Neutron characteristics of single-crystal magnesium fluoride

Abstract

A simple formula is proposed that permits the calculation of both the neutron reflectivity and attenuation of single-crystal magnesium fluoride (MgF2) as a function of wavelength at both room temperature and 77K. A computer program MgF2 written in FORTRAN-77, has been developed to carry out the required calculations for the single-crystal MgF2, when it is used as a neutron monochromator and/or filter.The monochromatic characteristics of single-crystal MgF2 are detailed in terms of the optimum crystal cutting plane, mosaic spread, thickness and reactor moderating temperature for efficient neutron reflectivity within the wavelength band from 0.1nm up to .45nm. Calculation shows that a 7mm thick MgF2 single-crystal cut along (111) plane having 0.50 FWHM on mosaic spread has the optimum parameters when it is used as a monochromator at neutron wavelengths shorter than 0.2nm. However, the integrated neutron intensity of 2nd and 3rd orders from a thermal reactor flux is even higher than that of the 1st order one at neutron wavelengths longer than 0.2nm. While, from a cold reactor flux, the use of the single-crystal MgF2 (110) as a neutron monochromator free from higher order contaminations at λ⩾0.38nm is more appropriate.A feasibility study of using single-crystal MgF2 as a neutron filter is also detailed in terms of the optimum crystal orientation, mosaic spread, thickness and temperature for efficient transmission of thermal-reactor neutrons. Calculation shows that a 100mm thick MgF2 single-crystal cut along (110) plane cooled to liquid nitrogen temperature, with 0.05o FWHM on mosaic spread, is a good thermal neutron filter, with high effect-to-noise ratio.