Gamma-induced positron spectroscopy at a superconducting linear accelerator

A new and unique setup for Positron Annihilation Spectroscopy has been established at a superconducting linear electron accelerator at Helmholtz-Zentrum Dresden-Rossendorf (Germany). The accelerator runs in continuous wave mode with variable bunch repetition rates up to 26 MHz delivering pulsed bremsstrahlung with energies up to 16 MeV. After collimation the photon beam impinges onto the sample where positrons are generated by means of pair production throughout the entire volume. Short gamma bunches below 5 ps duration allow for positron lifetime spectroscopy using the accelerator’s radiofrequency as time reference. Positron lifetime and Doppler broadening Spectroscopy are employed by a coincident measurement (Age-Momentum Correlation) of the time-of-arrival and energy of annihilation photons which in turn significantly reduces the background of scattered photons resulting in spectra with high signal to background ratios. Monte-Carlo simulations of the entire setup using the GEANT4 framework have been performed in order to yield optimum positron generation rates for various sample materials and improve background conditions.
The production of positrons inside the sample allows for experiments using bulk samples, gases, fluids, and even samples with high intrinsic radioactivity which would be hampered by accidental coincidences in source-based lifetime spectroscopy systems. Positron lifetime spectroscopy results will be presented for water, lead, activated reactor steel, and biological samples, as well.