Two-neutron transfer in nuclei close to the drip line

We investigate the two-neutron transfer modes induced by $(t,p)$ reactions in neutron-rich oxygen isotopes. The nuclear response to the pair transfer is calculated in the framework of continuum quasiparticle random phase approximation (cQRPA). The cQRPA allows a consistent determination of the residual interaction and an exact treatment of the continuum coupling. The $(t,p)$ cross sections are calculated within the distorted wave Born approximation approach and the form factors are evaluated by different methods: macroscopically, following the Bayman and Kallio method, and fully microscopically. A significant part of the cross section corresponds to a high-lying collective mode built entirely upon continuum quasiparticle states.

Figure 1

The QRPA response for the two-neutron transfer on ${}^{18,20,22}\mathrm{O}$. The exact continuum calculations are in solid lines whereas the calculations with box boundary conditions are in dashed lines. The results are displayed as functions of $E^{*}$, the excitation energy with respect to the final nucleus ground state.

Figure 2

The response function for the two-neutron transfer on ${}^{22}\mathrm{O}$. The unperturbed response is in solid line and the QRPA response in dashed line.

Figure 3

DWBA calculations for the reaction ${}^{22}\mathrm{O}(t,p)$ at $15\text{MeV}∕\text{nucleon}$ calculated with a macroscopic form factor (see text for details).

Figure 4

The microscopic form factors for the lowest energy state in the ${}^{22}\mathrm{O}+2n$ system. The solid line is obtained using the Bayman and Kallio method, and the dashed line with the unperturbed ${\mathbf{G}}_0$ response.

Figure 5

$15\text{MeV}∕\text{nucleon}$ ${}^{22}\mathrm{O}(t,p)$ DWBA calculations using the form factors shown in Fig. 4.

Figure 6

The QRPA form factors for the $8.6\text{MeV}$ state and for the giant pairing vibration region. The form factors obtained with box boundary conditions are in solid lines whereas the form factors obtained with the exact continuum treatment are displayed in dashed lines. The calculations are done for the ${}^{22}\mathrm{O}+2n$ system.

Figure 7

DWBA calculations for the $8.6\text{MeV}$ state. The solid (dashed) line corresponds to the QRPA results obtained with box (exact) boundary conditions. The calculations correspond to the system ${}^{22}\mathrm{O}+2n$.