Development of a New Positron Lifetime Spectroscopy Technique for Defect Characterization in Thick Materials

The recent development of Accelerator-based Gamma-induced Positron Annihilation Spectroscopy (AGPAS) has shown the possibility of probing residual stress in thick materials by Doppler broadening measurements [1]. In those measurements, the residual stresses were reflected in the line shape parameters of the 511 keV annihilation peak. However, since positron lifetime spectroscopy is a powerful tool to distinguish between different types of defects, such as dislocations and vacancy clusters [2], it is crucial to enable positron lifetime measurements in AGPAS. In this work, a new method is developed to conduct positron lifetime measurements on thick engineering materials using accelerators. By focusing 1.5 MeV protons from a Van de Graff accelerator on a thin Al window, coincident γ-rays of 2.8 MeV and 1.78 MeV are induced through (p,γ) reactions. The 1.78 MeV quantum provides a start signal for the positron lifetime spectrometer, where as the 2.8 MeV quantum bombards the material under investigation. This, in turn, creates a positron, which annihilates with one of the material electrons emitting two 511 keV photons. The stop signal for the positron lifetime is provided by the detection of one of the two 511 keV photons. The measured positron lifetime spectrum depends on the electron densities and hence provides information about the size of open volume defects. The method enables positron lifetime measurements in thick engineering materials up to tens of gm/cm, a thickness not accessible by conventional positron lifetime spectroscopy.