Anomalous Scattering of Fast Neutrons

Abstract

The cross sections of a number of elements for neutron scattering were measured as a function of neutron energy throughout a range of 0.5 Mev. Anomalous cross sections were found in the cases of magnesium and aluminum. It was possible to account quantitatively for the anomaly in magnesium on the assumption that the effect was due to resonance in Mg 26 .

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“…If it is assumed that the observed change in cross section is not due to a number of unresolved narrow levels but to a single broad level and that the resonance occurs in only one isotope (the most abundant) it becomes possible to attempt a comparison of the experimental cross section with that expected according to the one-body theory for light nuclei (Breit and Wigner 1936, Bethe and Placzek 1937, Macphail 1940, Feshbach, Peaslea and Weisskoff 1947. According to this theory the total cross section can be represented by 2J+1 sin2Su…”

“…(see Goldsmith, Ibser and Feld 1947, p. 271, for a list of references). Aoki (1939), Zinn et al (1939, Macphail (1940) andAllen et al (1947) have made measurements in the range from 2 MeV. to 4~e v .…”

“…As pointed out by Macphail (1940), the ground state of 27A1 has a spin of 512 and the scattering cross section for a given state is now determined not only by the corresponding values of the orbital quantum number but also by the transition probabilities between the various components of that state, and, as these are not known, an interpretation of the experimental results is not possible.…”

“…However, broad resonances have since been observed by Macphail (1940) in magnesium and aluminium and by Allen, Burcham and Wilkinson (1947) in beryllium and aluminium in the range of neutron energies from 2 M e~. to 4~e v .…”