Studies on the double-β decay nucleus ⁶⁴Zn using the (d,²He) reaction

The (d,²He) charge-exchange reaction on the double-β decay (ββ) nucleus ⁶⁴Zn has been studied at an incident energy of 183 MeV. The two protons in the ¹S₀ state (indicated as ²He) were both momentum analyzed and detected simultaneously by the BBS magnetic spectrometer and its position-sensitive detector. ²He spectra with a resolution of about 115 keV (FWHM) have been obtained allowing identification of many levels in the residual nucleus ⁶⁴Cu with high precision. ⁶⁴Zn is one of the rare cases undergoing a ββ decay in β⁺ direction. In the experiment presented here, Gamow-Teller (GT⁺) transition strengths have been extracted. Together with the GT^- transition strengths from ⁶⁴Ni(³He,t) data to the same intermediate nucleus ⁶⁴Cu, the nuclear matrix elements of the ββ decay of ⁶⁴Zn have been evaluated. Finally, the GT^± distributions are compared with shell-model calculations and a critical assessment is given of the various residual interactions presently employed for the pf shell.