Evaluation of gadolinium oxy-sulphide (P43) phosphor used in CCD detectors for electron microscopy

One of the most important components of a CCD camera for electron microscopy is the thin layer of phosphor or scintillator which converts the energy of the incident electron into light which is subsequently recorded by the CCD. We have previously evaluated a number of phosphors: gadolinium oxy-sulphide doped with terbium (P43) and zinc cadmium sulphide doped with silver (P20) and a single crystal scintillator (yttrium aluminium garnet, YAG). Our conclusions were that P43 met most closely with the quality criteria used in the analysis and was therefore used in our cameras. In the present work, the light output from P43 is evaluated for a range of coating weight (2.5–30 mg/cm2) for electron energies between 20 and 120 keV. The optimum thickness of the phosphor is strongly dependent on the incident electron energy and at 120 keV the optimum thickness of the phosphor is ∼10 mg/cm2. At the lower energies, e.g. at 20 keV, the optimum is below the minimum coating weight tested, i.e. is <2.5 mg/cm2. For energies between these values the optimum coating weight has intermediate values. According to a simple model of light transmission through the phosphor, increasing thickness results in an increase in total light generated in proportion to energy deposition, but at a certain thickness, dependent on the depth of penetration due to incident electron energy, the self-absorption of light within the phosphor leads to a reduction in the overall transmitted light to the recording medium.