High accuracy measurement of the prompt neutron decay constant in CROCUS using Gamma Noise and bootstrapped uncertainties

Abstract

We experimentally demonstrate the advantages of gamma detection for noise measurements to determine the prompt neutron decay constant α of a nuclear reactor using the power spectral density (PSD) method, coupled to a new uncertainty estimation scheme based on bootstrapping. We compare the relative precision of gamma noise measurements and neutron noise measurements performed in the CROCUS research reactor. The reference neutron noise experiment employs two large 235U fission chambers, whereas the new gamma noise experiment is based on two CeBr3 scintillators. The results demonstrate that gamma noise is superior to neutron noise with respect to precision, giving an advantage with regards to measurement time to reach desired uncertainty levels: the uncertainty budget shows a relative uncertainty of 3.6% and 1.3% on the prompt neutron decay constant with neutron and gamma noise, respectively. Moreover, thanks to bootstrapping, the reporting of full distributions – rather than assuming Gaussian spreads – of kinetic parameters is shown to be relevant in order to provide an accurate representation of the experimental results in some cases. In addition, we notably discuss the lower level discrimination threshold and its effect on the gamma PSDs. Gamma ray cross PSDs exhibit a behaviour contrary to that of a standard neutron acquisition system, and we discuss the implications. Gamma noise could prove to be a cheaper, more precise, and more flexible method to determine reactor kinetics.