Higher-harmonics contamination in slow-neutron spectra measured with the Fourier time-of-flight method

Abstract

A thorough theoretical analysis is carried out in order to comprehensively assess the real significance of the higher-harmonics contamination eventually occurring in neutron Fourier time-of-flight spectra measured with standard saw-tooth choppers. The resulting general conclusion is that neither in the frequency domain nor in the actual time-of-flight domain can these distortions lead to any serious problems, even when a typical 4-channel phased detection system, representing the worst alternative in practice, is used for data acquisition. The predictions of the explicit formulae derived are found to agree well with the non-analytical and experimental evidence available of the higher-harmonics influence in both the above domains. A new result is the discovery that frame overlapping, as caused by a discrete frequency step between experimental Fourier data, can produce some negative apparent features in synthetized spectra, which may become detectable especially in certain high-accuracy diffraction applications of the standard Fourier time-of-flight method.