Cloud chamber photograph of the nuclear beta decay of a helium-6 nucleus. Helium-6 has two more neutrons than ordinary helium, & one of these extra neutrons decays in less than a second. The picture shows the results of this decay: the short thick track at top left is the recoiling nucleus, & the lighter, curving track is an electron. The two tracks are not back-to-back, hinting at the invisible presence of the third, neutral decay product - a neutrino. The picture was taken by S. Szalay & J. Csikay at the Nuclear Research Institute in Debrecen, Hungary, in 1957.