Positron annihilation lifetime spectroscopy: When is it feasible to decompose the spectrum?

Positron annihilation lifetime spectroscopy (PALS) has the potential to determine open volume defect identities and concentrations only if the spectrum can be accurately decomposed into its constituent parts. The intrinsic difficulty of decomposing PALS spectra into their constituent lifetimes and intensities is demonstrated, and it is shown that the global minimum of the objective function does not represent the true solution for a range of typical experimental scenarios. We show that the function currently employed in standard fitting methods cannot be improved upon with alternate weighting schemes. Resolution function width minimally impacts fit decomposition quality but errors are reduced with higher counts. A regression model is developed based on the experimental count, intensity of the defect component, and difference between the defect and bulk lifetime which predicts the anticipated intrinsic error of the objective function global minimum in estimating the fraction of positrons which annihilate in the bulk. This can be employed to determine whether a given PALS spectrum can be successfully decomposed into defect types and lifetimes.