Quasifree ($p$, $2p$) Reactions on Oxygen Isotopes: Observation of Isospin Independence of the Reduced Single-Particle Strength

Quasifree one-proton knockout reactions have been employed in inverse kinematics for a systematic study of the structure of stable and exotic oxygen isotopes at the ${\mathrm{R}}^3\mathrm{B}/\mathrm{LAND}$ setup with incident beam energies in the range of $300–450\mathrm{MeV}/\mathrm{u}$. The oxygen isotopic chain offers a large variation of separation energies that allows for a quantitative understanding of single-particle strength with changing isospin asymmetry. Quasifree knockout reactions provide a complementary approach to intermediate-energy one-nucleon removal reactions. Inclusive cross sections for quasifree knockout reactions of the type ${}^A\mathrm{O}(p,2p){}^{A-1}\mathrm{N}$ have been determined and compared to calculations based on the eikonal reaction theory. The reduction factors for the single-particle strength with respect to the independent-particle model were obtained and compared to state-of-the-art ab initio predictions. The results do not show any significant dependence on proton-neutron asymmetry.

Figure 1

Correlations of polar ($\vartheta$) and azimuthal ($\phi$) angles of two protons detected in the CB for the reaction ${}^{16}\mathrm{O}(p,2p){}^{15}\mathrm{N}$ measured in coincidence with the ${}^{15}\mathrm{N}$ fragment.

Figure 2

Projection $P_y$ of the momentum distribution of ${}^{15}\mathrm{N}$ after one-proton removal from ${}^{16}\mathrm{O}$, compared to the sum of theoretical distributions for the $0p_{1/2}$ and $0p_{3/2}$ orbits (solid curve) and the one scaled to the experimental cross section (dashed-dotted curve with shaded $2\sigma$ uncertainty range).

Figure 3

Doppler-corrected single-$\gamma$ spectrum measured in coincidence with ${}^{15}\mathrm{N}$ and two protons in CB. The simulated decays of the ${3/2}^{-}$ states at 6.32 and 9.93 MeV were fitted to the experimental data together with the background contribution. The total fit is displayed by the solid curve.

Figure 4

Reduction factor $R$ deduced from ($p$, $2p$) cross sections (circles and square) as a function of $S_p-S_n$ compared to theoretical SFs calculated with SCGF (triangles). The shaded area indicates the trend from an analysis of intermediate-energy one-nucleon removal cross sections.