A new jet gas target system has been developed for the Felsenkeller 5 MV underground ion accelerator for nuclear astrophysics. It provides either a 1.5 x 1018 atoms/cm2 thick cylindrical jet or a 8 x 1017 atoms/cm2 thick wall of nitrogen gas, with a surface of 10 x 10 mm2 to be seen by the ion beam. The system includes a de Laval type nozzle and altogether five pumping stages: In addition to the jet catcher and the jet chamber surrounding it, there are three stages connecting the jet to the ion accelerator. Behind the jet chamber, as seen from the ion beam, a windowless static-type gas target and, subsequently, a beam calorimeter have been installed. This work describes the offline tests of the gas target system prior to its installation on the beam line of the Felsenkeller accelerator. The thickness of the jet has been determined using three different methods: By computational fluid dynamics simulations, with a Mach-Zehnder interferometer, and by alpha-energy loss using a mixed alpha source. The three methods were shown to be in agreement. For 0-6 bar inlet gas pressure, a linear relationship between inlet pressure and jet thickness has been found. Different shapes of de Laval type inlet nozzles, both circular and slit-type, have been manufactured from fused silica glass or stainless steel and tested using measurements and simulations. The power and stability of the beam calorimeter have been tested. The interferometry has been shown to work reliably and to give two-dimensional projections of the gas jet with sub-mm resolution.

Nuclear Physics in Astrophysics studies the production of the chemical elements in astrophysical scenarios ranging from primordial nucleosynthesis to stellar explosions and mergers. This field has seen impressive progress in recent years, both in theory and experiment, as evidenced by the Nuclear Physics in Astrophysics conference series. This Topical Collection assembles invited contributions drawn from the Nuclear Physics in Astrophysics XI (NPA-XI) conference in Dresden in September 2024, as well as additional papers invited by the Guest Editors. They cover the following four thematic areas: cosmic nucleosynthesis, element synthesis processes in stars, latest stages of stellar evolution, and special topics. A short review of new European facilities for nuclear astrophysics is given, from the FAIR facility in Darmstadt to underground ion accelerators to accelerator mass spectrometers.

In the Felsenkeller shallow-underground site, protected from cosmic muons by a 45 m thick rock overburden, a research laboratory including a 5 MV Pelletron ion accelerator and a number of radioactivity-measurement setups is located. The laboratory and its installations are described in detail. The background radiation has been studied, finding suppression factors of 40 for cosmic-ray muons, 200 for ambient neutrons, and 100 for the background in germanium γ -ray detectors. Using an additional active muon veto, typically the background is just twice as high as in very deep underground laboratories. The properties of the accelerator including its external and internal ion sources and beam line are given. For the radioactivity counting setup, detection limits in the 10 ^-4 Bq range have been obtained. Practical aspects for the usage of the laboratory by outside scientific users are discussed.

GERDA has been a pioneering experiment in the search for the still undetected neutrinoless double beta (0 nu beta beta) decay of Ge-76 and will be even surpassed by its successor experiment LEGEND. The discovery of this extremely rare process would verify the theory of neutrinos being Majorana particles and that at least one of the neutrino masses has to be non-zero [1]. In addition, this process violates the lepton number by 2 which gives rise to explanations regarding the matter-antimatter asymmetry in the universe by leptogenesis. For an explicit identification of a signal caused by the 0 nu beta beta decay, which corresponds to an energy of 2039 keV for Ge-76, a precise understanding of the background contributions in the region of interest (ROI) is crucial. Previous experiments indicated gamma rays, produced by neutron activation (n, p) and neutron scattering (n,n') processes on Ge-76 and Ge-74 but until now, no significant indications for their existence were found. In order to confirm the existence of the gamma rays, a Ge sample enriched in 76Ge is alternately irradiated by neutrons from a deuterium tritium (DT) generator and measured by a high-purity germanium (HPGe) detector. The present work describes the experimental procedure and the preliminary analysis of a reference sample consisting of natural Ge.

High-resolution neutron transmission measurements of natural iron have been performed in the fast neutron range from 100 keV to 12 MeV using the nELBE time-of-flight experiment at the Helmholtz-Zentrum DresdenRossendorf (HZDR) with a flight path of 868.34(30) cm. The measurements were carried out using three different iron samples of 20, 50 and 90mm thickness. The neutron transmission and effective total cross section were determined as a function of neutron energy, and the results were compared with existing evaluations and previous measurements. The measurements revealed significant shortcomings in the resolved resonance region of current evaluations, particularly in the energy range from 100 to 700 keV. The results of this work have already been used to improve the INDEN evaluation for the Fe-isotopes, which have been adopted by the ENDF/B-VIII.1 library and proposed for JEFF-4. The high-resolution time-of-flight measurements provide valuable new data for the development of more accurate nuclear data evaluations for natural iron.

Neutrinoless double-beta decay of nuclei represents one of the most promising methods for uncovering physics beyond the Standard Model. In this context, $^{76}$Ge stands out as a particularly attractive candidate, as it can serve as an intrinsic component in semiconductor detectors. If the neutrinoless process occurs in $^{76}$Ge, its signature would appear as a distinct peak at the $Q$ value of 2039 keV. A neutron activation measurement was performed on a germanium sample isotopically enriched in $^{76}$Ge at the DT neutron generator of TU Dresden. The measurement confirmed the presence of $\gamma$ rays with energies of 2033.1$\pm$0.5 keV, 2035.5$\pm$0.4 keV, and 2040.22$\pm$0.26 keV originating from the decays of $^{74}$Ga and $^{76}$Ga. These $\gamma$ rays lie in close proximity to the expected neutrinoless double-beta decay signal of $^{76}$Ge.

Neutrinoless double-beta decay of nuclei represents one of the most promising methods for uncovering physics beyond the standard model. In this context, 76Ge stands out as a particularly attractive candidate, as it can serve as an intrinsic component in semiconductor detectors. If the neutrinoless process occurs in 76Ge, its signature would appear as a distinct peak at the Q value of 2039 keV. A neutron activation measurement was performed on a germanium sample isotopically enriched in 76Ge at the DT neutron generator of TU Dresden. The measurement confirmed the presence of gamma rays with energies of 2033.1 +/- 0.5 keV, 2035.5 +/- 0.4 keV, and 2040.22 +/- 0.26 keV originating from the decays of 74Ga and 76Ga. These gamma rays lie in close proximity to the expected neutrinoless double-beta decay signal of 76Ge.

The beta-decay properties of nuclei near the second nuclear "island of inversion" around neutron rich nuclei with neutron number 40 are important tests of nuclear structure models and interactions. In particular, the beta-delayed neutron emission branch (P-n), is useful for investigating beta-strength and neutron-gamma competition above the neutron separation energies of the daughter nuclei. We report new constraints for Pn values for three nuclei in the region: Cr-62 (Pn < 1%), Mn-64 (P-n = 1.5(6)%), and Fe-65 (P-n < 1%), measured with the Neutron Emission Ratio Observer (NERO) neutron long counter system and the Beta Counting Station (BCS) at the National Superconducting Cyclotron Laboratory (NSCL). Our results resolve the large discrepancy between previous direct and indirect measurements for Mn-64 and confirm the predictions of global theoretical models when a statistical treatment of the. and neutron decays of the daughter states is included. We also obtain improved half-lives for Cr-62 [206(5) ms] and the short-lived isomer in the Fe-62 daughter [112(7) ms] from beta-delayed. emission data obtained in the same experiment with the Summing NaI (SuN) total absorption spectrometer. Finally, we use gamma emission data to obtain a new upper limit for the Cr-62 beta-decay population of the long-lived isomeric state in Mn-62.

Nuclei around the N = 40 "island of inversion" exhibit interesting structure features that have been the focus of several experimental and theoretical studies. The present work presents the first complete study of the beta-decay feeding intensity distribution and Gamow-Teller distribution for the beta decay of 64Mn to 64Fe up to approximate to 10 MeV. The beta-decay intensity function was extracted from total absorption spectroscopy measurements made at the National Superconducting Cyclotron Laboratory with the Summing NaI(Tl) (SuN) detector. The experimental results are compared to shell model calculations with and without the inclusion of the nu g9/2 orbital. From this comparison it is clear that the nu g9/2 orbital is essential for the accurate description of the 64Fe beta-decay strength above approximate to 3 MeV, emphasizing once again the transitional nature of this nucleus into the N = 40 island of inversion.

Background: Type I X-Ray bursts (XRBs) are energetic stellar explosions that occur on the surface of a neutron star in an accreting binary system with a low-mass H/He-rich companion. The rate of the ^34Ar(α,p)^37K reaction may influence features of the light curve that results from the underlying thermonuclear runaway, as shown in recent XRB stellar modelling studies. Purpose: In order to reduce the uncertainty of the rate of this reaction, properties of resonances in the compound nucleus ^38Ca, such as resonance energies, spins, and particle widths, must be well constrained. Method: This work discusses a study of resonances in the ^38Ca compound nucleus produced in the ^34Ar(α,p) reaction. The experiment was performed at the National Superconducting Cyclotron Laboratory, with the ReA3 facility by measuring proton scattering using an unstable ^37K beam. The kinematics were designed specifically to identify and characterize resonances in the Gamow energy window for the temperature regime relevant to XRBs. Results: The spins and proton widths of newly identified and previously known states in ^38Ca in the energy region of interest for the ^34Ar(α,p)^37K reaction have been constrained through an R-Matrix analysis of the scattering data. Conclusions: Using these constraints, a newly estimated rate is applied to an XRB model built using Modules for Experiments in Stellar Astrophysics (MESA), to examine its impact on observables, including the light curve. It is found that the newly determined reaction rate does not substantially affect the features of the light curve.

For almost three decades it has been known that the study of astro-physically important nuclear reactions between stable nuclei requires the use of low-background, underground accelerator laboratories. The Felsenkeller shallow-underground laboratory in Dresden, shielded by a 45 m thick rock cover, hosts a 5 MV Pelletron ion accelerator with an external sputter ion source (mainly able to provide carbon and oxygen beams) and an internal radio-frequency ion source (providing proton and alpha beams). The reduced muon, neutron and gamma-ray background achieved both with natural and active shielding situate the laboratory well in line with deep underground accelerator labs worldwide and allows highly sensitive nuclear reaction experiments. Currently, measurements affecting the solar fusion and Big Bang nucleosynthesis are ongoing. In addition to in-house research by HZDR and TU Dresden, the lab is an open facility for scientific users worldwide, with beam time applications reviewed by an independent science advisory board. Furthermore, EU-supported transnational access is available via the ChETEC- INFRA network for nuclear astrophysics. A brief introduction to underground nuclear astrophysics, status of the Felsenkeller shallow-underground laboratory and some preliminary results are discussed.

The rate of the final step in the astrophysical αp process, the ^{34}Ar(α,p)^{37}K reaction, suffers from large uncertainties due to a lack of experimental data, despite having a considerable impact on the observable light curves of x-ray bursts and the composition of the ashes of hydrogen and helium burning on accreting neutron stars. We present the first direct measurement constraining the ^{34}Ar(α,p)^{37}K reaction cross section, using the Jet Experiments in Nuclear Structure and Astrophysics gas jet target. The combined cross section for the ^{34}Ar,Cl(α,p)^{37}K,Ar reaction is found to agree well with Hauser-Feshbach predictions. The ^{34}Ar(α,2p)^{36}Ar cross section, which can be exclusively attributed to the ^{34}Ar beam component, also agrees to within the typical uncertainties quoted for statistical models. This indicates the applicability of the statistical model for predicting astrophysical (α,p) reaction rates in this part of the αp process, in contrast to earlier findings from indirect reaction studies indicating orders-of-magnitude discrepancies. This removes a significant uncertainty in models of hydrogen and helium burning on accreting neutron stars.

For direct cross section measurements in nuclear astrophysics, in addition to suitable ion beams and detectors, also highly pure and stable targets are needed. Here, using a gas jet as a target offers an attractive approach that combines high stability even under significant beam load with excellent purity and high localisation. Such a target is currently under construction at the Felsenkeller underground ion accelerator lab for nuclear astrophysics in Dresden, Germany. The target thickness will be measured by optical interferometry, allowing an in-situ thickness determination including also beam-induced effects. The contribution reports on the status of this new system and outlines possible applications in nuclear astrophysics.

The rate of the final step in the astrophysical αp process, the ^{34}Ar(α,p)^{37}K reaction, suffers from large uncertainties due to a lack of experimental data, despite having a considerable impact on the observable light curves of x-ray bursts and the composition of the ashes of hydrogen and helium burning on accreting neutron stars. We present the first direct measurement constraining the ^{34}Ar(α,p)^{37}K reaction cross section, using the Jet Experiments in Nuclear Structure and Astrophysics gas jet target. The combined cross section for the ^{34}Ar,Cl(α,p)^{37}K,Ar reaction is found to agree well with Hauser-Feshbach predictions. The ^{34}Ar(α,2p)^{36}Ar cross section, which can be exclusively attributed to the ^{34}Ar beam component, also agrees to within the typical uncertainties quoted for statistical models. This indicates the applicability of the statistical model for predicting astrophysical (α,p) reaction rates in this part of the αp process, in contrast to earlier findings from indirect reaction studies indicating orders-of-magnitude discrepancies. This removes a significant uncertainty in models of hydrogen and helium burning on accreting neutron stars.

A new ultra low-level counting setup has been installed in the shallow-underground laboratory Felsenkeller in Dresden, Germany. It includes a high-purity germanium detector (HPGe) of 163\% relative efficiency within passive and active shields. The passive shield consists of 45m rock overburden (140 meters water equivalent), 40 cm of low-activity concrete, and a lead and copper castle enclosed by an anti-radon box. The passive shielding alone is found to reduce the background rate to rates comparable to other shallow-underground laboratories. An additional active veto is given by five large plastic scintillation panels surrounding the setup. It further reduces the background rate by more than one order of magnitude down to 116$\pm$1 kg$^{-1}$ d$^{-1}$ in an energy interval of 40-2700 keV. This low background rate is unprecedented for shallow-underground laboratories and close to deep underground laboratories.

The 12C(p, & gamma; ) 13N reaction is the onset process of both the CNO and hot CNO cycles that drive massive star, red and asymptotic giant branch star, and novae nucleosynthesis. The 12C(p, & gamma; )13N rate affects the final abundances of the stable 12,13C nuclides with ramifications for meteoritic carbon isotopic abundances and the s-process neutron source strength. Here, an underground measurement of the 12C(p, & gamma; ) 13N cross section is reported. The present data, obtained at the Felsenkeller shallow-underground laboratory in Dresden (Germany), encompass the 320-620 keV center of mass energy range to include the wide and poorly constrained E = 422 keV resonance that dominates the rate at high temperatures. This work's S-factor results, lower than literature by 25%, are included in a comprehensive R-matrix fit, and the energy of the 1 + first excited state of 13N is found to be 2369.6(4) keV with a radiative and proton width of 0.49(3) eV and 34.9(2) keV, respectively. A reaction rate, based on the present R-matrix fit and extrapolation, is suggested.

The electric E 1 and magnetic M 1 dipole responses of the N=Z nucleus ^24 Mg were investigated in an inelastic photon scattering experiment. The 13.0 MeV electrons, which were used to produce the unpolarised bremsstrahlung in the entrance channel of the ^24 Mg( γ ,γ ^' ) reaction, were delivered by the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf. The collimated bremsstrahlung photons excited one J^π=1^- , four J^π=1^+ , and six J^π=2^+ states in ^24 Mg. De-excitation γ rays were detected using the four high-purity germanium detectors of the γ ELBE setup, which is dedicated to nuclear resonance fluorescence experiments. In the energy region up to 13.0 MeV a total B(M1)↑ = 2.7(3) μ _N^2 is observed, but this N=Z nucleus exhibits only marginal E 1 strength of less than ∑ B(E1)↑≤ 0.61 × 10^-3 e ^2 fm ^2 . The B( 1, 1^π_i → 2^+_1)/B( 1, 1^π_i → 0^+_gs) branching ratios in combination with the expected results from the Alaga rules demonstrate that K is a good approximative quantum number for ^24 Mg. The use of the known ρ ^2(E0, 0^+_2 → 0^+_gs) strength and the measured B(M1, 1^+ → 0^+_2)/B(M1, 1^+ → 0^+_gs) branching ratio of the 10.712 MeV 1^+ level allows, in a two-state mixing model, an extraction of the difference β _2^2 between the prolate ground-state structure and shape-coexisting superdeformed structure built upon the 6432-keV 0^+_2 level.

The 12C(p,{\gamma})13N reaction is the onset process of both the CNO and Hot CNO cycles that drive massive star, Red and Asymptotic Giant Branch star and novae nucleosynthesis. The 12C(p,{\gamma})13N rate affects the final abundances of the stable 12,13C nuclides, with ramifications for meteoritic carbon isotopic abundances and the s-process neutron source strength. Here, a new underground measurement of the 12C(p,{\gamma})13N cross-section is reported. The present data, obtained at the Felsenkeller shallow-underground laboratory in Dresden (Germany), encompass the 320-620 keV center of mass energy range to include the wide and poorly constrained E = 422 keV resonance that dominates the rate at high temperatures. This work S-factor results, lower than literature by 25%, are included in a new comprehensive R-matrix fit, and the energy of the 1+ first excited state of 13N is found to be 2369.6(4) keV, with radiative and proton width of 0.49(3) eV and 34.9(2) keV respectively. A new reaction rate, based on present R-matrix fit and extrapolation, is suggested.

We measured the Coulomb dissociation of 16 O into 4 He and 12 C within the FAIR Phase-0 program at GSI Helmholtzzentrum für Schwerionenforschung Darmstadt, Germany. From this we will extract the photon dissociation cross section 16 O(α,γ) 12 C, which is the time reversed reaction to 12C(α,γ) 16 O. With this indirect method, we aim to improve on the accuracy of the experimental data at lower energies than measured so far. The expected low cross section for the Coulomb dissociation reaction and close magnetic rigidity of beam and fragments demand a high precision measurement. Hence, new detector systems were built and radical changes to the R 3 B setup were necessary to cope with the high-intensity 16 O beam. All tracking detectors were designed to let the unreacted 16 O ions pass, while detecting the 12 C and 4 He.