Reconfigurable Positron Annihilation Lifetime Spectrometer Based On Multi-Channel Digitizer Card

Positron is the antiparticle of electron.Positron annihilation spectroscopy is a powerful tool to study the microstructure of materials.Normally used techniques include positron annihilation lifetime(PAL)spectroscopy,coincidence Doppler broadening(CDB)spectroscopy,angular correlation of annihilation radiation(ACAR),age-momentum correlation(AMOC),and slow positron beam techniques.Among them,PAL spectroscopy is the basic and most commonly used one.There are two kinds of PAL spectrometers with different data acquisition methods:conventional PAL spectrometer and digital PAL spectrometer.In this paper,a reconfigurable PAL spectrometer based on conventional nuclear electronic modules and commonly used A/D converter card was developed.Several online data processing algorithms were designed to improve the function and performance of PAL spectrometer.PAL spectra simulation program and the relative mathematical models are also developed to study the influences of spectra distortion by Monte Carlo simulation.Some valuable conclusions from the simulation works which cannot be studied by experiments have also been achieved.Based on multi-parameter acquisition techniques,the energy windows of detectors can be precisely set by software.So we can study the influences of energy windows to the PAL spectra,and find the proper setting methods to care both the precision and efficiency of PAL measurement.Some issues related to PAL spectra analysis have also been discussed.Besides,the principles of PAL spectra distortion were studied to establish the mathematical models of timing and energy deposition for simulations.Reasonable simulation results corresponding to the experimental results were got.In this paper,we developed the symmetrical PAL spectrometer that two detectors provide both start and stop signals to double the counting rate.Zero-channel calculation methods based on the "raw spectrum" were developed.The influences of zero-channel difference and FWHM difference were simulated.Besides,we tested PAL spectra by using Kapton film as reference sample,which can be called "single-sided" PAL spectra.The influences of high and unknown intensity of source components were simulated.The time-energy correlation(2D)positron annihilation spectra were also tested.This paper introduces a method to measure the absolute activity of 22Na source by using reconfigurable PAL spectrometer,without adding more devices.The activity can just be acquired together with the PAL spectra.We designed the correction methods for real experiments to diminish the system error of activity calculation,so the accuracy can be good enough to meet our requirements.The new methods introduced in this work provide good references for developing digital PAL spectrometers.We believe that digital reconfigurable PAL spectrometer can achieve better performance.And also,the combination of PAL spectra simulation,energy deposition simulation,and pulse distortion simulation is expected to provide better accuracy of simulation works in the future.