Study On Positron Lifetime Spectrometer Based On Signal Coincidence Technique

Positron annihilation technique?PAT?is a unique means to characterize the microscopic defects in the atomic scale of materials.Typical measurement techniques include positron annihilation lifetime spectrum?PALS?,positron annihilation doppler broadening spectrum,positron annihilation angle correlation spectrum and slow positron beam.Which are widely used in metal alloys and semiconductor materials science.PALS is an experiment method which is most commonly used to study the material internal defect scale and defect concentration,and the traditional measuring structure is a sandwich structure of the two pieces of the same sample under test package the redioactive source,which requires the samples must be clingy positron source and measuring environment cannot be interfered with gamma rays.Or it will lead to more cases of accidental coincidence and influence the correct measurement of positron lifetime spectrum.These limit the universality of sample measurement and make it impossible to measure the lifetime spectrum of easily contaminated radioactive sources such as liquids and materials with radionuclides.This paper uses techniques of signal anticoincidence and triple coincidence,we designed and developed the positron detector based on the plastic scintillator and triple coincidence detector based on yttrium doped Ba F₂ scintillation crystals.Then positron lifetime spectra measurement system was set up with three detectors.In order to achieve the positron lifetime spectrum measurement for liquid,such as non-contact classes and radioactive samples,we carried out a series of experimental studies.A plastic scintillation sheet was used to replace one of the samples,and the annihilation lifetimes of positrons in the scintillation sheet were removed by using the anode output signals of the anti-coincidence detector as the anti-coincidence signals.The triple coincidence of signals is realized by using Ortec 567 plug-in unit to set the coincidence time window for the annihilation gamma photon pair,so as to eliminate the false coincidence cases caused by different intensity and different energy gamma rays.The experimental results show that:in PALS measurement system,anticoincidence detector uses a plastic scintillator of 2 mm thickness coupling a type of Hamamatsu R1450 photomultiplier tube which can realize the lifetime spectrum measurement of the single sample.When we measure the lifetime spectrum of the liquid sample,the measurement results are valid while the upper surface of the sample is within 1mm of the radioactive source.This article also gives the PAL of some liquid scintillator and grease,etc.In addition,Yttrium doped Ba F₂ detector,as the triple coincidence detector,can slow down the detector signal stack phenomenon and improve the time resolution of the spectrometer system in the high intensity measurement environment.The positron lifetime spectrum of radionuclide materials can be accurately measured by setting a coincidence time window of ns magnitude.The results also show that in the three-detector measurement system,the counting rate of the start detector?detection of 1.28Me V gamma rays?has the greatest influence on the counting coincidence of the PAL.In order to improve the counting efficiency of the lifetime spectrum,the position of the start detector away from the radioactive source should be given priority.