Positron annihilation lifetime and Doppler broadening spectroscopy at the ELBE facility

Abstract

The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA with bunch charges up to 120 pC. The electron beam is employed to produce several secondary beams including X-rays from bremsstrahlung production, coherent IR light in a Free Electron Laser, superradiant THz radiation, neutrons, and positrons. The secondary positron beam after moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials science are performed. The adjustable repetition rate of the continuouswave electron beams allows matching of the pulse separation to the positron lifetime in the sample under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to perform depth resolved defect spectroscopy and porosity studies especially for thin films. Bulk materials, fluids, gases, and even radioactive samples can be studied at the unique Gamma-induced Positron Source (GiPS) where an intense bremsstrahlung source generates positrons directly inside the material under study. A 22 Na-based monoenergetic positron beam serves for offline experiments and additional depth-resolved Doppler-broadening studies complementing both accelerator-based sources.

Voltage-driven motion of nitrogen ions: a new paradigm for magneto-ionics

“…The corresponding relative intensities (I i ) reflect to a large extent the concentration of each defect type. In general, positron lifetime (τ i ) is directly proportional to defect size, i.e., the larger the open volume, the lower the probability and the longer it takes for positrons to be annihilated with electrons [42][43][44][45][46][47] . The positron lifetime and its intensity are probed as a function of positron implantation energy E p or, in other words, implantation depth (thickness).…”

Vacancy-Hydrogen Interaction in Niobium during Low-Temperature Baking

“…Measurements in the presented work were taken in a collaborative effort using four different setups, which were used to cross-check individual measurements but also complemented each other. Depth resolved VEPAS studies of sub-surface region were carried out at the Helmholtz Zentrum Dresden-Rossendorf on a continuous slow positron beam 'SPONSOR' 45 and ' AIDA' 46 employing DB spectroscopy (DB-VEPAS) as well as on a pulsed positron beam MePS 47,48 of the ELBE facility employing positron lifetime measurement (VEPALS). The spot size diameter of the positron beams were in the order of 3-5 mm and the energy of incident positrons was variable in the range 0.03-36 keV for DB-VEPAS (it corresponds to the mean positron penetration depth into Nb from 0.02 to 1380 nm) and 1-10 keV for VEPALS (it corresponds to the mean positron penetration depth into Nb from 4.7 to 394 nm).…”

Positron production using a 9 MeV electron linac for the GBAR experiment

“…Furthermore, the half-life of 22 Na is 2.6 years, thus the lowenergy beam intensity reduces over time and the source requires periodic and complicated replacement. Devices using nuclear reactors [27,28,29,30] or large accelerator facilities [31,32] can potentially provide a much higher positron flux. However, for the GBAR experiment, where a high positron intensity is crucial, a dedicated facility, with moderate size and cost, is the only feasible solution.…”