To enable a critical accident to be detected by a single type of detector by achieving sensitivity to neutrons as well as sensitivity to γ rays and by detecting both radiations of γ rays and neutrons without any difference.

The critical detector comprises: a γ-ray detector 10 for detecting incident γ rays; a neutron moderator 12 for moderating neutrons entering from the outside for converting to thermal neutrons while being installed so that the γ-ray detector is covered; and a thermal neutron-captured γ-ray converting material 14 interposed at a boundary between the neutral moderator and the γ-ray detector so that the entire γ-ray detector can be surrounded. Detected output from the γ-ray detector is converted to current by a current conversion section (photomultiplier tube 18), and the converted current is measured by a current measurement section (ammeter 20) for operating in a current mode.

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