Radiative Proton Capture on6He

Abstract

Radiative capture of protons is investigated as a probe of clustering in nuclei far from stability. The first such measurement on a halo nucleus is reported here for the reaction 6He(p,gamma) at 40 MeV. Capture into 7Li is observed as the strongest channel. In addition, events have been recorded that may be described by quasifree capture on a halo neutron, the alpha core, and 5He. The possibility of describing such events by capture into the continuum of 7Li is also discussed.

Halos and related structures

“…15 C The one-neutron binding energy is here just above 1 MeV, but the last neutron seems to be a good single-particle s-state and the system is therefore a good example of a state in between normal nuclei and well developed one-neutron halos. This is reflected both in its interaction cross-section and in the width of the longitudinal momentum distribution after one-neutron removal, note that this nucleus and 22 N appear to have a quite similar behaviour in the general surveys [81,82]. 4.1.10.…”

“…Shortly after longitudinal momentum distributions of the core fragment [80] became available, these are less affected by the reaction and -if the reaction mechanism is disregarded -can be easily interpreted (a large spatial extension corresponding to a narrow momentum width). Surveys of longitudinal momentum distributions have been carried out for many neutron-rich nuclei [81,82]. The reaction mechanism of course needs to be taken into account as well in order to extract quantitative information.…”

The cluster–core model for the halo structure of light nuclei at the drip lines

“…In these nuclei, the neutron(s)-cluster minimum in the PES for each case is deepest at the 2n+core configuration and, except for 12 Be and 27 F , the S 2n is also the lowest. For both the 12 Be and 27 F , the lowest neutron separation energy is for one-neutron separation S 1n , whereas the PES for these nuclei show the 2n+core configuration lying clearly lowest (for 12 Be, also both S 1n and S 2n >> 1 MeV). On the other hand, 29 F is clearly a 2n-halo nucleus from the point of view of the separation energy (S 2n is lowest, compared to S 1n and S 3n ) but in the PES, the minima at 3n+ and 4n+core are almost as deep as for the 2n+core configuration (see Fig.…”

“…2 )) between the x protons (here x = A 2 = Z 2 ). The above definitions for the binding energies of n-or p-clusters mean that the nucleons in these clusters are taken to be unbound, just as is the case in the model of Hansen and Jonson [45], the few-body theories [19] or as is suggested by the recent experiments [11,12].…”

“…Based on these characteristic properties, the experimentally observed 1nhalo nuclei are 11 Be and 19 C [2, 3,4,5] and the 2n-halo nuclei are 6,8 He, 11 Li, 14 Be and 17 B [6,7,8,9,10]. Since the existence of a bound di-neutron in the ground-state is not yet observed [11,12] (a free di-neutron is also unbound by about 70 KeV), apparently, we are dealing here with two-body (n+core) and three-body (n+n+core) structures, respectively, for the 1n-and 2n-halo nuclei. Table 1 shows that many other cases have been investigated, both theoretically and experimentally [13,14,15,16,17], and in some calculations [18] even a 3n-halo structure is predicted (for 26 F ) which means a four-body system.…”