PALSfit3: A software package for analysing positron lifetime spectra

The present report describes a Windows based computer program called PALSfit3. The purpose of the program is to carry out analyses of spectra that have been measured by positron annihilation lifetime spectroscopy (PALS). PALSfit3 is based on the well tested PATFIT and PALS fit programs, which have been used extensively by the positron annihilation community.

The present document describes the  mathematical foundation of the PALSfit3 model as well as a number of features of the program. The cornerstones of PALSfit3 are two least squares fitting modules: POSITRONFIT and RESOLUTIONFIT. In both modules a model function will be fitted to a measured lifetime spectrum.

This model function consists of a function representing the physics of the positron decay which is convoluted with the experimental time resolution function, plus a constant background.  The ‘physics function’ consists of a sum of decaying exponentials each of which may be broadened by convolution with a log -normal lifetime distribution.

The time resolution function is described by a sum of Gaussians which may be displaced with respect to each other. Various types of constraints may be imposed on the fitting parameters. In the POSITRONFIT module, the fitting parameters to be extracted from a measured spectrum are for each lifetime component its mean lifetime and its broadening as well as its intensity.

A correction for positrons annihilating outside the sample can be made as part of the analysis. In the RESOLUTIONFIT module, parameters determining the shape of the time resolution function can be fitted. The extracted resolution function may then be used in POSITRONFIT. Graphics displays are provided to ease the selection of some of the input parameters and to display results of spectrum analysis.

The results are also available in a text window. PALSfit3 is verified on Windows XP and Windows 7, 8 and 10. The PALSfit3 software can be acquired from the Technical University of Denmark (http://PALSfit.dk)