X. Tarrago

Émission d'hélions dans les réactions de spallation

Abstract In the course of studies on spallation reactions in thorium targets, it was found that the emission of alpha particles could explain high cross sections for Ac isotopes. A systematic study of both secondary reactions induced by α fragments, and Ac cross sections in the energy range 40–150 MeV has given the following results: 1) the α particles are emitted mainly in the forward direction; 2) the absolute cross sections of α fragments are 20 mb at 65 MeV, 35 mb at 85 MeV and 75 mb at 150 MeV; 3) these data were used in the calculation of cross sections for (p, α xn) reactions and good agreement was found with the experimental cross sections of Ac isotopes; 4) a mechanism is proposed of knock-out of α structures by prompt nucleons on the nucleus boundary. Spectra of prompt cascade nucleons and α particles were compared in two different cases (incident energies 82 and 150 MeV) and the results show a constant ratio of α particles for all intervals of energies. It is believed that these results show some evidence that a large proportion of nucleons on the surface of heavy nuclei belong to alpha particles (0.5 to 0.9).

Réactions secondaires de spallation

Abstract In the spallation nuclear reactions induced by 150 MeV protons om bismuth and thallium, light fragments are produced. Alpha particles induce secondary reactions. By radio-chemical study of astatine isotopes, cross sections for (α,2n) to (α,6n) reactions are measured. A cross section of 60 ± 10 mb is calculated for the production of helion fragments, and the energy distribution given. An upper limit is estimated for the production cross sections of lithium and beryllium fragments.

Application de la théorie statistique à la désexcitation de noyaux composés Xe de grands moments angulaires

Abstract Angular and energy distributions have been established of protons, deuterons and alpha particles produced in the bombardment of a natural silver target by 74 MeV nitrogen ions. The experimental results have been compared with theoretical values deduced from the evaporation mechanism taking into account angular momentum effects (Ericsons theory). A new type of calculation, which is analytical and not based on Monte-Carlo methods, was performed for this purpose. It is shown from this calculation and also in relation to angular distributions observed that emission of alpha particles is more probable at the end of the evaporation chain than at its beginning. The cross section for the formation of the compound nucleus is also discussed.

Protons d'interaction directe émis à partir des cibles Ag et Au bombardées par des protons de 156 MeV et de 91 MeV

Abstract Angular and energy distributions have been measured on prompt protons emitted between 4 and 150 MeV after the bombardment of Ag and Au targets by 156 and 91 MeV protons. The experimental device is a telescope made of 4 d E /d x and E solid state counters. The energy distributions obtained in the forward angles are compared with the results obtained after Monte-Carlo calculations on a large number of nucleon-nucleon quasi-elastic collisions inside the nuclear matter. Three models have been used to start on the calculation. The impulse approximation is assumed for all three. The first one is based on a square well potential (carre). Both model 2 and model 3 assume a diffuse edge nucleus. Model 2 (step) takes account of transmission coefficients by a classical refraction approximation. Model 3 (step no) does not include any effect of refraction or reflexion. The comparison with experimental data shows that prompt cascade protons are emitted at a large rate even for an incident energy of 91 MeV. The proportion of compound nucleus is much lower than the calculated value by model 2. In many respects the best fit seems to be obtained with model 3, so called “step no”. The complete set of experimental data might stimulate more elaborated calculations on the direct process.

Fragments alpha produits par spallation de bismuth par des protons de 240 à 600 MeV

Abstract From secondary reaction measurements on Bi nuclei bombarded by high energy protons the cross sections for α-particle emission were deduced. The values are 110, 220 and 320 mb at incident energies of 240, 420 and 550 MeV, respectively. The energy distributions of α-particles emitted in the forward or in the backward directions have been determined, and it could be shown that the following two processes are involved: 1. a)An isotropic emission in the centre-of-mass system the cross section of which increases from 25 to 210 mb when the incident energy goes from 240 to 550 MeV. 2. b)A process in which α-particles are emitted mainly in the forward direction. Calculations were carried out with a IBM 7094 computer, assuming that α substructures are “knocked-out” from the surface of the nucleus by cascade nucleons. The probability for the occurrence of such α structures was found to be 0.7 ± 0.1.