The principle of the branched fiber bundle sensor has been known for many years. The apparatus which is described in this paper is distinguished by simple fabrication, low cost, high sensitivity, and a fast response time. It is used for measuring the velocity of the back surface of an irradiated aluminum target. In spite of intense consecutive bremsstrahlung, the plastic fiber probe gives good results, whereas with glass fiber a large scintillation impulse appears which covers the signal.