Development of positron annihilation induced X-ray spectroscopy

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In conventional Auger-electron spectroscopy and characteristic X-ray spectroscopy, particles with sufficient energy are incident on specimens to remove core electrons by collisional ionization. When positrons with the energy lower than the binding energy of the core electrons are incident on the specimens, core electrons are removed by matter-antimatter annihilation and not by collisional ionization. The Auger-electrons from this process have been detected and used to characterize the elemental content of solid surface. The purpose of this project is to detect the X-rays from this process.A slow positron beam was incident on a specimen and emitted X-rays were detected. The beam was produced using a new positron moderator made with a stack of electro-polished tungsten meshes. The moderation efficiency was measured to be 7.5×10^<-4>. This value is higher than any other transmission moderators except for inert gas solids. This moderator has the advantages over the inert gas solids that the treatment is easier, the cost is lower, and the long term stability is higher.The yield of the X-rays is considerably low because the probability of creation of inner core holes by positron annihilation is small. Therefore, the background must be reduced sufficiently. There are two kinds of background : characteristic X-rays induced by the positron annihilation γ-rays and signals created by Compton scattering in the X-ray detector. Thin specimens are used to reduce the former background. A detector with thin Si (Li) crystal is developed to reduce the latter background. It is also reduced by the measurement under the condition where the γ-rays are not incident on the X-ray detector by coincidence measurements with a scintillation detector of the γ-rays.However, we did not succeeded in detecting the X-rays in the term of the project.