Triple Correlations in theNi60(p, γγ)Cu61Reaction

Abstract

Triple-directional-correlation measurements have been performed for the ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}(p, \gamma \gamma ){}_{}{}^{61}{}_{}{}^{}\mathrm{Cu}$ reaction in order to determine the following assignments for the low-lying states: the excitation energy (in MeV), the spin and probable parity, and the $\frac{E2\mathrm{}}{M1\mathrm{}}$ amplitude ratio (for the transition from the intermediate state to the ground state) as follows: 0.47, (${\mathrm{½}}^{-}$), indeterminate; 0.95, ${\frac{5}{2}}^{-}$, 0.35±0.03; 1.37, ${\frac{5}{2}}^{-}$, ${3.78}_{-0.13\mathrm{}}^{+0.15\mathrm{}}$; 1.63, (${\frac{3}{2}}^{-}$), -0.29 to -1.68, or (${\frac{5}{2}}^{-}$), -0.61 to -3.01; 1.89, ${\frac{3}{2}}^{-}$, -0.08 to -0.42 or -1.19 to -2.48; and 2.07, (${\mathrm{½}}^{-}$), indeterminate. Correlations for the first three excited states were observed at each of the resonances at proton energies of 1588-, 1599-, and 1620-keV, while those for the next three excited states were observed only at the 1620-keV resonance. The correlations were all in agreement with previously measured ${\frac{3}{2}}^{-}$ assignments to each of these capturing levels and to the ground state of ${}_{}{}^{61}{}_{}{}^{}\mathrm{Cu}$. In addition to the above, higher levels for which no spin assignments are made were observed at excitation energies (in MeV) of: (2.4), 2.64, 2.83, and 3.02. All energy measurements of this report are believed to be accurate to ±30 keV. The double sum-coincidence arrangement which allowed the simultaneous measurement of up to 24 correlation functions and which is here described for the first time is discussed in some detail. The results of these experiments are compared with other experimental results and found to be in good agreement. The implications of a reported doublet at about 1.35 MeV are considered with relation to the unusually high quadrupole admixture observed for the 1.37-MeV state in the present experiment. It is shown that the other member of the doublet is not observed in the ($p, \gamma \gamma$) reaction at the resonances studied, and that the undetected presence of such transitions could not produce the results observed for the 1.37-MeV state. The results of the present experiment are compared with the reported structure of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ and ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$ and with the core-excitation model which has been applied to these nuclides. This model does not appear to adequately explain the observations for the 1.37-MeV state, if the observed large value of the quadrupole admixture is a proper index of a high degree of collective motion for this state.