Pockels effect setup for internal electric field mapping in CZT radiation detectors. Polarizer is set at an angle of 45° with the direction of electric field. IR transmission images of unbiased detector are recorded with analyzer parallel to polarizer then with cross analyzer both biased and unbiased. Brightness of transmission images serves to calculation of internal electric field.
Schematic diagram of charge transients drift timing setup. A source is mounted on a 0.75 mm hole collimator at 7 mm above the cathode side of the CZT detector and can be moved along a line. Alpha scanning is done at increments of 0.5 mm along this line.
The geometry of the combined Pockels effect-charge transients techniques in the internal electric field mapping of the CZT detector used in our experiments. Brightness of Pockels image at pixel centered in (y,z) (illumination is along x axis) is function of the average of the electric field in all points on the red line. Charge transients caused by alpha particle incident in point of coordinates (x,y,0) allow calculation of electric field in any point on the green line
Pockels images of a CZT detector in planar configuration: (a) parallel polarizers, 0 V; (b) crossed polarizers, 0 V; (c) crossed polarizers, 0 V (higher sensitivity to emphasize the regions of residual stress); (d) crossed polarizers, −500 V applied at the bottom surface; (e) crossed polarizers, ; and (f) image average of (d) and (e).
(a) Charge transient in a CZT detector biased at 500 V and irradiated with 5.5 MeV alpha particles at cathode. Signal is amplified by an A250CF charge sensitive preamplifier and recorded by a digital picoscope. (b) Time derivative of from (a), basis of calculation of electric field along the line between the irradiation spot and its normal projection on anode using Eq. (8).
This figure shows the internal electric field in a CZT detector determined using the Pockels effect (triangles) and alpha-induced charge transients (circles).
Pockels images of a CZT detector biased at 1000 V with (a) planar and (b) dual anode contact configurations show the internal electric field of the crystal. In excellent agreement with simulations, the planar detector shows a uniform field throughout the volume, while the DAD exhibits a stronger field closer to the anode contacts.
Simulation of internal electric potential inside a CZT detector in the dual anode configuration. Likeness of this simulation with the Pockels image in Fig. 7(b) is obvious.
Internal electric field calculated from the Pockels images shown in Fig. 7. The first (a) shows the field along a median line in the planar detector of Fig. 7(a). The second (b) plots electric field along two lines, one directly below an anode strip (squares) and one below the gap between the dual anodes (triangles).
Pockels images of a CZT detector in the planar configuration taken along directions 1 and 2 with a bias of −1500 V applied at the bottom.
Spectral responses for a CZT detector in the DA configuration with anode strips along two directions and in planar configuration for comparison. Direction 2 (solid line) provides superior energy resolution at 2.9% compared to 4.6% for direction 1 (dotted line), consistent with the superior electric field uniformity of direction 2. Both spectra were collected from flood illumination by a source centered 17 mm below the cathode. Spectrum obtained in planar configuration does not resolve the 662 photopeak due to holes trapping.
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