A detector for use in cementing a tubular string in a wellbore includes: a tubular mandrel; an electronics package fastened to an outer surface of the mandrel; a first transducer: fastened to the mandrel outer surface, in communication with the electronics package, and operable to generate ultrasonic pulses; a second transducer: fastened to the mandrel outer surface, in communication with the electronics package, and operable to receive the ultrasonic pulses; and an antenna fastened to the mandrel outer surface and in communication with the electronics package.