Cross-shell excitation in two-proton knockout: Structure of ${}^{52}\mathrm{Ca}$

The two-proton knockout reaction ${}^9\mathrm{Be}$(${}^{54}\mathrm{Ti}$,${}^{52}\mathrm{Ca}+\gamma$) has been studied at 72 MeV/nucleon. Besides the strong feeding of the ${}^{52}\mathrm{Ca}$ ground state, the only other sizeable cross section proceeds to a $3^{-}$ level at 3.9 MeV. There is no measurable direct yield to the first excited $2^{+}$ state at 2.6 MeV. The results illustrate the potential of such direct reactions for exploring cross-shell proton excitations in neutron-rich nuclei and confirms the doubly-magic nature of ${}^{52}\mathrm{Ca}$.

Figure 1

Longitudinal momentum distribution of the ${}^{52}\mathrm{Ca}$ knockout residues and the unreacted ${}^{54}\mathrm{Ti}$ projectiles (momentum acceptance of the A1900 restricted to 1%). The ${}^{52}\mathrm{Ca}$ data are compared to a calculated shape given by the convolution of the separate distributions for two uncorrelated protons removed from the $f_{7/2}$ orbit (dashed line) and to the same curve, but folded with the momentum profile of the ${}^{54}\mathrm{Ti}$ beam (solid line).

Figure 2

γ-ray spectrum (2.75% detection efficiency at 1 MeV) in coincidence with the ${}^{52}\mathrm{Ca}$ knockout residues. The two γ-ray transitions establish the cascade shown in the inset. The arrows indicate the feeding profile by the reaction [$\sigma (3^{-})=0.11(3)$ mb and $\sigma (0^{+})=0.21(3)$ mb]. For this part of the experiment, the A1900 fragment separator was operated without momentum restriction.

Figure 3

Systematics of the first $2^{+}$ and $3^{-}$ excitations along the even-even Ca isotopic chain (data from Refs. [17, 18, 19]).