Development of positron annihilation induced X-ray spectroscopy

在传统的俄歇电子能谱和特征X射线能谱中，具有足够能量的粒子入射到样品上以通过碰撞电离去除芯电子。当能量低于核心电子结合能的正电子入射到样品上时，核心电子通过物质-反物质湮灭而不是碰撞电离被去除。从这个过程中的俄歇电子已被检测到，并用于表征固体表面的元素含量。本研究的目的是探测这一过程中产生的X射线，用慢正电子束入射到样品上，探测出发射的X射线。该束是使用一种新的正电子减速器产生的，该减速器由一堆电抛光的钨网制成。测量的慢化效率为7.5×10^<-4>。该值高于除惰性气体固体之外的任何其他传输减速剂。与惰性气体固体慢化剂相比，这种慢化剂具有处理简单、成本低、长期稳定性好等优点，但由于正电子湮没产生核内空穴的几率小，X射线产额相当低。因此，背景必须充分减少。背景有两种：正电子湮没γ射线产生的特征X射线和X射线探测器中康普顿散射产生的信号。薄的样品被用来减少以前的背景。研制了一种薄硅（锂）晶体探测器，以减少后者的背景。在γ射线不入射到X射线探测器上的情况下，用γ射线闪烁探测器进行符合测量，也可以降低γ射线的辐射强度，但在项目期间，我们没有成功地探测到X射线。

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.