Analysis On Components Of Characteristic X-ray Yield Of Pure Thick Targets By Positron Impact Near Threshold Energy

Experimental atomic inner-shell ionization cross sections induced by positron can not only help people understand the interaction mechanism between positron and substances better,but also improve the database of software related to nuclear science,and it has been widely used in materials science,radiotherapy and other fields.Atomic ionization cross section can be obtained by measuring characteristic X-ray yield and auger electron yield method.Among them,the characteristic X-ray yield method is used widely.Therefore,the accurate acquisition of experimental characteristic X-ray yield helps to obtain accurate atomic ionization cross section induced by positron,thereby promoting the development of the above-mentioned fields.In the process of low-energy positron impact with pure thick target,the characteristic X-ray is not only produced by the incident positrons whose energy is completely deposited in the target,but also by the other particles,namely annihilation photons,secondary electrons and backscattered positrons.At present,the contribution of the above particles to the characteristic X-ray yield is ignored when using the thick target method to measure the characteristic X-ray yield then obtain the atomic ionization cross sections.In addition,a part of backscattered positrons may collide with the target again when the non-uniform electromagnetic field in the target chamber is used to accelerate the positron beam to the required energy by adding negative high voltage.Therefore,when using the thick target method to obtain the positron ionization cross section from the experimental characteristic X-ray yield,it is necessary to analysis and correct the experimental characteristic X-ray yield.Based on the status,the main work carried out in this paper are:(1)Using PENELOPE-2008 to obtain the contribution of annihilation photons,secondary electrons and backscattered positrons to the characteristic X-ray yields of pure thick targets(Al?Ti?Ag?W)induced by positron below 10keV.(2)Using Geant4.10.04 to obtain the influence of electromagnetic field in the target chamber to the characteristic X-ray yields of pure thick targets(Al?Ti?Ag?W)induced by positron below 10keV.(3)The experimental characteristic X-ray yields of pure thick targets by positron impact below 10 keV were obtained.Then the experimental yields were corrected by combining the contributions of the above particles,and the experimental characteristic X-ray yield and the corrected yield are compared with the theoretical yield based DWBA theory.The conclusions of this work are as follows:(1)When low energy positrons collide with pure thick targets(Al,Ti,Ag,W),the characteristic X-rays are not only contributed by incident positrons,but also by annihilation photons,backscattered positrons and secondary electrons.With the increase of incident positron energy,the contribution of annihilation photon decreases.If the positron beam is accelerated by adding negative high voltage,not only the above particles contribute to the yield,but also the electromagnetic field in the target chamber will affect the characteristic X-ray yield.(2)When using the characteristic X-ray yield of thick target induced by low-energy positrons to obtain the ionization cross section,the contribution of annihilation photons,backscattered positrons and secondary electrons is not taken into account,which will lead to a serious overestimation of the result.(3)The influence of electromagnetic field on the characteristic X-ray yield of thick target impacted by low-energy positron depends on the size of the target.The larger the target radius is,the more significant the contribution is.For example,the estimation results obtained by the method developed in this work show that when the 5-9kev positron collides with pure thick W large target(radius=1.25cm),the contribution of the electromagnetic field is about 11%,and the contribution to the small W target(radius=0.75cm)is about 6%.(4)The trend of the corrected experimental characteristic X-ray yield of pure thick targets(Al?Ti?Ag?W)induced by positron is consistent with the shape of the predicted value curve based on the DWBA theory.