The neon-sodium cycle of hydrogen burning occurs in several astrophysical sites, such as asymptotic giant branch stars and novae, affecting the production of neon and sodium isotopes. To enhance the accuracy of predicted nucleosynthesis yields, there is a pressing need for new experimental investigations of the cross sections of the reactions involved in this cycle at energies relevant to astrophysics. The 400 kV accelerator at the Laboratory for Underground Nuclear Astrophysics (LUNA) provides a unique advantage relative to above-ground laboratories thanks to its deep underground location within the Gran Sasso National Laboratory (INFN-LNGS) in Italy. We performed experiments at LUNA on two of the reactions of the NeNa cycle: ^20 Ne(p, γ )^21 Na and ^21 Ne(p, γ )^22 Na using a high-purity gas target system for isotopically enriched gases coupled with two high-resolution germanium detectors, surrounded by copper and lead shielding to further reduce the natural background at LUNA. We describe the detailed characterization of the experimental setup performed through Monte Carlo simulations, and the method for the precise determination of resonance energies, giving improved values of E_r = 127.3 ± 0.5 keV, 271.4 ± 0.4 keV, 272.3 ± 0.4 keV, 291.5 ± 0.5 keV and 352.6 ± 0.4 keV. Additionally, decay branching ratios for the E_x = 7016.4 keV excited state in ^22 Na, and three new transitions ( R→ 4770 keV, R→ 3059.4 keV and R→ 583.05 keV) in the E_r = 291.5 keV resonance, are also reported.

The neutron-rich rare isotope 190W is discussed as a candidate for a prolate-oblate transitional nucleus with maximum γ-softness. The collectivity of this isotope is assessed for the first time by the measurement of the reduced E2 transition probability of its first 2+ state to the ground state. The experiment employed the FAst TIming Array (FATIMA), comprised of 36 LaBr3(Ce) scintillators, which was part of the DESPEC setup at GSI, Darmstadt. The 41+ and 21+ states of 190W were populated subsequently to the decay of its 127(12) μs isomeric Jπ=10− state. The mean lifetime of the 21+ state was determined to be τ=274(28) ps, which corresponds to a B(E2;21+→01+) value of 95(10) W.u. The results motivated a revision of previous calculations within an energy-density functional-based interacting boson model-2 approach, yielding E2 transition properties and spectroscopic quadrupole moments for tungsten isotopes. From comparison to theory, the new data suggest that 190W is at the transition from prolate to oblate structure along the W isotopic chain, which had previously been discussed as a nuclear shape-phase transition.

A novel system has been developed to detect simultaneous double-alpha emission from purified and weightless sources. The system includes the collection of 224Ra low-energy recoils in purified helium buffer gas from the decay of 228Th. The recoil products are thermalized and collected in a cryogenic buffer gas cell and extracted into an RF-ion guide for mass selection. The mass-separated ions are implanted at low kinetic energy into a thin carbon foil placed between two large-area double-sided silicon strip detectors to observe correlated alpha-particle emission. The apparatus is described in detail, including insights into its experimental performance.

The CNO cycle is one of the most important nuclear energy sources in stars. At temperatures of hydrostatic H-burning (20  MK<T<80  MK) the ^{17}O(p,γ)^{18}F reaction rate is dominated by the poorly constrained 64.5 keV resonance. Here, we report on the first direct measurements of its resonance strength and of the direct capture contribution at 142 keV, performed with a new high sensitivity setup at LUNA. The present resonance strength of ωγ_{(p,γ)}^{bare}=(30±6_{stat}±2_{syst})  peV is about a factor of 2 higher than the values in literature, leading to a Γ_{p}^{bare}=(34±7_{stat}±3_{syst})  neV, in agreement with the LUNA result from the (p,α) channel. Such agreement strengthens our understanding of the oxygen isotopic ratios measured in red giant stars and in O-rich presolar grains.

Carbon fusion reactions ^12 C( ^12 C,p) ^23 Na and ^12 C( ^12 C, α ) ^20 Ne play a key role in the evolution of massive stars and in explosive scenarios such as type-Ia supernovae and super-bursts in binary stars. A direct determination of their cross sections is extremely challenging and discrepancies exist between different data sets in the literature. Here we report the results of a direct measurement performed at the CIRCE Tandem Accelerator Laboratory in Caserta (Italy), using E-E detectors for unambiguous charge identification. Cross sections were measured in the energy range E_c.m. =2.51-4.36 MeV with energy steps between 10 and 25 keV in the centre of mass. To our knowledge these represent the finest energy steps to date. Results are presented in the form of partial and summed astrophysical S̃ -factors for individual proton- and α -particle channels. Branching ratios of individual proton- and α -particle groups were found to vary significantly with energy. Angular distributions, albeit limited to three angles, were also found to be non-isotropic, which could be a potential explanation for the discrepancies observed among different data sets. Further efforts are ongoing to extend measurements to lower energies.

Reduced transition probabilities have been extracted between excited, yrast states in the N=Z+2 nucleus 94Pd. The transitions of interest were observed following decays of the Iπ=14+, Ex=2129-keV isomeric state, which was populated following the projectile fragmentation of a 124Xe primary beam at the GSI Helmholtzzentrum für Schwerionenforschung accelerator facility as part of FAIR Phase-0. Experimental information regarding the reduced E2 transition strengths for the decays of the yrast 8+ and 6+ states was determined following isomer-delayed Eγ1−Eγ2−△T2,1 coincidence method, using the LaBr3(Ce)-based FATIMA fast-timing coincidence gamma-ray array, which allowed direct determination of lifetimes of states in 94Pd using the Generalized Centroid Difference (GCD) method. The experimental value for the half-life of the yrast 8+ state of 755(106) ps results in a reduced transition probability of B(E2:8→+6+) = 205−25+34 e2 fm4, which enables a precise verification of shell-model calculations for this unique system, lying directly between the N=Z line and the N=50 neutron shell closure. The determined B(E2) value provides an insight into the purity of (g9/2)n configurations in competition with admixtures from excitations between the (lower) N=3pf and (higher) N=4gds orbitals for the first time. The results indicate weak collectivity expected for near-zero quadrupole deformation and an increasing importance of the T=0 proton-neutron interaction at N=48.

The D(p, gamma ) 3He reaction, where 'D' denotes '2H', is of primary importance in cosmology and nuclear physics since it offers a unique opportunity to test predictions based on the description of nuclear interactions. We present a novel approach to extract the angular distribution of the photons emitted by the D(p, gamma ) 3He reaction inferred from the shape of the experimental gamma spectra. The measurement was performed in the Gran Sasso Laboratories by the LUNA collaboration. By using a HPGe detector faced to an extended deuterium gas target, it was possible to derive the angular distribution in the 30-280 keV kinetic energy range and to compare the results with recent ab initio predictions.

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.

Lifetime measurements of low-lying excited states in the semimagic (N=50) nucleus Rh95 have been performed by means of the fast-timing technique. The experiment was carried out using γ-ray detector arrays consisting of LaBr3(Ce) scintillators and germanium detectors integrated into the DESPEC experimental setup commissioned for the Facility for Antiproton and Ion Research () Phase-0, Darmstadt, Germany. The excited states in Rh95 were populated primarily via the β decays of Pd95 nuclei, produced in the projectile fragmentation of a 850 MeV/nucleon Xe124 beam impinging on a 4g/cm2Be9 target. The deduced electromagnetic E2 transition strengths for the γ-ray cascade within the multiplet structure depopulating from the isomeric Iπ=21/2+ state are found to exhibit strong deviations from predictions of standard shell model calculations which feature approximately conserved seniority symmetry. In particular, the observation of a strongly suppressed E2 strength for the 13/2+→9/2+ ground state transition cannot be explained by calculations employing standard interactions. This remarkable result may require revision of the nucleon-nucleon interactions employed in state-of-the-art theoretical model calculations, and might also point to the need for including three-body forces in the Hamiltonian. Published by the American Physical Society 2024

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.

Background: Among the several inhomogeneities in the composition of globular cluster stars, an overabundance of 23 Na is interpreted as the signature of the operation of the neon-sodium (NeNa) cycle. One of the hypothesis to explain the observed O-Na anticorrelation invokes massive asymptotic giant branch stars as the main agents. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars the 22 Ne( p , gamma ) 23 Na reaction rate has been the most uncertain so far, giving rise to considerable experimental efforts in recent years. While overall there is a good agreement between reported cross section results, some tensions still remain on the branching ratios of resonance gamma-ray modes and direct capture to excited sates. Purpose: The present paper offers full details and a partial analysis of the high sensitivity study, of both direct capture and low-energy resonances in the 22 Ne( p , gamma ) 23 Na reaction, performed at LUNA, and whose results were previously published in abbreviated form [F. Ferraro et al. , Phys. Rev. Lett. 121 , 172701 (2018)]. Methods: During the LUNA measurement an intense proton beam was delivered to a 22 Ne gas target. The gamma rays from the 22 Ne( p , gamma ) 23 Na reaction were detected by a high efficiency 4 pi , sixfold segmented bismuth germanate (BGO) detector. In the present paper the data from individual detector segments were combined with simulated detector responses to obtain cascade branching ratios. Results: For the three resonances at E p = 156 . 2 and 259.7 keV new gamma-decay branchings are provided. Moreover, partial cross sections for the direct capture to different states of 23 Na are reported down to E p = 188 keV, the lowest energy measured to date. Conclusions: A revised reaction rate has been calculated based on a new R-matrix fit of the recent 22 Ne( p , gamma ) 23 Na S-factor data and results for the resonances. The thermonuclear reaction rate is provided in tabular form to be used in stellar models.

For the first time a measurement of 8B + 64Zn reaction has been performed at HIE-ISOLDE at CERN at energies around the Coulomb barrier, to understand how the debated halo structure of the light nucleus can affect reaction dynamics.

The C-13(alpha,n)O-16 reaction is the main neutron source of the s-process taking place in thermally pulsing AGB stars and it is one of the main candidate sources of neutrons for the i-process in the astrophysical sites proposed so far. Therefore, its rate is crucial to understand the production of the nuclei heavier than iron in the Universe. For the first time, the LUNA collaboration was able to measure the C-13(alpha,n)O-16 cross section at E c.m. = 0.23-0.3 MeV drastically reducing the uncertainty of the S(E)-factor in the astrophysically relevant energy range. In this paper, we provide details and critical thoughts about the LUNA measurement and compare them with the current understanding of the C-13(alpha,n)O-16 reaction in view of future prospect for higher energy measurements. The two very recent results (from the University of Notre Dame and the JUNA collaboration) published after the LUNA data represent an important step forward. There is, however, still room for a lot of improvement in the experimental study of the C-13(alpha,n)O-16 reaction, as emphasized in the present manuscript. We conclude that to provide significantly better constraints on the low-energy extrapolation, experimental data need to be provided over a wide energy range, which overlaps with the energy range of current measurements. Furthermore, future experiments need to focus on the proper target characterisation, the determination of neutron detection efficiency having more nuclear physics input, such as angular distribution of the C-13(alpha,n)O-16 reaction below E alpha < 0.8 MeV and study of nuclear properties of monoenergetic neutron sources and/or via the study of sharp resonances of C-13(alpha,n)O-16. Moreover, comprehensive, multichannel R-matrix analysis with a proper estimate of uncertainty budget of experimental data are still required.

The CNO cycle is one of the most important nuclear energy sources in stars. At temperatures of hydrostatic H-burning (20 MK

Studies of charged-particle reactions for low-energy nuclear astrophysics require high sensitivity, which can be achieved by means of detection setups with high efficiency and low backgrounds, to obtain precise measurements in the energy region of interest for stellar scenarios. High-efficiency total absorption spectroscopy is an established and powerful tool for studying radiative capture reactions, particularly if combined with the cosmic background reduction by several orders of magnitude obtained at the Laboratory for Underground Nuclear Astrophysics (LUNA). We present recent improvements in the detection setup with the Bismuth Germanium Oxide (BGO) detector at LUNA, aiming to reduce high-energy backgrounds and increase the summing detection efficiency. The new design results in enhanced sensitivity of the BGO setup, as we demonstrate and discuss in the context of the first direct measurement of the 65 keV resonance (E (x) = 5672 keV) of the O-17(p,gamma)F-18 reaction. Moreover, we show two applications of the BGO detector, which exploit its segmentation. In the case of complex gamma-ray cascades, e.g. the de-excitation of E (x) = 5672 keV in F-18, the BGO segmentation allows to identify and suppress the beam-induced background signals that mimic the sum peak of interest. We demonstrate another new application for such a detector in form of in situ activation measurements of a reaction with beta (+) unstable product nuclei, e.g. the N-14(p,gamma)O-15 reaction.

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.

Neutron-rich lanthanides were produced via in-flight fission of a U-238 primary beam at the RIBF, RIKEN Nishina Center to measure half-lives (T-1/2) and beta-delayed neutron emission probabilities (P-n) in order to constrain r-process abundance calculations. Pm159-166, Sm161-168, Eu165-170, and Gd167-172 ions were implanted in the Advanced Implantation Detector Array (AIDA), and beta-delayed neutrons and gamma-rays were detected by the surrounding detector array (BRIKEN). For the validation of T-1/2 values derived from implantation-beta (i-beta) time correlations, gamma-spectroscopic methods were used as well. The experimental results of the beta-delayed gamma-spectroscopy of Pm-162 are presented here as an example. A half-life value from gamma-decay curves was derived with a comparable uncertainty to the result from the i-beta method, and a mean value well within the 1 sigma range.

The ISOLDE Scientific Infrastructure at CERN offers a unique range of post-accelerated radioactive beams. The scientific program can be improved with the "Isolde Superconducting Recoil Separator" (ISRS), an innovative spectrometer able to deliver unprecedented (A, Z) resolution. In this paper we present an overview of the physics and ongoing technical developments.

Ever-lasting interest in the structure of 100 Sn and neighbouring nuclei is still well justified by the fact that it is the heaviest doubly-magic nucleus with N=Z. State-of-the-art experimental techniques involving stable and radioactive beam facilities have enabled access to these exotic nuclei. In particular, the analysis of experimental data obtained in two DESPEC experiments at GSI Darmstadt extends the information on the shell structure and its evolution towards N = Z = 50, and allows the study of seniority conservation and proton-neutron interaction in the g9/2 orbit. Several theoretical approaches for shell-model investigations are discussed and their predictive power assessed. The calculated systematics of the reduced transition probabilities for high- to medium-spin states in N∼Z isotopes with active g9/2 orbit is presented for the first time.

The C12/C13 ratio is a significant indicator of nucleosynthesis and mixing processes during hydrogen burning in stars. Its value mainly depends on the relative rates of the C12(p,γ)N13 and C13(p,γ)N14 reactions. Both reactions have been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy down to the lowest energies to date (Ec.m.=60 keV) reaching for the first time the high energy tail of hydrogen burning in the shell of giant stars. Our cross sections, obtained with both prompt γ-ray detection and activation measurements, are the most precise to date with overall systematic uncertainties of 7%–8%. Compared with most of the literature, our results are systematically lower, by 25% for the C12(p,γ)N13 reaction and by 30% for C13(p,γ)N14. We provide the most precise value up to now of 3.6±0.4 in the 20–140 MK range for the lowest possible C12/C13 ratio that can be produced during H burning in giant stars.Received 23 January 2023Revised 3 May 2023Accepted 13 July 2023DOI:https://doi.org/10.1103/PhysRevLett.131.162701© 2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasH & He burningH & He induced nuclear reactionsNuclear astrophysicsProperties6 ≤ A ≤ 19Nuclear PhysicsAccelerators & BeamsGravitation, Cosmology & Astrophysics