An apparatus receives signals generated by a detector (100) sensitive to ionizing radiation such as x-rays. A differentiator (204) generates an output indicative of the rate of change of the detector signal. A discriminator (206) classifies the amplitude of the differentiator (204) output. An integrator (208) triggered by the output of the discriminator (206) generates outputs indicative of the detected photons. One or more correctors (24a, 24b) corrects for pulse-pileups, and a combiner (25) uses the outputs of the correctors (24a, 24b) to generate an output signal indicative of the number and energy distribution of the detected photons.