Study And Development Of A New LaBr₃ Digital Spectrometer Based On FPGA

Information acquisition of radioactivity and its processing have been widely applied in the fields of the industry and research,and the spectrum detection is one of the important means.The demand for spectrometers for radioactive monitoring is also rising with the development of the nuclear energy industry.A digital spectrometer system has many advantages compared to analog one and it is the development direction.In this paper,a digital spectrometer system using LaBr₃ detector has been developed by digital signal processing technology based on the FPGA.It is expected to be used for outdoor radioactivity monitoring and other applications.In order to solve the problem of temperature shift for LaBr₃ detector used in outdoor radioactive monitoring,a temperature shift correction module has been designed in the FPGA.Hence,the digital multi-channel analysis system the temperature shift correction ability.The signal processing module designed in this paper used the structure of ADC and FPGA.Pulse processing and control systems were both implemented by the FPGA.The internal RAM of the FPGA was also used.These designs were expected to improve system integration and to enhance equipment stability.The digital signal processing circuit board based on FPGA was designed.The input signal firstly passes an S-K filter and an amplifier.Then,the 100 MHz high-speed sampling was realized directly by the ADC circuit.The digital pulse was then processed in the FPGA to acquire spectrum.The communication between FPGA and PC was achieved by USB technology.The tests showed that each module functioned properly and the level of noise was suppressed effectively.Multi-channel algorithms based on the FPGA were mainly built up by the trapezoid shaper,pile-up rejection,baseline recovery,amplitude extraction and energy spectrum cache.We used Verilog hardware language to realize each module and used Modelsim for function and timing simulation.The Qualtus timing constraint tools has been used for solving the timing closure problems.LaBr₃,NaI?T1?and CZT detectors have been applied to verify the system.The test results show that the system clocks run stably and the trapezoid shaper system outputted no error.The designed digital spectrometer system realizes the acquisition of spectrums,which have good energy resolution and energy linear.This design used a temperature feedback method for the LaBr₃ detector to reduce the spectrum peak-shift caused by the temperature variation.The real-time temperature peak-shift correction was realized by directly adding a temperature compensation algorithm in the FPGA.The temperature compensation module received the ambient temperature data from the temperature sensor.The temperature peak-shift characteristics of the LaBr₃ detector was studied to obtain the curves of the relative peak shift.Then,the curves were transformed into FPGA algorithms.The algorithms were added to the multi-channel system and were tested.The results show that the developed digital spectrometer has a good performance of temperature peak-shift correction for LaBr₃ detector.Although there is still slight shift in the corrected energy spectrums,the improvement is obvious compared to the uncorrected spectrums.Measurement accuracy of the energy spectrums from the LaBr₃ detector in outdoor ambient temperature can be improved by this method.