An apparatus for detecting at least one of neutron and gamma ray reception and outputting a signal indicative of the reception. A detector is responsive to the at least one of neutron and gamma ray reception. The detector has a cathode, an anode separated by a space from the cathode, and a gas within the separating space. Charge is generated within the gas upon the at least one of neutron and gamma ray reception at the cathode and the charge passes to the anode as a detection. A processing arrangement is operatively connected to the anode for outputting the signal indicative of the detection. A light irradiation arrangement for introducing a light irradiation causes charge within the gas that replicates the charge generated upon detection and that causes output of a signal that replicates the signal indicative of the detection.