PURPOSE:To detect an integral dosage of radiant ray and to generate an alarm by monitoring the charcteristic parameter of an MOSFET. CONSTITUTION:A processing circuit 40 is stored previously with the irradiation characteristic of the N-MOSFET14. A gate voltage setter 30 controls a voltage generator 20 corresponding to a set integral dosage for alarm generation and applies a gate voltage to an FET14. Consequently, the processing circuit 40 detects a change of the load resistance voltage of the drain from a nonconductive state to a conductive state to generate an alarm and also display the generation of the alarm on a display part 50 simultaneously.
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