The present invention provides a sensor for detecting the build up of frozen deposits in a cryogen vessel housing a superconducting magnet, comprising a tensioned wire, a source of variable frequency alternating current and a voltage sensor. The tensioned wire is oriented perpendicular to the direction of the stray magnetic field produced by the superconducting magnet. By varying the frequency of the applied current, the resonant frequency of the tensioned wire may be detected as the frequency at which the voltage across the wire is a maximum. Any variation in the frequency or magnitude of the resonant peak may be interpreted as an indication of a frozen deposit hampering the free oscillation of the tensioned wire.