Lifetime Measurements with a Pulsed Slow Positron Beam

Abstract

The lifetimes of positrons annihilating in solids at the surface or close t o the surface are measured by a pulsed positron beam of variable energy. A time resolution of 240 ps FWHM at a coincidence count rate of 70 s-l is obtained. For stainless steel and nickel the lifetime associated to the annihilation in surface states is of the order of 500 ps. The lifetimes for the annihilation in trapping centers close t o the surface are in complete agreement with the lifetimes observed in conventional lifetime measurements. Mit einem gepulsten Positronenstrahl von variabler Endenergie werden die Lebensdauern vonPositronen gemessen, die in oberfliichennahen Schichten annihilieren. Die Zeitauflosung des Gesamtsystems betragt 240 ps FWHM bei einer Zahlrate von etwa 70 pro Sekunde. Untersucht werden Edelstahlproben und heliumimplantiertes Nickel. In beiden Fallen erhalt man eine langere Lebensdauer von etwa 500 ps, die der Annihilation in Oberflachenzustiinden zugeschrieben wird. Die kurzeren Lebensdauern, die von der Zerstrahlung an oberflachennahen Einfangzentren herruhren, sind in bester ubereinstimmung mit den Werten aus konventionellen Lebensdauermessungen.

Defect identification in semiconductors with positron annihilation: Experiment and theory

“…The above difficulties related to the source-originated background level and to the source corrections can be largely avoided by performing the positron lifetime experiment with a timestamped positron beam (Schödlbauer et al, 1987;Suzuki et al, 1992). The data analysis is, however, faced with other challenges such as an inherently non-Gaussian resolution function (even if very narrow) in pulsed beams (Reurings and Laakso, 2007).…”

“…Hence, either a positron-fly-by-detecting sensor must be installed or the beam must be tagged (e.g., by detecting secondary electrons ejected from the sample surface by positron impact) or modulated in time in order to retrieve a timing signal. The modulation of the positron beam has been shown to be the approach of choice, but requires radio-frequency beam bunching and chopping that have their own complications (Mills, 1980;Schödlbauer et al, 1987;Suzuki et al, 1992;Tashiro et al, 2001;Reurings and Laakso, 2007). The advantage is a time resolution good enough for studying semiconductors and metals where the positron lifetimes are an order of magnitude shorter than in molecular matter.…”

Positron depth profiling in solid surface layers

“…However, this latter lifetime technique requires more sophisticated beam setups, where a clock starting signal relies on either i) a pulsed positron flux formed by combination of chopping and bunching elements [45][46][47] or ii) the emission of secondary electrons induced by positron impinging the target [48]. Table I summarizes the main features, for both bulk and sub-surface measurements, to be considered when a certain positron method has to be selected for materials analysis.…”

Advances in positron annihilation spectroscopy of Si, Ge and their alloys

“…To date a few successful positron beam projects have been completed [14][15][16] with a good enough time resolution for defect studies in crystalline materials.…”