&beta;<SUP>+</SUP>-&gamma; Coincidence Positron Lifetime Spectrometer with Positron Energy Selection by Electromagnetic Lens

Abstract

A b ＋  g coincidence positron lifetime spectrometer equipped with two electromagnetic lenses was constructed. The electromagnetic lens was used to have energy selected positron beam. It accomplished a time resolution of 385 ps (FWHM) for the total system with a Pilot U plastic scintillator. By using a silicon avalanche diode instead of Pilot U plastic scintillator to initiate start timing signals, the time resolution was improved to 345 ps (FWHM). With this method, in situ positron lifetime measurements at high temperatures or under high stresses can be attained easily.

Optimization study of a new β +-γ coincidence positron annihilation lifetime spectrometer using Geant4

“…Moving the sample away from the plastic scintillator and obtaining the stop signal of the PAL by coincidence measurement of the two annihilation 𝛾 photons can effectively reduce 𝑃 SC [18,22]. On this basis, controlling the energy of positrons [20,21,23] or using an avalanche diode as the start signal detector [24,25] can improve the CTR to some extent, and the current best CTR is 240 ps [24]. However, these approaches often result in significant loss of the ultra-high CCR advantage (for example, 20 cps [23]).…”

High-temperature in-situ measurements of thermal vacancies in a TiAl intermetallic compound using a desktop positron beam

“…However, both methods have disadvantages such as the large size of the experimental ap-paratus (monoenergetic positron beam method) or difficulty in sample preparation (internal positron source method). Shirai and co-workers proposed a third method in which fast positrons emitted from a 68 Ge sealed positron source are directly irradiated into a sample at high temperatures with magnetic lenses to eliminate low-energy positrons which cause a degradation of the time resolution in positron lifetime measurements [6][7][8][9]. This third method allows us to separate the positron source from the heated sample like in the monoenergetic positron beam method.…”

Positronium as a probe of small free volumes in crystals, polymers and porous media

“…Instead of a thin scintillator Shirai et al [25,26] propose to use a silicon avalanche diode. The signal produced when the positron penetrates the diode has the rise time of about 1.5 ns, i.e similar to that from the fast scintillator, thus diode detector does not deteriorate the resolution of the spectrometer.…”