Arrays of position-sensitive virtual Frisch-grid CdZnTe (CZT) detectors offer an economical approach to make high-efficiency and high energy-resolution gamma cameras for spectroscopy and imaging of radioactive sources. There are many application areas for such instruments including gamma-ray astronomy, medical and industrial imaging, environmental cleanup, nonproliferation and nuclear safeguards. Here, we present the design and results from testing of a 4x4 array mounted on a fanout substrate coupled to a front-end ASIC. The current array houses up to 16 detectors with a cross section of 6x6 mm3 and thickness of 2 cm. However, the array’s design provides flexibility to extend its dimensions in conjunction with the opportunity to replace faulty individual detectors or higher-performing detectors with thicknesses potentially increased up to 4 cm. Each detector is encapsulated inside an ultra-thin polyester shell and furnished with 5 mm-wide charge-sensing pads placed near the anode. For each gamma-ray event the signals on the pads are converted into X-Y coordinates and combined with the cathode signals (for the Z coordinates) to give 3D positional information of all interaction points, which provides a high-spatial-resolution imaging capability for the array. Moreover, the positional information can be used to correct for the detectors’ response non-uniformities due to the presence of crystalline defects, which, in turn, allows the developers to use relatively economical standard-grade (unselected) CZT crystals, while retaining the high spectroscopic performance comparable to the large-volume pixelated detectors produced from more expensive monolithic CZT crystals.
We present new results from testing a small array of position-sensitive virtual Frisch-grid gamma-ray detectors. Such arrays provide high-detection efficiency, excellent energy and position resolution. They can be used in compact hand-held instruments or in large-area gamma ray imaging cameras. The high granularity position sensing enables these detectors to correct the response non-uniformity caused by crystal defects. This important feature allows one to achieve high detection performance while using standard-grade (unselected) CZT crystals, which is expected to reduce the overall cost of field deployable high-resolution CZT gamma ray detection instruments. Here, we report the results of testing several array prototypes with configurations designed for different applications.
High-resolution position-sensing has been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) gamma ray detectors by virtually subdividing the area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. The main hurdle in achieving high sub-pixel position resolution is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. Previous results have shown the benefit of using a focused laser beam to study position resolution in 3D pixelated detectors. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with different pixel dimensions ranging from 0.5 mm to 1.72 mm.
High-resolution position-sensitive detectors have been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) crystals by virtually subdividing the detectors area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. One recent innovation proposed for pixelated detectors is to use the induced (transient) signals from neighboring pixels to achieve high sub-pixel position resolution while keeping large pixel sizes. The main hurdle in achieving this goal is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with the pixel dimensions of approximately 2 mm. We also present the sub-pixel resolution measurements at comparable energies from various gamma emitting isotopes.
The traditional method for electron lifetime measurements of CdZnTe (CZT) detectors relies on using the Hecht equation. The procedure involves measuring the dependence of the detector response on the applied bias and applying the Hecht equation to evaluate the mu-tau product, which in turn can be converted into the carrier lifetime if the mobility is known. Despite general acceptance of this technique, which is very convenient for comparative testing of different CZT materials, the assumption of a constant electric field inside a detector is unjustified. In the Hecht equation, this assumption means that the drift time would be a linear function of the drift distance. This condition is rarely fulfilled in practice at low applied biases where the Hecht equation is most sensitive to the mu-tau product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low mu-tau material, <10-3 cm2/V, but give significantly underestimated values for the case of high mu-tau crystals. In this work, we applied the time-of-flight (TOF) technique to measure the electron lifetimes in long-drift-length (3 cm) standard-grade CZT detectors produced by Redlen Technologies. The TOF-based techniques are traditionally used for monitoring the electronegative impurity concentrations in noble gas detectors by measuring the electron lifetimes. We found the electron mu-tau product of tested crystals is in the range 0.1-0.2 cm2/V, which is an order of the magnitude higher than any value previously reported for CZT material. In this work, we reported the measurement procedure and the results. We will also discuss the applicability criteria of the Hecht equation for measuring the electron lifetime in high mu-tau product CZT.
KEYWORDS: Sensors, Analog electronics, Signal detection, Multiplexing, Thallium, 3D metrology, Image resolution, Application specific integrated circuits, Spectral resolution, Spectroscopy
A CMOS application specific integrated circuit (ASIC) was developed for 3D Position Sensitive
Detectors (PSD). The preamplifiers were optimized for pixellated Cadmium-Zinc-Telluride (CZT)
Mercuric-Iodide (HgI2) and Thallium Bromide (TlBr) sensors. The ASIC responds to an ionizing
event in the sensor by measuring both amplitude and timing in the pertinent anode and cathode
channels. Each channel is sensitive to events and transients of positive or negative polarity and performs
low-noise charge amplification, high-order shaping, peak and timing detection along with analog
storage and multiplexing. Three methodologies are implemented to perform timing measurement
in the cathode channel. Multiple sparse modes are available for the readout of channel data. The ASIC
integrates 130 channels in an area of 12 x 9 mm2 and dissipates ~330 mW. With a CZT detector
connected and biased, an electronic resolution of ~200 e- rms for charges up to 100 fC was measured.
Spectral data from the University of Michigan revealed a cumulative single-pixel resolution of ~0.55
% FWHM at 662 KeV.
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