Positron Annihilation Lifetime Spectrometer


The Positron Annihilation Lifetime Spectrometer (PALS) is one of the first techniques to directly measure molecular free volume and may be the only one which can assess the size of free volume sites as well as the concentration of these sites; hence PALS can assess the distribution of molecular free volume or nanoscale defects in a material.  It is nondestructive and has an amazing range of application as shown in the figure below.

Immediately obvious from this figure, is that, with respect to free volume or defect site size, PALS is a perfect complement to many conventional techniques.  At the same time, with respect to the free volume or defect concentration, it encompasses all these other techniques.  Combining the size and concentration of free volume sites provides the distribution of free volume which is not available by any other technique, making PALS an extremely powerful means of characterizing a broad range of materials and material properties.


CNSE’s PALS uses sub-atomic anti-matter particles to probe the molecular free volume in almost any substance. The probing entity is a positron, which is the anti-matter particle of the electron and is consequently extremely sensitive to differences in electron density.


Click play to see an animation of how the spectrometer works.

The electron density in a material is affected by the packing density of atoms or molecular chains, the degree of interaction in nanoscale interfacial regions, and the population of nanoscale defects and molecular dislocations.  The material can be a gas, liquid, or solid of almost any other technique.

Click play to see an animation of what happens when a positron enters a sample.

Molecular free volume fundamentally and profoundly impacts many physical properties and processes.  Theory has been developed that links free volume to:

  • Phase transitions, even in highly crystalline materials
  • Changes in mechanic properties as a function of temperature
  • Degree of crystallinity in polymers
  • Polymer blend and interpenetrating polymer network morphologies
  • Influence of molecular weight on mechanical properties in polymers
  • Permeation and diffusion
  • Nano-structural changes due to deformation
  • Plasticization and anti-plasticization
  • Aging and weathering effects
  • Crosslink density and degree of cure
  • Intrinsic viscosity