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Double beta decay
Pairing makes even-even nuclei lighter than their odd-odd neighbours, so along an even mass number the masses lie on two parabolas. For most candidates here the even-even parent lies below its odd-odd neighbour and above the next even-even nucleus: a single β step would climb, two at once go down. In 48Ca and 96Zr the single step is open by energy but strongly hindered by the change of spin. Two neutrons become two protons, with two antineutrinos (2νββ) or, if the neutrino is its own antiparticle, with none (0νββ, never seen). 2νββ is measured in all eleven nuclei listed: in counting experiments for most, through the geochemical 130Te/128Te ratio for 128Te, and in a single radiochemical experiment for 238U, which counts the daughter atoms and so cannot tell the 2ν from the 0ν mode.