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We report on the thermal neutron flux measurements carried out at the Laboratorio Subterráneo de Canfranc (LSC) with two commercial 2 ″ × 2 ″ CLYC detectors. The measurements were performed as part of an experimental campaign at LSC with 3 He detectors, for establishing the sensitivity limits and use of CLYCs in low background conditions. A careful characterization of the intrinsic α and γ -ray background in the detectors was required and done with dedicated measurements. It was found that the α activities in the two CLYC crystals differ by a factor of three, and the use of Monte Carlo simulations and a Bayesian unfolding method allowed us to determine the specific α activities from the 238 U and 232 Th decay chains. The simulations and unfolding also revealed that the γ -ray background registered in the detectors is dominated by the intrinsic activity of the components of the detector such as the aluminum housing and photo-multiplier and that the activity within the crystal is low in comparison. The data from the neutron flux measurements with the two detectors were analyzed with different methodologies: one based on an innovative α /neutron pulse shape discrimination method and one based on the subtraction of the intrinsic α background that masks the neutron signals in the region of interest. The neutron sensitivity of the CLYCs was calculated by Monte Carlo simulations with MCNP6 and GEANT4. The resulting thermal neutron fluxes are in good agreement with complementary flux measurement performed with 3 He detectors, but close to the detection limit imposed by the intrinsic α activity.

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 Δ 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 S ~ -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.

The evolutionary path of massive stars begins at helium burning. Energy production for this phase of stellar evolution is dominated by the reaction path 3 α → 12 C ( α , γ ) 16 O and also determines the ratio of 12 C/ 16 O in the stellar core. This ratio then sets the evolutionary trajectory as the star evolves towards a white dwarf, neutron star or black hole. Although the reaction rate of the 3 α process is relatively well known, since it proceeds mainly through a single narrow resonance in 12 C, that of the 12 C ( α , γ ) 16 O reaction remains uncertain since it is the result of a more difficult to pin down, slowly-varying, portion of the cross section over a strong interference region between the high-energy tails of subthreshold resonances, the low-energy tails of higher-energy broad resonances and direct capture. Experimental measurements of this cross section require herculean efforts, since even at higher energies the cross section remains small and large background sources are often present that require the use of very sensitive experimental methods. Since the 12 C ( α , γ ) 16 O reaction has such a strong influence on many different stellar objects, it is also interesting to try to back calculate the required rate needed to match astrophysical observations. This has become increasingly tempting, as the accuracy and precision of observational data has been steadily improving. Yet, the pitfall to this approach lies in the intermediary steps of modeling, where other uncertainties needed to model a star’s internal behavior remain highly uncertain.

A detailed knowledge of the decay properties of the so called Hoyle state in the 12 C nucleus ( E x = 7.654 MeV, 0 + ) is required to calculate the rate at which carbon is forged in typical red-giant stars. This paper reports on a new almost background-free measurement of the radiative decay branching ratio of the Hoyle state using advanced charged particle coincidence techniques. The exploitation, for the first time in a similar experiment, of a bidimensional map of the coincidence efficiency allows to reach an unitary value and, consequently, to strongly reduce sources of systematic uncertainties. The present results suggest a value of the radiative branching ratio of Γ rad / Γ tot = 4.2 ( 6 ) · 10 - 4 . This finding helps to resolve the tension between recent data published in the literature.

. The present contribution aims to present some general features of the experimental approaches in Nuclear Astrophysics. After a general introduction on light elements nucleosynthesis and on how to determine the reaction rates in a stellar environment, we will focus our attention on underground experiments aimed to directly measure nuclear cross sections of astrophysics interest. We will discuss the 14 N(p, γ ) 15 O and 12 C + 12 C reactions, underlying the advantages in approaching these measurements in a deep underground laboratory, as the Laboratori Nazionali del Gran Sasso.

For the measurement of reactions with small cross sections as aimed for in the Laboratory for Underground Nuclear Astrophysics (LUNA) at the Gran Sasso National Laboratory (LNGS), low detector background levels are compulsory. Background measurements with a BGO detector used at LUNA have been evaluated to develop a model of the contributions in the region of interest for ongoing and planned measurements. Conclusions from this model and implications for further background reduction for this detector setup are discussed. A customized lead shielding against environmental backgrounds is described.

The properties of the Hoyle state in 12C (7.654 MeV, 0+) affect the rate at which carbon, one of the most abundant elements in the Universe, is forged in stars. Recent experiments reported values of its radiative decay branching ratio that are in tension, posing major implications especially in the astrophysical domain. This work reports on an almost background-free measurement of the radiative decay branching ratio of the Hoyle state that exploits charged particle coincidence techniques. The experiment adopts several methodologies to minimize the background and identify the rare signal associated with the radiative decay. Large care is devoted to having under full control two of the major sources of systematic errors in particle-coincidence experiments: the coincidence efficiency and the spurious coincidence rate. We find a radiative decay branching ratio of Γrad/Γtot = 4.2(6) · 10−4. The new finding helps to resolve the tension between recent data published in the literature.

We study the s-process abundances at the epoch of the Solar-system formation as the outcome of nucleosynthesis occurring in AGB stars of various masses and metallicities. The calculations have been performed with the Galactic chemical evolution (GCE) model presented by [1, 2]. With respect to previous works, we used updated solar meteoritic abundances, a neutron capture cross section network that includes the most recent measurements, and we implemented the s-process yields with an extended range of AGB initial masses. The new set of AGB yields includes a new evaluation of the 22Ne(α, n)25Mg rate, which takes into account the most recent experimental information.

18O(p, γ)19F plays an important role in the AGB star scenarios. The low energy cross section could be influenced by a hypothetical low energy resonance at 95 keV and by the tails of the higher energy broad states. The 95 keV resonance lies in the energy window corresponding to the relevant stellar temperature range of 40-50 MK. Measurements of the direct cross section were performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), including the unobserved low energy resonance, the higher energy resonances and the non-resonant component, taking advantage of the extremely low environmental background. Here we report on the experimental setup and the status of the analysis.