Simulation Of Nitrogen Fluorescence Induced By Energetic α Particles In Air

The stand-off alpha imaging based on the fluorescence of nitrogen is a promising large-scale radiation detection technology,of which the fundamental principal is:alpha particles,which has strong ionization properties,producing a large number of electron-ion pairs in air,these secondary electrons with a certain energy can excite some of N₂ molecule energy levels,making them transit to their excited states.These excited states recover to ground state by collisions or radiation,and photons emitted when radiation occurred,hence,the position and intensity of alpha sources can be obtained by detecting and imaging those photons.The yield of photons which is mostly in the 300-400nm band is approximately proportional to the energy deposition in air ofαparticles,and the band distribution is decided by the Einstein coefficient,so the distribution about wavelength of the photons can be calculated this way.In this paper,Monte Carlo software GEANT4 is used to build a model of air fluorescence and simulate the physical process ofαparticles ionizing in air and generating photons of related wavelengths,the data which the simulation process needed are extracted from other author’s references.The photon generated was simulated imaging according to the imaging rule of the keyhole,and the accuracy of the simulated imaging was proved to be excellent by comparing with the actual imaging results in the literature.Meanwhile the SNR of the imaging were calculated to obtain its variation over imaging time,surface activity of the source.Someαradiation sources of common shapes and types in nuclear sites are simulated by using the method of simulated imaging.And the precision and the surface activity bottom level of discrimination in imaging at a specific distance is determined in the angle perpendicular to the source plane.Excellent precision was obtained in energy discrimination of point source and size discrimination of plane source by using the method of normalized brightness gradient analysis,and the relativity of distance limitation and surface activity ratio limitation was obtained through analysis of brightness masking effect among multiple sources over distance and surface activity of them.For multiple point sources on the plane,the distance limitation of the discriminating is about21mm,and the limitation of activity ratio between certain distance is 14:1,and increases with the distance of the two sources increasing.For multiple surface sources with the same shape,the distance limitation of discrimination accuracy is similar to that of point sources,and the activity ratio limitation also increases with the distance increasing between the two sources.There is also a certain limit for the size discrimination of the source,which requires that the diameter of the hole or the width of the gap be greater than a certain value,and the value has no obvious relationship with the size of its own but varies over the energy of the alpha particles emitted.For SNR,the bottom limitation of the surface activity is 330-1000Bq/cm~2.The analysis of the imaging characteristics above is in good agreement with the most experimental data,which proves that the method which can provide certain guidance and prediction for the stand-off alpha imaging detection technology of radiation sources in practical nuclear sites and facilities has a quite wide prospect.