Assessment of detector calibration materials for SANS experiments

The properties of incoherent-scattering samples that are used as calibrants for small-angle neutron scattering (SANS) experiments have been investigated using time-of-flight analysis of the wavelengths of the scattered neutrons. Such samples are employed both as secondary intensity standards and to calibrate the relative efficiency of detectors. The data show that care must be taken to allow for wavelength dependence of the detector efficiency if the measured results are to be understood or compared with respect to different instruments. At ambient temperatures, glassy polymer sheets of poly(methyl methacrylate) or polycarbonate offer better performance as secondary standards than water. The results reveal the limits for the use of transmission measurements to estimate incoherent scattering cross sections. Use of uniform scattering as a basis for a measure of the effective cross section is, in practice, instrument dependent, owing to extensive inelastic multiple scattering.