GAMPIX: a Gamma Camera for Homeland Security

Localization of radioactive hot spots is nowadays a major issue for Homeland Security to avoid illegal transit of radiological material or manage post-accidental events. In this context, CEA LIST Institute designed a new gamma camera, called GAMPIX, based on the association of a semiconductor detector with a coded mask. GAMPIX is currently under industrialization by AREVA CANBERRA under the name of iPIX. The detection system consists in a 1 mm thick CdTe substrate bump bonded to a Timepix readout chip developed by CERN and containing 256 × 256 pixels, each 55 μm side and including independent shaping and processing chains. Coded mask is MURA type and its choice is a trade-off between sensitivity (time required to localize a hot spot) and angular resolution. Our current work focuses on the implementation of spectrometric capabilities enabling radionuclides identification. The Timepix chip offers two methods for performing spectrometry. The first one is based on the size of the impact on the detection matrix and the other one uses the Time over Threshold (ToT) mode. In ToT mode, a threshold is set on the pulses at the output of the charge sensitive preamplifiers and the energy information is given by the time spent by pulses over this threshold. Evaluation of sensitivity and discrimination ability proved the superiority of the ToT mode. A combination of both cluster size and ToT methods could be considered in the future. A numerical and experimental study about the impact of the pixel pitch on spectrometric performances proved that the optimum in terms of energy resolution without deteriorate angular resolution is reached for pixel pitch ranged from 300 μm to 1 mm. In case of proved improvement in terms of energy resolution, the replacement of the Timepix chip by a new generation of chips (Medipix3 or Timepix3) could be studied.