Mirror reflection method for neutron spectral measurement

A mirror reflection method (MR) was developed and tested for analyzing the wavelength distribution of a thermal neutron beam. The working principle of this method is that, at a given angle of incidence, a neutron mirror totally reflects neutrons whose wavelengths are longer than the critical wavelength while weakly reflects neutrons whose wavelengths are shorter than the critical wavelength. As the critical wavelength is proportional to the incident angle, the incident angle can be scanned at small intervals so that the critical wavelength sweeps through the entire spectrum of the beam. It is shown that the spectral density at the critical wavelength is proportional to the derivative of the count rate with respect to the incident angle. Reflection experiments were carried out at the MIT research reactor using a nickel-coated glass plate and the results were compared with the time-of-flight spectrum. It was shown that the mirror method is capable of measuring a thermal spectrum with a wavelength resolution of 1% at 3 إ and 10% at 0.5 إ. Potential applications of this method may be found in neutron and X-ray scattering spectroscopies. The method may apply to energy analysis of cold neutrons and X-rays used in materials research since they also experience total reflection at small angles.