Thermal Neutron Spectra in Graphite

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Abstract
A pulsed, high-current, electron linear accelerator is used to excite thermal-neutron spectra in a graphite assembly. The steady-state energy spectra of neutrons are measured at several temperatures by pulsed-beam time-of-flight techniques. We compare the measured spectra with theoretical predictions which use free- and bound-carbon scattering kernels.The scattering kernel for carbon bound in graphite is obtained through a realistic treatment of the neutron-phonon interactions. With this kernel, theoretical calculations of spectra agree extremely well with the experimental results. Predictions derived from a scattering law in which the carbon atoms are treated as free differ markedly from the measured spectra, even up to a temperature of 810°K. Additional calculations show that the effects of chemical binding are significant in problems of reactor design physics.

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