Direct Observation of Two Protons in the Decay of Zn-54

The two protons emitted in the decay of {sup 54}Zn have been individually observed for the first time in a time projection chamber. The total decay energy and the half-life measured in this work agree with the results obtained in a previous experiment. Angular and energy correlations between the two protons are determined and compared to theoretical distributions of a three-body model. Within the shell model framework, the relative decay probabilities show a strong contribution of the p{sup 2} configuration for the two-proton emission. After {sup 45}Fe, the present result on {sup 54}Zn constitutes only the second case of a direct observation of the ground state two-proton decay of a long-lived isotope.