Graphene-based Positron Charge Sensor

We utilize a graphene field-effect transistor to measure back-gate charging by positrons. The device consists of an exfoliated graphene flake transferred on to hexagonal Boron Nitride, placed on a 1 cm2 substrate of 500um thick conducting p-Si capped by 285 nm-thick SiO2. It is placed at close proximity to a 1u Curie 22Na positron source emitting a constant flux of positrons which annihilate within the back-gate. We demonstrate that when the back-gate is allowed to float, the charging current of ~40fA 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.