Ionizing atoms and molecules can be achieved by any one of a number of means that have been in vogue in the past, and are well understood. These include heating up the medium in the vapor state to high temperatures so that thermal collisions may eliminate some of the electrons. The least-bound of the atomic electrons are naturally the most likely to be removed. Removal of inner shell electrons, particularly of heavy atoms, requires temperatures that are not normally reached especially for any meaningful length of time. Exposing the medium to extremely intense electromagnetic radiation is an alternate technique. These are represented by photons, which are generally of low energy, and there is little possibility that inner-shell electrons are removed from the atoms by absorption of these photons. Yet, M. D. Rosen et al (Physical Review Letters, Vol. 54, 1985, page 106) describe an exploding foil technique by which Se atoms are highly ionized in an uncontrolled manner by irradiating a microfoil of selenium with an extremely powerful burst of laser light. Synchrotron radiation offers photons of a higher range of energy, yet the possibility of producing inner shell ionization at any significant level is very limited. Hard X rays or gamma radiation could create inner-shell ionization via photoelectric effect or internal conversion, but applying the technique to a large assembly of atoms or molecules is beset with practical problems. Yet another possibility is the use of charged particle beams. Charged particle interactions at high energies can create vacancies in the inner shells, but occurring rather rarely.
The most common process wherein a positron incident on a material is annihilated takes place when the positron has come to rest in the material; and is called annihilation at rest. The positron gets annihilated along with an outer-shell electron of the atom at near zero momentum, and two 511-keV photons are emitted in mutually opposite directions. The strongly bound inner shell electrons are not involved in positron annihilation at rest. However it has been known for decades that a positron may be annihilated also while it is in flight, although relatively rarely, in which case a core electron of an atom can be involved. The annihilation of an electron-positron pair during the flight of the positron shall occur with emission of a single photon or a multiple of photons. Annihilation with emission of a single photon takes place in the Coulomb field of the nucleus via interaction of a bound electron. Owing to the proximity of the K electron with the nucleus, the process produces vacancies predominantly in the K shell, followed in decreasing order of probability by the L, M, and the other atomic shells. Various aspects of the phenomenon have been studied recently, and the trends clearly established. Annihilation in flight with two or more photons however occurs differently, wherein all electrons of an atom are equally affected. This process is significant only for emission of two photons, emission of higher number of photons being negligibly rare.
By a recent detailed experimental studies of single-quantum annihilation, a particularly significant component of positron annihilation in flight, it has been observed by J. C. Palathingal et al (Physical Review, Vol. 51, 1995, pages 2122-2130) that the cross section depends on the atomic number Z of the element as roughly Z.sup.5. Two-quanta annihilation has a cross section dependance that is proportional to Z in first order, and presents approximately the same cross section per electron irrespective of the shell it belongs to. This cross section per electron is also more or less invariant between the elements, but depends on the positron energy. Although the cross section per atom for two-quanta annihilation in flight is several times larger than for single quantum annihilation, the combined cross section per electron for annihilation in flight is largest for the K electron and decreases in an orderly manner for electrons in the outer shells, as seen in Table 1. Annihilation in flight as a process of ionization hence favors the elimination of electrons from the innermost shells, especially for the heaviest atoms.