L. Dorikens-Vanpraet

Evolution of the positron annihilation lifetime for ageing in β phase Cu-Al-Ni-(Ti)-(Mn) shape memory alloys

During the last years, the positron annihilation technique has proven to be a sensitive tool for the characterization of phase transitions, and in particular of the martensitic transformation in Cu-based alloys. The differences in structure between the high temperature phase (cubic) and the martensite (monoclinic) produce distinctive positron annihilation characteristics which allow the determination of the transformation temperature range. In the present article the influence of these parameters on the positron annihilation characteristics of three Cu-Al-Ni shape memory alloys is discussed. Two of the chosen compositions contain Ti and Mn, which have been added to the ternary Cu-Al-Ni alloy in order to improve its mechanical properties. The effect of those elements has also been evaluated.

Positron-annihilation study of defects in the cyclically transformed martensite phase in a cu-zn-al alloy

Positron annihilation lineshape and lifetime measurements showed that by thermal cycling through the martensitic phase transformation, defects are introduced in Cu-Zn-Al alloys. It was concluded that mobile and immobile defects were introduced, most probably vacancies and dislocations.

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

Monte‐Carlo simulation of the positron production at a linac‐based slow positron beam

At accelerator‐based slow positron beams the slow positron yield depends on the efficiency of the primary production of positrons by the pair‐production process in the converter. This efficiency depends on the electron energy and the converter thickness. No simple relation exists which correlates the converter thickness to the energy of the impinging electrons. We performed Monte‐Carlo simulations as a function of the electron energy and the thickness of the converter to obtain the optimum thickness of the converter. The angular spread and the energy distribution of the outcoming positrons was also studied.

Positron Doppler broadening measurements with a LINAC-based slow positron beam

All the LINAC-based slow positron beams are pulsed. In order to perform Doppler broadening measurements one needs a continuous distribution of the slow positron intensity. This intensity has to be kept as constant as possible. A smearing out of the positron intensity with a Penning trap is described. The first depth profiling results are presented.