Marc Lefort

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).

Do we have actually strong motives for studying nuclear collisions in the energy range 10 to 100 MeV.A

Abstract This introduction tries to express the general idea that results on nucleus-nucleus collisions at medium energies are very often complementary of those obtained at barrier or even sub-barrier energies. The subjects examined are: elastic scattering and nucleus-nucleus potentials — complete fusion, neck formation, “extra push”, and fast fusion-fission processes — partial linear momentum transfer, incomplete fusion or nucleonic cascades — Energy deposit and its consequences — particle emissions and search for emission source — evolution of deep inelastic scattering as a function of η 1 = Z p Z T e 2 h v 1 — and production of neutron rich exotic nuclei through various mechanisms (N/Z equilibration at low energies, fragmentation of high energies). Finally, the main equipments built at GANIL are very briefly presented.

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.

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.