3-D defect density plots of large scale structural materials with positron annihilation spectroscopy: feasibility and optimization

We investigated the feasibility of the pair-production based positron annihilation spectroscopy (PAS) technique to generate 2-D and possibly 3-D defect density images (defect density plots) of structural materials widely used in industry. After assessing the possible PAS techniques and performing MCNPX simulations on different positron production mechanisms, we chose to employ Doppler broadening analysis of the annihilation peak coupled with the pair-production method of positron creation using bremsstrahlung photon beams from a linear accelerator. First, to optimize the important parameters of the proposed experimental setup, MCNPX simulations were performed. These parameters are: thickness of the bremsstrahlung beam hardener, detector orientation, thickness of the bremsstrahlung converter, beam energy and current, etc. These parameters were studied to maximize the number of 511 keV counts in the detector and minimize the low energy background photons that Compton scatter into the detector. A Figure of Merit (FOM) was used to quantitatively track effectiveness (or ineffectiveness) of each value for all the parameters. We conducted experiments with the simulated setup and these results were compared to MCNPX calculations.