Ortho-positronium lifetime measurement – positron source activity and statistics

As a non-destructive method positron lifetime annihilation spectroscopy is used to investigate porous materials. The long lifetime component in the spectrum which is caused by the annihilation of positronium provides information about the pore size. The result of the analysis of the long lifetime component is affected by the activity of the positron source. Monte-Carlo simulations were done to find the suitable positron source activity for different positronium lifetimes. First of all, spectra with different total count numbers were simulated. A total count number of 4 × 106 was found to be a good compromise. Secondly, spectra with different positron source activities (i.e. different background) were simulated for several lifetimes. The differences between the known expected (simulated) lifetime and the analysed mean lifetime obtained in the fit were evaluated. The aim was to achieve a standard deviation Δτ that complies with Δτ/τ reaches a maximum of 1%. As conclusion we found the maximum source activity that complies with our aim as a function of the lifetime τ.