ATTENUATION OF THERMAL-NEUTRONS BY AN IMPERFECT SINGLE-CRYSTAL

A semi-empirical formula is given which allows one to calculate the to tal thermal cross section of an imperfect single crystal as a function of crystal constants, temperature and neutron energy E, in the energy range between 3 meV and 10 eV. The formula also includes the contribu tion of the parasitic Bragg scattering to the total cross section that takes into account the crystal mosaic spread value and its orientatio n with respect to the neutron beam direction. A computer program (ISCA NF) was developed to calculate the total attenuation of neutrons using the proposed formula. The ISCANF program was applied to investigate t he neutron attenuation through a copper single crystal. The calculated values of the neutron transmission through the imperfect copper singl e crystal were fitted to the measured ones in the energy range 3-40 me V at different crystal orientations. The result of fitting shows that use of the computer program ISCANF allows one to predict the behaviour of the total cross section of an imperfect copper single crystal for the whole energy range.