Knockout from ${}^{46}\mathrm{Ar}:\ell =3$ neutron removal and deviations from eikonal theory

The ${}^9\mathrm{Be}({}^{46}\mathrm{Ar},{}^{45}\mathrm{Ar}+\gamma )X$ one-neutron removal reaction has been studied in inverse kinematics at 70 MeV/nucleon. Coincidences with γ rays served to disentangle knockout events leading to the ${}^{45}\mathrm{Ar}$ ground state. The measured partial cross section corresponds to a spectroscopic factor of 4.9(7). The residue momentum distribution is compared with new calculations based on eikonal theory and represents the first case of an $\ell =3$ neutron removal, as is expected for populating a $0f_{7/2}$ hole in the $N=28$ projectile. However, the measured ${}^{45}\mathrm{Ar}$ momentum distribution has a marked low-momentum tail suggestive of dissipative effects whereas the eikonal model predictions are symmetric. The angular distribution of the residues confirms that there is a deviation from the model.

Figure 1

Coincident γ rays referred to the rest system of the ${}^{45}\mathrm{Ar}$ residues. Only peak structures that are clearly visible in the Doppler-reconstructed spectra of both rings of SeGA are attributed to the decay scheme of the knockout residue ${}^{45}\mathrm{Ar}$.

Figure 2

(a) Experimental momentum distribution (black triangles) in coincidence with the 0.532-MeV γ ray. The contribution from γ rays of higher energy, determined from a gate set just above the peak, has been subtracted. The theoretical distributions [19] are shown for $\ell =0$ (dot-dashed), $\ell =1$ (dashed), and $\ell =2$ (solid). They have been transformed to the laboratory system and folded with the momentum profile shown in Fig. 3. (b) Inclusive momentum distribution (black dots), shown on the same relative scale as the non coincident events in Fig. 3. The errors are smaller than the size of the points.

Figure 3

The lower panel shows the measured momentum distribution of the unreacted ${}^{46}\mathrm{Ar}$ fragments (black squares) and an analytical fit (dashed). The distribution of the ${}^{45}\mathrm{Ar}$ ground state residues (black circles) obtained with a different field setting to exclude the unreacted ${}^{46}\mathrm{Ar}$ beam particles is compared with theoretical calculations for $\ell =1$ (dot-dashed), $\ell =2$ (dashed), and $\ell =3$ (solid). The upper panel shows the fraction of events at each momentum detected within 1.1° and a comparison (solid line) with a calculation based on eikonal theory.