New positron lifetime spectrometer with the coincidence of energy selected positron and the annihilation photon

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In order to achieve in-situ studies of condensed matter under extremeconditions, new beta^+-gamma conicidence lifetime spectrometer with the coincidence of energy-selected positron and the annihilation photon has been constructed. The whole apparatus is desk-top and low cost, since it does not need any accelerators but a small sealed positron-source and a pair of magnetic lenses. Specimens can be exposed to any extreme surrounding conditions without the risk of contamination by radioactive materials, since the specimen part is independent and well-separated from the sealed positron-source. In order to overcome the inevitable deterioration of time resolution due to the spread of TOF (time of flight) of positrons emitted from radioisotopes, positrons in a certain energy band are selected using magnetic field. The total efficiency (more than 10^<-3> of positrons from the source can be focused on a specimen) is much superior to that expected for the case of re-acceleration of moderated positrons. The time resolution of the whole system with a 10mCi^<22> Na positron source is about 385ps (FWHM) as yet, which is worse than that of the conventional gamma-gamma coincidence method (around 200ps FWHM typically) but bears comparison with former beta^+-gamma coincidence lifetime spectrometers using close geometry between the positron source and a specimen.