G. Gervino

Laboratory for underground nuclear astrophysics(LUNA)

A compact high-current 50 kV ion accelerator facility including a windowless gas target system, a beam calorimeter, and detector telescopes in close geometry has been built and tested. The data acquisition and analysis involved a multiparameter system and a Monte Carlo program. The LUNA facility, presently installed at the Gran Sasso underground laboratory, is a pilot project focused initially on cross section measurements of the ^3He(^3He, 2p)^4He reaction within the thermal energy region of the sun. To achieve this goal, the experimental sensitivity has been improved by more than four orders of magnitude over that of previous work.

Electron screening effect in the reactions 3 He(d, p) 4 He and d( 3 He, p) 4 He ☆ ☆ Supported in part by INFN, BMBF (06BO812), DFG (436UNG113-146) and OTKA (T025465).

The cross section of the reactions 3He(d, p)4He and d(3He, p)4He has been measured at the center-of-mass energies E=5 to 60 keV and 10 to 40 keV, respectively. The experiments were performed to determine the magnitude of the electron screening effect leading to the respective electron-screening potential energy Ue=219±7 and 109±9 eV, which are both significantly higher than the respective values from atomic physics models, Ue=120 and 65 eV.

Limits on light-speed anisotropies from Compton scattering of high-energy electrons

The possibility of anisotropies in the speed of light relative to the limiting speed of electrons is considered. The absence of sidereal variations in the energy of Compton-edge photons at the European Synchrotron Radiation Facilitys GRAAL facility constrains such anisotropies representing the first nonthreshold collision-kinematics study of Lorentz violation. When interpreted within the minimal standard-model extension, this result yields the two-sided limit of 1.6×10(-14) at 95% confidence level on a combination of the parity-violating photon and electron coefficients (κ(o+))(YZ), (κ(o+))(ZX), c(TX), and c(TY). This new constraint provides an improvement over previous bounds by 1 order of magnitude.

Absolute measurement of the total photoabsorption cross section for carbon in the nucleon resonance region

Abstract We present the results of an absolute measurement of the total nuclear photoabsorption cross section for carbon in the higher nucleon resonance region. In the Δ region our results agree with existing data, while at higher energies they show a damping in the excitation of the higher nucleon resonances, clearly seen in the photon absorption on proton and deutron.

Preparation and characterisation of isotopically enriched Ta2O5 targets for nuclear astrophysics studies

The direct measurement of reaction cross-sections at astrophysical energies often requires the use of solid targets of known thickness, isotopic composition, and stoichiometry that are able to withstand high beam currents for extended periods of time. Here, we report on the production and characterisation of isotopically enriched Ta2O5 targets for the study of proton-induced reactions at the Laboratory for Underground Nuclear Astrophysics facility of the Laboratori Nazionali del Gran Sasso. The targets were prepared by anodisation of tantalum backings in enriched water (up to 66% in 17O and up to 96% in 18O. Special care was devoted to minimising the presence of any contaminants that could induce unwanted background reactions with the beam in the energy region of astrophysical interest. Results from target characterisation measurements are reported, and the conclusions for proton capture measurements with these targets are drawn.

Precision study of ground state capture in the N-14(p, gamma)O-15 reaction

Previous extrapolations for the ground state contribution disagreed by a factor 2, corresponding to 15% uncertainty in the total astrophysical S-factor. At the Laboratory for Underground Nuclear Astrophysics (LUNA) 400kV accelerator placed deep underground in the Gran Sasso facility in Italy, a new experiment on ground state capture has been carried out at 317.8, 334.4, and 353.3 keV centerof-mass energy. Systematic corrections have been reduced considerably with respect to previous studies by using a Clover detector and by adopting a relative analysis. The previous discrepancy has been resolved, and ground state capture no longer dominates the uncertainty of the total Sfactor.

PROBING THE LIGHT SPEED ANISOTROPY WITH RESPECT TO THE COSMIC MICROWAVE BACKGROUND RADIATION DIPOLE

We have studied the angular fluctuations in the speed of light with respect to the apex of the dipole of Cosmic Microwave Background (CMB) radiation using the experimental data obtained with GRAAL facility, located at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The measurements were based on the stability of the Compton edge of laser photons scattered on the 6 GeV monochromatic electron beam. The results enable one to obtain a conservative constraint on the anisotropy in the light speed variations Δc(θ)/c<3×10-12, i.e. with higher precision than from previous experiments.

Phenomenological approach to baryon resonance damping in nuclei

Abstract We analyse the total photonuclear cross section in the energy range from 0.2 to 1.2 GeV, recently measured in Frascati on C, Be and U. By employing a simple resonance-hole model for the various baryonic resonances, which exist in the above range, we find that the experimental cross sections can be fairly well reproduced if one assumes that the nuclear medium strongly increases the width of all resonances above the Δ.

Improved Direct Measurement of the 64.5 keV Resonance Strength in the O 17 (p,α) N 14 Reaction at LUNA

The ^{17}O(p,α)^{14}N reaction plays a key role in various astrophysical scenarios, from asymptotic giant branch stars to classical novae. It affects the synthesis of rare isotopes such as ^{17}O and ^{18}F, which can provide constraints on astrophysical models. A new direct determination of the E_{R}=64.5  keV resonance strength performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) accelerator has led to the most accurate value to date ωγ=10.0±1.4_{stat}±0.7_{syst}  neV, thanks to a significant background reduction underground and generally improved experimental conditions. The (bare) proton partial width of the corresponding state at E_{x}=5672  keV in ^{18}F is Γ_{p}=35±5_{stat}±3_{syst}  neV. This width is about a factor of 2 higher than previously estimated, thus leading to a factor of 2 increase in the ^{17}O(p, α)^{14}N reaction rate at astrophysical temperatures relevant to shell hydrogen burning in red giant and asymptotic giant branch stars. The new rate implies lower ^{17}O/^{16}O ratios, with important implications on the interpretation of astrophysical observables from these stars.

A Monte Carlo code for nuclear astrophysics experiments

Abstract A Monte Carlo code, suited for nuclear astrophysics experiments, is described. The code has been developed in the frame of the LUNA pilot project at the Laboratori Nazionali del Gran Sasso. An accurate evaluation of ion energy and angular straggling, and Doppler broadening has been implemented, which are important at subCoulomb energies. The considered effects are compared with experimental data.