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Some applications use a huge amount of irradiation of ionizing radiation, and to employ detectors in such an environment, radiation hardness (or radiation tolerance) is required. Ionizing radiations can cause ejections and ionizations of atoms. When we irradiate high energy photons, such as X‐ and γ‐rays, ionization is a main phenomenon, and energetic particles mainly cause an ejection. In the case of e⁻, p, and n, if their energy exceeds a threshold energy, generation of the Frenkel defect, which consists of hole and interstitial ion, starts. In such ejection‐induced damage, generation of defects will occur, and will reduce scintillation light yield by trapping.

The most typical radiation damage is a degradation of transmittance, and coloration is often observed. If the transmittance decreases, emitted photons inside the scintillator cannot reach the photodetectors, and cause a reduction of signal output intensity from the photodetectors. In the case of PbWO₄, which is often used for experiments in high energy physics, p‐irradiation with ~10⁵ Gy causes a degradation of transmittance which induces a reduction of light yield [71]. When the coloration occurs due to exposure of ionizing radiation, it sometimes recovers by annealing (thermal annealing, thermal bleaching), UV‐irradiation (UV‐annealing), and keeping the sample under room temperature for a long time (spontaneous recovery). In typical interpretation, ejected ions stay near to the incident position, and by these perturbations of thermal or light energy, they can return to the incident position. On the other hand, damage by hadron beams are not or less recovered by these phenomena. It can be understood that the amount of damage is largely higher than that generated by X‐ or γ‐rays.

There are several ways of evaluating the effects of radiation damage; one of the typical ways is to measure optical transmittance before (T₀ (λ)) and after T(λ) of the irradiation. By using the thickness of the sample d, the relationship before and after irradiation is

(1.26)

where μ_ir is an irradiation‐induced absorption coefficient. By this μ_ir, we can evaluate the radiation damage quantitatively. In actual experiments of radiation hardness, after the irradiation, many small signals can be detected if you directly observe the output signal of scintillator + photodetector by oscilloscope, and these small signals are the afterglow induced by a huge amount of irradiation.

One of the relating phenomena is positive hysteresis (sometimes called radiation drift), which is a phenomenon in which the light yield of the scintillator is increased after exposure to ionizing radiation. From the viewpoint of solid‐state physics, it is an interesting phenomenon, but from the engineering and operation of actual detectors, it is problematic because it causes a temporal change of the detector gain. The positive hysteresis was discovered in Tl‐doped CsI [72] and Ce‐doped GSO [73, 74]. After these pioneering works, we found Ce/Zr co‐doped GSO [75] and Ce‐doped GAGG [76] also exhibited the positive hysteresis. Up to now, several interpretations are proposed, and our interpretation is a generation of tentative excitation states [75, 76].