Development Of HPGe Digital Multi-channel Pulse Amplitude Analysis System

The high purity germanium?HPGe?detector has an excellent energy resolution,which plays an important role in the discrimination of the complex ? energy spectrum.The performance of multi-channel pulse amplitude analyzer?MCA?,as an indispensable component in the process of nuclear energy spectrum measurement,has a direct impact on the accuracy of energy spectrum measurement.The traditional analog MCA and digital MCA have some disadvantages,such as large noise in the analog circuit,low resolution ADC,large dead-time and sub-optimal digital signal processing algorithms,so they cannot be used for HPGe detector to achieve high energy resolution spectrum readout.Therefore,according to the performance of the HPGe detector,this paper design an analog signal conditioning circuit with ultra-low noise and a high-precision analog to digital converter?ADC?circuit.For the digital signal processing algorithms,a dual-channel pulse shaping algorithm are developed.The fast channel selects the finite cusp-like shaper with the optimal filtering effect to improve the precision of pulse amplitude extraction,and the fast channel use symmetrical zero-area trapezoidal shaper to improve the accuracy of pulse triggering and the ability of pulse discrimination.This study was supported by the National Natural Science Foundation of China?the construction of nuclear pulse signal chain and the research of high speed and real time digital reconstruction technique,No.41474159?.According to the current research situations and theoretical basis,to overcome the difficulties in the development of the systems,we have completed the process of theoretical analysis,simulation verification and the actual debugging of this systems.Finally,a practical,complete and high-precision DMCA system is developed.The main research results of this paper are as follows:1.The low noise power supply for the HPGe and the high voltage power supply of the HPGe detector.The power supply of preamplifier can be turned on and off by digital way.The parameter of the high voltage power supply: the range of 0 to 5000 V,digital adjusting precision ±2V,the ripple within 10 mV.High voltage power supply also has the function of automatic shutdown in high temperature for HPGe detector.2.The analog signals conditioning circuit with ultra-low noise.The functions of the flexible analog signal adjusting circuit include the front-end differential amplifier,the adjustment of time constant,the adjustment of hardware gain,the adjustment of DC offset and single-end to differential driver.The noises of the whole analog circuit are within 200 ?V.3.The high-precision and high-speed digital signal processing circuit.The ADC chip use the ADI's AD9266-80 with 16-bits resolution,80 MSPS sampling rate.The 16-bits resolution guarantee that the HPGe detector can obtain a high-resolution energy spectrum with 16384 channels.The 80 MSPS sampling rate can reduce the digital noise and improve the signal-to-noise ratio?SNR?of the subsequent filtering.4.The FPGA+ARM digital signal processing approach.It can remarkably reduce the difficulty of design and the cost of the systems.The low cost EP4CE22 series,FPGA chips,can achieve the pulse shaping and the amplitude extraction algorithms.The memory of the ARM controller using STM32 chip are designed for double buffer structure with 64 KBytes,and ARM controller is also responsible for the data exchange and the control of the power supply system of the detector.5.The data transmission use the high-speed USB 2.0 and high-speed Ethernet.The USB bridge,FT232 H chip,can achieve the communication of high-speed USB 2.0,and Ethernet controller,W5500 chip,can achieve TCP/IP transmission.The chip of the high-speed data transmission using whole hardware protocol can guarantee that the 32 KBytes data of energy spectrum can be transmitted to the computer reliably and improve the speed of data acquisition.6.The slow and fast dual-channel digital pulse shaping algorithms.The slow channel has excellent SNR,which is mainly used for amplitude extraction of the pulse signals.The fast channel has excellent ability of time resolution,which is mainly used for pulse triggering,pile-up discrimination and count rate correction.7.The slow channel use the finite cusp-like shaping algorithm with the optimal filtering effect.The finite cusp-like shaping algorithm have studied and implemented,which can greatly improve the effect of the filtering and shaping.In addition,by adding a flat-top to cusp-like shaper can against the ballistic deficit so as to obtain a higher energy resolution of energy spectrum.8.The simple and efficient symmetrical zero-area trapezoidal pulse shaping algorithm.This algorithm can automatically baseline restoration,which can achieve the accurately triggering of the 1 mV minimum pulse amplitude.At the same time,for the wide range of the amplitude of pulse signals,the accuracy of time triggering within 25 ns can be achieved.9.The overall performance of the HPGe DMCA systems are excellent.Its pulse throughput is up to 1 Mcps,which can guarantee the HPGe detector can be used in environment with a high particle fluence rate.In addition,the linearity is up to 0.99993,which can guarantee the accuracy of spectrum measurement.10.The measurement of ¹³⁷Cs ? ray source achieve the energy spectrum readout with 1.8 KeV FWHM.The ¹³⁷Cs ? ray radiation source was measured by using P-type coaxial GEM40P4-76 HPGe detector of the ORTEC company,and the measured resolution of energy spectrum is 0.271%.