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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.

International audience; [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.

International audience; The Coulomb excitation of Ru102 was performed with beams of C12 and O16 ions. The beam particles scattered at forward angles were momentum analyzed with a magnetic spectrograph. The resolution achieved enabled the populations of the 21+ state, the unresolved 22+/41+, and 24+/31−, doublets of states, and the 32− state to be determined as a function of the scattering angle. These populations are compared with gosia calculations, yielding B(E2;21+→01+)=41.5±2.3 W.u., B(E2;22+→01+)=1.75±0.11 W.u., B(E3;31−→01+)=31.5±3.5 W.u., and B(E3;32−→01+)=6.8±0.5 W.u. The B(E3;31−→01+) value is significantly larger than previously measured. The weakly populated 23+ state, presumed to be a member of the band built on the 02+ state, was observed clearly for a single angle only, and a fit to its population results in B(E2;23+→01+)=0.053±0.011 W.u. Using the known γ-ray branching ratios for the 23+ level, the B(E2;23+→02+) value is calculated to be 18±4 W.u., substantially less than the B(E2;21+→01+). This suggests that the deformation of the 02+ state is lower than that of the 01+ state. The results are compared with beyond-mean-field calculations with the Gogny-D1S interaction using the symmetry-conserving configuration-mixing method.

In a high-energy fragmentation experiment at GSI an I=π(6+) isomer and its γ-decay are identified in $^{102}$Sn, the two-neutron neighbour of the doubly-magic $^{100}$Sn. Its half-life is measured to be T=1/2367(11) ns. The possible existence of further isomers is discussed in the framework of large-scale shell model (LSSM) calculations including up to five particle-hole excitations of the $^{100}$Sn core. From the precise B(E2; 6+→4+) strength and the recently remeasured value for B(E2; 8+→6+) in the two-proton hole neighbour $^{98}$Cd effective E2 polarization charges for protons and neutrons were inferred including LSSM corrections within the full N=4 0ħω space. The results are discussed in comparison to predicted and empirically determined effective operators.

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

International audience

Astrophysical studies on extraterrestrial samples often involve long-lived radionuclides to study the evolution of the solar system and the galaxy on long timescales. Dyadic radionuclide/stable nuclide systems such as U-Pb, Sm-Nd or Rb-Sr are well-established and widely used for dating and characterization of meteorites and lunar rocks. Such systems rely on the decay of a long-lived radionuclide and the resulting isotopic anomalies of the daughter isotope compared to natural abundances. In contrast, in this talk, the two live radionuclides Fe-60 and Mn-53 are shown to be versatile tools to study meteorites and past supernova activity. Both radionuclides with half-lives around 3 Myr are produced in massive stars as well as by spallation in extraterrestrial materials, planetary surfaces and cosmic dust. Measured and modelled production rates [1,2] and the first use as a dyadic system to disentangle the origin of supernovaproduced Fe-60 on the surface of the moon [3] and in Antarctic snow [4] will be presented amongst recent discoveries of the individual radionucides in geological archives. [1] Leya et al., Meteoritics & Planetary Science 55, 818–831 (2020) [2] Merchel et al., Nucl. Instr. Meth. Phys. Res. B 172, 806-811 (2000) [3] Fimiani et al., Phys. Rev. Lett. 116, 151104 (2016) [4] Koll et al., Phys. Rev. Lett. 123, 072701 (2019)

A bentonite engineered barrier will be used in deep geological repositories in granitic host rock to retard the transport of radionuclides possibly released from corroded canisters to the geo- and biosphere. Thus, it is relevant to investigate the diffusion of radionuclides through bentonite and their consequent migration through water-conductive features in granitic rock. Such scenario in the case of glacial melt water intrusion has been simulated at the Grimsel Test Site with the Long-Term in-situ Test (LIT). A packer-system containing bentonite rings spiked with radionuclide tracers was emplaced in the crystalline rock in contact with a water-conducting shear zone [1]. The bentonite pore-water mixing with Grimsel groundwater was collected at sampling points 5.6 cm from the bentonite for ca. 4.5 years. In these samples, we have determined the concentration of the actinide tracers 233U, 237Np, 242Pu, 241Am and 99Tc. The expected ultra-trace radionuclide concentrations could only be addressed via the high analytical sensitivity of Accelerator Mass Spectrometry. Actinide tracers were analyzed at the 3 MV tandem accelerator of VERA, while 99Tc at the 14 MV tandem accelerator of TUM, enabling quantification at the level of 25 at/g and 0.5 fg/g, respectively [2, 3]. In order to identify radionuclides originating from the LIT, an extensive analysis of the background originating from previous in-situ radionuclide tracer tests [2] was undertaken. First results suggest, that 242Pu release from the bentonite could not be detected. Concentrations of 233U, 237Np and 241Am were found slightly above background for samples collected within 600 days. The levels of 99Tc were always above background indicating the possible release of ca. 8.5 × 10-3 % of the total 99Tc within the experimental time frame. Such early appearance of tracers is unexpected. Further analysis is ongoing in order to identify the possible transport mechanism. References: [1] NAB 14-48. GTS Phase VI – CFM Phase 3. February 2015 [2] Quinto et al., (2017) Anal. Chem. 2017, 89, 7182-7189 [3] Quinto et al., (2019) Anal. Chem. 2019, 91, 4585-4591 Acknowledgments: German Federal Ministry for Economic Affairs and Energy, BMWi, grant number 02E11759B & international consortium of the CFM project

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.

ispartof: PHYSICS LETTERS B vol:809 status: published

Excited states in $^{98}$Ru were investigated using γ-ray spectroscopy following the β-decay of $^{98}$Rh, and via the $^{100}$Ru(p,t) reaction. Combining the results from the two experiments, two states were revised to have spin-parity of 4+ and subsequently assigned to the 02+ and “γ” bands, respectively. The observed structures in $^{98}$Ru are suggested to be deformed and rotational, rather than spherical and vibrational, and fit well into the systematics of these excitations in the Ru isotopes. The 02+ excitation is suggested as a shape coexisting configuration. This observation eliminates some of the last remaining candidates for nearly harmonic vibrational nuclei in the Z≈50 region. Beyond-mean-field calculations are presented that support shape coexistence throughout the Ru isotopes with N=52–62, and suggest a smooth evolution of the shapes.

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., 9 pages, 4 figures

We used a high-resolution magnetic spectrograph to study neutron pair-correlated 0+ states in $^{136}$Ba, produced via the Ba138(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 (0νββ) decay. We demonstrate for the first time an evaluation of part of a 0νββ decay NME by use of an experimental observable, presenting a new avenue of approach for more accurate calculations of 0νββ decay matrix elements.