| Pulse signal shaping technology is one of the key technologies in the measurement of ray energy spectrum.Using the traditiona analog filtering and shaping circuit,the output voltage pulse signal of the radiation detectors can be shaped as gaussian waveform,or trapezoidal waveform.It is the key method to reduce electronic noise,and improve energy resolution of radial energy spectrometer.It is difficult to improve temperature stability and simple the complexity of circuit of analog circuit.The hardware of the analog circuit must be adjusted when adjusting the parameters,the flexibility and stability are limited.With the development of modern micro-electronics and computer technology,the acquisition of nuclear pulse signal is digitized gradually.The research on digital processing method and technology of nuclear pulse signal has become a hot topic in the measurement of the ray energy spectrum.The paper focuses on the output signal of the detector.The characteristics of output signal and noise of the scintillation counter and semiconductor detector are mainly analyzed.A variety of digital shaping algorithm for nuclear pulse signal is studied and real-time implementation research of the algorithm on FPGA(field programmable gate array)is also conducted.The topics of this paper are from the National High Technology Research and Development Program(863 Program)environmental technology resources issues relating to the project " High-precision radial energy spectrometer research and development"(project number:2012AA061803)and the National Natural Science Fund Project " Mathematics construction of the nuclear pulse signal chain and research on high-speed real-time digital reconstruction technology "(project number:41474159).The main contents and results are as follows:(1)Current integral deconvolution(I deconvolution)and voltage differential deconvolution(V deconvolution)of digital shaping algorithm(referred to as VI double deconvolution digital shaping algorithm)are presented.The voltage pulse waveform output by analog circuit of a ray energy spectrometer is decomposed into the current response of the detector output current impulse signal in the preamplifier circuit(integral signal of current,the rising phase of the voltage waveform)and the voltage response of the current integrated signal in the preamplifier or the main amplifier RC equivalent circuit(formed by RC discharge,the falling phase of the voltage waveform).The VI double deconvolution digital shaping algorithm eliminate the influence of voltage differential and current integral and the original current impulse of the detector can be obtained.Then the digital shaping system(such as trapezoidal shaping system)is designed to shaping the current impulse signal.The VI double deconvolution shaping algorithm solves the problem of the effective suppression of the ballistic losses and the deformation and smearing of the shaped waveform in the smaller shaped pulse width.(2)The digital gaussian shaping system,the digital trapezoidal shaping system and the digital triangle shaping system are designed based on the time domain,frequency domain and the s domain transform.Based on the analysis of the analog shaping circuit in time domain,frequency domain and the s domain,the conversion methods of system function of the analog shaping circuit into the digital domain,that is,impulse response invariant method and bilinear transform method are respectively used to transform the system function of the analog shaping circuit to the digital domain.The IIR(Infinite Impulse Response)and FIR(Finite Impulse Response)digital filter is designed to shape the voltage differential signal into gaussian waveform based on the frequency domain analysis of the frequency response of the analog shaping system.Based on the analysis of the voltage differential signals in the frequency domain and s domain,digital gaussian shaping system,digital trapezoid shaping system and digital triangle shaping system are respectively designed by using Fourier transform and Laplace transforms to shape the voltage differential signals into gaussian waveform,trapezoid waveform and triangle waveform.The ratio of the SNR(signal-to-noise)of the ideal optimum filter and the SNR of actual filter can respectively reach at 1.07,1.16 and 1.16 for the digital gaussian shaping system,the digital trapezoid shaping system and the digital triangle shaping system.(3)The voltage differential signals output by the scintillation counter or semiconductor detector is a fast-rising and slow-decaying exponential signal,that is,f(t)=(e-t/τ-e-t/τ2)u(t).Digital shaping system can be designed based on FFT(Fast Fourier Transformation)to shape the voltage differential signals into gaussian waveform,trapezoid waveform and triangle waveform or any other waveform without knowing the value of τ1 and τ2.The ratio of the SNR(signal-to-noise)of the ideal optimum filter and the SNR of actual filter can reach at 1.07.(4)A new digital pole-zero cancellation method is proposed.There often exist the problem of the overshoot of the shaping waveform when the voltage differential signals is shaped.A method of convolution between the truncated impulse response of the digital shaping system and the voltage differential forming signals is proposed.The method maintains the amplitude characteristics of the voltage differential signal and shows the better eliminating overshoot effects.(5)The shaping algorithm is implemented on FPGA in real time by using convolution and difference equations.The convolution is achieved by using delayers,multipliers and adders.The difference equations is achieved by using recursive method.The comparison of the shaping results implemented by the FPGA and the MATLAB respectively shows that peak quantization error of convolution method is 0.5%and peak quantization error of difference equations is 0.17%。The Ⅵ double deconvolution shaping algorithm used for digital multi-channel pulse amplitude analyzer is applied to a 20mm×20mm×20mm CsI(T1)scintillator detector to carry out the spectrum measurements,the energy resolution of 137Cs is reached at 4.9%(relative full width at half maximum). |
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