Measurement of Reactor Fluctuation Spectra and Subcritical Reactivity

Fluctuation spectra of the current from a neutron-sensitive ionization chamber located in a reactor have been determined experimentally with increased precision. The cold and clean Bulk Shielding Reactor core was held critical and at various subcritical reactivities up to ρ = −10β. The amplified current fluctuations were processed on-line in a ten-channel spectrum analyzer utilizing analog-computer components. The shapes of the spectra, and the values of neutron generation time and reactivity inferred therefrom, were in good agreement with theoretical predictions and with values measured using the pulsed-neutron technique. The results were the same for the two detector locations investigated.