Implementation of an imaging spectrometer for localization and identification of radioactive sources

Spatial localization of radioactive sources is currently a main issue interesting nuclear industry as well as homeland security applications and can be achieved using gamma cameras. For several years, CEA LIST has been designing a new system, called GAMPIX, with improved sensitivity, portability and ease of use. The main remaining limitation of this system is the lack of spectrometric information, preventing the identification of radioactive materials. This article describes the development of an imaging spectrometer based on the GAMPIX technology. Experimental tests have been carried out according to both spectrometric methods enabled by the pixelated Timepix chip used in the GAMPIX gamma camera. The first method is based on the size of the impacts produced by a gamma-ray energy deposition in the detection matrix. The second one uses the Time over Threshold (ToT) mode of the Timepix chip and deals with time spent by pulses generated by charge preamplifiers over a user-specified threshold. Both energy resolution and sensitivity studies demonstrated the superiority of the ToT approach which will consequently be further explored. Energy calibration, tests of different pixel sizes for the Timepix chip and use of the Medipix3 chip are future milestones to improve performances of the newly implemented imaging spectrometer.