Halo Structure of the Neutron-Dripline Nucleus B19

Abstract

The heaviest bound isotope of boron 19 B has been investigated using exclusive measurements of its Coulomb dissociation, into 17 B and two neutrons, in collisions with Pb at 220 MeV=nucleon. Enhanced electric dipole (E1) strength is observed just above the two-neutron decay threshold with an integrated E1 strength of BðE1Þ ¼ 1.64 AE 0.06ðstatÞ AE 0.12ðsysÞ e 2 fm 2 for relative energies below 6 MeV. This feature, known as a soft E1 excitation, provides the first firm evidence that 19 B has a prominent two-neutron halo. Three-body calculations that reproduce the energy spectrum indicate that the valence neutrons have a significant s-wave configuration and exhibit a dineutronlike correlation.

Three-body structure of B19 : Finite-range effects in two-neutron halo nuclei

“…The computed dB(E1)/dε agrees rather well with the available data after convolution with the experimental resolution. The integrated B(E1) strength (up to 6 MeV) is 1.53 e 2 fm 2 , also in good agreement with the experimental value [16].…”

“…3 together with the experimental data from Ref. [16]. Again, we focus on the case of the GPT interaction plus the three-body force (thick black line) and the contact-like Gaussian 1 potential plus the density-dependent term (dot-dashed red line).…”

“…These curves include the convolution with the experimental resolution reported in Ref. [16], but for completeness we include also the purely theoretical results (thin lines). From this figure it is clear that the B(E1) distribution obtained using the realistic, finite-range GPT interaction, which provides 53.2% of s-waves in the ground state, gives a better description of the data.…”

“…Finally, note that the present calculations with a simple central Gaussian dependence of the n-n interaction are not fully equivalent to the contact potential in Refs. [9,16]. First, we simulate a δ(r nn ) dependence, whereas in Refs.…”

“…Following Ref. [16], the spin of the core is neglected, and the core-neutron potential is taken as a Woods-Saxon shape, whose parameters are adjusted to the scattering length a s = −50 fm and the d 5/2 resonance energy predicted by shell-model calculations [28]. For the n-n interaction two different tensor potentials have been considered: the GPT potential [29] and the GFJ parametrization in Ref.…”