Design Of Front-End Readout Circuit Based On MCSA In In-Beam PET System

Compared with traditional cancer treatment methods,heavy ion radiotherapy tech-nology has unique physical,biological and medical characteristics.In recent years,its position in the field of tumor treatment research has risen rapidly,and it is currently considered to be the most cutting-edge,most effective and most promising cancer treat-ment.IMP relied on a large scientific facility（HIRFL+CSR）to carry out multiple clin-ical experimental studies of heavy ion radiotherapy and became the first unit in the country to master heavy ion therapy technology.PET imaging technology is used for real-time monitoring of positional information,ranging information and dose distributional information of heavy ion beam incident.It is a key technology to ensure the accuracy of heavy ion radiotherapy.In order to pro-mote the domestication of the main chip of the in-beam PET imaging system in the Heavy-Ion Medical Machine（HIMM）,the paper developed a front-end readout circuit based on the four-channel filter forming application specific integrated circuit（ASIC）chip-MCSA developed by the research group.The circuit is composed of a front-end preprocessing unit（PPU）and a data acquisition and processing unit（DAPU）.It has 8charge measurement channels（energy chain）and 2 time measurement channels（time chain）.The structure of the energy chain is:voltage Pre-amplification circuit,MCSA-based filter shaping circuit,single-ended-differential signal conversion circuit and ana-log-to-digital conversion circuit;the structure of the time chain is:fast amplifying cir-cuit,ultra-high-speed discrimination circuit,PECL-LVDS level conversion circuit and time-to-digital conversion（TDC）logic.DAPU takes Field Programmable Logic Gate Array（FPGA）as the core,and completes the processing and calculation of digital data flow through logic modules such as data framing,integral area algorithm and TDC,and then measures the information of energy,time and location of nuclear events accurately.A series of testing and verifying experiments on the front-end readout circuit has been done.In the test of basic electronics performances,the linearity of the circuit reached 0.78%,the average noise（RMS）in the input range of-600mV～-100mV was less than 0.827mV,and the time resolution was 246.5ps（FWHM）.The detection system which composed of LYSO crystal detector and front-end circuit has been carried out a joint test on the ²²Na point source signal,the energy spectrum resolution of 511keV ga-mma ray is 5.2%（FWHM）,the position resolution in the X and Y direct ions are 0.36mm and 0.49mm respectively.Each pixel in the two-dimensional position spectrum is clear,which has a short radius,and is easily to be distinguished from each other.In tests of other performances,the rate of inter-channel crosstalk and inter-channel difference were 0.13%and 4.5%respectively,the maximum change of integral nonlinearity（INL）and noise（RMS）@-100mV input under long-term continuous working condition are0.075%and 0.028mV respectively,and the maximum change of INL and RMS in the industrial temperature range are 0.252%and 0.093mV respectively.The performance indicators of the circuit has achieved to the level of front-end readout circuit based on commercial filter forming chips,which meets applicational requirements and proves the practical feasibility of MCSA chips.