Physicists have created a pumpkin-shaped nucleus that has broken the proton emission record with the shortest measured half-life for a proton emitter.

In Physical Review Letters, an international team reports the half-life of a nucleus that sports proton emission, a rare type of radioactive decay where an atom emits a proton. The team estimate that the half-life of such emission was about 450 nanoseconds, the shortest ever measured of any proton emitting isotope.

Every known element has isotopes, versions of itself that have the same number of protons and electrons (so the chemistry is the same) but a different number of neutrons, which can make it heavier or lighter and thus affect its physical properties.

The varying number of neutrons in the nucleus also plays a role in its radioactivity. Protons are positively charged and they should just repel each other, making the formation of anything other than basic hydrogen impossible. But thanks to neutrons – and the strong nuclear force among the quarks that make protons and neutrons – nuclei stay together. At least for a little while.

In the new discovery, physicists have created a rare isotope of lutetium. Proton emission doesn’t happen in naturally occurring isotopes, so scientists need to create some really odd nuclei.

Lutetium has 71 protons, and its most common isotope has 104 neutrons. In this configuration, the atom is not radioactive. The team instead used a technique to create an isotope of Lutetium-149, which has only 78 neutrons. The much smaller number of neutrons makes it unstable. It also produces quite the odd shape. The nuclear forces pushed the protons and neutrons in a very oblate configuration: basically, it's pumpkin-shaped.

The team has observed 14 events from this atom. They report that lutetium-149 is also the most oblate proton emitter ever measured as well as being the one with the highest ground-state proton-decay energy ever recorded.

The physicists, who work at the Accelerator Laboratory of the University of Jyväskylä, Finland, created the peculiar isotope by shooting nickel-58 atoms into a thin target of ruthenium-96. The lutetium-149 atoms were then implanted withing a silicon strip detector where they could be studied. The isotope of lutetium-149 decays into ytterbium-148, which is also radioactive but decays in the more traditional beta decay, by emitting a positron (the positive anti-matter version of the electron).

Now, the team has a few avenues for further research. The properties of lutetium-149 might be further studied using gamma-rays although it won’t be easy. They might also try to create lutetium-148, which may have a longer half-life. The current and future studies are useful to test models for proton emissions in truly exotic atoms, and will lead to better models that can predict the properties of nuclei.