Prominence of Pairing in Inclusive $\mathbf{(}p,2p\mathbf{)}$ and $\mathbf{(}p,pn\mathbf{)}$ Cross Sections from Neutron-Rich Nuclei

Fifty-five inclusive single nucleon-removal cross sections from medium mass neutron-rich nuclei impinging on a hydrogen target at $\sim 250\mathrm{MeV}/\mathrm{nucleon}$ are measured at the RIKEN Radioactive Isotope Beam Factory. Systematically higher cross sections are found for proton removal from nuclei with an even number of protons as compared to odd-proton number projectiles for a given neutron separation energy. Neutron removal cross sections display no even-odd splitting, contrary to nuclear cascade model predictions. Both effects are understood through simple considerations of neutron separation energies and bound state level densities originating in pairing correlations in the daughter nuclei. These conclusions are supported by comparison with semimicroscopic model predictions, highlighting the enhanced role of low-lying level densities in nucleon-removal cross sections from loosely bound nuclei.

Figure 1

Chart of the nuclides showing existing data (blue) for inclusive single nucleon-removal cross sections from exotic nuclei near $200\mathrm{MeV}/\mathrm{nucleon}$ (see [12, 15, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55]) and data from this work (red). Parent nuclei are indicated. Stable nuclides are shown in black, and major proton and neutron shell closures are indicated by gray lines.

Figure 2

(a) Particle identification plot of reaction products detected in the ZeroDegree spectrometer for ${}^{96}\mathrm{Kr}$ incident on target. $Z$ is the proton number of the nucleus, whereas $A/q$ is the mass to charge ratio. The nuclides are assumed to be fully stripped, although charge states are visible for $Z=36$ beyond $A/q=2.75$. (b) Ratio of BigRIPS dispersive focal distributions for ${}^{96}\mathrm{Kr}(p,pn){}^{95}\mathrm{Kr}$, including the fit used to extract the daughter/parent ratio $N_d/N_p$. See text for details.

Figure 3

(a) Inclusive $(p,2p)$ cross sections measured in this work (black circles) compared with intranuclear cascade (INCL) predictions (blue squares). Even-$Z$ projectiles are shown as open symbols, odd-$Z$ projectiles are shown as filled symbols. (b) Odd-even splitting in the $(p,2p)$ data compared with INCL (blue squares) and modified INCL (red triangles) calculations. Regressions shown with standard residual uncertainty bands. See text for details.

Figure 4

(a) Inclusive $(p,pn)$ cross sections measured in this work (black circles) compared with INCL predictions (blue squares). Even-$N$ projectiles are shown as open symbols, odd-$N$ projectiles are shown as filled symbols. Adjacent isotopes are connected by lines. (b) Odd-even splitting in the $(p,pn)$ data compared with INCL (blue squares) and modified INCL (red triangles) calculations. Regressions are shown with standard residual uncertainty bands. See text for details.