Scintillation probe instability stabilized by automatic gain control circuit

Variation of ambient temperature of a scintillation probe, beside the variation of output pulse amplitudes by some constant coefficient, introduce also non-linear distortion of differential spectrum of the radiation. Other sources of non-linear distortion of the spectrum can be variation of amplification of fotomultiplier, when it operates with high pulse current densities. Variation of amplification of the probe by a constant coefficient can be stabilized by a circuit of automatic gain control (AGC), but non-linear distortion introduces deformation of the radiation spectrum shape, which results in variation of count rate at different discrimination levels. Measurements performed with scintillation probes containing plastic scintillators and controlled by AGC circuits showed that instabilities (0.1% - 0.2%)/100C are caused by non-linear distortion of radiation spectrum. This imposes limits to the achievable stability of AGC controlled probe. (author)