Count-loss effect due to time interval between counting-gates in Feynman-α method

Abstract

The Feynman-α (variance-to-mean ratio) method is a neutron noise analysis based on the statistical characteristics of neutron counts accumulated within counting-gate time (gate width). The kinetic parameters of reactor physics are evaluated from the gate width dependence of these characteristics, so that we need to accumulate many neutron count data for various gate widths. Hence huge measuring time is required to get the kinetic parameters with good accuracy. To save measuring time, we make use of the bunching technique proposed by T. Misawa, in which we accumulate time series data of neutron counts within a fundamental gate width T by using a multi-channel scaler device having dwell time T, and synthesize any data for longer gate widths by bunching these data. When the multi-channel scaler device is used in this bunching technique, count loss time exists between adjacent channels because of channel advance time Δ. Although this channel advance time is generally small, it seems that loss of correlated neutron counts brings about a serious problem. In this report, therefore, the rigorous formula containing the channel advance time was derived for a single core system based on the multi-gate Pal-Bell equation without delayed neutrons, in order to investigate the effect due to the loss of correlated neutron counts. From numerical study for a light water moderated-core, it was concluded that the influence of the loss of correlated counts depended on only the channel advance time-to-fundamental gate width ratio Δ T , and it became prominent as Δ T value increased.