Design Of Spectrum Measurement System Based On Width Spectrum

With the development of electronic technology,digital spectrum system has become the mainstream choice of nuclear radiation detection because of its advantages,such as stability,flexibility,high precision and convenient for digital processing.The digital spectrum systems have been able to obtain satisfactory results with the excellent performance of high-speed ADC and microprocessors.However,the traditional digital multi-channel system is still too large in deep space exploration,radiation imaging and other large array,multi-channel radiation detection applications.Many ASICs,such as ADC,need to be used in the digital multi-channel detection,which will cause more cost,volume,and control difficulty.Therefore,this paper designs an energy spectrum system based on width measurement with fewer resources,which convert the measurement of amplitude to the measurement of width.It is a more simple way to realize spectrum measurement,and suitable for simplified large area detection system.Firstly,signal for amplitude measurement was converted to rectangular pulse of the appropriate level,the conversion process was realized by the comparator circuit,which is set with different threshold level.A segment of the pulse was intercepted as an effective signal,whose width was measured in FPGA.Altera's EP4CE10F17C8 N chip was employed to achieve FPGA digital circuit,including clock control module,width measurement module,data buffer module,multi-channel polling module,USB read and write control module.Determine the high and low level of the rectangular pulse,and then count the effective level according to the clock.Take the time corresponding to the count value as the width of the pulse,transfer the width data through the USB bus to the upper computer.On the other hand,a large array energy spectrum measurement system was designed based on the multi pixel measurement system.To process the data accurately,framing mechanism and double buffer mechanism were realized in FPGA.All simulation of FPGA digital circuit modules were implemented in the ModelSim.Cypress' s CY7C68013 A chip was used to achieve USB communication between the FPGA and the host computer: FPGA sends the width data to the host computer,and the host computer controls the working state of the FPGA through the USB.USB firmware was developed in Keil to control the USB device.The PC software was in charge of data processing,spectrum rendering,spectrum processing,file management functions in the form of single document: in addition to traditional spectrum curve,the software also gives photon distribution chart,which is calculated according to the data,to characterize the location information of radiation source;in order to avoid flicker,memory buffer was used to refresh the spectrum curve;OpenCV computer vision library was applied to achieve smooth curve,peak position calibration;the Software set the Excel document as the system file format,to manage the spectrum data and the photon distribution data.The experiment measured the 2 x 2 pixel CZT detector and 8 x 8 orthogonal grid CZT detector with the 8)241 source,and analyzed the energy spectra briefly,the results show that the system can achieve the function of spectrum measurement.In addition,two different kinds of photon counting mode were compared: summation count and peak count.The results showed the summation of photon counts can reflect the position of source more closely.