Two-proton decay of 12O and its isobaric analog state in 12N

Following neutron knockout from an O-13 beam, O-12 fragments were created and the three decay products following two-proton decay were detected. A new ground-state mass was determined by the invariant mass method implying a decay kinetic energy of 1.638(24) MeV, and the width was found to be less than 72 keV. The latter is inconsistent with previous measurements with lower experimental resolutions but consistent with theoretical estimates. The isobaric analog of O-12 in N-12 was produced from proton knockout reactions with the same beam and decayed by two-proton emission to the isobaric analog state in B-10 with a decay kinetic energy of 1.165(29) MeV. It represents only the second case of an analog state where two-proton decay is the only isospin- and energy-conserving particle decay mode. With our measurements of the mass excesses of O-12 and its analog, the quadratic form of the isobaric multiplet mass equation was found to fit the A = 12 quintet and any deviations are less than the magnitude found for the A = 8 quintet and A = 7 and 9 quartets.