Construction of a Positron Lifetime Spectrometer with beta^+-gamma coincidence for in-situ studies of lattice Defect

正电子寿命谱已成功地应用于固体中原子级缺陷的研究。寿命测量中的常规正电子源是^<22>Na。正电子的发射伴随着一个合适的起始信号，1.28MeV的光子来自产生的^ Ne核的去激发<22>。正电子源通常是通过将几微居里的NaCl水溶液蒸发<22>到薄金属或塑料箔（通常为1 mg cm 2）上<-2>并用相同的箔覆盖来制备的。然后将源夹在两块相同的样品材料中间，然而，用这种方法进行高温高压下的寿命测量是相当困难的，甚至是几乎不可能的。因此，我们建造了一个新的正电子寿命谱仪，采用β ^+-γ符合技术，使用正电子本身提供启动信号。当正电子通过正电子探测器时，它们会向符合电路发出启动信号。正电子探测器由一个薄的塑料闪烁体和一个连接到快速光电倍增管的光导组成。通过起始探测器的正电子被引导并注入样品材料中。整个仪器是台式的。整个系统的时间分辨率约为300 ps（FWHM），比传统的γ-γ符合寿命谱仪的时间分辨率差，但优于国际上以往的β ^+-γ符合寿命谱仪。

Positron lifetime spectrometry has been successfully applied to the studies of atomicscale defects in solids. The conventional positron source in the lifetime measurements is ^<22>Na. The positron emission is accompanied by a suitable start signal, the 1.28MeV photon from de-excitation of the resulting ^<22>Ne nucleus. The positron source is usually prepared by evaporating a few microcuries of aqueous ^<22>NaCl onto a thin metal or plastic foil (typically 1 mg cm^<-2>) and covering it with the same foil. The source is then sandwiched by two identical pieces of sample material.However, it is quite difficult or almost impossible to carry ont lifetime measurements at high temperatures or under high pressures with the sandwich configulation. We, therefore, constructed a new positron lifetime spectrometer adopting the beta^+-gamma coincidence technique which uses positrons themselves for supplying the start signal. Positrons give a start signal to the coincidence circuit when they pass a positron detector, which consists of a thin plastic scintillator connected with a light guide to a fast photomultiplier. The positrons passed through the start detector are guided and implanted into a sample marterial. The whole apparatus is desk-top. The time resolution of the whole system is about 300ps (FWHM) as yet, which is worse than that of the conventional gamma-gamma coincidence method but is better than any former beta^+-gamma coincidence lifetime spectrometers in the world.