Gamma-Gamma Angular Correlation Measurements With GRIFFIN

The assignment of spins and parities to excited nuclear states plays an important role in determining nuclear structure. There is a directional asymmetry in the emitted radiation from a $\gamma-\gamma$ cascade that depends on the sequence of spin values for the nuclear states, the multipolarities, and the mixing ratios of the emitted $\gamma$-rays. These $\gamma$-ray angular correlations are used for the assignment of spins and parities to the nuclear states, and thus provide a powerful means to elucidate the structure of nuclei away from stability. The first in-beam test of gamma-gamma angular correlation measurements with the GRIFFIN spectrometer at TRIUMF-ISAC were performed with a radioactive beam of $^{66}$Ga. In the daughter nucleus $^{66}$Zn, mixing ratios were measured for various cascades, and are in excellent agreement with literature. The sensitivity to a pronounced 0$^+$-2$^+$-0$^+$ angular correlation was also measured. The ability to assign spins for a 0$^+$-2$^+$-0$^+$ cascade is important for the case of $^{62}$Ga superallowed Fermi $\beta$-decay where a recent measurement was made to clarify two conflicting measurements for the spin assignment of the 2.34 MeV excited state in the daughter.