A system for detecting photon emissions wherein a detector serves to derive electrical parameter signals having amplitudes corresponding with the detected energy of the photon emissions and other signal generating events. Two comparator networks are employed within an energy window defining function to develop an output, L, when an event based signal amplitude is equal to or above a threshold value, and to develop an output, H, when such signal amplitude additionally extends above an upper limit. Improved reliability and accuracy is achieved with a discriminator circuit which responds to these outputs L and H to derive an event output upon the occurrence of an output L in the absence of an output H. This discriminator circuit is an asynchronous, sequential, fundamental mode discriminator circuit with three stable states.