<!--intro-->After a 40-year search, physicists have finally caught a glimpse of a rare form of radioactivity in which a nucleus spits out two protons at the same time. The researchers believe the protons emerge stuck together as a pair--a unique configuration that will give a an important insight into the strong nuclear force that glues the particles in nuclei together. "This might provide us with a new way of looking at how nucleons interact," says Philip Woods, a nuclear physicist at the University of Edinburgh.<!--/intro--> <img SRC="http://www.newscientist.com/ns_images/2263/226327F1.JPG" align=right>There are only a few ways in which unstable nuclei can transform themselves. They can either split apart or chuck out one of a small repertoire of particles, such as a neutron or a helium-4 nucleus--two protons and two neutrons. Decades ago, nuclear theorists predicted that this list should include di-proton decay, where two protons fly away stuck together as a helium-2 nucleus. But researchers could never be sure they were seeing such decays. Physicists hope that by studying di- proton decay they will learn about the arrangement of protons inside the nucleus and how they escape as a pair. "It could tell us something about the strength of the pairing interaction in the strong nuclear force," says Bertram Blank of the Centre for Nuclear Studies in Bordeaux-Gradignan, France. "Right now we know very little about this." Researchers have known since the 1980s that decaying radioactive nuclei emit two protons. But they could not confirm whether they came out at the same time, or just very quickly one after the other. Now a team using the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory in Tennessee has found a way to make sequential emission impossible. They did this by firing a beam of fluorine-17 atoms at a thin plastic surface rich in hydrogen atoms. The fluorine grabs hydrogen from the plastic and is converted into neon-18, which then decays to oxygen-16. If the neon-18 is in a particular excited state, it is energetically impossible for it to decay by emitting only one proton. "That's the clever part," says team member Alfredo Galindo-Uribarri. "There's no intermediate step of the ladder." So the atoms are forced to emit two protons at once to become oxygen-16. There is still a possibility that the protons could be leaving simultaneously but separately in a process called democratic emission (see below). Since these protons probably weren't "living together" inside the nucleus, says Galindo-Uribarri, they would tell us little about the strong nuclear force. But his colleague Jorge Gomez del Campo has few doubts. "I'm convinced we're seeing helium-2 emission." <center ><img SRC="http://www.newscientist.com/ns_images/2263/226327F2.JPG"></center> Galindo-Uribarri says they'll need a bigger detector--due to be up and running early next year--to decide for sure. The team has submitted a paper to Physical Review Letters. Nicola Jones From New Scientist magazine, 04 November 2000.