Characteristics of unerasable and reappearing photostimulated luminescence signals in over-irradiated imaging plates and a new complete erasing method

Characteristics of unerasable and reappearing photostimulated luminescence (PSL) signals in an imaging plate (IP) made of europium-doped BaFBr were investigated. It has been empirically known that the phenomena are observed remarkably in IPs irradiated with X-ray doses exceeding 1 mGy. In these over-irradiated IPs, quite stable latent images remain unerasable after fully erasing with visible light and latent images reappear after a while, even after they seem to have been completely erased. The results showed that the amount of reappearing PSL signals was proportional to the delivered dose when irradiated at a fixed dose rate, and it was not dependent on dose rate when irradiated with a fixed dose. The effect of heat on unerasable and reappearing latent PSL signals was examined. Unerasable PSL signals almost disappeared under a temperature of 120 °C over 28 elapsed days; however, a long heating period has also caused deterioration in IP samples. In a study on the effect of light sources on PSL signals using halogen light and fluorescent light, it was found that ultraviolet light around 290 nm promoted the occurrence of reappearing PSL signals that could be easily erased with visible light. This indicates that electrons localized in short wavelengths can be excited optically by an atomic line of mercury and shifted towards longer wavelengths, around 633 nm, causing reappearing latent images. From this result, a new method to completely erase unerasable latent images, i.e., by simultaneously exposing IP samples with an ultraviolet light and a fluorescent light, has been developed. After repeated treatment by this method, unerasable latent images were completely erased without reappearing PSL signals.