Low-temperature x-ray-induced optical properties of Bi4Ge3O12 scintillators

Undoped Bi4Ge3O12 single crystals exposed to a moderate dose (less than 15 krad) of x-radiation at 12 K show a rapid decay in x-ray-induced light emission intensity and very little increase in optical absorption. The emission spectrum may be recovered by thermally storing the sample between 12 and 300 K, although the spectral distribution obtained depends on the storage temperature. The emission decay does not correspond to light lost by increase in optical absorption; the origin of this decay is attributed to the depopulation of pre-existing electronic traps in the material. Higher doses (6 Mrad or above) of x-radiation at 12 K induce an isotropic electron paramagnetic resonance spectrum formed by a band centred at g = 2.0036 with a half-width of 9 G; this signal has been-attributed to the creation of trapped holes. On heating the sample to above 12 K after irradiation, broad intense optical absorption increases in the visible and ultraviolet regions; this optical absorption has been attributed to new hole-related centres (probably holes trapped in the oxygen sublattice). These centres are formed after the thermal release of the trapped holes initially induced by the x-rays. The defects induced by x-rays in undoped Bi4Ge3O12 crystals are unstable above 200 K but Mn impurities induce a stable colouration at room temperature. Fe and Cr impurities modify the colouration process at temperatures below 220 K but do not show stable x-ray-induced colouration at room temperature. (author)