A 2 × 2 square pixel In0.5Ga0.5P p+-i-n+ mesa photodiode array was fabricated and investigated for its suitability in photon counting X-ray and γ-ray spectroscopy. Each pixel, which had an area of 200μm×200μm and a 5μm thick i layer, was coupled to a low-noise charge-sensitive preamplifier and standard onwards readout electronics to form an X-ray and γ-ray photon counting spectrometer. The pixels were illuminated in turn with an 55Fe radioisotope X-ray source, an 241Am radioisotope X-ray and γ-ray source, and a 109Cd radioisotope X-ray and γ-ray source. The mean value (across all pixels) of the best energy resolution (Full Width at Half Maximum, FWHM) at 20 °C was 770 eV ± 30 eV at 5.9 keV, 840 eV ± 20 eV at 22.16 keV, and 870 eV ± 30 eV at 59.54 keV. The spectroscopic response of one of the pixels was then investigated at temperatures up to 100 °C; noise analysis was performed and the different noise contributions were identified. The FWHM at 100 °C was 1.29 keV ± 0.04 keV at 5.9 keV, 1.32 keV ± 0.06 keV at 22.16 keV, 1.34 keV ± 0.06 keV at 59.54 keV, and 1.43 keV ± 0.08 keV at 88.03 keV. The results indicate that the detector did not suffer from incomplete charge collection, and that the spectrometer had better energy resolution at 100 °C than any other multi-pixel radiation spectrometer so far reported
