HAVAR foils are used in the medical industry as a window material for the production of 18FDG for PET scans. First comparative measurements of HAVAR foils, 25 μm thick, are presented. Three samples were measured: cold rolled (CR), annealed (AN), and proton irradiated (IR). These HAVAR foils were studied by means of Slow Positron Implantation Spectroscopy (SPIS), Positron Annihilation Lifetime spectroscopy (PAL), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). TEM and XRD results show that HAVAR has a fcc structure with a small amount of dislocations in the AN sample and a high density of dislocation nets in the CR sample. The positron diffusion lengths, extracted from the SPIS measurements, are ∼8 nm and ∼66 nm in the CR and AN samples, respectively, in agreement with TEM observations. The results of PAL measurements show significant differences between positron mean lifetimes in the three samples. Differences of ∼50 ps and ∼70 ps were measured between the mean lifetime in the AN sample and these in the CR and IR samples, respectively. GEANT4 simulations were used for the first time in PAL analysis. The simulation method and its benchmarking against previous measurements are described. Lifetime results obtained using conventional PAL analysis and GEANT4 based analysis are consistent within uncertainties for both the HAVAR and a Si reference sample.