On the Applicability of 48V in Positron Annihilation Lifetime Spectroscopy

The applicability of the 48V isotope as a source of positrons for positron annihilation lifetime spectroscopy (PALS) measurements is discussed. It has been demonstrated that using such a positron source, the classical PALS setup with two detectors does not accurately determine the positron annihilation parameters of the samples being studied. This issue arises when one of the two nuclear gamma rays (of energies of 983 and 1312 keV) that are emitted almost simultaneously with the creation of a positron triggers a start signal, while the other nuclear gamma triggers a stop signal instead of the 511-keV annihilation quanta. These events manifest as prompt coincidences in the start-stop histogram, rendering the analysis of PALS spectra unreliable. To address this problem, a modification to the classical PALS spectrometer was proposed and tested. This modification involved incorporating a logic branch that significantly reduced the undesired prompt coincidences between the 983- and 1312-keV gamma rays. By conducting measurements on a series of samples utilizing 2- and 25- $\mu $ m-thick Ti foils enriched with 48V, it was demonstrated that the altered setup reliably extracts accurate information on the positron annihilation states within the samples being examined.