Pair neutron transfer in ${}^{60}\text{Ni}+{}^{116}\text{Sn}$ probed via $\gamma$-particle coincidences

We performed a $\gamma$-particle coincidence experiment for the ${}^{60}\text{Ni}+{}^{116}\text{Sn}$ system to investigate whether the population of the two-neutron pickup channel leading to ${}^{62}\text{Ni}$ is mainly concentrated in the ground-state transition, as has been found in a previous work [D. Montanari et al., Phys. Rev. Lett. 113, 052501 (2014)]. The experiment has been performed by employing the PRISMA magnetic spectrometer coupled to the Advanced Gamma Tracking Array (AGATA) demonstrator. The strength distribution of excited states corresponding to the inelastic, one- and two-neutron transfer channels has been extracted. We found that in the two-neutron transfer channel the strength to excited states corresponds to a fraction (less than 24%) of the total, consistent with the previously obtained results that the $2n$ channel is dominated by the ground-state to ground-state transition.

Figure 1

TKEL spectra obtained for the elastic (+inelastic) $\left(0n\right)$ and one-neutron $\left(1n\right)$ and two-neutron $\left(2n\right)$ pickup channels in the ${}^{60}\text{Ni}+{}^{116}\text{Sn}$ reaction at ${\mathrm{E}}_{\mathrm{lab}}=245$ MeV and ${\theta }_{\mathrm{lab}}={70}^{\circ }$, corresponding to a distance of closest approach of 14.5 fm. The dashed lines correspond to the position of the ground-state $Q$ values: $Q_{\mathrm{gs}}^{1n}=-1.7$ MeV and $Q_{\mathrm{gs}}^{2n}=+1.3$ MeV.

Figure 2

: Top panels: Doppler-corrected $\gamma$ spectra for ${}^{60}\text{Ni},{}^{61}\text{Ni}$, and ${}^{62}\text{Ni}$ detected in PRISMA. Bottom panels: Doppler-corrected spectra for the heavy binary partners ${}^{116}\text{Sn},{}^{115}\text{Sn}$, and ${}^{114}\text{Sn}$ (the bin is 2 keV wide). The strongest transitions are labeled with the $\gamma$-ray energy and the spin and parity of the initial and final states. The broader peaks, corresponding to the wrongly Doppler-corrected reaction partner, are also labeled (red color) in each frame.

Figure 3

Differential cross sections, from coupled-channels or DWBA calculations (see text), for the indicated states in ${}^{60,61}\text{Ni}$ and ${}^{116,115}\text{Sn}$ in the ${}^{60}\text{Ni}+{}^{116}\text{Sn}$ reaction at ${\mathrm{E}}_{\mathrm{lab}}=245$ MeV. The red (gray) line is the elastic over Rutherford ratio. The hatched area corresponds to the PRISMA angular acceptance.

Figure 4

Experimental (symbols) and microscopically calculated (lines) transfer probabilities for the one- and two-neutron pickup channels plotted as a function of the distance of closest approach $D$. Open symbols correspond to the presently obtained angular distribution in direct kinematics while solid symbols refer to the excitation function previously performed in inverse kinematics [11].

Figure 5

Experimental TKEL spectrum (histogram) for the $2n$ channel, together with the Gaussian fits (lines). The three Gaussians (dashed lines) have the same standard deviation of 1 MeV and are centered, respectively, at $-1.3,-0.14$, and 1 MeV, corresponding to the transitions to the ground state, to the $2^{+}$ state, and the higher-energy states in ${}^{62}\text{Ni}$.