A solid scintillator member is provided in the internal space of a container. The scintillator member is an aggregate of a plurality of pellets. The internal space also confines a gas produced through the vaporization of a liquid sample containing a radioactive substance. When radiation emitted from a plurality of particles within the gas reaches the scintillator member, light is generated. That light is detected by a pair of photomultipliers. A plurality of particles may be produced outside of the container and introduced into the container.