Use of CZT as 3D detectors for gamma-ray astronomy

Recent progress in the CdZnTe (CZT) based detectors prototypes suggests the possibility of an efficient use as 3D detectors. In this paper we discuss how the z coordinate of the interacting photon can be obtained in thick pixellated CZT crystals with a spatial resolution equal to the linear dimension of the implemented pixel (mm). Two different approaches will be described. The first considers a fine segmentation of multi anode and cathode electrodes arranged in orthogonal strips and biased with an electric field transversal to the photons arrival line (PTF = Planar Transversal Field): this solution tends to simplify the readout electronics although at the expense of some complexity in detectors assembling. The second one is based on a more classical configuration, single cathode and multi electrodes anode (exploiting the small pixel effect) biased with a field parallel to the impinging photons direction (PPF = Parallel Transversal Field). This approach needs a more complex electronics to analyze the signal shape of each interaction in order to evaluate, among other parameters, the interaction depth. Conversely in this configuration the focal plane can be assembled with relative simplicity. Both the configurations can be disposed in a stack to improve the working energy range through an effective use of the Compton effect.