Text: In the region of higher energies, neutron classifications based on energy are somewhat less sharply defined. Ordinarily, high-energy neutrons are assumed to have energies in the region from 0.5 to 10 Mev. Accelerator for producing particles of 50 Mev and above, often called the very-high-energy range, are not yet numerous. Relatively few sources of neutrons with this range of energies are available. Therefore much remains to be learned regarding the properties of very-high-energy neutrons. As efforts multiply in the development of accelerators to generate charged-particle radiations of energies greatly exceeding 50 Mev, the production of neutrons in the ultra-high-energy range becomes impossible. It is impossible to predict the ultimate limit of the upward extension of neutron energies. Enough exploratory work in the 1-to-5 Bev region has been done to indicate the general tendencies of interactions of ultra-high-energy neutrons with nuclei. Fission and spallation of nuclei with atomic numbers below bismuth, as well as above it, are produced in this region. Also most nuclei appear to be relatively transparent to neutrons in the Bev range of energy. The cosmic radiation is also a source of neutrons with energies well above those which are likely to be produced by accelerators. However these cosmic-ray neutrons are not as accessible for study as neutrons generated by accelerators.

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