Construction of a desk-top-sized and mobile positron lifetime spectrometer with β^<+->γ coincidence and positron energy selection for in-situ studies of lattice defects

正电子寿命测量是一种无损检测和鉴别材料中纳米级晶格缺陷的方法。在传统的正电子寿命测量中，由于从正电子源向各个方向发射的所有正电子都必须与样品材料中的电子湮灭，因此必须在两个相同的样品材料之间夹一个正电子源。因此，在样品存在的地方测量正电子寿命是相当困难的，除非从样品材料中切割两块。为了实现对正电子寿命的原位测量，我们构建了一台具有正电子能量选择的β^<+->γ符合寿命谱仪。在光谱仪中，正电子源被放置在离样品很远的地方。从正电子源发射的正电子通过磁透镜选择能量，只有具有一定能量的正电子被引导到先进的光电二极管。当正电子通过二极管时，它给重合电路一个启动信号。通过正电子探测器的正电子再次被磁透镜聚焦在样品上。正电子在样品中与电子湮灭。湮灭光子被检测用作停止信号。具有快速正电子束的新型正电子寿命谱仪是桌面大小，可以方便地携带。

The positron lifetime measurement is a non-destructive method for sensitively detecting and differentiating crystal lattice defects with size of nanometer order in materials. In the conventional positron lifetime measurement it is necessary to sandwich a positron source between two identical pieces of sample material, because all positrons emitted in all directions from the positron source have to annihilate with electrons in the sample material. It is therefore quite difficult to measure positron lifetime in the place where the sample exists, without cutting two pieces out of the sample material.We have constructed a new β^<+->γ coincidence lifetime spectrometer with positron energy selection in order to achieve in-situ measurement of positron lifetime. In the spectrometer a positron source is placed well apart from the sample. The positrons emitted from the positron source are selected in energy by a magnetic lens and only the positrons with a certain energy are led to an advanced photo diode. When the positron passes through the diode, it gives a start signal to the coincidence circuit. The positron that passed through the positron detector is again focused on the sample by a magnetic lens. The positron annihilates with an electron in the sample. The annihilation photon is detected to use as a stop signal. The new positron lifetime spectrometer with fast positron beam is desk-top-sized and can be easily carried.