Apparatus for detecting radiation, including a detector which is configured to generate electrical charges responsive to incidence of a photon on the detector. The apparatus includes a plurality of detector circuits coupled to the detector at different respective locations. Each detector circuit consists of an amplifier which is configured to generate a pulse in response to the charges, and a first pulse shaper, having a first time constant, which is configured to produce a metric representative of an energy of the pulse. Each detector circuit also has a second pulse shaper, having a second time constant greater than the first time constant, which is configured to produce an indication that the metric is representative of an energy of the photon. The apparatus also includes a summing device which is coupled to sum the metric of each of the detector circuits in response to the respective indication.

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