Lan Zhang

PERFORMANCE IMPROVEMENT OF CZT DETECTORS BY LINE ELECTRODE GEOMETRY

Due to their high efficiency and high energy resolution, applications of CdZnTe (CZT) detectors have spread into many areas such as room temperature detectors. To improve the performance and decrease the effects of hole trapping and crystal non-uniformity, special contact geometries are applied to the detectors, such as Coplanar, 3D pixel, Frisch grid, Capture and so on. In this paper, we introduce a new line electrode geometry to greatly improve CZT detector performance. A line anode collects electrons, while a planar cathode collects holes. Due to the low electric field close to the cathode and the low hole μτ value, this geometry is also a single charge sensitive structure. The calculated energy resolution of a 20 mm x 20 mm x 15 mm detector could be improved up to 1.51% for 662 keV gamma rays. Both simulations and experimental results are presented here.

Effects of sample processing on the performance of CdZnTe crystals

Due to the outstanding properties of CdZnTe materials, CdZnTe detectors have been the research focus for X- and gamma ray applications for many years. For CdZnTe detector fabrication heat treatments are often desirable. In order to provide detailed information of the CdZnTe crystal temperature tolerance during the detector fabrication procedure, and determine if performance degradations are from elevated temperatures or other processing steps, we tested the leakage current variation of raw CdZnTe crystals with temperature and during the crystal processing. The studies were conducted on (111)-oriented CdZnTe crystals provided by Yinnel Tech. Due to the 111 orientation, the CdZnTe crystals show a Cd rich side (A side) and a Te rich side (B side). These two sides revealed entirely different character variation. Preliminary experimental results are provided here

Multi-energy detection using CdZnTe semiconductor detectors

The CdZnTe semiconductor detectors have been widely applied in radiation detection, space physics, medical imaging systems and security inspection field. In order to improve the imaging quality and to provide material identification, dual energy and multi-energy detection technology are often necessary. Here we provide a CdZnTe multi-energy detection technology. Micro-strip anode are separated to several segments along the X ray incidence direction. These different segments of electrodes are used as different energy detection regions, from low energy regions to high energy regions. Under this contact geometry the multi-energy information could be obtained, at the same time the detection efficiency of CdZnTe detectors could be improved by the increase of the detector size, not the thickness. So the charge collection efficiency and the operation voltage would be kept invariable. This technology will also solve the problem that the low energy detector is too thin to fabricate for traditional dual energy solid detectors. The data correction in different energy regions could be performed according to the simulation results of the energy deposition distribution in CdZnTe detectors of different energy X rays. This paper provides the details of the multi-energy detection technology.

Signal processing of a strengthened electric field line anode CdZnTe detector

Most methods to improve the performance of CZT detectors focus on the single polarity charge sensitivity technology, such as Coplanar grid, small pixel effect and so on. Our new design of line anode geometry also applies the single polarity charge sensitivity. With the simple electrode geometry the energy resolution of a 10mm*10mm*10mm CZT detector was improved to 2.17% for 662keV gamma rays. In order to solve the high energy tail and the signal loss of low energy rays incidence from the side wall a revised structure - a Strengthened Electric Field Line Anode was presented. The performance of CZT detectors was improved largely. To obtain better energy resolution kinds of digitized signal processing and analysis methods were tested. The signal peak time difference and the signal amplitude ratio of anode to cathode signals were used to give the relationship with the line anode signal. We could select different value ranges to see the energy resolution variation. Through the cutting of some signals the 662keV gamma ray energy resolution of the 10mm*10mm*10mm CZT was improved further up to about 1.6% without obvious loss of the total energy peak area.

Simulation and measurement of pixelated CZT detector

A GUI application named CZT Simulator based on ROOT is developed to study pixelated CZT detector. The parameters taken into account include detector size, pixel size, electronic noise, threshold and so on. Ansoft Maxwell is used to calculate the electric field and the weighting potential of the detector. Geant4 is used to obtain the initial energy deposition data. Induced Current Calculator package is used to simulate the response of the detector. Compton scattering effect, carrier trapping, corner and edge effect are investigated to understand how the spectrum is formed. A 4 × 4 pixelated, 10 × 10 × 10mm3 CZT detector is simulated, fabricated and tested.

Study on the performance of HgI 2 semiconductor detectors

The room temperature semiconductor detector HgI2 is well known for its high detection efficiency and low leakage current. But the defects created during the growth degrade the performance of HgI2 detectors. The low crystal rigidity and the surface instability lead to the difficult operation in crystal processing and further more make the detector performance very sensitive to those processing procedures. When measuring the 241Am spectrum with planar contact HgI2 detectors we found that the 60keV peak suffered the variation from multi peaks to single peak and slow improvement of the spectrum peak shape. These multi peaks reveal the relative serious hole trapping sites inside the HgI2 crystals and the ununiformity of the crystals. Our research provides the results on the ununiformity, the long time stability of HgI2 detectors and the improvement from the carbon glue contact material.

Study on the performance of HgI2 semiconductor detectors

The room temperature semiconductor detector HgI2 is well known for its high detection efficiency and low leakage current. But the defects created during the growth degrade the performance of HgI2 detectors. The low crystal rigidity and the surface instability lead to the difficult operation in crystal processing and further more make the detector performance very sensitive to those processing procedures. When measuring the 241Am spectrum with planar contact HgI2 detectors we found that the 60keV peak suffered the variation from multi peaks to single peak and slow improvement of the spectrum peak shape. These multi peaks reveal the relative serious hole trapping sites inside the HgI2 crystals and the ununiformity of the crystals. Our research provides the results on the ununiformity, the long time stability of HgI2 detectors and the improvement from the carbon glue contact material.

Signals from guard rings on side wall of CZT detectors

Researches on CZT detectors emphasize on how to correct the incomplete charge collection due to the defects in CZT crystals and to improve the performance of CZT detectors. Normally to reduce the surface leakage current and to shape the weighted electric field the guard ring on side wall or on the anode surface surrounding the pixel is applied. Because the guard ring is on the side wall of CZT detectors and is sensitive to the transfer of induced electrons and holes, we expected that the signals from the guard ring could provide the charge transfer information. At first we simulated the electric field inside planar CZT detectors and pixel CZT detectors with single guard ring covering the whole side walls or double guard rings (one is close to the pixel surface and another one is close to the cathode surface). We also changed the electrode surface size and kept the thickness unchanged to compare the variation of the inside field. Preliminary results are provided here

Effects of different orientation on CdZnTe detectors

The traditional orientation used for CdZnTe detectors is (111) surface. To improve the single crystal yield and effectively utilize one oven crystal ingot, we tested CdZnTe detector speciality whose (115) orientation was taken as contact surfaces. Through the same processing as that of (111) orientation we made three pixel CdZnTe detectors. Their performance were tested and compared with that of (111) orientation CdZnTe detectors. The preliminary results were provided here.

A Multi-Channel Front-End ASIC for Pixellated Detectors

A 16-channel front-end ASIC is designed for pixellated detectors. It consists of a charge-sensitive-preamplifier(CSA), a CR-(RC)2 shaper and a class-AB buffer for each channel. A novel continuous reset scheme for low noise and large linear range is presented. The equivalent feedback resistance can be digitally programmed by 3 decades. The shaper output width can also be adjusted from 1mus to 10mus, allowing it to be used both for low noise and high counting rate applications. The chip is implemented in 0.6mum CMOS process. The ENC is measured to be 1413e+20e/pF @ 1mus pulse width and 1402e+11e/pF @ 10mus pulse width respectively. The large zero-capacitance ENC is dominant by the FETs used as resistors in the shaper. The nonlinearity is less than 1.1% over an input charge range of 0-28fC using 0.1pF feedback capacitor.