Quasifree Neutron Knockout Reaction Reveals a Small $s$-Orbital Component in the Borromean Nucleus ${}^{17}\mathrm{B}$

A kinematically complete quasifree $(p,pn)$ experiment in inverse kinematics was performed to study the structure of the Borromean nucleus ${}^{17}\mathrm{B}$, 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 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 ${}^{17}\mathrm{B}$. 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 a neutron halo.

Figure 1

Angular correlation between recoil protons and neutrons for events with recoil neutrons detected by WINDS (a) and reconstructed from momentum conservation (b). The black line indicates the kinematical simulation assuming quasifree $(p,pn)$ off ${}^{17}\mathrm{B}$. (c) Doppler-corrected $\gamma$-ray spectrum, fitted using two $\gamma$ rays (1327 and 1407 keV) and a two-exponential background. The inset presents the $\gamma$-gated $E_{\mathrm{rel}}$ spectrum, together with the inclusive one for comparison.

Figure 2

(a) ${}^{16}\mathrm{B}$ $E_{\mathrm{rel}}$ spectra gated by $0\mathrm{MeV}/c\le P\le 60\mathrm{MeV}/c$ (red) and $60\mathrm{MeV}/c\le P\le 160\mathrm{MeV}/c$ (blue) in comparison to the inclusive one (black). (b) Ratios of the momentum-gated spectrum to the inclusive one. The gray dashed line, with a constant value of 1, stands for the scenario that the entire spectrum is associated with knockout of $0d_{5/2}$ neutrons. (c) The fitting with a sum of four resonances. The inset is an enlarged view of the 0–1 MeV region.

Figure 3

Observed ${}^{16}\mathrm{B}$ states compared to GSM, VS-IMSRG, AMD, and SM calculations. The $1n$-separation energies are shown in parentheses (in MeV).

Figure 4

Transverse momentum ($P_x$) distributions for different ${}^{16}\mathrm{B}$ states. The vertical error bars stand for the statistical error, while the horizontal for the bin size. In (a) and (b), DWIA calculated curves for knockout of $1s_{1/2}$ (red dashed) and $0d_{5/2}$ (black dotted) neutrons are normalized to the peak of the experimental spectrum; in (c) and (d), the blue solid curves represent the fitting with a combination of $1s_{1/2}$ (red dashed) and $0d_{5/2}$ (black dotted). All DWIA curves have been convoluted with the experimental resolution.