Boxuan Shi

Dynamic Time Over Threshold Method

The time over threshold (TOT) method has several advantages over direct pulse height analysis based on analog to digital converters (ADCs). A key advantage is the simplicity of the conversion circuit which leads to a high level of integration and a low power consumption. The TOT technique is well suited to build multi-channel readout systems for pixelated detectors as described in our previous work that also exploits the Pulse Width Modulation (PWM) method. The main limitation of the TOT technique is that the relation between the input charge to be measured and the width of the encoded pulse is strongly non-linear. Dynamic range limitation is also an issue. To address these aspects, we propose a new time over threshold conversion circuit where the threshold of the comparator is dynamically changed instead of being constant. We call this scheme the “dynamic TOT method”. We show that it improves linearity and dynamic range. It also shortens the duration of measured pulses leading to higher counting rates. We present a short analysis that explains how the ideal linear input charge to TOT transfer function can theoretically be obtained. We describe the results obtained with a test circuit built from discrete components and present several of the spectrums obtained with crystal detectors and a radioactive source. The proposed method can be used for applications like Positron Emission Tomography (PET) that require moderate energy resolution.

Novel Front-End Pulse Processing Scheme for PET System Based on Pulse Width Modulation and Pulse Train Method

The architecture of a multi-channel front-end system is important for realizing a high-resolution PET system. We propose a novel front-end pulse processing scheme with pulse width modulation (PWM) and pulse train method for PET systems. Each channel of the proposed system consists of a preamplifier, a shaping amplifier, a comparator, and a digital circuit that generates a pulse train for each event. The preamplifier-shaper-discriminator module first generates a trigger pulse with time-over-threshold (ToT), which contains the energy information. The trigger pulse is then processed through a digital circuit that adds subsequent pulses to form a pulse train. These additional pulses encode channel information, timing information, etc. The digital signal output of each channel can be connected by simple wired-OR logic, and the output is read in one transmission line. This multi-channel, low power consumption front-end scheme can acquire enough pulse height (energy) and position information to realize a PET system with a significantly smaller number of output pins in the front-end ASIC. The pulse width encoding also simplifies the digital processing system. We designed a new ASIC based on this concept. The proposed architecture can be applied to high-resolution PET systems with multi-channel ASICs.

Multi-Level Time-Over-Threshold Method for Energy Resolving Multi-Channel Systems

Pixellated radiation detectors require an energy resolving multichannel system. The time-over-threshold (ToT) method provides an inexpensive way to energy-resolving signal processing. However, the conventional, single-threshold ToT method suffers from a small dynamic range and poor linearity. We have studied linearity in a ToT system for typical shaping filters and propose a new, multi-level, ToT scheme. This method provides higher linearity and wider dynamic range. We have analyzed the typical signal cases of triangular, CR-RC, and Semi-Gaussian filters, and achieved a linearity of less ≤3% in Integral Non-Linearity (INL).

Application of Time-over-Threshold readout method to Micro Strip Gas Chamber

Linearity problem has restrained the application of Time-over-Threshold (ToT) on Micro Strip Gas Chamber (MSGC). Here, we proposed an improved ToT method, Dynamic ToT, which could achieve better linearity in a large dynamic range. Both normal ToT and Dynamic ToT module has been assembled with discrete circuits and tested with Multi-Grid MSGC (M-MSGC). In our experiments, Dynamic ToT method has shown a significant linearity improvement, whose coefficient of determination (R-Squared) is ~ 0.94 over 10fC to 100 fC input charge. With a FPGA based DAQ system, the energy spectrum has been reconstructed. Utilizing Dynamic ToT readout, both 5.9 keV peak and argon escape peak could be clearly captured.

Development of Waveform Multiplexing Method With Decay Time Modulation for Front-End ASIC of Pixelated Detectors

This paper proposes a novel waveform multiplexing method to realize high channel-density front-end application specific integrated circuit (ASIC) design. In the proposed method, multiplexing is achieved through connecting multiple preamplifiers to a single analog-to-digital converter (ADC). These preamplifiers have different decay time (90%-10%) which will be used as the tag information. The channel information can be identified by analyzing the decay time using digital signal processing. Based on this method, two prototype ASICs have been designed with 2-to-1 multiplexing. They are a 36-channel multiplexing charge sensitive preamplifier ASIC (MCSP) and a 24-channel multiplexing waveform sampling front-end ASIC (MWSFE).

2-Dimensional He-3 M-MSGC with floating pads

M-MSGC has been successfully fabricated for neutron detector at Japanese spallation neutron source, J-PARC.

Application of dynamic time-over-threshold method to pixellated CdTe detector

We have designed, fabricated and tested a 64channel ohmic-type CdTe detector for the gamma camera application with aconventional PHA (Pulse Height Analysis) circuit dynamic ToT based circuit. The pixel size of the detector is 2mm × 2mm and it consists of 8×8 arrays. The thickness of detectors is 1mm. Our group has been proposing the energy resolving Time over Threshold method with a dynamically changing threshold (dynamic ToT). Although the ToT is suitable for the readout circuits for multi-pixel detectors because of the binary readout and circuit simplicity, the linearity of the ToT versus the input charge is rather poor. The dynamic ToT method greatly improves the linearity and dynamic range. We have tested 64channel CdTe detectors with a PHA method and dynamic ToT method and confirmed that dynamic ToT method is usable as a energy resolving system, and more suitable as a multi-channel individual readout system.

Pulse train multiplexing method for pixellated gamma detectors

Architecture of high-density Front-End electronics is very important for high-resolution pixcellated gamma detector system such as PET or gamma camera. We propose a novel front-end pulse processing scheme with Pulse Width Modulation (PWM) and Pulse Train Method. Each channel of the proposed front-End consists of a preamplifier, a shaping amplifier, a comparator and digital/analog circuit which generates a pulse train. Preamplifier/ Shaper/ Discriminator module generates a first trigger pulse using Time over Threshold (ToT) which contains energy information. Then the additional pulse which expresses the group or ASIC chip address is generated after the trigger pulse to form the pulse train. This additional pulse is unique in each ASIC chip to indicate the chip address where the signal comes from. Output digital pulse train of each ASIC can be connected simply by Wired-OR logic and the number of transmission line in a whole system can be greatly reduced. The pulse width encoding also simplifies the following digital processing parts. This pulse train multiplexing method is useful for pixellated gamma detectors.

Development of waveform multiplexing CMOS ASIC with decay time modulation for pixellated detectors

A novel waveform multiplexing method has been proposed to implement mixed-signal design in large multichannel front-end electronics systems. By multiplexing analog channels with decay time modulation, this new approach is able to increase readout channels without increasing the number of analog to digital converters and digital circuits. Therefore, the die area usage and power consumption per-channel could be greatly reduced. With 2-to-1 waveform multiplexing implementation, a 36-channel multiplexing preamplifier ASIC chip and a 24-channel multiplexing waveform sampling ASIC chip have been designed and fabricated with Rohm 0.35 μm CMOS process. For the 36-channel ASIC, the multiplexed preamplifiers have been tested to have an optimum ENC of about 2600 e- FWHM, which degrades from 1000 e- FWHM ENC of original preamplifier component. The 24-channel ASIC has successfully digitized waveforms from multiplexed channels with different decay time.

Characteristics of 16-Channel ASIC Preamplifier Board for Microstrip Gas Chamber and Animal PET

A 16-channel ASIC preamplifier board has been designed for microstrip gas chamber (MSGC) and animal position emission tomography (PET) detectors. The highly integrated ASIC chips can be used for individual readouts from a large number of channels to improve the spatial resolution and counting rate. The preamplifier board was tested to have a low optimum equivalent noise charge (ENC) of ~ 1400 e− FWHM at a shaping time of 0.1 μs. The output voltage to input charge gain is 0.96 V/pC, and the nonlinearity is ~ 2:0% over a range of −500 fC to 1000 fC in input charge. The rise time (10%–90%) with no input capacitor is about 54 ns. The power consumption of this preamplifier board is ~ 100 mW. The preamplifier board has been used to read out a 3 × 3 cm MSGC plate and an optimum FWHM energy resolution of 19.1% (5.9 keV peak of Fe-55) was obtained.