Preliminary measurements of the prompt neutron decay constant in MASURCA

Abstract Pulse counting techniques have been used to measure the prompt decay constant α = (β - ϱ) / Λ in the MASURCA reactor of CEA at critical state. The data has been analyzed in time domain using Rossi-α and Feynman-α techniques, and in frequency domain using the cross power spectral density. The Rossi-α technique has been studied using one and two detectors. Due to the strong inherent spontaneous fission source, the one-detector variant gives a very strong white-noise signal, which is absent in the two-detector method. Because each neutron detected recorded not only a pulse, but also an echo after 120 ns, corrections had to be made to the theory applied. The Feynman-α technique is even more sensitive to the echo in the signals, and quite large corrections had to be made. Nevertheless the results obtained are in reasonable agreement with those of the correlation methods. For both measurement techniques, experiments of long duration are needed to get accurate results. The results obtained agree within 10% with calculations. The prompt decay constant has also been measured with a continuous current technique. From the cross power spectral density thus obtained, the α-value is in agreement with that of the pulse counting techniques.