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

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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.