Inverse-kinematics one-neutron pickup with fast rare-isotope beams

Abstract

Measurements and reaction model calculations are reported for single-neutron pickup reactions onto a fast $^{22}\mathrm{Mg}$ secondary beam at 84 MeV per nucleon. Measurements made on both carbon and beryllium targets, having very different structures, were used to investigate the likely nature of the pickup reaction mechanism. The measurements involve thick reaction targets and $\ensuremath{\gamma}$-ray spectroscopy of the projectile-like reaction residue for final-state resolution, which permit experiments with low incident beam rates compared to traditional low-energy transfer reactions. From measured longitudinal momentum distributions we show that the ${}^{12}\mathrm{C}({}^{22}\mathrm{Mg},{}^{23}\mathrm{Mg}+\ensuremath{\gamma})X$ reaction largely proceeds as a direct two-body reaction, with the neutron transfer producing bound $^{11}\mathrm{C}$ target residues. The corresponding reaction on the $^{9}\mathrm{Be}$ target seems to largely leave the $^{8}\mathrm{Be}$ residual nucleus unbound at excitation energies high in the continuum. We discuss the possible use of such fast-beam one-neutron pickup reactions to track single-particle strength in exotic nuclei and also their expected sensitivity to neutron high-$\ensuremath{\ell}$ (intruder) states, which are often direct indicators of shell evolution and the disappearance of magic numbers in the exotic regime.