An international team of nuclear physicists has created zinc-54 for the first time. They have confirmed that it can undergo two-proton decay, a rare process that has only been seen in one other isotope before. The results should shed more light on how protons are bound together in the nucleus. 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.
An atomic nucleus is an ensemble of nucleons - protons and neutrons. To bind these nucleons and to form a stable nucleus, a subtle equilibrium of the number of protons and the number of neutrons is needed. For light species, a stable nucleus is formed from an equal number of protons and neutrons. Above the nucleon number A = 40, more neutrons than protons have to be added to form a stable atomic nucleus to overcome the Coulomb repulsion of the charged protons.
If the equilibrium between protons and neutrons is disturbed, a nucleus becomes unstable and decays; for a slight imbalance it decays by b-decay (transformation of a proton into a neutron or vice versa), whereas for a large disequilibrium it will emit nucleons. This kind of particle emission from a nucleus was first observed for heavy nuclei, which may emit a particles (helium nuclei) to gain stability. For lighter very proton-rich nuclei with an odd number of protons (Z), proton emission was observed for the first time during the early 1980s at the GSI laboratory in Darmstadt, Germany. According to theoretical predictions, simultaneous two-proton emission from nuclear ground states should occur for even-Z nuclei. This two-proton radioactivity is only observable if the sequential emission of two independent protons is energetically forbidden. This is the case for medium-mass proton-rich nuclei around A = 40-50. Due to the gain of stability from the pairing energy, the mass of the even-Z two-proton emitter is smaller than the mass of the odd-Z one-proton daughter, and therefore one-proton emission cannot occur. The only open decay branch is simultaneous two-proton decay. This process was seen for the first time in 2002 in iron-45, (containing 26 protons and 19 neutrons), created by firing a beam of nickel-58 ions onto a nickel or beryllium target.
Bertram Blank and colleagues of the CENBG laboratory in France 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 10^17 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.
