Magic Numbers in Boson 4He Clusters: The Auger Evaporation Mechanism

The absence of magic numbers in bosonic 4He clusters predicted by all theories since 1984 has been challenged by high-resolution matter-wave diffraction experiments. The observed magic numbers were explained in terms of enhanced growth rates of specific cluster sizes for which an additional excitation level calculated by diffusion Monte Carlo is stabilized. The present theoretical study provides an alternative explanation based on a simple independent particle model of the He clusters. Collisions between cluster atoms in excited states within the cluster lead to selective evaporation via an Auger process. The calculated magic numbers as well as the shape of the number distributions are in quite reasonable agreement with the experiments.

Shell closure at N = 34 and the 48Si nucleus

By using a nonrelativistic independent particle model, we investigate the mechanism promoting 34 as new magic number. We carried out Hartree–Fock plus Bardeen–Cooper–Schrieffer and Quasi-particle Random Phase Approximation calculations by consistently using the same finite-range interaction in all the three steps of our approach. We used four Gogny-like interactions, with and without tensor terms. We find that the shell closure for [Formula: see text] neutrons appears in isotones with [Formula: see text] protons. The smaller the proton number, the more evident is the shell closure at [Formula: see text]. An ideal nucleus to investigate this effect should be [Formula: see text]Si, as it has been recently suggested. However, some discrepancies occur between the results obtained with the four effective interactions we used concerning the position of the two-neutron drip line and, therefore, the existence of [Formula: see text]Si. The experimental identification of this nucleus could shed light about the shell evolution in nuclei far from the stability valley and put stringent tests on nuclear structure theories.

Study on anisotropies and momentum densities in AlN, GaN and InN by positron annihilation

The independent particle model (IPM) coupled with empirical pseudopotential method (EPM) was used to compute the thermalized positron charge densities in specific family of binary tetrahedrally coordinated crystals of formula ANB8-N. Initial results show a clear asymmetrical positron charge distribution relative to the bond center. It is observed that the positron density is maximum in the open interstices and is excluded not only, from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001,110) planes. The results are used to analyze the positron effects in AlN, GaN and InN compounds. Our computational technique provides the theoretical means of interpreting the k-space densities obtained experimentally using the twodimensional angular correlation of annihilation radiation (2D-ACAR).