Observation of a proton halo in8B

Abstract

Abstract. The longitudinal momentum distribution of ZBe was measured after the break-up reaction of s B in C, A1, and Pb targets at 1471 A.MeV. We observed a narrow distribution with a FWHM of (814-6) MeV/c in all targets. The experimental results indicate an extended spatial distribution of the loosely bound proton in s B, and agree with QRPA calculations. PACS: 27.20+n; 21.10.Ft; 29.30_h; Pioneering experiments with secondary nuclear beams [1, 2] produced via projectile fragmentation at relativistic energies have lead to the discovery of neutron-halo nuclei. These first experiments at relativistic energies (790 A.MeV) have demonstrated that nuclear structure information can be deduced from the total interaction cross sections of halo nuclei, and from the momentum distributions of the break-up products. It was shown [3,4] that the longitudinal momentum distribution which results from the fragmentation of weakly-bound light projectiles is insensitive to the break-up interaction and provides a reliable probe of the internal momentum wavefunction of the system. The nucleus SB has an extemely low binding energy (137 keV) for the valence proton and is considered to be a proton halo candidate. However, the existence of nuclei with a proton halo is controversial based on present experimental and theoretical results [5,6,7,8]. Inspired by the lack of a consistent picture we launched an experiment to investigate this nucleus by'mcasuring the longitudinal momentum distribution of ~'Be created in the break-up reaction at relativistic energies in different target materials. were used at the central and at the final focalplane for momentum an, d cross-section measurements, respectively. The fields of the first t#vo dipole stages of the FRS were set to select SB. The SB nuclei were identified by a combination of their magnetic rigidity (Bp) and energy loss measured by a position sensitive scintillation detector located in front of the secondary target at the central focal plane. In order to measure the parallel momentum distribution of 7Be fragments the FRS was operated as an energy-loss spectrometer [9]. The incident phase space of the SB ions was reduced by slits at the dispersive first focal plane and at the entrance of the FRS in order to reduce transmission losses for the break-up products. The FRS magnets behind the secondary target were set to transmit ZBe nuclei produced from SB break-up. A valid SB identification at the central focal plane was required in coincidence with a ZBe ion at the final focal plane. Targets of 4.40 g/cm 2 C, 5.45 g/cm 2 A1, and 8.00 g/crn 2 Pb were used. The particle identification of ZBe was performed by coincident time-of-flight and energy-deposition measurements combined with the Bp analysis deduced from position measurements (x) at the final focal plane. The Z-identification and the position measurement at the final focal plane were done with two ionisation chambers (ICI,IC2). The peculiar situation that both neighboring isotopes of SB and ;'Be are particle unstable allowed reaction cr...

Probing the exotic structure of 8B by its elastic scattering and breakup reaction on nuclear targets

“…However, as it has been pointed out in Ref. [21], the ground state of 8 B is more complex than simply a proton around an inert 7 Be core and nuclear structure calculations with an appropriate treatment of continuum effects can describe the measured narrow momentum distribution and cross sections.…”

“…The latter model used to interpret the experimental data in Ref. [18], as well as quasiparticle RPA calculations [21] and Serber model applied to data in Ref. [22], demonstrate the necessity to have a rms 7 Be-p distance of 4-4.5 fm in order to reproduce the small momentum distribution width.…”

“…The narrow momentum distributions of 7 Be fragments in the breakup of 8 B measured in C, Al, and Pb targets at 1471 MeV/nucleon with full width at half maximum (FWHM) of 81 ± 6 MeV/c in all targets have been interpreted in terms of a largely extended proton distribution for 8 B and have implied a radius of 2.78 fm [21]. Here we should mention also the results of the experiments at lower energies for the breakup of 8 B in the collisions with Be and Au targets at 41 MeV/nucleon (81 ± 4 and 62 ± 3 MeV/c FWHM for Be and Au targets, respectively) [22] and for C target at 36 MeV/nucleon [23] with FWHM 124 ± 17 and 92±7 MeV/c for the stripping and diffraction components, correspondingly.…”

“…The halo nature of 8 B nucleus through studies of its breakup has been mostly tested with cluster models pre-suming simple two-cluster structure that consists of 7 Be core and valence proton (for instance, Refs. [18,21,24], and Ref. [25] presenting a Fortran code to calculate cross sections for the core plus one-nucleon case) or extended three-body model (α+ 3 He+p) [26].…”

Hartree-Fock calculations in the density matrix expansion approach

“…In extreme dripline nuclei as e.g. 8 B [1] and 11 Li [2] evidence for a strong competition of interactions between individual nucleons and mean-field dynamics is found.…”