Excitation of Luminescent Materials by Ionizing Radiation

The buildup of light emission under irradiation by x-rays and fast electrons was determined for several inorganic phosphors at room temperature and at -70°C. Measurements were performed under excitation by x-rays of 35 kev and 3.5 kev and by ${\mathrm{Sr}}^{90}$ $\beta$ rays. From a knowledge of these rise curves of light emission, the effective volume of phosphor excited by a single ionizing event can be calculated. For the three types of irradiation used, it was found that the effective volumes of these channels of excitation are proportional to the amount of energy transferred to the phosphor by each ionizing particle and that the ratio between the initial and final values of the light emission is almost independent of the mode of excitation. This implies that the excited electrons diffuse out from the path of the ionizing particle until they are stopped by traps. Evaluation of this diffusion process indicates that only a small percentage of all available deep traps are filled within the effective channel volume by one single event and that a nonbimolecular process participates in the initial light emission. In the above cases, the electrostatic field produced by the positively charged core of the channels is too small to interfere with the diffusion of the excited electrons. In the case of irradiation by high-energy $\alpha$ particles, this field is large enough to inhibit the diffusion process completely. Only a small part of the energy dissipated by the $\alpha$ particle is expended in filling all the traps in a narrow channel, the rest being available for radiative processes. This channel model thus accounts for the almost instantaneous rise of light emission which is observed under irradiation by $\alpha$ particles.