Abstract: The detector system includes three substantially similar detectors each of which includes an ionization chamber having a pedestal therein mounting a charged electret. Two chambers are completely sealed except for an equalization aperture. The third detector has filtered inlet openings, which permit entry of radon gas, and responds to gamma rays and radon gas. Each detector is a canister including a lid having a round base surrounded by a cylindrical wall. The canister body is a circular base surrounded by a cylindrical wall that nests within the lid with a telescopic fit. A cylindrical pedestal assembly rests on the lid base and concentrically sandwiches the canister body between the pedestal assembly and lid. A press-fit holds these three elements together. The charged electret is mounted on the platform by a retaining collar that holds the electret with a press-fit.