J.P. Ribeiro

Enhanced electron screening in d(d, p)t for deuterated Ta

Abstract:The recent observation of a large electron screening effect in the d(d, p)t reaction using a deuterated Ta target has been confirmed using somewhat different experimental approaches: Ue = 309±12 eV for the electron screening potential energy. The high Ue value arises from the environment of the deuterons in the Ta matrix, but a quantitative explanation is missing.

A new setup for the underground study of capture reactions

For the study of astrophysically relevant capture reactions in the underground laboratory LUNA a newsetup of high sensitivity has been implemented. The setup includes a windowless gas target, a 4p BGO summing crystal, and beam calorimeters. The setup has been recently used to measure the d(p,g) 3 He cross-section for the first time within its solar Gamowpeak, i.e. dow n to 2.5 keV c.m. energy. The features of the optimized setup are described. r 2002 Elsevier Science B.V. All rights reserved.

Electron screening in d(d, p)t for deuterated metals: temperature effects

The electron screening in the d(d, p)t reaction has been studied for the deuterated metal Pt at a sample temperature T = 20 °C–340 °C and for Co at T = 20 °C and 200 °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 T = −10 °C–200 °C: above 50 °C, the hydrogen solubility dropped to values far below 1 and a large screening effect became observable. Similarly, all metals of groups 3 and 4 and the lanthanides showed a solubility of a few per cent at T = 200 °C (compared to T = 20 °C) and a large screening also became 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.

Experimental study of proton-induced nuclear reactions in 6,7 Li

The 6Li(p,α)3He and 7Li(p,α)4He reaction cross sections were obtained for E = 90–580 keV and E = 90–1740 keV, respectively. R-matrix and polynomial fits to the bare astrophysical S-factor confirmed, with improved accuracy, previous work data, yielding Sb(0) = 3.52 ± 0.08 MeV b, and 55.6+0.8−1.7 keV b for the 6Li and 7Li reactions, respectively. Therefore, the astrophysical consequences related to these two isotopes remain essentially unchanged. With the present work Sb(E) data, a reanalysis of the low energy data for different environments—Li2WO4 insulator, Li metal, and PdLix alloys—confirms that the large electron screening effects can be explained by the plasma model of Debye applied to the quasi-free electrons in the metallic samples.

Excitation function and cross-sections of the reaction 19F(p,p′γ)19F

Abstract The excitation function and cross-sections of the reaction 19F(p,p′γ)19F, for the two exit channels γ 1 =110 keV, γ 2 =197 keV for protons in the energy range 0.75–2.8 MeV were measured, with an accurate method not dependent on the absolute values of the number of incident protons (beam charge collection) and of the target thickness. Accurate values of cross-sections are needed to implement a code to determine the concentration of fluorine in thick and intermediate samples analysed by proton-induced γ-ray emission (PIGE).

Proton-induced γ-ray analysis of lithium in thick samples

Abstract A method for proton-induced γ-ray analysis (PIGE) of Li (either elemental or isotopic composition) in thick samples is presented in this work, which is based on a code that integrates the nuclear reaction excitation function, measured at the same experimental conditions, along the depth of the sample. The energy steps needed to define accurately the excitation function are used as energy intervals for the integration procedure. The excitation function and cross-sections of the reaction 7 Li (p,p′γ)7Li, for protons in the energy range 0.60–2.5 MeV, were obtained and used as input to calculate lithium concentrations in thick and intermediate samples. The calculated thick target yields of samples with several amounts of lithium were compared with the respective measured yields, showing that this is a reliable and accurate method for PIGE analysis of Li in thick samples.

L-shell X-ray production cross sections for PIXE analysis of elements from Ag to U

Abstract Absolute proton-induced L X-ray production cross sections at incident proton energies ranging from 0.5 to 1.75 MeV are presented for Ag, Sb, Cs, Eu, W, Au, Pb, and U. It is argued that accurate cross section values for the elements not measured in this work may be obtained by Z -interpolation. Relative intensities are given separately for the lines or group of lines which differ by more tnan 200 eV; these results may be used to introduce constraints reliable enough to improve the fitting accuracy in complex PIXE spectra.

M-subshell ionization of U by light-ion bombardment

Abstract M X-rays of U were produced by proton, deuteron and alpha-particle bombardment in the energy range of 0.20–1.00 MeV/u. N 6,7 → M 5 (M α ), N 6 → M 4 (M β ), N 5 → M 3 (M γ ), N 4 → M 2 and N 2 → M 1 line yields were obtained from a least-squares fit to the spectra and used to convert M X-ray production into M-subshell ionization cross sections. The uncertainty induced by the atomic parameters (X-ray fluorescence yields, Coster-Kronig and radiative transition rates) used in the conversion is discussed. The subshell ionization cross sections are then compared to PWBA values corrected for Coulomb deflection and energy loss according to Brandt and Lapicki, to the semiclassical theoretical values of Kocbach and to relativistic PWBA results, corrected for Coulomb and binding effects, of Chen et al. Intrashell transitions induced by the projectile and multiple ionization are suggested as causes of disagreement between theory and experiment, especially for alpha-particles. It is concluded that theory must go beyond the simple picture of the first-order perturbation approximation to explain M-subshell results and that care must be taken in the choice of wave functions.

Influence of target thickness on resonant elastic scattering of protons by 19F

Abstract The excitation function and cross-sections of the reaction 19 F(p,p) 19 F, for protons in the energy range 0.75–2.8 MeV, were measured with an accurate method not dependent on the absolute values of the number of incident protons (beam charge collection) and of the target thickness. Corrections for target thickness and beam energy dispersion effects were introduced and their importance are discussed.