Positron Annihilation Lifetimes Spectroscopy

Positron Annihilation Lifetime spectroscopy (PALS) is a powerful tool for studying vacancy-type defects in solids. It is used in many different fields of materials science such as semi-conductors to determine vacancies in the microstructure or polymeric materials in order to measure the size of micropores.

The positrons from a sodium-22 (Na-22) source enter the materials and interact until they lose energy. When the positron has an energy of 100eV, it will combine with an electron from the materials to form a positronium. The positronium has a short life and will release two gamma rays of 511 keV. The time it takes to release the gamma rays is directly related to the size of the hole in the material.

Why use PALS?

The sizes of the holes in materials can be readily determined using a number of routine techniques, such as transmission electron microscopy (TEM), BET surface analysis, and positron annihilation lifetime spectroscopy (PALS).

TEM can give information at the atomic level but involves extensive sample preparation, and many materials of interest to this program are unstable under electron beam irradiation and high vacuum conditions. The BET technique uses inert gases to probe the porosity and surface area of materials. It gives information on pore sizes from 3 - 100 nanometres.  BET also requires high vacuum conditions, and can give information about pore connectivity provided it is connected to surface pores.

PALS can provide information on holes from as small as an atomic vacancy (0.2 nm) to those up to 10 nm in bulk materials or in films. The advantage of PALS is the minimal sample preparation, making it ideal for comparing changes in the properties of materials under going various processes.

The high end technologies used to characterise porosity in materials include: small angle neutron and X-ray scattering (SANS and SAXS), neutron and X-ray reflectrometry, and slow positron spectroscopy. The different nuclear probes used in these techniques (neutrons, x-rays, positrons and electrons) can yield valuable additional information. At ANSTO's Nuclear Sensors facility, we have a digital PALS system using BaF2 scintillators coupled to photomultiplier tubes.  We are developing two types of systems incorporating two and three detectors.