Abstract: A method for determining X-ray, or gamma ray photon energy by, irradiating a high resistance n-type or p-type cadmium telluride alloy crystal with X-ray or gamma ray photons respectively providing first and second electrical contacts which are in electrical communication with the crystal, which are respectively negatively and positively biased and which provide free flow of electrons from the negatively biased contact to the crystal, and wherein holes, generated by absorption of photons in the crystal, recombine with the electrons, and determining the photon energy by measuring the amount of charge generated per photon absorbed be the crystal.