Nuclei decay when they contain too many neutrons or too many protons to be stable. The most commons forms of decay are nuclear fission and alpha, beta and gamma decay. However, some nuclei that contain more protons than neutrons can also decay by emitting a proton — a process that was first observed about 20 years ago.

The set-up

Single-proton emission is observed in nuclei with an odd number of protons. However, theorists also predicted that some nuclei that contain an even number of protons and/or neutrons could undergo two-proton emission. This process was seen for the first time in 2002 in iron-45, which contains 26 protons and 19 neutrons. The iron-45 nuclei were created by firing a beam of nickel-58 ions onto a nickel or beryllium target.

The detector

Now, Bertram Blank of the CENBG laboratory in France and colleagues have created zinc-54 — which contains 30 protons and 24 neutrons — in a similar experiment involving nickel-58 ions and a nickel target at the GANIL laboratory. The zinc-54 nuclei were created in about 1 in 10¹⁷ of the collisions, and Blank and co-workers found that the proton energy and decay half-life of about 3.7 milliseconds both agreed with predictions.

“Having a second two-proton emitter allows for a first real comparison between theory and experiment, which is somewhat difficult to make with just one case,” Blank told PhysicsWeb. “Two is better than one and hopefully there will be more.”

The group now plans to search for other two-proton emitters. It will also study iron-45 and zinc-54 in more detail with a time projection chamber that will be able to follow the paths of the two protons. “In this way we can study the correlations between the two protons and better understand the emission process itself,” says Blank.