AVS 63rd International Symposium & Exhibition
    Electronic Materials and Photonics Thursday Sessions
       Session EM+AC+SS+TF-ThM

Invited Paper EM+AC+SS+TF-ThM10
Position-Sensitive 3D CZT Gamma-Ray Detectors with Thickness Up to 50 mm

Thursday, November 10, 2016, 11:00 am, Room 102A

Session: Radiation Detection Materials and Devices
Presenter: Ralph James, Brookhaven National Laboratory
Authors: R.B. James, Brookhaven National Laboratory
A.E. Bolotnikov, Brookhaven National Laboratory
G.S. Camarda, Brookhaven National Laboratory
Y. Cui, Brookhaven National Laboratory
G. De Geronimo, Brookhaven National Laboratory
J. Fried, Brookhaven National Laboratory
A. Hossain, Brookhaven National Laboratory
G. Mahler, Brookhaven National Laboratory
U. Roy, Brookhaven National Laboratory
E. Vernon, Brookhaven National Laboratory
G. Yang, Brookhaven National Laboratory
Correspondent: Click to Email
High-granularity position-sensitive detectors allow for accurate charge-signal corrections to overcome non-uniformities in the devices’ responses caused by crystal defects. The operational principle of position-sensitive detectors is analogous to the well-known drift ionization chambers used for tracking charged particles and detecting the interaction events generated by gamma rays. Advantages of the position-sensitive designs were realized in a number CZT detectors, including CAPtureTM, hemispherical, Frisch-ring, capacitive Frisch-grid and even pixel detectors in which pixel contacts act like shielding electrodes. In our virtual Frisch-grid (VFG) devices, the sensing strips are separated from the crystal surfaces by a thin insulating layer, as it was originally done in other Frisch-grid designs. The amplitudes of the signals readout from the strips are used to measure the coordinates of the interaction points and correct the response non-uniformities. The drift time and the cathode-to-anode ratio were used to independently evaluate the location of the interaction points in Z directions, correct for electron loss, and identify and reject the events for which the charge losses caused by defects are so great that they cannot be corrected accurately. Combining these two techniques allows us to significantly enhance the spectral responses of position-sensitive VFG detectors, and to significantly improve their performance. Such high-granularity position-sensitive detectors open up the opportunity for using thicker, less expensive crystals. We demonstrated that today’s CZT material is suitable for detectors with up to 40-50-mm drift distances, provided that the detectors have the ability to correct their response non-uniformities on a scale comparable to the sizes of electron clouds, which is ~100 m m. We employed an ASIC and data-acquisition system developed by BNL’s Instrumentation Division for arrays of VFG detectors. For each detector we used 6 ASIC channels to read the negative signals from the cathode and from four position-sensing pads, and the positive signals from the anode. For each interaction event, the anode signal correlates with the X and Y values converted from the 4 strip signals and Z coordinate evaluated from the cathode signal. This relationship allows us to correct each anode signal in accordance with the location of the interaction point. We selected the voxel sizes to achieve the best performance, typically ~30x30 pixels in XY-space and ~100 segments in the Z-direction. The performance of thick position-sensitive VFG detectors fabricated from CZT crystals will be reported for a variety of radioactive sources and testing conditions.