A sensor for detecting particles is presented comprises a silicon wafer substrate and a charge detection layer mounted on the silicon wafer substrate, wherein the charge detection layer comprises a plurality of discrete pixel sensors. The sensor further comprises a converter material, wherein the converter material is operable to interact with a first type of particle to generate a reaction, wherein the reaction produces charged particles, wherein the charge detection layer is configured to detect the charged particles produced by the reaction. Further, the sensor comprises a substrate layer operable to filter a second type of particle, wherein the converter material is coated on an underside of the substrate layer such that the converter material faces the charge detection layer and an air gap is formed between the converter material and the charge detection layer.