Nuclear structure of ${}^{76}\mathrm{Ni}$ from the ($p,2p$) reaction

The nuclear structure of the ${}^{76}\mathrm{Ni}$ nucleus was investigated by ($p,2p$) reaction using a NaI(Tl) array to detect the deexciting prompt $\gamma$ rays. A new transition with an energy of 2227 keV was identified by $\gamma \gamma$ and $\gamma \gamma \gamma$ coincidences. According to these coincidence spectra the observed transition connects a new state at 4147 keV and the previously known $4_1^{+}$ state at 1920 keV. Two weaker transitions were also obtained at 2441 and 2838 keV, which could be tentatively placed to feed the known $2_1^{+}$ state at 990 keV. Our shell-model calculations using the Lenzi, Nowacki, Poves, and Sieja interaction produced good candidates for the experimental proton hole states in the observed energy region, and the theoretical cross sections showed good agreement with the experimental values. Although we could not assign all the experimental states to the theoretical ones unambiguously, the results are consistent with a reasonably large $Z=28$ shell gap for nickel isotopes in accordance with previous studies.

Figure 1

Particle identification of the incoming cocktail beam in coincidence with outgoing ${}^{76}\mathrm{Ni}$ ions tagged in the ZeroDegree spectrometer.

Figure 2

Particle identification of the incoming cocktail beam in coincidence with outgoing ${}^{76}\mathrm{Ni}$ ions tagged in the ZeroDegree spectrometer and protons detected in the MINOS device.

Figure 3

Doppler-corrected $\gamma$ ray spectra of ${}^{76}\mathrm{Ni}$ using vertex reconstruction (requiring only a single proton in the TPC) and addback procedure (upper panel: singles, middle panel: projection of $\gamma \gamma$ coincidences, lower panel: projection of $\gamma \gamma \gamma$ coincidences). The data with error bars and shaded area represent the experimental spectrum, the red line is the simulation plus an exponential background, and the latter function (exponential background) is also plotted separately as a blue line. Magenta stands for spectra of coincidence events ($\gamma \gamma$ and $\gamma \gamma \gamma$) with cuts on the background close to the peaks.

Figure 4

Partial level and decay scheme of ${}^{76}\mathrm{Ni}$ showing the two known, lowest-lying transitions with energies of 930 and 990 keV. The newly established proton hole state at 4147 keV (magenta) decaying with a $\gamma$ ray of 2227 keV to the second-excited state ($4_1^{+}$) is also drawn. The two other observed $\gamma$ rays are tentatively placed to feed the $2_1^{+}$ state, resulting in excited proton hole states at 3431 keV (red) and 3828 keV (blue). The numbers on the right-hand side of the levels indicate the extracted exclusive cross section of the states in the ${}^{77}\mathrm{Cu}(p,2p){}^{76}\mathrm{Ni}$ reaction. The experimental data are compared with our shell-model calculations. The three lowest excited levels can be unambiguously matched with the experimental states. The calculation provides four states ($2_2^{+}$, $4_2^{+}$, $5_2^{+}$, $6_2^{+}$) associated with mainly neutron excitations in the energy region of the observed states not populated in the $(p,2p)$ reaction. The colored (green, brown, cyan) theoretical levels are proton hole states that are good candidates for the experimental states.