An interaction of a positron, e.g. an antielectron, and an electron results in annihilation and gamma radiation is produced. Before this happens, the positron has been slowed down in the material. The positron life time, i.e. the time it takes until the positron combines with an electron in a material, depends on the properties of the material. If there are material defects present, e.g. vacancies or cracks, the positrons may be caught there and survive for a longer time. The positron life time can thus be used as a measure of micro defects in a material.

The positron beam

At the department of Nuclear Engineering, Chalmers University of Technology, a positron beam is under continuous development. This beam, which is one of at the most five in the world, is a pulsed positron beam with adjustable energy. The pulsing makes the positrons well defined in time, which makes direct life time measurements possible.

The energy of the positron beam is adjusted through an acceleration stage just before the sample chamber. In this way, the penetration depth in the material can be varied and the material defects can be analysed as a function of depth. The deposition depth is in the range of µm.

With the positron beam, it is possible to measure the positron life time spectrum, i.e. the number of annihilations, positron-electron combinations, as a function of time. A typical time spectrum consists of a sum of several exponentials.

The life time in the material without defects is the dominating exponential. The defects can be seen as superimposed exponentials. The intensity depends on the defect density and the decay constant depends on the defect kind. The interpretation of the time spectrum is not quite simple and demands both mathematical modelling of the positron transport (transport equations and Monte Carlo calculations) and experimental data on materials without defects.

Use

Positron technique can be used to analyze various material defects, from vacancies, empty atom positions in metallic crystalline structures (0.1 – 0.2 nm, a few hundred ps of life time) to macroscopic cavities and cracks. Internationally, positron measurements have earlier been used for

• Irradiated steel from nuclear power tanks
• Material samples exposed to mechanical stretching
• Polymers and semi conductors
• Micro porosity in zeolites (ion exchangers)

The beam is currently used for investigation of radition induced damage in steel within the framework of the VR project Genius and the EU project Getmat.

Anders Nordlund Nuclear Engineering
Chalmers University of Technology
Tel. +46 (0)31 772 3083 or 772 3080
E-mail: anders at nephy dot chalmers dot se