59 electrons. To explain this peculiar observation one has to take into account the conservation of the total angular momentum, since only particular spin orientations of the nucleus and of the captured electron can contribute to the allowed decay., Comment: 4 pages, 3 figures