Observation of water luminescence for diagnostic 120-kV X-rays by using PMT and CCD camera

Luminescence emitted from water was recently observed by Yamamoto et al. for X-rays with lower energy than the Cerenkov-light threshold. This phenomenon contradicts the conventional consensus that such low-energy photons cannot generate luminescence. Because properties and mechanism of this phenomenon have not been revealed yet, we carried out a series of observations of water luminescence during irradiation of diagnostic 120-kV X-rays by using a photomultiplier tube (PMT) and a charge-coupled device (CCD) camera. As a result of PMT measurement, the light yields owing to luminescence per unit dose deposition to water decreased almost linearly with the increase in the pulse duration time; approximately 25 % reduction of light yield was observed as the duration time increased from 0.025 s to 0.5 s. Therefore, the luminance intensity was not simply proportional to the dose deposition to water. In addition to water, sulfuric acid was used for luminescence imaging by a CCD camera for comparison. The profile curve of luminance along the beam center axis for sulfuric acid showed a somewhat different tendency in comparison to that for water; a notable bump appeared in the entry region of the X-rays. Based on these observations, the possibility of the contribution of sonoluminescence to the water luminescence can be discussed.