New Type of Nuclear Decay Demonstrated

Physicists predicted democracy--in atomic decay--but researchers have caught neon atoms doing something that might turn out to be very undemocratic. If they are correct, they will have discovered a new form of nuclear decay, and given scientists a powerful way of investigating the structure of the nucleus.

Radioactive nuclei are unstable by nature, and they perform all kinds of tricks to settle down. For instance, they might spit out a helium nucleus (also known as an alpha particle) or a proton. In some cases, a nucleus may wish to spit out two protons. There are several ways physicists imagine this can happen. The nucleus can emit a single proton and then a second in rapid succession, or it can emit them both at the same time--a process dubbed "democratic decay" by the Soviet theorists who predicted it. Strangest of all, it ought to be able to spit out a bound packet of two protons--essentially a helium-2 nucleus, which is never spotted in nature. The helium-2 nucleus then splits into two separate protons.

To try to demonstrate that helium-2 decay can exist, physicist Alfredo Galindo-Uribarri of Oak Ridge National Laboratory in Tennessee and colleagues smashed a stream of fluorine-17 into a hydrogen-rich target. When a fluorine atom hit a hydrogen (which is just a proton) hard enough, the fluorine-17 became neon-18--the nucleus the researchers were hoping to study. Neon-18 cannot emit one proton at a time, because it's forbidden by quantum theory. But it can lose two protons at a time. Thus, neon-18 must decay either via democratic decay or by emitting a helium-2, the team reports in the 1 January issue of Physical Review Letters.

"I think the result is really quite exciting," says Philip Woods, a nuclear physicist at the University of Edinburgh, particularly because the data suggest that the helium-2 decay is going on, rather than democratic decay. If the results hold up, "... it would allow us, in theory, to understand better phenomena within the nucleus," says Bertram Blank of the Center for Nuclear Studies in Bordeaux-Gradignan, France, including how protons inside the nucleus pair up.