Spectroscopy of strongly deformed ${}^{32}\mathrm{Ne}$ by proton knockout reactions

Low-lying states of neutron-rich ${}^{32}\mathrm{Ne}$ were populated by means of one- and two-proton knockout reactions at the RIKEN Radioactive Isotope Beam Factory. A new transition is observed at 1410(15) keV and assigned to the $4_1^{+}\to 2_1^{+}$ decay. With this energy the $R_{4/2}$ ratio is calculated to be 2.99(6), lying close to the rigid rotor limit and suggests a high degree of collectivity and strongest deformation among neutron-rich neon isotopes. Comparisons of experimental inclusive and exclusive reaction cross sections with shell-model and eikonal reaction dynamical calculations reveals considerable quenching for this highly asymmetric system and contributes to systematic trends.

Figure 1

The ZeroDegree spectrometer particle identification of predominantly fully stripped reaction residues after the ${}^9\mathrm{Be}$ secondary reaction target.

Figure 2

Doppler-reconstructed $\gamma$-ray energy spectrum of two-proton and (a) one-proton knockout reactions leading to ${}^{32}\mathrm{Ne}$. A least-squares fit (red solid line) of a global function composed of simulated responses of dali2 (dotted blue) and a two-component exponential background (dotted cyan) is applied. (b) Background-subtracted $\gamma \gamma$ coincidence spectrum of combined one- and two-proton knockout reactions for a 1410-keV transition (1300–1500-keV gate and 1600–3000-keV background region).

Figure 3

(a) Energy levels from the present experiment compared to (b) EEdf1 [42] and (c) SDPF-M [28] effective interaction shell-model calculations for ${}^{32}\mathrm{Ne}$. The extrapolated neutron separation energy $\left(S_n\right)$ of 2250(570) keV [43] is shown in red. The uncertainty is shown as the width.

Figure 4

Systematics of experimental first excited $2_1^{+}$ and $4_1^{+}$ levels across $16\le N\le 26$ for (a) silicon (black, triangle), (b) magnesium (blue, square), and (c) neon (red, circle). Open circles are measured in this Rapid Communication. Connecting lines are drawn to guide the eye. Horizontal bars are calculations with EEdf1 effective interaction [42]. (d) $R_{4/2}$ values. Dashed horizontal lines for vibrational (2.0) and rotational limits (3.33) are drawn. Experimental data were taken from Refs. [45, 46, 47, 48, 49, 50, 51].

Figure 5

(a) Two-proton knockout experimental inclusive and $0_{\mathrm{g}.\mathrm{s}.}^{+}\to 0_{\mathrm{g}.\mathrm{s}.}^{+}$ cross sections $\left(\sigma \right)$. Data taken from Refs. [32, 34, 35]. (b) $R_s\left(2N\right)$ suppression factors utilizing EEdf1 and SDPF-M interactions. The theoretical inclusive cross sections to ${}^{26}\mathrm{Ne}$ and ${}^{28}\mathrm{Ne}$ are calculated with the eikonal reaction framework, described in this Rapid Communication, to be 2.82 and 1.28 mb. The associated inclusive $R_s\left(2N\right)$'s for ${}^{26}\mathrm{Ne}$ and ${}^{28}\mathrm{Ne}$ are 0.53(4) and 0.38(4).

Figure 6

Experimental exclusive cross sections and theoretical predictions from shell-model calculations. Exclusive theoretical cross sections are scaled by the inclusive $R_s$ value for the visualization of the populated ratio. (a) Two-proton knockout reactions to ${}_{10}^{32}\mathrm{Ne}$ [35]. The theoretical cross section to the $2_1^{+}$ state was conjectured to include unobserved feeding from a $2_2^{+}$ state. This combined cross section is also plotted. (c) One-proton knockout reactions to ${}_{10}^{32}\mathrm{Ne}$.