Detector characteristics of CCDs for high resolution gamma ray imaging

We are developing a high resolution gamma ray imager for use in Nuclear Medicine at energies up to ∼150 keV. The imager is based upon a charge coupled device (CCD) coated with a polycrystalline scintillator to improve the detection efficiency. Three CCDs from E2V Ltd, bearing 100 μm, 300 μm and 500 μm layers of Gd₂O₂S(Tb) (GADOX) have been tested. These CCDs have a pixel size of 44 μm. The modulation transfer function (MTF) and relative detection efficiency of the CCDs have been measured using a ²⁴¹Am (59.5 keV) source. The MTFs were estimated using the edge spread response function (ESRF) to derive the line spread function (LSF). The measurements show that significant improvements in detector efficiency, together with minimal loss of spatial resolution, follow from the use of a CCD with a 500 μm thick GADOX scintillator rather than the standard 100 μm scintillator thickness. The 500 μm GADOX coated CCD thus has highly promising characteristics for use in a small, high resolution gamma ray imager.