Neutron Pair Transfer in ${}^{60}\mathrm{Ni}+{}^{116}\mathrm{Sn}$ Far below the Coulomb Barrier

An excitation function of one- and two-neutron transfer channels for the ${}^{60}\mathrm{Ni}+{}^{116}\mathrm{Sn}$ system has been measured with the magnetic spectrometer PRISMA in a wide energy range, from the Coulomb barrier to far below it. The experimental transfer probabilities are well reproduced, for the first time with heavy ions, in absolute values and in slope by microscopic calculations which incorporate nucleon-nucleon pairing correlations.

Figure 1

Mass spectra of Ni isotopes in the ${}^{60}\mathrm{Ni}+{}^{116}\mathrm{Sn}$ system at two representative energies, above and below the Coulomb barrier.

Figure 2

Matrix of mass vs total kinetic energy loss (TKEL) for Ni isotopes at $E_{\mathrm{lab}}=410\mathrm{MeV}$. One sees the clear separation between quasielastic and ($1n$) and ($2n$) transfer channels and the concentration of the TKEL distributions in a narrow range close to $\sim 0\mathrm{MeV}$.

Figure 3

Top: Ratio between the quasielastic and the Rutherford cross section. Symbols represent the experimental values, solid line is the theoretical calculation with the GRAZING code. Bottom: Experimental (points) and microscopically calculated (lines) transfer probabilities for the one- (${}^{61}\mathrm{Ni}$) and two-neutron (${}^{62}\mathrm{Ni}$) pickup plotted as a function of the distance of closest approach $D$ (the entrance channel Coulomb barrier is estimated to be at 12.13 fm [4]). We also report (top) the reduced distance $d_0=D/(A_1^{1/3}+A_2^{1/3})$. The shown errors are only statistical and in most cases are smaller than the size of the symbol.

Figure 4

TKEL spectra obtained for the quasielastic and one- and two-neutron transfer channels at three representative bombarding energies, above (upper), near (middle), and below (lower) the Coulomb barrier. The corresponding distances of closest approach (in fm) are indicated. The dashed lines correspond to the position of the ground state $Q$ values: $Q_{\text{gs}}^{(1n)}=-1.7\mathrm{MeV}$ and $Q_{\text{gs}}^{(2n)}=+1.3\mathrm{MeV}$.