NOVELTY - The sensor has a housing (11) comprising flat vibration and reflection plates (9, 7) perpendicular to side walls (12) and parallel to each other so that the housing forms a cavity (10) between the plates. Fluid connection (3) connects the cavity to an inner space (2) of the housing so that gas (20) is contained in the housing and the cavity during operation of sensor. A transducer (5) is coupled to the vibration plate for generating an acoustic signal to vibrate the plate (9) and gas in the cavity and for detecting acoustic characteristic response signal of the plate (9) and the gas.    USE - Acoustic sensor for continuous and real time measurement of physical parameter i.e. behavior, of gas e.g. fission gas such as helium, krypton, xenon, contained in an inner space of a chamber i.e. nuclear fuel pencil (claimed), of radiation reactor to produce energy. Can also be used for measuring molar mass of gas to deduce mass composition of helium-xenon mixture in the fission gas in the nuclear fuel cell in experimental reactor and production reactor.    ADVANTAGE - The sensor has reduced size so as to be used in nuclear fuel pencil holder of irradiation reactor. The sensor is used in a simple manner, and avoids a need of analysis laboratory of fission products in reactor protecting installation. The sensor permits characterization of release of fission gas in the nuclear fuel pencil in continuous and in real time and measures the behavior of gas in the fuel pencil in real radiation conditions of fuel in electric reactor. The sensor allows maintenance of nominal pressure conditions and space in the fuel pencil and cooling of the fuel pencil and assures physical separation between the fission gas and fuel pencil environment.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of an assembly comprising a gas chamber and a sensor connected to chamber.    Inner space (2)    Fluid connection (3)    Transducer (5)    Reflection plate (7)    Vibration plate (9)    Cavity (10)    Housing (11)    Sidewalls (12)    Gas (20)