Quasi-free Neutron Knockout Reaction Reveals a Small $s$-orbital Component in the Borromean Nucleus $^{17}$B

TL;DR

This study used a quasi-free scattering experiment to analyze the structure of $^{17}$B, revealing a surprisingly small $s$-orbital component and providing new insights into neutron halo phenomena in nuclei.

Contribution

It presents the first direct measurement of the $s$-orbital component in $^{17}$B, challenging previous assumptions about halo nuclei and linking experimental results with theoretical models.

Findings

Small $s$-orbital component of 9(2)% in $^{17}$B

Evidence of a definite but not dominant neutron halo

Largest $s$- or $p$-orbital component among known halo nuclei

Abstract

A kinematically complete quasi-free $(p,pn)$ experiment in inverse kinematics was performed to study the structure of the Borromean nucleus $^{17}$B, which had long been considered to have 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 percentage of 9(2)$\%$ was determined for $1s_{1/2}$. Our finding of such a small $1s_{1/2}$ 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 $^{17}$B. 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 neutron halo.