Design Of Portable Digital Nuclear Spectroscopy Instrument Based On FPGA

It is very important to extract and process nuclear information for the purpose of basic and applied researches.In nuclear spectroscopy measurement,multi-channel pulse height analysis is a commonly-adopted method,and the nuclear spectroscopy instrument is the key equipment.On account of the restrictions of ADC and digital signal processor development,traditional nuclear spectroscopy instrument is an analog equipment,which samples and holds the peak of analog pulse signal,and then converts it to digital pulse signal.The rapid development of high-sample-rate and high-resolution ADC and digital processing technology has made it feasible to design a digital nuclear spectroscopy instrument,which can sample the pulse signal output by employing preamplifier of nuclear detector,and then extract the pulse height via using digital signal processing method.The author of this paper has accomplished the design of a portable digital nuclear spectroscopy instrument by utilizing ultra-wideband amplifier,125Msps high-speed ADC and high-speed FPGA with the function of logic control and data processes.A digital nuclear spectroscopy instrument sets a high demand for broad bandwidth in analog amplifier circuit and high sample rate in ADC,which are usually realized at the cost of reducing the signal-to-noise ratio of analog-to-digital conversion.Consequently,the correct and accurate extraction of the digital pulse height in the background of intense noise becomes the foremost problem to be solved. Otherwise,the digital nuclear spectroscopy instrument,not superior to an analog one in terms of correction and accuracy,is not worthy of more researches.This paper, applying curve-fitting method to the extraction of the digital pulse height,firstly analyzes the pulse's curve feature output by various types of nuclear detectors,and then the high-speed curve-fitting algorithm is adopted to extract the pulse's height. With a simulation of the ideal pulse curve output via semiconductor detector when the signal-to-noise ratio is 31.4dB,the curve-fitting algorithm is utilized,and the noise-curve's height is extracted.The comparative study of the height to the maximum in the noise-curve's sample-point data reveals that the height's correction and accuracy extracted by curve fitting method is 1 magnitude better than that of the maximum.If the signal-to-noise ratio is improved—for example,it is up to 45.3dB, which is the instrument's signal-to-noise ratio designed by the paper—the height's correction and accuracy measured by the instrument via curve fitting method will be further improved by 1 magnitude.Another important task of this paper is to design a high signal-to-noise ratio data acquisition system so as to sample the random-pulse output by the nuclear detector and plan a logic unit to discriminate the pulse and noise and then only record the pulse. The author of this paper designs a low noise and ultra-wideband amplifier circuit with phase-compensated technology and uses a 14bit,125Msps ADC to accomplish a data acquisition system,which can convert the analog signal output by nuclear detector to digital signal with high fidelity.A logic unit is then ingeniously invented to capture the conditional pulse and record it at real time.In this paper,some parameters,e.g.signal-to-noise ratio,accuracy and energy resolution of the instrument,have been tested,which is better than that of analog nuclear spectroscopy instrument.To sum up,the following points are innovative:Firstly,the instrument is designed based on FPGA,which can simultaneously do logic control,data process and data recording.In contrast to CPLD+RAM(or FIFO)+DSP design,the new scheme can decrease power and reduce printed circuit area,which contributes to the realization of portability.Secondly,curve-fitting is employed to extract nuclear detector's pulse height. The algorithm is not only of easy feasibility,but also help improve the correction and accuracy on the pulse height measurement.Thirdly.the ingenious algorithm is successful in compensating the ballistic deficit of digital pulse while the analog nuclear spectroscopy instrument fails to do so. In the sight of this,digital nuclear spectroscopy instrument is better than an analog one.