Two-Proton Radioactivity of Kr 67

In an experiment with the BigRIPS separator at the RIKEN Nishina Center, we observed two-proton (2p) emission from Kr67. At the same time, no evidence for 2p emission of Ge59 and Se63, two other potential candidates for this exotic radioactivity, could be observed. This observation is in line with Q value predictions which pointed to Kr67 as being the best new candidate among the three for two-proton radioactivity. Kr67 is only the fourth 2p ground-state emitter to be observed with a half-life of the order of a few milliseconds. The decay energy was determined to be 1690(17) keV, the 2p emission branching ratio is 37(14)%, and the half-life of Kr67 is 7.4(30) ms.