Correlated two-nucleon stripping reactions

The cross sections for the stripping of two correlated nucleons from light and medium-mass nuclei are considered. Such reactions are of interest both as a means for populating and identifying low-lying excited states of very exotic nuclear species and as a potential direct spectroscopic probe of two-nucleon correlations in such systems. A calculation scheme that combines the full shell model two-nucleon spectroscopic amplitudes with eikonal reaction theory is presented. The theoretical predictions of the method, and of more approximate schemes, are compared with new data on two-proton removal from ${}^{28}\mathrm{Mg}$. The combined full shell model structure amplitudes and reaction dynamics predictions are in good agreement with the available measurements. First indications of the sensitivity of the reaction mechanism to the spatial and angular momentum structure of the stripped two-nucleon wave functions are also discussed and clarified.

Figure 1

Schematic of the angular momentum couplings used in the description of the two-nucleon knockout reaction.

Figure 2

Energy diagram of the neutron-rich $N=16$ isotones ${}^{28}\mathrm{Mg}$, ${}^{27}\mathrm{Na}$, and ${}^{26}\mathrm{Ne}$, showing the single-neutron $\left(\nu \right)$ and proton $\left(\pi \right)$ separation energies for each nucleus. The diagram shows that nondirect population of the bound states of ${}^{26}\mathrm{Ne}$, by one-proton removal to excited ${}^{27}\mathrm{Na}$ followed by proton evaporation, would involve states high above the (much lower) neutron evaporation threshold and so is expected to be negligible.

Figure 3

The analog of Fig. 2 for the neutron-deficient ${}^{30,29,28}\mathrm{S}$ isotopes, showing the single-neutron $\left(\nu \right)$ and proton $\left(\pi \right)$ separation energies for each nucleus. The diagram shows that nondirect two-neutron removal to bound states in ${}^{28}\mathrm{S}$, by one-neutron removal to excited ${}^{29}\mathrm{S}$ and then neutron evaporation, would involve states far above the proton evaporation threshold and is expected to be negligible.