Intrusion detecting device

An intrusion detecting device comprises a grid of tubes spaced apart by a distance d sufficient to impede an intruding body. Each tube contains a radioactive luminescent source which transmits radiation that is confined to a narrow beam by a series of aligned apertured masks. A detector receives the radiation and provides an input to an AND-gate connected to alarm circuitry. The radiation is diverted from the detector if the respective tube is deformed or deflected hence providing a signal to indicate an intrusion. The tubes may form part of a device for preventing an intruder from gaining access to a security zone. Alternatively, the tubes may be flexible to enable the passage of, e.g. fish to be counted. An alternative embodiment employs intercommunicating tubes with mirrors for guiding radiation from a common source to a common radiation detector. (author)