Operation of the CARTOGAM portable gamma camera in a photon-counting mode

The CARTOGAM portable gamma camera, of particularly compact size (8 cm in d iameter, 15 kg including the shielding), has been developed for gamma-imagi ng applications in nuclear facilities. The detector is composed of a CsI(TI ) scintillator, an image intensifier, and a charge-coupled device (CCD) mat rix. The ordinary mode for image acquisition with such a detector is based on integration, and the main sources of noise in this mode are the photocat hode thermoelectronic emission and the CCD pixels that have an abnormally h igh leakage current. We have developed an alternative acquisition mode based on a morphological processing of the elementary images at the video frequency (25 Hz). In this mode, gamma events are individually identified and the noise due to errati c thermoelectrons or white pixels is efficiently filtered, thus leading to an important gain in sensitivity. We present here experimental results obta ined in this photon-counting mode in terms of signal-to-noise ratio (SNR), spatial resolution, saturation limit in dose rate, and sensitivity. A compa rison is made with the integration mode. We also present a brief analysis o f the problem of measuring the SNR on real images.