Single crystal filters for neutron spectrometry

Abstract

Measurements of the effective absorption coefficient I" have been made, for a range of neutron energies, of large single crystal specimens of quartz, silicon and sapphire, at 300 K and 80 K. The effectiveness of such filters is given by the ratio of I" for (unwanted) epithermal neutrons to that for the desired thermal neutron beam. Our results indicate that sapphire is significantly more effective than either quartz or silicon for a wide range of neutron energies.

The neutron transmission of single-crystal sapphire filters

“…They have found that a 90 mm-long sapphire-crystal filter has a transmission of about 0.8 for wavelengths in the range 0.12-0.24 nm and 0.07 for epithermal neutrons. These transmission results are in good agreement with those obtained by Nieman et al (1980). However, unlike Nieman et al, who suggest that the effective attenuation coefficient can be minimized by fine-tuning the crystal orientation, Born et al have found that there is no need to tune the crystals for every wavelength.…”

“…There is good agreement between the calculated 0021-8898/93/030438-10506.00 © 1993 International Union of Crystallography neutron cross sections and the experimentally determined cross sections for several materials, e.g. single-crystal silicon at both 300 and 77 K. There are, however, substantial differences between his results for sapphire and the experimental results of Nieman et al (1980), especially at neutron energies greater than about 100 meV. Freund suggests that improvements in the quality of sapphire single crystals will reduce these differences and thereby decrease the problems associated with elastic scattering of the higher-energy thermal neutrons.…”

“…5 is a comparison to the data of Tennant (1988) taken at specific wavelengths at both 300 and 80 K. The present data show a slightly lower attenuation than the 300 K data of Tennant, which is perhaps indicative of the degree of perfection of the crystals. The transmission data of Nieman et al (1980) were probably taken using single-crystal sapphire with a much larger mosaic and lattice distortion than is found in the presently available crystals that are used for filter material. Our results show that the elastic Bragg peaks are drastically reduced for high-quality sapphire.…”

“…It is also an effective filter of thermal neutrons with wavelengths less than about 0.1 nm, since there is a great density of high-order reflections available to scatter the incident beam. Nieman, Tennant & Dolling (1980) have measured the wavelength dependence of the total neutron cross section for single-crystal sapphire. They have shown that high-quality single-crystal sapphire at room temperature is a significantly more efficient thermalneutron filter, with higher transmissions for wavelengths longer than 0.1 nm, than either single-crystal silicon or quartz (SIO2), even when the latter materials are cooled to liquid-nitrogen temperature (77 K).…”

Adapting a triple-axis spectrometer for small angle neutron scattering measurements

“…A sapphire filter is optionally used for reducing the presence of "fast neutrons." 15 Therefore, depending on the chosen wavelength, either a sapphire ͑ Յ3.99 Å͒, or Be ͑ Ͼ3.99 Å͒ filter ͑component 1 in Fig. 1͒, cooled to liquid nitrogen temperature is placed upstream of the PG monochromator ͑component 2 in Fig.…”