Monte Carlo calculations have been made of neutron capture probabilities in solution-filled phantoms, including cylinders of 7 diameters and several human-phantom models. For some phantoms, probabilities were calculated at 20 energies from thermal to 14 MeV; for others at only a few energies. The effects of wall thickness on the specific activity of Na in solution in the phantom were studied and wall-correction factors were derived for various neutron energies, wall thicknesses and cylinder diameters. Neutron capture in Na at epithermal energies was shown to have little effect on the activation produced. The calculated capture probabilities are compared with a wide variety of experimental results far cylinders and human phantoms. With a few exceptions, agreement is within the experimental errors of about ± 15%. For a human phantom irradiated from the front and side, the ratio of 24Na activities produced is shown to depend strongly on neutrons scattered from the floor. For an average person exposed to reactor spectra, the variation of Na activity per unit fluence with orientation is considerably greater than typical variations with spectrum or body size.
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