The only direct "sensor" of ionizing radiation I know if the Geiger-Müller tube. These are generally only available used or NOS (New, Old Stock) parts on the surplus electronic market. The tubes are often designed to only measure beta and/or gamma particles.
GM tubes are powered by a high voltage (low current) power source which makes them modestly complex if you do not have experience working with HV supplies.
The only other "sensor" I can imagine would be to adapt the ionizing smoke detector's ionization chamber, but as it is designed to detect a disruption of alpha particles (emitted from americium-241) caused by smoke absorbing the alpha particles, I don't know is such an approach is very feasible to detect all but high levels of ambient radiation (assuming you removed the Am-241).