As the masses of the elementary particles forming exotic atoms are large compared to that of the electron, the dimensions of their orbits in these atoms are much smaller and the energies required to eject them from the atom are much larger than for electrons in ordinary atoms. This makes exotic atoms suitable for studying the properties of exotic particles such as their mass, charge, spin, magnetic moment and the strength of their interaction with nuclear matter, Hartmann reported. Exotic atoms are formed when particles are shot into and slowed down in matter. When they reach energies comparable to the binding energies of the outer atomic electrons, they are captured by an atom to form highly excited exotic-atom states. Because it achieves this lowly excited state so rapidly, the exotic particle dumps an energy of up to 1 GeV into the nucleus, several times the energy released during a nuclear fission reaction.
Antiprotonic helium is a very special exotic atom.
He atoms exist in long-lived, metastable states with a microsecond lifetime, orders of magnitude longer than in the ordinary exotic atom. Irradiation of this atom with laser light of the appropriate wavelength brings the exotic atom to a short-lived level. The usual fast cascade follows and leads to states from which annihilation occurs. In another recent experiment, heavy antiprotonic atoms were used to probe the difference between neutron and proton densities at the nuclear periphery. Antiprotons are shot into samples which contain only one isotope of an element.