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

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 $\gamma$-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 ${}^{12}\mathrm{C}({}^{22}\mathrm{Mg},{}^{23}\mathrm{Mg}+\gamma )X$ 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-$\ell$ (intruder) states, which are often direct indicators of shell evolution and the disappearance of magic numbers in the exotic regime.

Figure 1

Particle identification spectrum (energy loss vs time of flight) for ${}^{23}\mathrm{Mg}$ produced in the ${}^{12}\mathrm{C}({}^{22}\mathrm{Mg},{}^{23}\mathrm{Mg}+\gamma )X$ one-neutron pickup reaction. The spectrum shows all projectile-like reaction residues from the interaction of ${}^{22}\mathrm{Mg}$ with the C target that entered the S800 focal plane in the same magnetic rigidity setting as ${}^{23}\mathrm{Mg}$.

Figure 2

Event-by-event Doppler reconstructed $\gamma$-ray spectra measured in coincidence with ${}^{23}\mathrm{Mg}$ populated in ${}^{12}\mathrm{C}({}^{22}\mathrm{Mg},{}^{23}\mathrm{Mg}+\gamma )X$ (top) and ${}^9\mathrm{Be}({}^{22}\mathrm{Mg},{}^{23}\mathrm{Mg}+\gamma )X$ (bottom), respectively. Left: The prominent $\gamma$-ray transition at 451 keV depopulates the $5/2_1^{+}$ excited state to the $3/2^{+}$ ground state; right: expansion of the higher energy part of the $\gamma$-ray energy spectrum. The other $\gamma$ rays correspond to transitions from the known $7/2^{+}$, $1/2^{+}$, and $9/2^{+}$ or $5/2^{+}$ excited states at 2052, 2359, and 2715 keV [14], respectively. There is some indication of a transition at about 2900 keV, which may depopulate the alleged $(3/2,5/2){}^{+}$ state at 2908 keV excitation energy [14].

Figure 3

Momentum distributions of the one-neutron pickup product ${}^{23}\mathrm{Mg}$ and momentum profile of the ${}^{22}\mathrm{Mg}$ projectile beam passing through the Be and C targets, respectively. The lines are to guide the eye and have been used for the ${}^{23}\mathrm{Mg}$ distribution to estimate acceptance losses on the low-momentum side caused by a beam blocker in the S800 focal plane that was necessary to prevent the direct beam from entering the detection system.

Figure 4

Laboratory-frame scattering-angle distribution, $d\sigma /d{\theta }_L$, of the heavy residues measured in the experiment for the carbon (upper panel) and the beryllium (lower panel) targets. For the carbon target a calculated laboratory-frame angular distribution is also shown (dashed curve) for pickup to a single final state, the 0.451 MeV, $5/2^{+}$ state (with ${}^{11}\mathrm{C}$ left in its $3/2^{-}$ ground state). The latter curve has been scaled to the measurements to aid comparison of the shapes but does not take into account small folding effects due to the finite emittance of the incident beam and from angular straggling in the target. The spectra show that the distributions fit well into the elliptical angular acceptance of the S800 spectrograph ($\pm$5${}^{°}$ $\times$ $\pm$3.5${}^{°}$) in agreement with the calculations.

Figure 5

Schematic of the CCBA channel-coupling scheme used for the ${}^{22}\mathrm{Mg}$(${}^{12}\mathrm{C}$,${}^{11}\mathrm{C}$)${}^{23}\mathrm{Mg}$($J^{\pi }$) and ${}^{22}\mathrm{Mg}$(${}^9\mathrm{Be}$,${}^8\mathrm{Be}$*)${}^{23}\mathrm{Mg}$($J^{\pi }$) neutron pickup reactions. The solid lines (from ${}^{22}\mathrm{Mg}$ to ${}^{23}\mathrm{Mg}$) represent one-step single-particle transfers to the final states indicated. The dashed lines show two-step pathways to ${}^{23}\mathrm{Mg}$ states, by neutron capture onto the ${}^{22}\mathrm{Mg}$$(2^{+})$ core. All of these involve a $1d_{5/2}$ neutron orbital.