Power Spectral Analysis For A Subcritical Reactor System Driven By A Pulsed Spallation Neutron Source In Kyoto University Critical Assembly

For a subcritical reactor system driven by a periodically pulsed spallation neutron source in Kyoto University Critical Assembly, a series of power spectral analysis on frequency domain is carried out to determine the prompt-neutron decay constant and quantitatively to confirm a non-Poisson characteristics of the neutron source. Not only the cross-power but also the auto-power spectral density has a considerable correlated noise component even at a deeply subcritical state, where no correlated component could be observed under a pulsed DT(14MeV) neutron source. The non-Poisson character of the spallation source must enhance the correlation amplitude of these power spectral densities. Moreover, the Degweker's factor (m (2)-m (1) (2))/m (1) (2) of 0.082 +/- 0.021, which is a quantitative indication of the non-Poisson character, is determined from the present analysis and is consistent with that obtained by a previous Rossi-alpha analysis for the same subcritical system driven by the spallation source. The non-zero value of the factor convinces us again that the present source has a different statistical distribution from the Poisson. The prompt-neutron decay constant obtained from the present analysis well agreed with that done from the above previous Rossi-alpha analysis.