Long-range versus short-range correlations in the two-neutron transfer reaction ${}^{64}\mathrm{Ni}({}^{18}\mathrm{O},{}^{16}\mathrm{O}){}^{66}\mathrm{Ni}$

Recently, various two-neutron transfer studies using the $({}^{18}\mathrm{O},{}^{16}\mathrm{O})$ reaction were performed with a large success. This was achieved because of a combined use of the microscopic quantum description of the reaction mechanism and of the nuclear structure. In the present work we use this methodology to study the two-neutron transfer reaction of the ${}^{18}\mathrm{O}+{}^{64}\mathrm{Ni}$ system at 84 MeV incident energy, to the ground and first $2^{+}$ excited state of the residual ${}^{66}\mathrm{Ni}$ nucleus. All the experimental data were measured by the large acceptance MAGNEX spectrometer at the Instituto Nazionale di Fisica Nucleare –Laboratori Nazionali del Sud (Italy). We have performed exact finite range cross section calculations using the coupled channel Born approximation (CCBA) and coupled reaction channel (CRC) method for the sequential and direct two-neutron transfers, respectively. Moreover, this is the first time that the formalism of the microscopic interaction boson model (IBM-2) was applied to a two-neutron transfer reaction. From our results we conclude that for two-neutron transfer to the ground state of ${}^{66}\mathrm{Ni}$, the direct transfer is the dominant reaction mechanism, whereas for the transfer to the first excited state of ${}^{66}\mathrm{Ni}$, the sequential process dominates. A competition between long-range and short-range correlations is discussed, in particular, how the use of two different models (Shell model and IBM's) help to disentangle long- and short-range correlations.

Figure 1

Excitation energy spectrum of ${}^{66}\mathrm{Ni}$ obtained from the ${}^{18}\mathrm{O}+{}^{64}\mathrm{Ni}$ reaction at 84 MeV and $21.5^{\circ }<{\theta }_{\text{lab}}<{31}^{\circ }$. The red-hatched area corresponds with the background that comes from ${}^{12}\mathrm{C}$ impurities in the target. Line corresponding to the one-neutron separation energy ($S_n$) is also indicated.

Figure 2

Coupling scheme of the projectile and target overlaps used in the direct (a) and sequential (b) two-neutron transfer reaction calculations.

Figure 3

Experimental angular distributions for the transitions to the ground (a) and first excited state (b) at 1.424 MeV in ${}^{66}\mathrm{Ni}$. The curve corresponds to the extreme cluster model results.

Figure 4

Comparison of the experiment angular distribution with the independent coordinate and two-step DWBA calculations for the reaction ${}^{64}\mathrm{Ni}({}^{18}\mathrm{O},{}^{16}\mathrm{O}){}^{66}\mathrm{Ni}$. The spectroscopic amplitudes from Tables 1, 2, and 3 are used.

Figure 5

Comparison between calculated and experimental low-lying spectra for the ${}^{64}\mathrm{Ni}$ and ${}^{66}\mathrm{Ni}$ isotopes.

Figure 6

Comparison between calculated and experimental low-lying spectra for nucleus ${}^{65}\mathrm{Ni}$ using the set I of parameters.

Figure 7

Comparison of the experiment angular distribution with the independent coordinate and two-step DWBA calculations for the reaction ${}^{64}\mathrm{Ni}({}^{18}\mathrm{O},{}^{16}\mathrm{O}){}^{66}\mathrm{Ni}$. The spectroscopic amplitudes from Tables 7, 8, and 9 were used.