Abstract
Wigner-Wilkins have analytically calculated the neutron spectrum in an infinitely large monatomic hydrogen gas moderator in which the absorption cross section has the usual inverse dependence on velocity, v. The above analytical methods were found to be not always applicable. A calculation is given for the neutron spectum in a monatomic hydrogen gas moderator which contains hypethetically a large amount of Pu/sup 239/ absorber or l/v law dependent absorber. The calculation is based on the assumptions that the temperature of the moderator T = 581 deg K, the scattering cross section of hydrogen is constant (50 barn), and the ratio of the number of atoms of Pu/sup 239/ or l/v law absorber to the hydrogen atoms are 0, 0.01, and 0.02. When the moderator contains no Pu/sup 239/. the distribution of the neutron velocities is approximately Maxwellian. However, when the concentration is 0.01 or 0.02, the spectrum is shifted in the direction of higher energies, i.e., the spectrum is hardened. There is a pronounced dip at the 0.3 ev resonance of Pu/sup 239/. In the case of the l/v law absorber, the distribution is Wigner-Wilkins spectrum and the spectrum is shifted in the direction of higher energies. (auth)
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