Electronic excitation, defect creation

It is discussed how the primary processes of defect formation during irradiation occur via electronic excitation. This can take the form of either the creation of electron-hole pairs, followed by trapping into localized energy states, or of exciton creation leading to the formation of stable vacancy and interstitial defects. Heating the sample after the irradiation causes the release of this stored energy in the form of phonons or photons. Photon emission, ie. luminescence, results from either electron-hole recombination or from vacancy-interstitial recombination. Several examples of both types are discussed for crystalline CaF and SiC. ... 

Further evidence for the importance of electronic excitation as a primary means of defect creation comes from studies of "sub-threshold" damage in which the energy of the incoming particle is less than that required for a "knock-on" collision. 

Fig. 3.7. Trap-controlled carrier recombination 1 - excitation of solid with creation of electron ( ) and hole (0) pair 2, 3 - their localization (trapping) by defects 4 - thermal ionization of electron from a trap 5 - its recombination with the recombination centre.

Figure 9. Conventional model of photorefraction in crystals iron impurity forms defect states of variable valence within the forbidden band gap of a lithium niobate crystal. Optical excitation of the divalent state leads to creation of a mobile electron in the conduction band. This is able to move and recombines with a trivalent iron impurity at another location which becomes divalent. The displacement of charge leads to an electric field and the Pockels electro-optic effect leads to local modification of the refractive index.

Radiation has been seen to produce in solids, and consequently at their surface, both structural imperfections and excited electronic states. The structure defects, which constitute new impurity levels, induce in a quasipermanent manner a new equilibrium repartition of the electronic population in the various levels. They modify the position of the Fermi level, and therefore the catalytic activity. On the contrary, the creation of excited electronic states and particularly of pairs of free carriers results transiently in a repartition of the electronic population, different from the thermal one. The potential energy, stored this way in the surface of the solid, may give rise to new catalytic processes. 

The ESR experiments primarily examine the microscopic nature of defects or other localized electronic states that lie below the energy gap. Coupled with optical excitation, ESR techniques also provide useful information concerning electronic metastabilities such as an optical rearrangement of the electrons in existing localized states or the optically induced creation of new states. These experiments often provide data for useful comparisons with transport or optical properties. 

Insufficient absorption of the X-ray flux incident on the scintillator could have various deleterious effects. First, the efficiency of the X-ray detection is diminished when X-ray photons are allowed to pass through the scintillator, without absorption and creation of excitons that excite luminescent centers. Second, in the common detector geometry where a photodiode is attached to the side of the scintillator opposite from where the X-rays enter, an X-ray that is not absorbed in the scintillator can be absorbed by the diode. This will cause the formation of electronic defect in the diode, so that an additional source of noise will be created, which in turn can degrade the performance of the whole detection system. Therefore, the scintillator material should be able to absorb aU the incident X-rays ideally. Various detector designs have been proposed to minimize this effect, e.g., by placing the diode surface away from the direct path of the incoming X-ray beams [73, 77-79]. 

---