We utilize a graphene field-effect transistor to measure back-gate charging by positrons. The device consists of an exfoliated graphene flake transferred onto hexagonal Boron Nitride, placed on a 1 cm2 substrate of 500 μm thick conducting p-Si capped by 285 nm-thick SiO2. It is placed at close proximity to a 25 μCi 22Na positron source emitting a constant flux of positrons, which during the measurement annihilate within the back-gate. We demonstrate that when the back-gate is allowed to float, the charging current of ≈20 fA causes the buildup of positive charge which capacitively couples to the graphene device and is detected as a variation in the two-terminal conductance. Furthermore, a prolonged exposure to positrons causes a shift in the graphene transport characteristics, associated with local charges at the immediate environment of the graphene flake. Our results demonstrate the utility of two-dimensional layered materials as probes for charging dynamics of positrons in solids.
A slow positron facility is being built in Israel, at the Hebrew University, for basic and applied research. It consists of a slow positron beam and a compact Positrons Annihilation Lifetime (PAL) spectrometer. The slow positron beam follows a traditional design, using a 22 Na source, of about ~1.5 GBq (40 mCi), a Tungsten moderator and a unique grounded target cell, with positrons energy that can vary between 0.03 keV and 30 keV. The detection system will be comprised of High Purity Germanium and BaF2 detectors, facing each other, for low background Doppler Broadening (DB) measurements. The target cell is designed to allow a combined measurement of sample conductivity and DB, with the flexibility to add more detection options in the future. The compact PAL spectrometer includes two fast scintillation detectors read by a fast digitizer (DRS4), with a sampling rate of 5.12 GS/s. A dedicated software package was developed to emulate analogue data acquisition. Lifetime measurements were performed using a ~25Ci 22 Na source. The time resolution was defined using a 60 Co source, to be 180-200 ps. First positron lifetime validation measurements of Ti resulted in positrons lifetime of 157±4 ps, consistent with previously published values.
The MUon Scattering Experiment, MUSE, at the Paul Scherrer Institute, Switzerland, investigates the proton charge radius puzzle, lepton universality, and two-photon exchange, via simultaneous measurements of elastic muon-proton and electron-proton scattering. The experiment uses the PiM1 secondary beam channel, which was designed for high precision pion scattering measurements. We review the properties of the beam line established for pions. We discuss the production processes that generate the electron and muon beams, and the simulations of these processes. Simulations of the π/μ/e beams through the channel using TURTLE and G4beamline are compared. The G4beamline simulation is then compared to several experimental measurements of the channel, including the momentum dispersion at the intermediate focal plane and target, the shape of the beam spot at the target, and timing measurements that allow the beam momenta to be determined. We conclude that the PiM1 channel can be used for high precision π , μ, and e scattering.
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