Diproton
Encyclopedia
A diproton is a hypothetical isotope of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 nucleus consisting of two proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s and no neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s, and is predicted to be less stable than 5He. Diprotons are not stable; this is due to spin-spin interactions in the nuclear force, and the Pauli exclusion principle
Pauli exclusion principle
The Pauli exclusion principle is the quantum mechanical principle that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is anti-symmetric with respect to exchange of the particles...

, which forces the two protons to have anti-aligned spins and gives the diproton a negative binding energy
Nuclear binding energy
Nuclear binding energy is the energy required to split a nucleus of an atom into its component parts. The component parts are neutrons and protons, which are collectively called nucleons...

.

Observations of unstable

In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once - perhaps a nucleus. The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the creation of elements inside stars
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...

. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce , which then decays into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a 'diproton' - a pair of protons bound together as a nucleus - which then decays into separate protons. Alternatively, the protons may be emitted separately but at the same time - so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place.

The best evidence of was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of ions was collided into a foil of beryllium. In this collision some of the neon ended up as nuclei. These same nuclei then collided with a foil of lead. The second collision had the effect of exciting the nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the nucleus decayed into an nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together before decaying into separate protons much less than a billionth of a second later.

Also, at RIKEN
RIKEN
is a large natural sciences research institute in Japan. Founded in 1917, it now has approximately 3000 scientists on seven campuses across Japan, the main one in Wako, just outside Tokyo...

 in Japan and JINR in Dubna, Russia, during productions of with collisions between a beam of nuclei and a cryogenic hydrogen target, it was discovered that the nucleus can donate all four of its neutrons to the hydrogen. This leaves two spare protons that may be simultaneously ejected from the target as a nucleus, which quickly decays into two protons. A similar reaction has also been observed from nuclei colliding with hydrogen.
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