Measurements of neutron spectra at depths in tissue equivalent material for fast neutron beams generated by deuterons on beryllium.

The threshold foil technique has been used to investigate the behaviour in tissue of the spectra of neutron beams produced by the bombardment of thick Be targets with 13-0 and 20-5 MeV deuterons. The results indicate a gradual "softening" with depth in tissue of the spectrum generated by 20-5 MeV deutrons which contrasts with a gradual hardening as reported by other authors. This is discussed in terms of the significance of field size and the methods used for data collection and analysis.