Two-dimensional Spectrum System Of Energy And Time For Gamma Ray

With the rapid development of technologys of multi-function digital processing chips?high speed ADCs and powerful CPUs,digital energy spectrum systems become the mainstream in the field of nuclear radiation detection.In the field of multi-channel and large-array detection,the traditional digital multi-channel system requires a large number of ADC chips,which increases the cost,volume and design difficulty of the system.On the other hand,current energy spectrum systems only measure a single parameter,such as pulse amplitude or generation time,and can only output one dimentional spectrum.This paper designs a two-dimensional spectrum system of photon energy and generation time,which converts the pulse amplitude into a pulse width,and performs the simultaneous measurement of two parameters of pulse width and generation time,so as to achieve two-dimensional spectrum analysis.This spectrum system not only eliminates the use of ADCs,but also realizes completely parallel signal processing,which makes the spectrum measurement system smaller,faster and easier to extend.In this paper,²⁴¹Am is used as the radiation source,a weak current pulse signal generated when the emitted photons hit on the four-pixel Cd Zn Te detector.The signal is amplified and filtered by the front-end conditioning circuit to become a voltage pulse signal that is easy to measure.This signal can be directly acquired by traditional pulse amplitude detection systems after ADC conversion.However,the voltage pulse signal is converted into a rectangular pulse signal through a comparator circuit,which has a high voltage level duration depending on the pulse amplitude and the comparator threshold,and can be processed by a FPGA system.The two-dimensional spectrum system for energy and time measurement mainly includes three parts:FPGA digital processing circuit,USB communication and host computer software.The pulse width and pulse time are measured in FPGA,the data is transmitted to the host computer through the USB data bus for processing and display.In addition,based on the four channels spectrum system,a 16-channel independent width spectrum system is designed,which expandes the number of the measurement channels up to 16.In the two-dimensional spectrum measurement system of energy and time,FPGA digital circuit includes 7 modules.Among them,the clock module provides the clock for each module,the pulse width and time measurement module simultaneously measures the pulse width and time of the signal,the frame recognition module realizes the synchronous detection of the signal;besides,there are data summary module,data separation module,USB control module and FIFO storage module.Synchronous measurement is realized mainly by judging whether the signals of four channels are generated by the same photon in the detector within a certain time diviation,so that they belong to the same frame of signal.The FPGA digital circuit of the 16-channel width spectrum independent measurement system mainly has 5 modules,that is a width measurement module,a multi-channel polling module,a USB control module,a FIFO storage module and a clock module.This system mainly performs pulse width measurement,roll polling of multiple channels,data synthetise and FIFO storage,after that,the data is transmitted through USB to the host computer for processing.The two systems use the same USB chip to achieve communication between the FPGA and the host computer.Because the output data formats of the two systems are different,the data processing methods in the host computer software are also different.The functions of the host computer include energy spectrum display,statistic analysis,file management and spectrum filter processing.The energy spectrum plotting of the two-dimensional spectrum system includes time spectrum array,width spectrum array,an integrated spectrum,a photon position density map and a two-dimensional spectrum of energy and time;while the 16-channel width independent system only displays the width spectrum array and an integrated spectrum.A series of experimental test are carried out using simulated pulse signals and real Gamma irradiation detection.First,the two systems are tested with simulated pulse signal from a signal gerator.The square wave signal with a frequency of 50k Hz and a sampling time of 10s is used to measure the spectrums of the two systems,the signal waveform is monitored with an oscilloscope,while the spectra of two systems are acquired and studied respectively.Then,the real Gamma ray signal from a ²⁴¹Am radioactive source is use for testing of the two-dimensional spectrum system,the four-channel energy spectrum resolution,total pulse count,FWHM of the total energy peak and other characteristics are analyzed,as well as the position density map of photons.Finally,the two-dimensional spectrum is analyzed with a three-dimensional histogram plotting,and the tendency of time spectra near the main peak is studied by curve fitting,which preliminariely obtains the time constant of nuclear reactions.