Functioning of the CARTOGAM portable gamma camera in a photon counting mode

The CARTOGAM portable gamma camera, particularly compact (15 kg including the shield, 8 cm in diameter), has been developed for gamma imaging applications in nuclear facilities. The detector is composed of a CsI(Tl) scintillator, an image intensifier and a CCD matrix. The ordinary mode for image acquisition with such a detector is an integrating mode and the main sources of noise are the photo-cathode thermo-electronic emission and the CCD leaking pixels. We have developed an alternative acquisition mode based on a morphological processing of the elementary images at the video frequency (25 im/s). In that mode, gamma events are individually identified and the noise due to isolated thermoelectrons or white pixels is efficiently filtered, thus leading to an important gain in sensitivity. We present here experimental results obtained in this photon counting mode concerning SNR, spatial resolution, saturation limit in dose rate, sensitivity and comparison with the integrating mode. We present also a short analysis of the problem of measuring the SNR in practice in such images