Practical aspects of reverse fourier time-of-flight analysis of slow neutrons with mechanical beam choppers

Abstract

In the accounts so far published of slow-neutron time-of-flight (TOF) analysis by the Fourier-type techniques, only limited specific attention has been paid to the practical implications of employing ordinary mechanical choppers as neutron beam modulators. However, mechanical modulation not merely imposes certain operational restrictions but may also notably influence the performance of actual measuring systems. This paper is devoted to the analysis of such chopper-related aspects of Fourier TOF spectrometry. The treatment is confined mainly to the so-called reverse Fourier method, as distinguished particularly for its inherent on-line capability, but many of the results apply to the conventional technique, as well.Apart from some new results concerning the properties of practical frequency “windows” applicable to the Fourier method in general, the main conclusions from the analysis are: (1) the statistical accuracy attainable depends not only on the modulation wave form but also substantially on the background-to-signal ratio, and in this respect the standard saw-tooth pattern actually turns out to be one of the best choices for a mechanical chopper, (2) due to the usually non-zero minimum frequency of modulation, the resolution function will have a small but broad negative component which may present difficulties especially in measuring slowly varying TOF spectra and (3) the finite absorber thickness of the chopper always implies a limited range of neutron wavelengths where the modulation function has the desired form, but outside which its amplitude either decreases and unwanted background results (for fst neutrons) or its shape becomes distorted, in addition to amplitude reduction (for slow neutrons, particularly at high modulation frequencies).