HELIUM-6 NUCLEI SHARE DI-NEUTRONS Helium-6 nuclei, formed into beams for the first time only last year, are thought to be "Borromean" structures (so named for the heraldic symbol of the Princes of Borromeo, and consisting of three interlinking rings which fall apart if any one ring is removed). The He-6 nucleus, theorists believe, is really a He-4 core surrounded by two extra, loosely bound neutrons which can reside in one of two configurations: (1) one neutron on either side of the He-4 core or (2) both neutrons close together (comprising a "di-neutron") far from the He-4 core. To test this theory and to demonstrate the existence of di-neutrons, Yuri Oganessian and his colleagues at the Joint Institute of Nuclear Research (JINR) near Moscow (oganessian@flnr.jinr.ru, 011-7-09621-62151) collided a He-6 beam with a He-4 target and observed that some of the He-4 nuclei had been converted into He-6, proving that in some of the high-energy collisions di-neutrons had jumped from one nucleus to the other. This also holds true when He-6 beams hit hydrogen targets (the target nucleus being a single proton). In this case a di-neutron joined the proton to form a tritium nucleus. These results seem to favor the picture in which di-neutrons are the rule rather than the exception in He-6 nuclei. Now the JINR scientists are using He-8 beams to study in more detail how neutrons correlate with each other within nuclei and to search for signs of "tetra-neutron" states. (Oganessian, Zagrebaev, and Vaagen, Physical Review Letters, 21 June 1999; figures at Physics News Graphics)