X-ray spectroscopy with an AlInP photodiode

A custom-made Al0.52In0.48P p+-i-n+ circular mesa X-ray photodiode (200 µm diameter; 2 µm i layer thickness) has been investigated for its use in photon counting X-ray spectroscopy within the energy range 4.95 keV to 21.17 keV. This the widest energy range so far used to characterise an AlInP X-ray detector. High-purity X-ray fluorescence calibration samples were excited by a Mo target X-ray tube to give characteristic energy photons with which to illuminate the detector. X-ray fluorescence spectra were accumulated with the Al0.52In0.48P X-ray detector coupled to a custom-made charge sensitive preamplifier (both operated at a temperature of 30 °C ± 2 °C). The charge output linearly increased with X-ray photon energy. The energy resolution (Full Width at Half Maximum, FWHM) degraded from 0.99 keV ± 0.02 keV (80 erms) at 4.95 keV to 1.12 keV ± 0.02 keV (89 erms) at 21.17 keV. The increase of the FWHM with increased energy exceeded the increase expected as a consequence of Fano noise. Hence, the presence of energy dependent incomplete charge collection noise was suggested. The count rate linearly increased with increasing incident X-ray flux across the investigated flux range, up to ~ 3.8 × 105 photons s-1 cm-2.