Quasifree Neutron Knockout Reaction Reveals a Small s-Orbital Component in the Borromean Nucleus B-17

A kinematically complete quasifree (p, pn) experiment in inverse kinematics was performed to study the structure of the Borromean nucleus B-17, which had long been considered to have a neutron halo. By analyzing the momentum distributions and exclusive cross sections, we obtained the spectroscopic factors for 1s(1/2) and 0d(5/2) orbitals, and a surprisingly small 1s(1/2) percentage of 9(2)% was determined for 1s(1/2). Our finding of such a small component and the halo features reported in prior experiments can be explained by the deformed relativistic Hartree-Bogoliubov theory in continuum, revealing a definite but not dominant neutron halo in B-17. The present work gives the smallest s- or p-orbital component among known nuclei exhibiting halo features and implies that the dominant occupation of s or p orbitals is not a prerequisite for the occurrence of a neutron halo