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Performance of a High-Resolution Digital Positron-Lifetime Spectrometer

100%

Bečvář F., Čížek J., Procházka I.

Acta Physica Polonica A

2008

vol. 113

issue 5

1279-1284

This work describes a digital positron-lifetime spectrometer based on two ultrafast digitizers. Performance of the digital spectrometer in conditions of a routine measurement is compared with an analogue setup connected to the same detectors. It was found that engagement of the digital setup leads to an improvement of timing resolution (FWHM for ^{22}Na) from 169 ps for the traditional analogue setup to 146 ps for the digital setup. The coincidence count rate for 1.2 MBq ^{22}Na source is 75-80 coinc./s for both configurations. These results clearly demonstrate the advantages of the digital timing over the traditional analogue setup.

Electrical Resistivity and Positron Lifetime Studies of Precipitation Effects in Al-Cu Alloy

76%

Αda P., Cieslar M., Vostrý P., Bečvář F., Νovotný I., Procházka I.

Acta Physica Polonica A

1995

vol. 88

issue 1

111-117

Decomposition of the supersaturated solid solution of 2 at.% Cu in Al was investigated by means of electrical resistivity and high-resolution positron lifetime measurements. The phase composition of this alloy was determined by transmission electron microscopy. Electrical resistivity measurements were performed by a classical four-point method. Positron lifetime spectra were measured by means of a spectrometer consisting of two BaF_{2} detectors and a standard fast-slow coincidence system. The specimens were first exposed to the solution heat treatment at 783 K for 19 hours with a subsequent quenching. Then the specimens were isochronally annealed in the temperature range 293 ÷ 573 K. Annealing responses of electrical resistivity and positron annihilation were studied. Resistivity measurements as well as transmission electron microscopy observations confirmed the well-known decomposition sequence of the Al-Cu alloy. The decomposition of the alloy was manifested by the decrease in the intensity of positron lifetime component τ_{2} = (207 ±2 ) ps correlated with simultaneous appearance and increase in the intensity of τ_{3} = (180 ± 5) ps component. Component τ_{2} originates from positron annihilation in vacancies trapped at the Guinier-Preston zones while τ_{3} comes from annihilation of the positrons localized in the misfit dislocations at coherent precipitates of the Al_{2}Cu phase. The shortest observed component τ_{1} apparently belongs to annihilation of untrapped positrons.

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2 Acta Physica Polonica A

1 2008

1 1995

2 Bečvář F.

2 Procházka I.

1 Cieslar M.

1 Vostrý P.

1 Čížek J.

1 Αda P.

1 Νovotný I.

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Performance of a High-Resolution Digital Positron-Lifetime Spectrometer

Electrical Resistivity and Positron Lifetime Studies of Precipitation Effects in Al-Cu Alloy