Jianqiang Fu

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.

Design of a neutron-TPC prototype and its performance evaluation based on an alpha-particle test

A neutron-TPC (nTPC) is being developed for use as a fast neutron spectrometer in the fields of nuclear physics, nuclear reactor operation monitoring, and thermo-nuclear fusion plasma diagnostics. An nTPC prototype based on a GEM-TPC (Time Projection Chamber with Gas Electron Multiplier amplification) has been assembled and tested using argon-hydrocarbon mixture as the working gas. By measuring the energy deposition of the recoil proton in the sensitive volume and the angle of the proton track, the incident neutron energy can be deduced. A Monte Carlo simulation was carried out to analyze the parameters affecting the energy resolution of the nTPC, and gave an optimized resolution under ideal conditions. An alpha particle experiment was performed to verify its feasibility, and to characterize its performance, including energy resolution and spatial resolution. Based on the experimental measurement and analysis, the energy resolution (FWHM) of the nTPC prototype is predicted to be better than 3.2% for 5 MeV incident neutrons, meeting the performance requirement (FWHM<5%) for the nTPC prototype.

Performance study of the neutron-TPC

Fast neutron spectrometers will play an important role in the future of the nuclear industry and nuclear physics experiments, in tasks such as fast neutron reactor monitoring, thermo-nuclear fusion plasma diagnostics, nuclear reaction cross-section measurement, and special nuclear material detection. Recently, a new fast neutron spectrometer based on a GEM (Gas Electron Multiplier amplification)-TPC (Time Projection Chamber), named the neutron-TPC, has been under development at Tsinghua University. It is designed to have a high energy resolution, high detection efficiency, easy access to the medium material, an outstanding n/? suppression ratio, and a wide range of applications. This paper presents the design, test, and experimental study of the neutron-TPC. Based on the experimental results, the energy resolution (FWHM) of the neutron-TPC can reach 15.7%, 10.3% and 7.0% with detection efficiency higher than 10?5 for 1.2 MeV, 1.81 MeV and 2.5 MeV neutrons respectively.

Hybrid reconstruction method at intermediate energy range

3D Pixel CZT detector based Compton imaging is an ideal candidate at energy above 250 keV, mechanical collimation technique must be used at lower energy range where the Compton scattering probability is low. We propose a hybrid imaging method to reconstruct gamma-ray source distribution at intermediate energy range. In this new method, both single and two interaction events are used, and a coded mask is brought into the system to concentrate the possible source direction of single interaction event. The feasibility of this method is verified by the data generated from the Monte Carlo simulation program of a 3D pixel CZT detector system, better imaging resolution and efficiency are reached than a single method.

Study of spectral distortion in pixel CZT detector

CZT detectors have attracted much interest as room-temperature solid-state X-ray and gamma ray detectors due to their high energy resolution and high spatial resolution. Pixel CZT detectors are widely applied. However, the observed spectrum of a single pixel is distorted towards lower energies. The possible reasons for the spectral distortion may be carriers trapping, charge sharing with the adjacent pixels, and scattering photons contributed by the other pixels. To find out the cause, Monte Carlo simulation codes were developed to get an insight of the reason for the spectral distortion. Simulation and measurement confirmed that the reason for high counts below 150 keV are not backscatter, but charge sharing. Another remarkable conclusion was that the low tail of full energy peak is mainly due to charge sharing rather than electron trapping. Measurements also showed anti-coincident readout is effective to mitigate the spectral distortion.

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.