[en] The present invention concerns a sensor for alfa particles, applicable for maintenance of a constant pressure. The sensor consists of a chamber connected to a predetermined volume, a monochromatic alfa source, hemispheric scintillation detector, a photomultiplier, recording device and an executive unit. The sensor operates on the following basis: the monochromatic alfa source is situated on a hemispherical surface concentrically to the scintillation detector at a distance equal to the alfa particles mean free path at the sensors working pressure