M. Fonseca

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.

Electron screening effects in nuclear reactions: still an unsolved problem

Electron screening for nuclear reactions in metals plays an unexpected and important role in enhancing reaction cross sections in the ultra-low energy region. Even though there are still some discrepancies between experimental data from different authors, the enhanced screening effect in metallic environments is well established, and attributed to the quasi-free valence electrons in the metals. However, there is still no solid theory which can describe quantitatively the observed enhancements. In the present work, experimental and theoretical results obtained so far will be overviewed, and a proposal to improve our knowledge on this subject.

Study of nuclear reactions producing 36Cl by micro-AMS

36Cl is one of several short to medium lived isotopes (as compared to the earth age) whose abundances at the earlier solar system may help to clarify its formation process. There are two generally accepted possible models for the production of this radionuclide: it originated from the ejecta of a nearby supernova (where 36Cl was most probably produced in the s-process by neutron irradiation of 35Cl) and/or it was produced by in-situ irradiation of nebular dust by energetic particles (mostly, p, a, 3He -X-wind irradiation model).The objective of the present work is to measure the cross section of the 37Cl(p,d)36Cl and 35Cl(d,p)36Cl nuclear reactions, by measuring the 36Cl content of AgCl samples (previously bombarded with high energy protons and deuterons) with AMS, taking advantage of the very low detection limits of this technique for chlorine measurements.For that, the micro-AMS system of the LF1/ITN laboratory had to be optimized for chlorine measurements, as to our knowledge this type of measurements had never been performed in such a system (AMS with micro-beam).Here are presented the first results of these developments, namely the tests in terms of precision and reproducibility that were done by comparing AgCl blanks irradiated at the Portuguese National Reactor with standards produced by the dilution of the NIST SRM 4943 standard material.