One-neutron removal reactions of ${}^{18}\mathrm{C}$ and ${}^{19}\mathrm{C}$ on a proton target

We have studied the one-neutron removal reactions of ${}^{18}\mathrm{C}$ at 81 MeV/nucleon and ${}^{19}\mathrm{C}$ at 68 MeV/nucleon on a liquid hydrogen target. Transverse-momentum distributions of the core fragments ${}^{17}\mathrm{C}$ and ${}^{18}\mathrm{C}$, and their partial cross sections, are measured using coincidences with the $\gamma$ rays. These are compared to theoretical calculations based on the continuum-discretized coupled-channels (CDCC) method. We have found that the transverse-momentum distributions of ${}^{17}\mathrm{C}$ are well described by the CDCC calculations, and the agreement provides evidence for the spin-parity assignments of $J^{\pi }=1/2^{+}$ for the 0.21-MeV state and $J^{\pi }=5/2^{+}$ for the 0.33-MeV state in ${}^{17}\mathrm{C}$. The $J^{\pi }=(2,3){}^{+}$ assignment for the 4.0-MeV state in ${}^{18}\mathrm{C}$ is obtained for the first time based on the momentum distribution of ${}^{18}\mathrm{C}$. The measured partial cross sections are consistent with the CDCC calculations combined with shell-model spectroscopic factors. The present study demonstrates that the transverse-momentum distribution obtained in a one-neutron removal reaction on a proton target is as useful as the conventional longitudinal-momentum distribution measured with the heavier Be or C targets for determining the orbital angular momentum of a removed neutron.

Figure 1

Level schemes of ${}^{17}\mathrm{C}$ and ${}^{18}\mathrm{C}$ from the previous experiments, together with the shell-model prediction (indicated by WBP). The dashed lines display the one-neutron separation energy. The arrows indicate the $\gamma$ transitions observed in the previous experiments [27, 37].

Figure 2

Energy spectrum of the $\gamma$ rays emitted from ${}^{17}\mathrm{C}$ in the one-neutron removal reaction of ${}^{18}\mathrm{C}$. The observed peaks are indicated by the arrows. The solid line shows the result of a fit using the response functions of the 0.21- and 0.33-MeV $\gamma$ rays (dashed lines) and an exponential background (dotted line).

Figure 3

Transverse-momentum distributions of ${}^{17}\mathrm{C}$ in coincidence with the (a) 0.21- and (b) 0.33-MeV $\gamma$ rays. The solid and dashed lines represent the CDCC calculations by assuming neutron removal from the $l=0$ and $l=2$ orbitals, respectively.

Figure 4

Energy spectrum of the $\gamma$ rays emitted from ${}^{18}\mathrm{C}$ in the one-neutron removal reaction of ${}^{19}\mathrm{C}$ (upper panel). The observed peaks are indicated by the solid arrows. The solid line shows the result of a fit using the response functions of the $\gamma$ rays (dashed lines) and an exponential background (dotted line). Level scheme and transitions are also shown in the lower panel.

Figure 5

Transverse-momentum distribution of ${}^{18}\mathrm{C}$ in coincidence with the 2.4-MeV $\gamma$ ray. The solid and dashed lines represent the CDCC calculations by assuming neutron removal from the $l=0$ and $l=2$ orbitals in ${}^{19}\mathrm{C}$.