Decay of the odd-odd N=Z nuclide ⁷⁸Y

The odd-odd N=Z nuclide, ⁷⁸Y has been produced in the ⁴⁰Ca(⁴⁰Ca,pn) reaction at 125 MeV. Recoiling fragments separated by their A/Q values were implanted onto the tape of a moving tape collector and transported to a shielded position between two plastic β detectors and two Ge γ-ray detectors where β-γ coincidences were recorded as a function of time. γ rays with energies of 279 (100%), 504 (90%), and 713 (40%) keV, previously identified as yrast transitions in daughter ⁷⁸Sr, were observed and found to decay with a half-life of 5.8 (6) s. From the relative intensities of the γ-rays, a spin and parity of 5⁺ and T=0 are assigned to the parent state in ⁷⁸Y undergoing β decay. A production cross section of 4±1 μb has been determined for ⁷⁸Y by comparison of the counting rates with those of other reaction products with known cross sections. An upper limit of 500 keV can be set for the energy of this level relative to a possible highly deformed T=1 0⁺ ground state. From this limit, it can be inferred that T=1 pn pairing is considerably quenched relative to such pairing in adjacent odd-odd N=Z ⁷⁴Rb. Two-quasiparticle rotor model calculations have been used to account for the structure of ⁷⁸Y and adjacent nuclides.