tetraneutron

The tetraneutron is considered an unbound isotope with a lifetime around 10−22 seconds. The stability of this cluster of four neutrons is not supported by current models of nuclear forces. Recent empirical evidence is "consistent with a quasi-bound tetraneutron state existing for a very short time".

== Marqués' experiment == Francisco-Miguel Marqués and co-workers at the GANIL accelerator in Caen used a particle accelerator to fire atomic nuclei at carbon targets and observed the "spray" of particles from the resulting collisions. In this case the experiment involved firing beryllium-14, boron-15 and lithium-11 nuclei at a small carbon target, the most successful being beryllium-14. This isotope of beryllium has a nuclear halo that consists of four clustered neutrons; this allows it to be easily separated intact in the high-speed collision with the carbon target. Current nuclear models suggest that four separate neutrons should result when beryllium-10 is produced, but the single signal detected in the production of beryllium-10 suggested a multineutron cluster in the breakup products; most likely a beryllium-10 nucleus and four neutrons fused together into a tetraneutron.