PURPOSE: To prevent a preamplifier from being saturated at a high DC input level by setting a DC gain through DC feedback of the output from a count diamond to an FET through the preamplifier. CONSTITUTION: A probe 10 comprises a preamplifier including a count diamond 12 and FET TR1, TR2. DC component of the output from preamplifier on the leads 20, 22 is amplified through a wide band amplifier 26 and fed, as a pulse mode signal, to a pulse counter 34 through a low-pass filter 28, an output amplifier 30 and a pulse peak comparator 32. It is also fed to a common mode removing stage 36 and a DC buffer amplifier 38 in order to indicate a conduction mode signal from the diamond 12. Output from the removing stage 36 fed back through a lead wire 24 to the FET TR1 and the DC gain of preamplifier is set while preventing the input stage from being saturated at the time of measuring a high level nuclear radiation.
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