Characterization of GaAs mesa photodiodes with X-ray and γ-ray photons

Results characterizing the performance of prototype thin (3 µm i layer) GaAs p⁺–i–n⁺ mesa photodiodes (one 200 µm diameter and one 400 µm diameter device) are presented showing the spectral performance of the devices at photon energies from 4.95 keV (V Kα₁ X-ray fluorescence) to 59.5 keV (²⁴¹Am γ-ray emission). The devices were operated uncooled at +33.3 °C. The energy resolution (full width half maximum) was measured to vary from 780 eV at 4.95 keV to 950 eV at 59.5 keV for the 200 µm diameter diode, and from 1.08 keV at 4.95 keV to 1.33 keV at 59.5 keV for the 400 µm diameter diode. The increased broadening of FWHM with increasing photon energy was found to be greater than can be explained by the expected energy dependence of the Fano noise, but the peak charge output from the devices varied linearly (R²_{200 µm}=0.99998, R²_{400 µm}=0.999998) with incident photon energy