Two custom-made In_0.5Ga_0.5P p⁺-i-n⁺ circular mesa spectroscopic X-ray photodiodes with different diameters (200um and 400um) and a 5um i layer have been characterized for their response to X-ray photons within the energy range 4.95keV to 21.17keV. The photodiodes, operating uncooled at 30°C, were coupled, in turn, to the same custom-made charge-sensitive preamplifier. X-ray fluorescence spectra of high-purity calibration foils excited by a Mo target X-ray tube were accumulated. The energy resolution (Full Width at Half Maximum) increased from 0.79±0.02keV at 4.95keV to 0.83±0.02keV at 21.17keV, and from 1.12±0.02keV at 4.95keV to 1.15±0.02keV at 21.17keV, when using the 200um and 400um diameter devices, respectively. Energy resolution broadening with increasing energy was attributed to increasing Fano noise (negligible incomplete charge collection noise was suggested); for the first time, the Fano factor for In_0.5Ga_0.5P was experimentally determined to be 0.13, suggesting a Fano limited energy resolution of 145eV at 5.9keV. The charge output of each system had a linear relationship with photon energy, across the investigated energy range. The count rate of both spectroscopic systems increased linearly with varying X-ray tube current up to 10⁵ photons s-¹cm-² incident photon fluences. The development of In_0.5Ga_0.5P based spectrometers is particularly important for hard X/y-ray astronomy, due to the material’s large linear X-ray and y-ray absorption coefficients and the ability to operate uncooled at high temperatures.

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