Characterization of GSO:Ce phosphorescence after low-dose-rate gamma-ray irradiation

Measurements of the phosphorescence of cerium-doped gadolinium oxyorthosilicate (GSO:Ce) crystals in a high-radiation environment with dose rates higher than 5 Gy/h have revealed the presence of a strong component of phosphorescent light. Such component, at times reaching values as high as the irradiation itself, could disturb planned orthogonal ray imaging systems, where megavoltage linacs deliver target doses of the order of 2 Gy/min. In two previous studies of orthogonal ray imaging a crystal of cerium-doped lutetium yttrium oxyorthosilicate (LYSO:Ce) was utilized for obtaining first orthogonal ray images with a single-pixel collimated detector. Because an orthogonal ray imaging device must comprise a multi-pixel system, the possibility of utilizing GSO as a scintillator suiting that purpose is currently under investigation. In order to rule out the possibility that the aforementioned phosphorescence of GSO disturbs future orthogonal ray imaging systems, we have performed spectroscopic measurements of a finger-like GSO crystals before, during, and after irradiation with both a 22Na and 60Co radioactive source with activities of 1.6 and 7.0MBq, respectively. A dosimetric Geiger detector positioned adjacent to the 60Co source revealed a dose rate of 1 mGy/h, i.e. more than 3 orders of magnitude lower than the aforementioned study. This value, nevertheless, is still above the expected dose rate value to be experienced by an orthogonal ray imaging detector since such detector is to be positioned behind a multi-hole or a multi-slit collimator. Cunha et al. calculated that the radiation dispersed onto such detectors is diminished by a factor of at least 105, which renders the dose-rate values measured in this work pertinent. We found no evidence of GSO phosphorescence at these very-low dose rates. Pulse shape analysis revealed nevertheless the existence of a very small amount of intrinsic radioactivity due to the alpha decay of 152Gd. GSO is therefore a suitable scintillator for planned orthogonal ray imaging systems.