Study On Measurement Method Of Annihilation Lifetime Of Slow Positron Burst

Positron annihilation technology is a characteristic characterization technique that studies material microstructures,especially atomic scale defects,and has been widely used in many fields such as metals and semiconductor polymers.The capture-based slow positron pulse beam technology provides a pulse beam method that can greatly improve the "brightness" of slow positron bunches.Each pulse contains a large number of slow positrons,and at the same time,they are cooled by mutual cooperation.Techniques such as radial compression,bunching and the like,can obtain low-energy monochromatic slow positron bunches with extremely low energy dispersion and a short time width.We know that positron annihilation lifetime spectroscopy is the most commonly used characterization method for characterization of defects using positrons.The capture-based slow positron pulsed beam technology provides the measurement of micro-nanosecond slow positron annihilation lifetime spectra.It's possible.A trapped pulse beam will simultaneously annihilate a large number of positrons in the sample.If it is possible to detect the time information of each gamma photon at the same time in the moment of annihilation,theoretically a pulse beam can acquire microscopic information such as defects in the material.It is also possible to study the dynamic annihilation behavior of positrons in matter.When a traditional gamma detector measures a slow positron bunch,due to the accumulation effect,the detector is difficult to distinguish each gamma signal,and it is easy to cause the detector signal to be over-saturated.This paper proposes a method for measuring the lifetime of slow positron bunches based on spatially distributed array detectors.After a reasonable space allocation for the detector unit,it can detect a gamma photon as much as possible while maintaining maximum detection efficiency.Taking the time of slow positron bunch generation as the starting zero,the time distribution of the timing signal of the detector unit is counted,and the cumulative measurement of multiple bunches is repeated,and the positron annihilation lifetime spectrum with enough statistical counts can be obtained.Silicon photomultipliers(SiPM)are being used in high-energy physical detection and other fields for their advantages of compact structure,anti-magnetic,low operating voltage,high gain,and high sensitivity.In order to test the feasibility of the method and try to find out the measurement law of the detector unit,this paper uses four small-size SiPM scintillation detectors and carries out relevant experimental exploration based on the new slow-beam system of the Institute of High Energy of Chinese Academy of Sciences.In this paper,firstly,the solid helium-based slow positron beam system is introduced in detail,and the current beam state is given.Then,based on the principle of the measurement method proposed in this paper,a simple test platform was designed and built,which mainly includes the sample chamber,negative high-voltage feed,detector placement,magnetic field coil and so on.As a digital information acquisition and analysis device,this article then introduces the working principle of the digital oscilloscope and its application in this experiment in more detail.Finally,the performance of each detector was tested and analyzed,and a preliminary measurement of slow positron bunches was explored.Theoretical and experimental results show that with the increase of the distance between the scintillation detector and the radiation source,the intrinsic detection efficiency will gradually increase,but the absolute detection efficiency will gradually decrease due to the limitation of the space angle.The detection efficiency of the detector on the bunch is mainly related to the three-dimensional space of the detector,the positron number of the bunch,and the intrinsic detection efficiency of the detector.When the distance between the fixed and annihilation points is fixed,the detection efficiency of the beam detection unit of the detection unit increases as the number of gamma photons incident on the surface of the detector increases(n).When n is about 40,the detector reaches the state of working saturation.The theoretical formula is in good agreement.However,the increase in n means that the probability of the detector unit outputting a stacked signal is greater.The calculations show that the probability of controlling the stacking signal needs to satisfy ?G?1/M,and the detection efficiency of the array detectors can satisfy binomial distribution.The distribution law of items,experimental results and theoretical results are also more consistent.For a stable pulsed beam,the detection efficiency of the whole array detector for bunches can be appropriately increased by increasing the number of detector units and the gamma detection efficiency of the detector.