A QTC-based signal readout for position-sensitive multi-output detectors

A QTC-based signal readout for position-sensitive multi-output detectors 1 2 3 first-author 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 3 1 2 3 1 2 3 1 2 3 1 2 corresp Corresponding author, weil@ihep.ac.cn Corresponding author, weil@ihep.ac.cn weil@ihep.ac.cn Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China Beijing Engineering Research Center of Radiographic Techniques and Equipment, Beijing 100049, China Beijing Engineering Research Center of Radiographic Techniques and Equipment, Beijing 100049, China University of Chinese Academy of Sciences, Beijing 100049, China University of Chinese Academy of Sciences, Beijing 100049, China A new signal readout method for position-sensitive multi-output detectors, such as those in high energy spectroscopy measurement and nuclear imaging, was developed by combining the charge division circuit, summing circuit and charge-to-time conversion (QTC) circuit. The 64 outputs of a Hamamatsu H8500 position-sensitive photomultiplier tube (PSPMT) were processed and three digital pulses were generated. The widths of digital pulses were determined using the time-to-digital converter (TDC) in an FPGA (Field Programmable Gate Array). The energy and position information of incident γ-rays is estimated based on the proportionality between the width of digital pulses and input charge created by γ photons. A prototype was built using discrete components and tested, and the energy and position resolutions were improved compared with that obtained with standard ADCs. This method greatly simplifies the front-end electronics and the digital interface. It enables a compact electronics system and an easy integration into an ASIC. Signal readout Charge-to-time conversion Time-to-digital converter H8500 PSPMT 1 Introduction 1 In recent years, position-sensitive multi-output detectors to record the energy and position information of nuclear radiations have been widely used in high energy physics [ 1 1 - 6 6 1-6 ] , PET/SPECT (positron emission tomography/single photon emission computed tomography), and gamma camera for radiation detection [ 7 7 - 12 12 7-12 ] . The Hamamatsu H8500 is a compact flat panel position-sensitive photomultiplier (PSPMT) based on 12-stage dynode electronic amplification and contains an anode matrix of 8×8 anodes spaced at 6 mm pitch. With an 89% effective active area coverage and high spatial resolution on a 52 mm×52 mm square panel, H8500 PSPMT is convenient for applications in small animal imaging, gamma camera, scinti-mammography and 2D radiation monitor [ 13 13 - 17 17 13-17 ] . Conventionally, Its current signals are processed by a preamplifier and a shaping circuit, and the shaped signals are converted to digital signals using Analog-to-Digital Converters (ADCs) before the digital signals are fed into back-end electronics such as a Field Programmable Gate Array (FPGA). The energy and positioning information of nuclear radiations can be obtained by a special algorithm installed in the FPGA or a computer. QTC (Charge-to-Time conversion) is an effective way of transforming the detected information. The amplitude of an input signal from the detector is transformed to width of the digital pulse, is then measured by time-to-digital converter (TDC) in the FPGA. Without external ADCs, QTC is advantageous in its lower power consumption and simplified front-end electronics. It contributes to higher levels of channel density, as the routing between the front-end and back-end electronics is simplified. Owing to these advantages of QTC and the implementation of high-resolution TDC in the FPGA [ 18 18 ] , different types of QTC circuits have been developed in laboratories, such as the TOF (Time-Of-Flight) detectors in BES (Beijing Spectrometer) III [ 19 19 ] , time-based readout ASIC [ 20 20 ] and the pulse width modulation for PET application [ 21 21 ] . In this paper, we propose a novel readout method based on combining the charge division circuit, summing circuit and QTC circuit for position-sensitive multi-output detectors. The Hamamatsu H8500 PSPMT, coupled with LYSO crystal (cerium-doped lutetium yttrium orthosilicate), is used as position-sensitive multi–output detector module. The readout method is described in detail, A prototype circuit is designed, and performance of the circuit is tested. This method can be applied in multi-channel readout of position-sensitive detectors used in high energy spectroscopy measurements and nuclear imaging, including industry imaging and medical imaging. Also, this readout method may be applicable for other position-sensitive detectors [ 10 10 ] , such as pixelated silicon photomultiplier (SensL-C-60035) for high energy spectroscopy. 2 Methods 1 2.1 Signal processing flow 2 A readout method is proposed to acquire energy and positioning information of incident γ photons. The concept of this method is shown in Fig.1 Fig.1 . The incident γ photons deposit energy in the LYSO crystal to create fluoresce immediately, which is converted to photo-electrons on  photocathode of the H8500 PSPMT to, generate current signals. The current signals from the PSPMT are processed by the charge division module (CDM) [ 22 22 ] , which works to reduce the number of readout channels from sixty-four to I A , I B , I C , and I D , which are fed into the summing module (SM) ( Fig.2a Fig.2a ) designed to convert input current signals to voltage and reduce the number of output channels from four to V X , V Y and V E . The V X , V Y and V E are fed into the charge-to-time conversion module (QTCM) ( Fig. 2b Fig. 2b ) composed of charge-to-time circuits and high speed comparators. The output digital pulses of T X , T Y and T E (LVTTL levels) are finally generated with pulse width proportional to the integral of the input signal. The pulse width is measured using a TDC in the FPGA, and the data are transmitted to the computer via a gigabit network. 2.2 Principle and circuit design 2 The SM circuit in Fig.2(a) Fig.2(a) is composed of current-sensitive preamplifiers, and inverting summing circuits with their gain being set to 1. So we can obtain Eqs.(1-3): <math display="block" id="M1"><mrow><msub><mi>V</mi><mi>E</mi></msub><mo>=</mo><mtext> </mtext><mo stretchy="false">(</mo><msub><mi>I</mi><mtext>A</mtext></msub><mo>+</mo><msub><mi>I</mi><mtext>B</mtext></msub><mo>+</mo><msub><mi>I</mi><mtext>C</mtext></msub><mo>+</mo><msub><mi>I</mi><mtext>D</mtext></msub><mo stretchy="false">)</mo><msub><mi>R</mi><mtext>S</mtext></msub><msub><mrow/><mo>,</mo></msub></mrow></math> <math display="block" id="M2"><mrow><msub><mi>V</mi><mi>X</mi></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><msub><mi>I</mi><mtext>A</mtext></msub><mo>+</mo><msub><mi>I</mi><mtext>B</mtext></msub></mrow><mo>)</mo></mrow><msub><mi>R</mi><mtext>S</mtext></msub><mo>,</mo></mrow></math> <math display="block" id="M3"><mrow><msub><mi>V</mi><mi>Y</mi></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><msub><mi>I</mi><mtext>B</mtext></msub><mo>+</mo><msub><mi>I</mi><mtext>C</mtext></msub></mrow><mo>)</mo></mrow><msub><mi>R</mi><mtext>S</mtext></msub><mo>.</mo></mrow></math> The QTCM circuit in Fig. 2(b) Fig. 2(b) is composed of charge-to-time conversion circuits and high speed discriminators. The charge-to-time conversion circuit is designed according to the original QTC principle of the TOF detectors in BESIII. [ 19 19 ] Some appropriate improvement is made for the application in position sensitive detector. The comparator (ADCMP602) output digital pulse (LVTTL) can be directly fed into FPGA without any level translation. For the signal flow of V E ~ V TE ~ T E , the negative input signal ( V E ) charges C T via R T and the constant source I T also discharges C T . The clamping diode ( D ) provides a DC route for charge-to-time conversion in order to clamp the output V TE to the baseline ( V B ) of about −300 mV at V E =0. The charge Q in fed into C T is proportional to the integral of the input signal ( V E ). So the discharge time interval T E can be described as: <math display="block" id="M4"><mrow><msub><mi>T</mi><mi>E</mi></msub><mo>=</mo><msub><mi>Q</mi><mrow><mtext>in</mtext></mrow></msub><mo>/</mo><msub><mi>I</mi><mi>T</mi></msub><mo>=</mo><mi>a</mi><mo>∫</mo><msub><mi>V</mi><mi>E</mi></msub><mtext>d</mtext><mi>t</mi><mo>+</mo><mi>b</mi><mo> </mo><mo>,</mo></mrow></math> where a and b are coefficients associated with I T , R T , V B and the threshold voltage of comparator V REF . Note that the output V TE will maintain V B and T E is zero, if charging current I E is less than the discharge current I T . So there is a tradeoff in pile-up at high event rates and the dynamic range. Specifically, the discharge current should be as small as possible for wide dynamic range which also lead to pile-up in high event rates. Assuming that Q A , Q B , Q C and Q D are integrals of the current signals I A , I B , I C and I D , we have: <math display="block" id="M5"><mrow><msub><mi>Q</mi><mi>E</mi></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><msub><mi>Q</mi><mtext>A</mtext></msub><mo>+</mo><msub><mi>Q</mi><mtext>B</mtext></msub><mo>+</mo><msub><mi>Q</mi><mtext>C</mtext></msub><mo>+</mo><msub><mi>Q</mi><mtext>D</mtext></msub></mrow><mo>)</mo></mrow><mo>∝</mo><mtext> </mtext><msub><mi>E</mi><mtext>γ</mtext></msub><mo> </mo><mo> </mo><mo>,</mo></mrow></math> where Q E is the total charge collected by PSPMT, which is proportional to energy of the incident γ photons ( E γ ). Combining Eqs. (1), (4) and (5), we can obtain: <math display="block" id="M6"><mrow><msub><mi>Q</mi><mi>E</mi></msub><mo>=</mo><mi>A</mi><msub><mi>T</mi><mi>E</mi></msub><mo>+</mo><mi>B</mi><mo>,</mo></mrow></math> where A and B are coefficients associated with the SM and QTCM, and Q E can represent the energy of incident γ photons. Similarly, we can obtain Eqs.(7) and (8): <math display="block" id="M7"><mrow><msub><mi>Q</mi><mi>X</mi></msub><mo>=</mo><mi>A</mi><msub><mi>T</mi><mi>X</mi></msub><mo>+</mo><mi>B</mi><mo>,</mo></mrow></math> <math display="block" id="M8"><mrow><msub><mi>Q</mi><mi>Y</mi></msub><mo>=</mo><mi>A</mi><msub><mi>T</mi><mi>Y</mi></msub><mo>+</mo><mi>B</mi><mo> </mo><mo>.</mo></mrow></math> So the γ photon position can be determined by Eqs.(9) and (10): <math display="block" id="M9"><mrow><mi>X</mi><mo>=</mo><mrow><mo>(</mo><mrow><msub><mi>Q</mi><mtext>A</mtext></msub><mo>+</mo><msub><mi>Q</mi><mtext>B</mtext></msub></mrow><mo>)</mo></mrow><mo>/</mo><msub><mi>Q</mi><mi>E</mi></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mi>A</mi><msub><mi>T</mi><mi>X</mi></msub><mo>+</mo><mi>B</mi></mrow><mo>)</mo></mrow><mo>/</mo><mrow><mo>(</mo><mrow><mi>A</mi><msub><mi>T</mi><mi>E</mi></msub><mo>+</mo><mi>B</mi></mrow><mo>)</mo></mrow><mo> </mo><mo>,</mo></mrow></math> <math display="block" id="M10"><mrow><mi>Y</mi><mo>=</mo><mrow><mo>(</mo><mrow><msub><mi>Q</mi><mtext>B</mtext></msub><mo>+</mo><msub><mi>Q</mi><mtext>C</mtext></msub></mrow><mo>)</mo></mrow><mo>/</mo><msub><mi>Q</mi><mi>E</mi></msub><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mi>A</mi><msub><mi>T</mi><mi>Y</mi></msub><mo>+</mo><mi>B</mi></mrow><mo>)</mo></mrow><mo>/</mo><mrow><mo>(</mo><mrow><mi>A</mi><msub><mi>T</mi><mi>E</mi></msub><mo>+</mo><mi>B</mi></mrow><mo>)</mo></mrow><mo> </mo><mo> </mo><mo>.</mo></mrow></math> The outputs (TE, TX and TY) are received directly by the FPGA and measured by the TDC (64ps resolution) in the FPGA. The data TE, TX and TY are transmitted to the computer through the network and the energy and position information of the γ photons can be obtained using Eqs. (6), (9) and (10). The experimental circuit was designed and tested with the H8500 PSPMT. Fig.3(a) Fig.3(a) is a photo of the circuit, with Parts 1, 2 and 3 being the CDM, SM and QTCM, respectively; while Fig.3(b) Fig.3(b) shows the VE, VTE and TE signals generated by the experiment circuit (captured by Tektronix DPO 3054 oscilloscope). 3 Experimental tests and results 1 Performance of the experimental circuit was tested with the Hamamatsu H8500 and an 11×11 array of LYSO crystals of 3.5 mm ×3.5 mm ×20 mm. Fig.4 Fig.4 shows the LYSO crystal array, H8500 PSPMT and the experimental platform including high- and low-voltage sources, the experimental circuit, data acquisition board for TDC and computer. 3.1 Linearity test of QTCM 2 In the linearity test of QTCM, pulses with adjustable amplitude (200 mV per step) from a function generator (AFG3252) were sent to the input stage of QTCM with the discharge current of 100 μA. A group of digital pulses was generated, and their time widths were obtained using TDC. As shown in Fig.5 Fig.5 , the QTCM has a large dynamic range from 300 mV to 4 V, with the maximum time width of about 9 μs, the fitting coefficient of R 2 = 0.9997, and the integrated nonlinearity of 1.31%. 3.2 Energy spectrum test 2 The H8500 PSPMT coupled with a single 3.5 mm ×3.5 mm ×20 mm LYSO crystal was used to obtain the energy spectrum with 137 Cs and 22 Na sources. The high voltage source was −750V and the discharge current was 100 μA. Under the same experimental conditions, we obtained the energy spectrum by using the commercial 12-bit 40-MHz ADC (ADS5282 from TI) with the signal V E being fed into the input of the ADC. Fig.6 Fig.6 compares the spectra obtained with the ADC and the QTC circuit, after appropriate scaling. From Table 1 Table 1 , one sees that energy resolutions of the peaks at 511 keV, 662 keV and 1.27 MeV by the two methods are close to each other. Although some distortion appears in low energy regions of both spectra, it can be seen that input signals cannot be detected by the QTC circuit if the input charge current is less than the discharge current I T , as explained in Section 2.2. So a tradeoff between dynamic range and dead time has to be found in practical spectrum measurements. 1 1 Gamma-rays 1 1 ADC system 1 1 QTC-based circuit 1 1 511 keV( 137 Cs) 1 1 14.62 1 1 15.42 1 1 662 keV( 22 Na) 1 1 13.05 1 1 13.86 1 1 1.27 MeV( 22 Na) 1 1 8.89 1 1 9.75 3.3 Positioning resolution test 2 The H8500 PSPMT coupled with an 11×11 array of 3.5 mm ×3.5 mm ×20 mm LYSO crystal, and a 137 Cs source, were used to test the position resolution. The high voltage was −750 V, and the discharge current was 100 μA. Fig.7(a) Fig.7(a) shows the flood histogram and profile histogram, with the peak-to-valley ratio of about 5:1. It can be seen that the 11×11 crystals in the array can all be identified clearly. In comparison, using the ADC method, with the ADS5282 and the same combination of the H8500 PSPMT and LYSO crystal array, the profile histogram ( Fig.7b Fig.7b ) has a peak-to-valley ratio of about 6:1. 3.4 Linearity test of system 2 Gamma-ray spectra of 137 Cs, 22 Na and 60 Co were measured with the system ( Fig.8a Fig.8a ), and linearity of the test pulses generated by the function generator (AFG3252) was checked ( Fig.8b Fig.8b ), with the fitting coefficient of R 2 =0.9989 and the integrated nonlinearity of 1.38%, while it is 0.95% in the energy range using the ADC-based system (ADS5282). 3.5 Stability test of system 2 To test stability and credibility, the system was measuring the 137 Cs source at room temperature for 48 hours, recording a γ-ray spectrum every three hours. From Fig.9 Fig.9 , the maximum deviation of peak position in 50 hours is 0.32%. 3.6 Discussion 2 The readout performance of the experimental circuit for position-sensitive detectors indicates that the QTC-based circuit has good linearity and energy resolution, which are close to the results of conventional ADC-based systems. We noticed some distortion in low energy region of the spectra using the QTC method, so the best tradeoff between dynamic range and dead time should be evaluated. In the position resolution test of QTC-based circuit, all the crystals can be identified with a peak-to-valley ratio of about 5:1 though the flood histogram distorted slightly compared to that of the ADC method. However, the proposed method does not outperform ADC-based systems in linearity, with integrated nonlinearities of 1.38% and 0.95% for the QTC and ADC methods, respectively. In fact, the QTCM output width T is not simply proportional to the input charge Q , some complicated calibrations should be performed for better energy and position resolutions, which means a more complicated system. The original QTC calibration method has been described in detail for BESIII’s TOF detector in Ref.[ 19 19 ]. To simplify the whole system including hardware and software design for the application in position sensitive detector, the QTCM output width T is considered to be proportional to the input charge Q in this paper and the results are acceptable. An advantage of the proposed method is that large number of interface lines between ADCs and FPGA can be simplified. 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DOI: 10.1109/23.507162 http://doi.org/10.1109/23.507162 Simple Charge Division Readouts for Imaging Scintillator Arrays using a Multi-Channel PMT The authors would like to thank Prof. Liu Yu and Dr. Qian Sen for their careful and useful comments of the manuscript. 10.1007/s41365-016-0124-4 This work was supported by the National Key Scientific Instrument and Equipment Development Project (No. 2011YQ120096), National Instrumentation Program (No. 2013YQ030629) and the National Natural Science Foundation of China (Nos. 11475206, 11175200 and 11205170) . 2015-10-09 2015-11-16 2015-12-09 2016-08-31 2016-10-20 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0124-4 http://dx.doi.org/10.1007/s41365-016-0124-4 . 2016 Nuclear Science and Techniques 05 27