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

The CARTOGAM portable gamma camera, of particularly compact size (8 cm in diameter, 15 kg including the shielding), has been developed for gamma-imaging applications in nuclear facilities. The detector is composed of a CsI(Tl) scintillator, an image intensifier, and a charge-coupled device (CCD) matrix. 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 photocathode thermoelectronic emission and the CCD pixels that have an abnormally high 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 erratic 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 in terms of signal-to-noise ratio (SNR), spatial resolution, saturation limit in dose rate, and sensitivity. A comparison is made with the integration mode. We also present a brief analysis of the problem of measuring the SNR on real images.