M. Dorikens

Positron lifetimes and lineshape factors in deformed copper

Measurements of positron annihilation lifetime and lineshape factors in deformed OFHC copper, for 14 different pairs of specimen, ranging up to 66% deformation, are discussed. The influence of annihilation in the source is carefully examined and the data are consequently corrected. The lifetime varies from (124±2) ps for annealed copper to (179±2) ps for dislocation saturated samples.Trapping cross sections and trapping efficiencies are calculated. The possibility of interference of vacancies formed during the deformation is discussed.

Analysis of double decay spectra by the SIMPLEX stepping method

The principle of a computer programme for the analysis of multiple decay spectra, as obtained in positron lifetime measurements, is described. It is based on the SIMPLEX algorithm. The performance of the analysis method is discussed.

Momentum distribution in metals from doppler-broadened annihilation lineshapes

Abstract The annihilation lineshape is measured with a Ge (Li) spectrometer for the elements Be, Mg, Au, C (graphite), Al, Ni and Cu. A method is described which corrects for the detector resolution. From the corrected data electron momentum distributions are calculated. The method offers the possibility of observing high momentum values.

Positron trapping in indium

The positron annihilation lineshape was measured in indium in the temperature range between 74 K and the melting point (430 K). A model based on self-trapping of positrons and trapping by vacancies was fitted through the data. A vacancy formation enthalpy of (0.48±0.03) eV was obtained.

Evidence for positron trapping in magnetism from dopplerbroadening measurements

Abstract Positron annihilation Dopplerbroadening measurements, display for the first time the effect of positron trapping in thermally induced vacancies in magnesium. The vacancy formation enthalpy is derived and is 0.85 eV.

The influence of geometry on the resolution of a positron annihilation lifetime spectrometer

A detailed study of the prompt resolution curve of a positron annihilation lifetime spectrometer shows that the geometry of the source-detector set-up has great influence on its shape.

Generation of slow positrons on an electron LINAC

A target and converter set-up for production of slow positrons on linear electron accelerators is described. It makes use not only of the pair-production processes but also of a positron source created on-line by nuclear reaction

Positron lifetime measurements inn- andp-type silicon

Positron lifetime measurements were carried out in four different samples of silicon, namelyn-type (P-doped) 75 Ωcm,n-type (Sb-doped) 0.018 Ωcm,p-type (B-doped) 60 Ωcm andp-type (B-doped) 0.02 Ωcm. The measurements were made at room temperature and at 77K. A positron lifetime of τ1=(230±2) psec was found for all samples, independant of dopant or temperature.

Lineshape factors and counting rate in annihilation radiation

Abstract Annihilation lineshape factor measurements are found to be influenced by the total counting rate observed by the Ge(Li) detector, and by the output pulse width of the main (semi-Gauss) amplifier. Counting rates have to be kept very low, typically below 2kHz.

Temperature dependence of positron annihilation parameters in solid gallium

Positron annihilation measurements between 80 and 300 K indicate that in gallium positrons are not trapped in thermally induced vacancies. Also no clear evidence is obtained for self-trapping of positrons in gallium. The slight temperature dependence of the lineshape measurements is probably due to thermal expansion.