A Multi-Channel Coincidence Digital Positron Annihilation Lifetime Spectrometer

Positron annihilation spectroscopy,as an important tool for studying the microstructure of materials,has the advantages of high sensitivity and no damage to the tested samples.With the development of positron science,positron annihilation lifetime spectrometer(PALS),angular correlation of annihilation radiation spectrometer,Doppler broadening spectrometer,and other experimental techniques came into being for measuring different parameters of positron annihilation.PALS is one of the most basic techniques in the experiment.Commonly used PALS are traditional PALS based on nuclear instrument modules(NIMs)and digital PALS based on high-speed sampling.However,whether it is a traditional PALS or a digital PALS,the way of one channel receiving the "start" signal and the other channel receiving the "stop" signal limits the improvement of the system's coincidence count rate.In addition,these two types of PALS also have problems such as high cost or complicated structure.In order to improve the coincidence count rate of the PALS,a multi-channel coincidence digital PALS is designed here.By improving the timing discrimination circuit and trigger circuit,and calibrating the measured time of the "start" and "stop"channels of various combinations,the delay differences between each channel are corrected to achieve the structural symmetry between the four channels.Each channel can receive "start" signal and "stop" signal.In theory,the coincidence count rate can be increased by up to 12 times.This thesis uses an analog-to-digital converter(ADC)and a time-to-digital converter(TDC)to perform high-precision waveform sampling and time measurement.A field programmable gate array(FPGA)is applied for digital process.The modules of the PALS are integrated into an electronic boards.The system structure is simple and the cost is much lower than the traditional PALS and the digital PALS based on high-speed sampling.In order to verify the performance of the multi-channel coincidence digital PALS,a series of electronics and spectra tests were conducted.The electronic performance test results show that the time resolution of our system reaches 56.2 ps.The integral nonlinearity of the TDC is(-0.49,0.10)LSB,and the differential nonlinearity is(-0.11,0.12)LSB.The effective number of bits of the ADC reaches 10.72 bit,and the amplitude measurement reaches a high accuracy and good linearity.Spectra tests include conventional spectra tests and multi-channel coincidence spectra tests,both using LaBr3:5%Ce3+as scintillator.The energy resolution of our system is 4.27%while the energy resolution of the traditiaonal PALS is 4.62%.The analysis results of the lifetime spectrum tested by our system is almost the same as the traditional PALS.However,the full half-maximum value(FWHM)of the lifetime spectrum tested by our system reaches 193.7 ps,while the FWHM of the lifetime spectrum measured by the traditional PALS is 212.8 ps.The multi-channel coincidence lifetime spectra tests with several different structures by the system were conducted.In the case of four-channel coincidence measurement,the FWHM of the lifetime spectrum tested by the system reaches 212.4 ps.The count rates of multi-channel coincidence measurements have increased significantly.The count rate of 4-channel coincidence measurement has reached 11 times that of the conventional one-to-one coincidence.The main innovations of this article are:1.A digital PALS is designed,which uses ADC and TDC to measure energy and time respectively.The energy and time resolutions of the PALS are better than the traditional PALS2.To increase the count rate of PALS,we have improved the discriminator circuit and trigger circuit,and corrected the time difference of all channels,so that the structure between each channel is symmetrical.The electronic board supports 4 channels input.Each channel can receive "start" and "stop" signals.There are a total of 12 different start-stop combinations.In theory,the count rate can be increased by 12 times.3.Reconfigurable,high-integration,and low-cost.The user can select the working channel and the channel to receive the start signal.The threshold,time and energy window can be set by the user through the control software.Each functional module of the spectrometer is integrated on an electronic board,the system structure is simple,easy to use,and the cost of the system is significantly reduced.