V. Roca

Activation measurement of the He3(α,γ)Be7 cross section at low energy

The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-gamma technique and with inferior precision. Using a windowless gas target, high beam intensity, and low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined at 127, 148, and 169 keV center-of-mass energy with a total uncertainty of 4%. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S factor to solar energies.

First Direct Measurement of the 17O(p,γ)18F Reaction Cross Section at Gamow Energies for Classical Novae

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive (18)F, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The (17)O(p,γ)(18)F reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes, but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range E(c.m.)~/= 200-370 keV appropriate to hydrogen burning in classical novae. In addition, the E(c.m.)=183 keV resonance strength, ωγ=1.67±0.12 μeV, has been measured with the highest precision to date. The uncertainty on the (17)O(p,γ)(18)F reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis.

A recoil separator for the measurement of radiative capture reactions

A recoil separator in combination with a windowless gas target has been designed for the measurement of the radiative capture reaction p(7Be, γ)8B. The separator consists of momentum and velocity filters and a ΔE−E detector telescope. The setup was tested quantitatively using the p(12C, γ)13N reaction at the effective energy Ecm = 841 keV. Projectile fluxes were measured directly with Faraday cups and indirectly with elastic backscattering into Si detectors, while the 13N recoil flux was measured directly with the ΔE−E telescope. A suppression of the 12C beam particles by a factor 2 × 10−10 was observed when the system was tuned for the recoil 13Ns. Special emphasis was given to the charge state probabilities of the 13N recoils. Possible improvements of the system for the measurement of other capture reactions are discussed.

The N14(p,γ)O15 reaction studied with a composite germanium detector

The rate of the carbon-nitrogen-oxygen (CNO) cycle of hydrogen burning is controlled by the N14(p,γ)O15 reaction. The reaction proceeds by capture to the ground states and several excited states in ...

New experimental study of low-energy (p,γ) resonances in magnesium isotopes

Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar H-shell burning. In particular, the strengths of low-energy resonances with E < 200 keV in 25 Mg(p, γ) 26 Al determine the production of 26 Al and a precise knowledge of these nuclear data is highly desirable. Absolute measurements at such low energies are often very difficult and hampered by γ-ray background as well as changing target stoichiometry during the measurements. The latter problem can be partly avoided using higher-energy resonances of the same reaction as a normalization reference. Hence the parameters of suitable resonances have to be studied with adequate precision. In the present work we report on new measurements of the resonance strengths ωγ of the E = 214, 304, and 326 keV resonances in the reactions 24 Mg(p,γ) 25 Al, 25 Mg(p,γ) 26 Al, and 26 Mg(p,γ) 27 Al, respectively. These studies were performed at the LUNA facility in the Gran Sasso underground laboratory using multiple experimental techniques and provided results with a higher accuracy than previously achieved.

The He3(α,γ)Be7 S-factor at solar energies: The prompt γ experiment at LUNA

The 3He(alpha,gamma)7Be process is a key reaction in both Big-Bang nucleosynthesis and p-p chain of Hydrogen Burning in Stars. A new measurement of the 3He(alpha,gamma)7Be cross section has been performed at the INFN Gran Sasso underground laboratory by both the activation and the prompt gamma detection methods. The present work reports full details of the prompt gamma detection experiment, focusing on the determination of the systematic uncertainty. The final data, including activation measurements at LUNA, are compared with the results of the last generation experiments and two different theoretical models are used to obtain the S-factor at solar energies.

Astrophysical S factor of the He3(α,γ)Be7 reaction measured at low energy via detection of prompt and delayed γ rays

Solar neutrino fluxes depend both on astrophysical and on nuclear physics inputs, namely on the cross sections of the reactions responsible for neutrino production inside the Solar core. While the flux of solar 8B neutrinos has been recently measured at Superkamiokande with a 3.5% uncertainty and a precise measurement of 7Be neutrino flux is foreseen in the next future, the predicted fluxes are still affected by larger errors. The largest nuclear physics uncertainty to determine the fluxes of 8B and 7Be neutrinos comes from the 3He(alpha,gamma)7Be reaction. The uncertainty on its S-factor is due to an average discrepancy in results obtained using two different experimental approaches: the detection of the delayed gamma rays from 7Be decay and the measurement of the prompt gamma emission. Here we report on a new high precision experiment performed with both techniques at the same time. Thanks to the low background conditions of the Gran Sasso LUNA accelerator facility, the cross section has been measured at Ecm = 170, 106 and 93 keV, the latter being the lowest interaction energy ever reached. nThe S-factors from the two methods do not show any discrepancy within the experimental errors. An extrapolated S(0)= 0.560+/-0.017 keV barn is obtained. Moreover, branching ratios between the two prompt gamma-transitions have been measured with 5-8% accuracy.

DETECTION OF BREAKUP FRAGMENTS IN INVERSE COULOMB DISSOCIATION EXPERIMENTS

In the framework of Coulomb dissociation experiments, performed to reconstruct the cross section of the inverse radiative capture process, the detection of breakup fragments is usually performed using separate detectors, which unavoidably are not sensitive to small relative angles. It is shown, by means of Monte Carlo simulations, that the phase-space constraints imposed by the detection geometry can bias, specially at very low relative energies, the extracted cross section, which turns out to be strongly dependent upon the assumed angular distribution of fragments in the intrinsic reference frame. A solution to the problem of the model-dependence of the response function is proposed, making use of a single three-stage detector for the identification and the relative energy measurement of the breakup fragments. A test measurement on the reaction 208Pb(16O, 12C-a)208Pb at 126 MeV incident energy has been performed, using a gas—Si—CsI tritelescope. The results indicate the feasibility of measuring very low relative energies in the Coulomb dissociation approach to the solution of important problems in nuclear astrophysics via the accurate determination of reaction rates at energies far below the Coulomb barrier. ( 1998 Elsevier Science B.V. All rights reserved.

A three-stage detector for heavy fragments and light particles emitted in heavy ion collisions

Abstract A low threshold charged particle three-stage detector has been designed, built and successfully tested. A truncated pyramidal geometry has been chosen to use the device as an element in a large solid angle array for heavy ion nuclear experiments. Angular resolution of 0.4° at 35 cm from the target, energy resolution of about 1% for the elastically scattered ions and Z resolving power of 63 for Z = 17 have been achieved in the 35 Cl+ 64 Ni reaction at 280 MeV incident energy.

3He(α,γ)7Be cross section at low energies

The flux of 7Be and 8B neutrinos from the Sun and the production of 7Li via primordial nucleosynthesis depend on the rate of the 3He(alpha,gamma)7Be reaction. In extension of a previous study showing cross section data at 127 - 167 keV center of mass energy, the present work reports on a measurement of the 3He(alpha,gamma)7Be cross section at 106 keV performed at Italys Gran Sasso underground laboratory by the activation method. This energy is closer to the solar Gamow energy than ever reached before. The result is sigma = 0.567 +- 0.029(stat) +- 0.016(syst) nbarn. The data are compared with previous activation studies at high energy, and a recommended S(0) value for all 3He(alpha,gamma)7Be activation studies, including the present work, is given.