Pulse shape measurement and experimental validation of noise theory developed under consideration of finite pulse width in D-T neutron generator driven subcritical system

Abstract

Degweker and Rana (2007) had postulated a noise theory assuming the external neutron source as exponentially correlated Gaussian character for finite pulse width comparable to prompt neutron decay constant of subcritical system. They considered various conditions of source correlation time (i.e. the decay time of correlated neutrons), accelerator pulse width and pulse period. This noise theory has been validated experimentally for the first time in D-T neutron generator coupled BRAHMMA subcritical system. The present work has been carried out under the consideration that the source correlation time is small compared to the pulse width and rectangular pulse shape. The pulse shape of the neutron generator was characterised through an AC Current Transformer (ACCT) placed in the D+ beam path of the neutron generator. Variations of source correlation time with pulse widths were studied; it has been observed that source correlation time decreases with smaller pulse width. In validation experiments, data have been analysed using Auto Covariance, Variance to Mean ratio, and Auto Power Spectral Density method. The measured prompt neutron decay constant was found to be in good agreement with the theoretical estimate of the system. Furthermore, the present research also discusses the modulation of the correlated and uncorrelated part in variance to mean ratio method for a wide range of accelerator frequency at fixed source strength. The present research findings also lay the criteria for choosing the proper accelerator frequency for reactivity measurement in ADS.