Search

The electron screening in the d(d,p)t reaction has been studied for the deuterated metal Pt at a target temperature $\ensuremath T = 20 ^{\circ}$ C to $\ensuremath 340 ^{\circ}$ C, and for Co at $\ensuremath T = 20 ^{\circ}$ C and $\ensuremath 200 ^{\circ}$ C. The enhanced electron screening decreases with increasing temperature, where the data agree with the plasma model of Debye applied to the quasi-free metallic electrons. The data represent the first observation of a temperature dependence of a nuclear cross-section. We also measured the screening effect for the deuterated metal Ti (an element of group 4 of the periodic table) at $\ensuremath T = -10 ^{\circ}$ C to $\ensuremath 200 ^{\circ}$ C: above $\ensuremath 50 ^{\circ}$ C the hydrogen solubility dropped to values far below unity and a large screening effect became observable. Similarly, all metals of groups 3 and 4 and the lanthanides showed a solubility of a few percent at $\ensuremath T = 200 ^{\circ}$ C (compared to $\ensuremath T = 20 ^{\circ}$ C) and a large screening became also observable. Within the Debye model the deduced number of valence electrons per metallic atom agrees with the corresponding number from the Hall coefficient, for all metals investigated. [ABSTRACT FROM AUTHOR]

The astrophysical S( E) factor of 14N(p,γ)15O has been measured for effective center-of-mass energies between Eeff = 119 and 367 keV at the LUNA facility using TiN solid targets and Ge detectors. The data are in good agreement with previous and recent work at overlapping energies. R-matrix analysis reveals that due to the complex level structure of 15O the extrapolated S(0) value is model dependent and calls for additional experimental efforts to reduce the present uncertainty in S(0) to a level of a few percent as required by astrophysical calculations. [ABSTRACT FROM AUTHOR]

The astrophysical S( E) factor of 14N(p,γ)15O has been measured for effective center-of-mass energies between Eeff = 119 and 367 keV at the LUNA facility using TiN solid targets and Ge detectors. The data are in good agreement with previous and recent work at overlapping energies. R-matrix analysis reveals that due to the complex level structure of 15O the extrapolated S(0) value is model dependent and calls for additional experimental efforts to reduce the present uncertainty in S(0) to a level of a few percent as required by astrophysical calculations. [ABSTRACT FROM AUTHOR]

The LUNA (Laboratory Underground for Nuclear Astrophysics) facility has been designed to study nuclear reactions of astrophysical interest. It is located deep underground in the Gran Sasso National Laboratory, Italy. Two electrostatic accelerators, with 50 and 400 kV maximum voltage, in combination with solid and gas target setups allowed to measure the total cross-sections of the radiative-capture reactions ^2H2H(p, ?)^3He3Heand ^14N14N(p, ?)^15O15Owithin their relevant Gamow peaks. We report on the gamma background in the Gran Sasso laboratory measured by germanium and bismuth germanate detectors, with and without an incident proton beam. A method to localize the sources of beam-induced background using the Doppler shift of emitted gamma rays is presented. The feasibility of radiative-capture studies at energies of astrophysical interest is discussed for several experimental scenarios. [ABSTRACT FROM AUTHOR]

We report on a new measurement of the 14N(p,γ)15O capture cross section at Ep=140 to 400 keV using the 400 kV LUNA accelerator facility at the Laboratori Nazionali del Gran Sasso (LNGS). The uncertainties have been reduced with respect to previous measurements and their analysis. We have analyzed the data using the R-matrix method and we find that the ground state transition accounts for about 15% of the total S-factor. The main contribution to the S-factor is given by the transition to the 6.79 MeV state. We find a total S(0)=1.7±0.2 keV b, in agreement with recent extrapolations. The result has important consequences for the solar neutrino spectrum as well as for the age of globular clusters. [Copyright &y& Elsevier]

A new detection array called GASTLY (GAs-Silicon Two-Layer sYstem) has been designed to detect and identify low-energy light particles emitted in nuclear reactions of astrophysical interest. Devoted to the measurement of nanobarn cross-sections, the system is optimised for large solid angle coverage and for low-energy detection thresholds. The array consists of eight modules, each comprising an ionisation chamber and a large area silicon strip detector. Its modularity and versatility allow for use in a variety of experiments. Here we report on the performance of the array as obtained during its commissioning phase with standard α-particle sources and during in-beam tests with an intense 12C beam. Typical energy resolutions ΔE( FWHM )/E of about 3% and 2% were obtained for the ionisation chambers and the silicon detectors, respectively. The status of the development of individual strip readout, based on ASIC technology, is also presented. [ABSTRACT FROM AUTHOR]

The 12C(12C,p)23Na and 12C(12C,α)20Ne fusion reactions are among the most important in stellar evolution since they determine the destiny of massive (M≃8-10M⊙) stars. However, experimental low-energy investigations of such reactions are significantly hampered by ubiquitous natural hydrogen and deuterium contaminants in the carbon targets. The associated beam-induced background completely masks the reaction products of interest thus preventing cross-section measurements at the relevant energies of astrophysical interest, E cm <2 MeV. In this work, we report about an investigation aimed at assessing possible deuterium reductions on both natural graphite and Highly Ordered Pyrolytic Graphite targets as a function of target temperature. Our results indicate that reductions up to about 80% can be attained on both targets in the temperature range investigated, T≃200-1200∘C. A further reduction by a factor of 2.5 in absolute deuterium content is observed when the scattering chamber is surrounded by a dry nitrogen atmosphere so as to minimise light-particles uptake within the chamber rest gas (and thus on target) through air leaks. The results from this study will inform the choice of optimal experimental conditions and procedures for improved measurements of the 12C + 12C reactions cross-sections at the low energies of astrophysical interest. [ABSTRACT FROM AUTHOR]

The NeNa and the MgAl cycles play a fundamental role in the nucleosynthesis of asymptotic giant branch stars undergoing hot bottom burning. The Na 23 (p , γ) 24 Mg reaction links these two cycles and a precise determination of its rate is required to correctly estimate the contribution of these stars to the chemical evolution of various isotopes of Na, Mg and Al. At temperatures of 50 ≲ T ≲ 110 MK , narrow resonances at E p = 140 and 251 keV are the main contributors to the reaction rate, in addition to the direct capture that dominates in the lower part of the temperature range. We present new measurements of the strengths of these resonances at the Laboratory for Underground Nuclear Astrophysics (LUNA). We have used two complementary detection approaches: high efficiency with a 4 π BGO detector for the 140 keV resonance, and high resolution with a HPGe detector for the 251 keV resonance. Thanks to the reduced cosmic ray background of LUNA, we were able to determine the resonance strength of the 251 keV resonance as ω γ = 482 (82) μ eV and observed new gamma ray transitions for the decay of the corresponding state in Mg 24 at E x = 11931 keV. With the highly efficient BGO detector, we observed a signal for the 140 keV resonance for the first time in a direct measurement, resulting in a strength of ω γ 140 = 1.46 − 0.53 + 0.58 neV (68% CL). Our measurement reduces the uncertainty of the Na 23 (p , γ) 24 Mg reaction rate in the temperature range from 0.05 to 0.1 GK to at most − 35 % + 50 % at 0.07 GK. Accordingly, our results imply a significant reduction of the uncertainties in the nucleosynthesis calculations. [ABSTRACT FROM AUTHOR]

Background: The reaction 17O(p, γ)18F influences hydrogen-burning nucleosynthesis in several stellar sites, such as red giants, asymptotic giant branch (AGB) stars, massive stars, and classical novae. In the relevant temperature range for these environments (T9 = 0.01-0.4), the main contributions to the rate of this reaction are the direct capture process, two low-lying narrow resonances (Er = 65.1 and 183 keV) and the low-energy tails of two broad resonances (Er = 557 and 677 keV). Purpose: Previous measurements and calculations give contradictory results for the direct capture contribution which in turn increases the uncertainty of the reaction rate. In addition, very few published cross section data exist for the high energy region that might affect the interpretation of the direct capture and the contributions of the broad resonances in the lower energy range. This work aims to address these issues. Method: The reaction cross section was measured in a wide proton energy range (Ec.m. = 345-1700 keV) and at several angles (θlab = 0°, 45°, 90°, 135°). The observed primary γ transitions were used as input in an R-matrix code in order to obtain the contribution of the direct capture and the two broad resonances to the low-energy region. Results: The extrapolated S factor from the present data is in good agreement with the existing literature data in the low-energy region. A new reaction rate was calculated from the combined results of this work and literature S-factor determinations. Resonance strengths and branchings are reported for several 18F states. Conclusions: We were able to extrapolate the astrophysical S factor of the reaction 17O(p, γ)18F at low energies from cross section data taken at higher energies. No significant changes in the nucleosynthesis are expected from the newly calculated reaction rate. [ABSTRACT FROM AUTHOR]

Abstract: In the present work we report on a new measurement of resonance strengths ωγ in the reaction 25Mg(p, γ)26Al at and 189 keV. This study was performed at the LUNA facility in the Gran Sasso underground laboratory using a 4π BGO summing crystal. For the first time the 92 keV resonance was directly observed and a resonance strength was determined. Additionally, the γ-ray branchings and strength of the 189 keV resonance were studied with a high resolution HPGe detector yielding an ωγ value in agreement with the BGO measurement, but 20% larger compared to previous works. [Copyright &y& Elsevier]

Abstract: In stars with temperatures above , hydrogen burning is dominated by the CNO cycle. Its rate is determined by the slowest process, the 14N(p, γ)15O reaction. Deep underground in Italy''s Gran Sasso laboratory, at the LUNA 400 kV accelerator, the cross section of this reaction has been measured at energies much lower than ever achieved before. Using a windowless gas target and a 4π BGO summing detector, direct cross section data has been obtained down to 70 keV, reaching a value of 0.24 picobarn. The Gamow peak has been covered by experimental data for several scenarios of stable and explosive hydrogen burning. In addition, the strength of the 259 keV resonance has been remeasured. The thermonuclear reaction rate has been calculated for temperatures , for the first time with negligible impact from extrapolations. [Copyright &y& Elsevier]

Abstract: Accurate knowledge of thermonuclear reaction rates is important in understanding the generation of energy, the luminosity of neutrinos, and the synthesis of elements in stars. Cross-section measurements for quiescent stellar H-burning are mainly hampered by extremely low counting rate and cosmic background. The LUNA Collaboration has shown that, by going underground and by using the typical techniques of low background physics, it is possible to measure nuclear cross-sections down to the energy of the nucleosynthesis inside stars. This paper reports an overview of the experimental techniques adopted in underground nuclear astrophysics through a summary of the main recent results and achievements. The future developments of the LUNA experiment are also given. [Copyright &y& Elsevier]