Halo signals in the beta decay of28 O

Abstract

Abstract, We present a study of the 'hypothetical', doubly magic and extremely neutron-rich nucleus 280. In our description 280 is a bound nucleus and exhibits some of the features of a neutron halo, where a particular channel for beta delayed deuteron emission is evident.PACS: 21.10. Dr; 21.10.Gv; 21.60.Cs; Nuclei at the valley of stability have very similar neutron an proton radii, even in situations (e.g. 48Ca, 2~ of large neutron excess. However, recent experimental data on light nuclei far from stability, where the binding energy of the last one or two nucleons is very small, show the existence of a new structure in which these loosely bound neutrons or protons, depending on which side of the valley of stability, extend themselves far out, forming a halo [1,21. Most of this experimental data comes from the studies of the 2n-halo nuclei 11Li. The main fingerprints seen here are:9 an abnormally large matter radius compared to the charge radius, and consequently large reaction cross sections [3-6]; 9 large cross sections for Coulomb dissociation, especially in the 2 n channel [3-6]; 9 a very narrow momentum distribution in the fragmentation channel [7].There have been recent efforts aiming to understand the peculiar behaviour of some very neutron rich nuclei in a shell model context [8][9][10][11]. These studies have shown that the intruder configurations play a prominent role in the onset of deformation around 31Na. The physical mechanisms behind these shape transitions may be the same as those responsible for the appearance of a neutron halo. In order to explore this hypothesis, we take as a very simple example the extremely neutron rich nucleus 280.* Permanent address: Instituto de Estructura de la Materia, CSIC, Serrano 119, E-28006 Madrid, SpainThe studies of the shape transition around N = 20 far from stability, have shown that the gap between (usually) occupied and (usually) empty configurations is greatly reduced with respect to what happens at stability. From a mean field point of view the situation can be interpreted as the bunching of the bound and (almost) unbound orbits at the top of the well. Among these orbits some may have large radial extension correlated with their quasiunbound nature [12,13]. Therefore, the residual interaction determines what configurations are energetically favoured. When valence protons are available the attractive Q,~. Qv interaction may lead to deformed shapes.The shell model apl~roach to the neutron halo has a serious shortcoming in the difficulty of extracting the structure of the wave function in Configuration Space from the properties of the effective interaction and wave function, defined in Fock Space. In spite of that, it is the natural framework to account for some of the expected features of the neutron halo which depend only implicitely on the radial form of the wave function.In our case study, 280, we shall analyse the following results of the shell model calculations; 9 the structure of the wave function 9 the one and two-neutron separation energies 9 the s...

Microscopic Studies on the Nucleus 28 O

“…22 ' The average binding energies in NRMF approaches also agree with Poves et a/. 's prediction' 15 ] -E/A = 6.125 MeV. For the theoretical results obtained in RMF approaches, the data are more dispersed than those in NRMF approaches.…”

Nuclear Halos

“…Poves et al (139) have suggested that the as yet undiscovered and very likely unstable nudeus 280 would have a large dineutron component in the ground state, decaying to a high-lying deuteron cluster in 28F.…”