Nuclear Instruments and Methods in Physics Research A 460 (2001) 113–118 Experimental tests of a hybrid pixellated detector for gamma imaging O. Gal a , B. Mikulec b , M. Million a, * a LETI (CEA - Technologies Avance ´es), DEIN, CEA/Saclay, 91191 Gif sur Yvette Cedex, France b EP Division, CERN, 1211 Geneva 23, Switzerland Abstract In the framework of the MEDIPIX Collaboration, a hybrid pixel detector has been developed primarily for X-ray radiography. This detector consists of a 64  64 pixel photon counting chip (PCC), bump bonded to a 200 mm thick GaAssubstrate.ThePCCisoptimisedforenergydepositionsintherangeofafewkeVtoafewtensofkeV.Theaimof this study is to evaluate the detector for applications in decommissioning of nuclear power plants where typical sources have energies in range of a few hundred keV. Tests were realised using a 137 Cs gamma source (660keV). At this energy, Monte-Carlosimulationspredictthat,onaverage,formorethan60%ofprimaryinteractions,thereisatleastonepixel on which the deposited energy exceeds 100keV. Simulations also allow modelling of the spatial energy spreading. The comparison of the simulation results with experimental data should indicate if there is a significant contribution of electrical cross-coupling between pixels to the cluster size of the detected hits. The results obtained demonstrate promising perspectives for this kind of detector towards gamma imaging applications. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Pixellated; Detector; g imaging 1. Introduction Semiconductor detectors provide an interesting alternative to scintillator-based systems for X- and g-ray imaging. Several such detectors have been reported recently in the field of medical imaging [1–3]. In the field of nuclear power plant decommis- sioning, a g camera called CARTOGAM [4,5] has been designed to remotely localise radioactive hot spots prior to human intervention in contaminated areas. However, the use of a detector which converts photons directly to electric charges may provide better sensitivity and would be a more compact solution. The hybrid pixel detector appears to be a good candidate for such applica- tions. We evaluate in this paper such an existing system, the Photon Counting Chip (PCC) which was developed at CERN in the framework of the MEDIPIX Collaboration 1 . The PCC electronics functions in counting mode, and whilst it is capable of counting at rates of up to 2  10 6 per *Corresponding author. Present address: Heimann Systems, 45 rue d’ Antony-silic 1171, 94533 Rungis Cedex, France. Tel.: +33-(0)1-45-12-8668; fax: +33-(0)1-46-87-10-10. E-mail address: m-million@heimannsystems.fr (M. Million). 1 The MEDIPIX collaboration consists of CERN, University of Freiburg, University of Glasgow and Universities and INFN of Pisa, Napoli and Sassari. 0168-9002/01/$-see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0168-9002(00)01104-9