Zhi Deng

Development of an Eight-Channel Time-Based Readout ASIC for PET Applications

An eight-channel readout ASIC has been developed for reading output signals from solid-state photomultipliers for positron emission tomography applications. This ASIC converts both the signal charge and occurring time to digital timing pulses so that only a time-to-digital converter is required for further signal processing. This provides the advantages of simplified circuit design, reduced power consumption, and suitability for applications that have a large number of readout channels. The ASIC uses a fully current mode preamplifier to achieve high bandwidth ( >; 100 MHz), high time resolution (better than ~ 1 ns FWHM), and low power consumption (a few mW/ch). The linear dynamic range of charge measurement is adjustable and can be extended up to 103 pC. The chip has been fabricated with 0.35 μm 2P4M CMOS technology. A test prototype board has been developed and used for ASIC functionality and performance evaluation. Our preliminary studies show that the targets have been successfully achieved.

Development of a prototype PET scanner with depth-of-interaction measurement using solid-state photomultiplier arrays and parallel readout electronics

In this study, we developed a prototype animal PET by applying several novel technologies to use solid-state photomultiplier (SSPM) arrays to measure the depth of interaction (DOI) and improve imaging performance. Each PET detector has an 8 × 8 array of about 1.9 × 1.9 × 30.0 mm(3) lutetium-yttrium-oxyorthosilicate scintillators, with each end optically connected to an SSPM array (16 channels in a 4 × 4 matrix) through a light guide to enable continuous DOI measurement. Each SSPM has an active area of about 3 × 3 mm(2), and its output is read by a custom-developed application-specific integrated circuit to directly convert analogue signals to digital timing pulses that encode the interaction information. These pulses are transferred to and are decoded by a field-programmable gate array-based time-to-digital convertor for coincident event selection and data acquisition. The independent readout of each SSPM and the parallel signal process can significantly improve the signal-to-noise ratio and enable the use of flexible algorithms for different data processes. The prototype PET consists of two rotating detector panels on a portable gantry with four detectors in each panel to provide 16 mm axial and variable transaxial field-of-view (FOV) sizes. List-mode ordered subset expectation maximization image reconstruction was implemented. The measured mean energy, coincidence timing and DOI resolution for a crystal were about 17.6%, 2.8 ns and 5.6 mm, respectively. The measured transaxial resolutions at the center of the FOV were 2.0 mm and 2.3 mm for images reconstructed with and without DOI, respectively. In addition, the resolutions across the FOV with DOI were substantially better than those without DOI. The quality of PET images of both a hot-rod phantom and mouse acquired with DOI was much higher than that of images obtained without DOI. This study demonstrates that SSPM arrays and advanced readout/processing electronics can be used to develop a practical DOI-measureable PET scanner.

Design and construction of muon tomography facility based on MRPC detector for high-Z materials detection

Muon tomography with cosmic ray muons is a novel technology for high-Z material detection. The tomographic imaging is based on multiple coulomb scattering of cosmic ray muons in matter, which requires large-scale, high-efficiency, high spatial resolution detectors for tracking of incoming and outgoing muons. From previous studies, the multi-gap resistive plate chamber (MRPC) would be an excellent and inexpensive choice for this application, which also offers the possibility of introducing energy information of muons by TOF measurement for image reconstruction of muon tomography. A prototype of muon tomography facility with 6 layers large-scale MRPC detectors has been designed and is under construction, the preliminary results of muon tomography and design details of the MRPC detectors and electronics were presented.

Limits on light WIMPs with a 1 kg-scale germanium detector at 160 eVee physics threshold at the China Jinping Underground Laboratory*

We report results of a search for light weakly interacting massive particle (WIMP) dark matter from the CDEX-1 experiment at the China Jinping Underground Laboratory (CJPL). Constraints on WIMP-nucleon spin-independent (SI) and spin-dependent (SD) couplings are derived with a physics threshold of 160 eVee, from an exposure of 737.1 kg-days. The SI and SD limits extend the lower reach of light WIMPs to 2 GeV and improve over our earlier bounds at WIMP mass less than 6 GeV.

TIMPIC-II: The second version time-based-readout ASIC for SSPM based PET applications

A second version ASIC for front-end detector readout, TIMPIC-II, has been developed for Solid-State Photomultiplier (SSPM) based PET applications. It uses the previously developed and evaluated time-based-readout (TBR) architecture. However, several major changes have been made to make TIMPIC-II more flexible and suited for PET applications, including adding a common energy trigger to select the true events and using a constant width integrator to improve linearity. A special logic unit is added to combine the energy and the timing pulses into one output signal that reduces half of the output pins. A 16channel chip has been designed and fabricated with 0.3S,.m 2P4M CMOS technology. The die area is 3mm × 3mm, and the chip is provided in a compact 14mm × 14mm BGA package. TIMPIC-II initial evaluated result shows that the trigger and TBR with control logic function works as designed. And the ASIC specifications including linearity, intrinsic noise and timing jitter, etc. are well achieved as the linear regression R > 0.999 in full dynamic range, intrinsic energy resolution is better than 0.1 % FWHM of SOOpC and the timing jitter standard deviation is 100-300ps for different input signal range. This ASIC is also tested and used for a PET front-end detector module with FPGAbased TDC and acquisition.

A 20ps resolution wave union FPGA TDC with on-chip real time correction

Benefit from wave union, the bins (especially the ultra-wide bins) are sub-divided by each other, making FPGA TDC achieve a resolution beyond its cell delay. At such high level resolution, delay chain becomes very sensitive to the environment disturbance, including power supply voltage, temperature and current surge. On chip calibration needs lots of events and hence cannot follow fast delay changes of the chain. On-chip real time correction method proposed in this article gives one correcting parameter for each sample, making the FPGA TDC stronger when exposed to fast disturbance. A fast encoding logic is also implemented in our design and the dead time can be reduced to 1 clock cycle in the best case. Test results show a typical RMS of 20ps and the max RMS is below 30ps.

Design of a low noise readout ASIC for CdZnTe detector

A low noise readout ASIC has been designed for CdZnTe detector. This chip contains 16 channels and each channel consists of a dual-stage charge sensitive preamplifier, 4th order semi-Gaussian shaper, leakage current compensation circuit (LCC), discriminator and output buffer. This chip has been fabricated in Globalfoudries 0.35 μm CMOS process, the preliminary results are presented here. The total channel charge gain can be adjusted from 100 mV/fC to 400 mV/fC and the peaking time can be adjusted from 1 μS to 4 μS. The minimum ENC at zero input capacitance measured at maximum charge gain and 1.33 μS peaking time is 70 e. The noise difference between FR4 and PTFE test board are analyzed. When connected with a 4 × 4 pixelated CdZnTe detector, energy spectrum from radioactive isotope has been measured with energy resolution of 2.74 keY FWHM for 241Am.

Development of a 64-Channel Readout ASIC for an

In this paper, we developed a 64-channel application specific integrated circuit (ASIC) named EXYT for 8 × 8 SSPM arrays for positron emission tomography (PET) and time-of-flight (TOF)-PET applications. It integrates individual fast readout for timing and resistive networks for energy and position measurements, which reduces the outputs down to 4 differential signals: Timing, Energy and X/Y positions. Gated summation circuits were implemented to suppress the dark count noise and to reduce the dead time. An automatic threshold calibration method was also introduced in this paper and the threshold difference among all the channels could be effectively reduced to <; 1 p.e. by adjusting a local 5-bit DAC for each channel. The chip has been fabricated with 0.18 μm CMOS technology. Evaluation system with SSPM array and LYSO crystals has been developed. All the functions of EXYT have been tested and verified. The power consumption is 3 mW per channel. Energy resolution of 12% and coincidence resolving time (CRT) of 363 ps (FWHM-full width at half maximum) have been achieved for 2 × 2 × 14 mm3 LYSO crystal(s). Sub-pixel resolution can be achieved when coupled to a 12 × 12 array of 2.7 × 2.7 × 14 mm3 LYSO crystals.

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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.

SSPM Array for PET and TOF-PET Applications

Fast neutron spectrometers with high energy resolution are required in some research fields such as nuclear physics and thermo-nuclear fusion plasma diagnostics. In this study, a fast neutron spectrometer based on GEM-TPC has been proposed, which uses the energy of recoiled protons and the recoil angles from reconstructed proton tracks to derive the neutron energy. Compared to the traditional spectrometers, this new method has merits such as high detection efficiency and excellent discrimination ability for neutron from gamma.