A. Fubini

First Measurement of the 3 He ( 3 He , 2 p ) 4 He Cross Section down to the Lower Edge of the Solar Gamow Peak

We give the LUNA results on the cross section measurement of a key reaction of the proton-proton chain strongly affecting the calculated neutrino luminosity from the Sun: He3+He3-->He4+2p. Due to the cosmic ray suppression provided by the Gran Sasso underground laboratory it has been possible to measure the cross section down to the lower edge of the solar Gamow peak, i.e. as low as 16.5 keV centre of mass energy. The data clearly show the cross section increase due to the electron screening effect but they do not exhibit any evidence for a narrow resonance suggested to explain the observed solar neutrino flux.

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.

Stopping power, electron screening and the astrophysical S(E) factor of d(He-3,p)He-4

Abstract The d( 3 He,p) 4 He cross section has been measured at E =4.2 to 13.8 keV using the LUNA underground accelerator facility. The experiment was performed to determine the magnitude of the atomic screening effect and to establish values for the energy loss used in data reduction. The observed stopping power of the 3 He ions in the D 2 target is in good agreement with the standard compilation. Using these stopping power values the data lead to an electron-screening potential energy U e = 132±9 eV, which is significantly higher than the estimated value of 65 eV from an atomic-physics model.

Measurement of the 3He(3He,2p)4He cross section within the solar Gamow peak

We report on the status of the 3He(3He,2p)4He experiment at the underground accelerator facility LUNA (Gran Sasso). The lowest projectile energies for the measured cross section correspond already to the energies at the center of the solar Gamow peak (21 keV). The data provide no evidence for the existence of a hypothetical resonance in the energy range investigated. Although no extrapolation is needed anymore (except for energies at the low-energy tail of the Gamow peak), the data must be corrected for the effects of electron screening, leading tentatively to S(0) = (5.1 ± 0.1) MeVb.

The cross section of 3He(3He,2p)4He measured at solar energies

We report on the status of the 3He(3He,2p)4He experiment at the underground accelerator facility LUNA (Gran Sasso). The lowest projectile energies for the measured cross section correspond already to energies below the center of the solar Gamow peak (E0=22 keV). The data provide no evidence for the existence of a hypothetical resonance in the energy range investigated. Although no extrapolation is needed anymore (except for energies at the low-energy tail of the Gamow peak), the data must be corrected for the effects of electron screening, clearly observed the first time for the 3He(3He,2p)4He reaction. The effects are however larger than expected and not understood, leading presently to the largest uncertainty on the quoted S(0) value for bare nuclides (=5.40 MeVb).

A method to improve the energy calibration of a Ge(Li) spectrometer

Abstract A method is described which improves the precision of the energy calibration of a Ge(Li) spectrometer taking into account the relationship between cascade gamma rays.

Status of the LUNA experiment

Luna is a pilot project initially focused on the 3 He( 3 He, 2p) 4 He cross section measurement within the thermal energy region of the Sun (15–27 keV). A compact high current 50 kV ion accelerator facility including a windowless gas target system, a beam calorimeter and four detector telescopes has been built, tested and installed underground at Laboratori Nazionali del Gran Sasso. In these conditions, thanks to the cosmic ray suppression, we could attain a background level of less than 1 event per week, a rate similar to the one expected from 3 He( 3 He, 2p) 4 He at the lower edge of the Sun thermal energy region.

The program LUNA2 at the gran sasso

The scientific program of nuclear astrophysics at the INFN underground laboratories of Gran Sasso for the next five years (LUNA phase II or LUNA2) is presented and discussed.

Laboratory for underground nuclear astrophysics (LUNA): First results of the measurements of the reaction 3He(3He,2p)4He at solar energies

A compact 50 kV accelerator facility has been installed in the underground laboratory of the Laboratori Nazionali del Gran Sasso (LNGS) in Italy in the framework of the LUNA—project. Because of the specially designed accelerator setup and the suppression of the cosmic ray background in the underground laboratory the sensitivity of the set up gives the possibility to measure the cross sections of nuclear reactions at energies far below the coulomb barrier, where cross sections are as low as a few fbarn. Currently the reaction 3He(3He,2p)4He, which is important for the Solar Neutrino Puzzle, is being investigated at energies relevant in our sun (E=15–27 keV). These are the first direct measurements of a nuclear cross section covering the whole thermal energy range of the solar plasma. This contribution reports on the measurements covering the energy range between E=20.7 keV and 25 keV.

Further direct approaches to the nuclear reactions in the Sun

Abstract Nuclear fusion reactions play a key role in the understanding of energy production, neutrino emission and nucleosynthesis of the elements in stars. The direct measurement of the cross section of these reactions at the relevant energies is usually hampered by cosmic radiation, beam induced background and/or the radioactivity of the nuclei involved. The measurements are expected to be completed by fall 1996.