Fast neutron spectra at the surface of spherical shells of natural uranium

Energy spectra have been measured by the time of flight method, of fast neutrons leaking from the surface of spherical shells of natural uranium. Source neutrons were generated at the centre of the shells by electron bombardment of spherical natural uranium and also lead targets. New measurements have been made of the neutron spectra generated by these targets in order to specify the source to be used in S_n transport theory calculations. Shell thicknesses of 9·9 cm and 3·0 cm were used; spectra were measured at 22° and 45° to the shell surface normal and covered the energy range 10 keV–6 MeV. The measurements were compared with S_n theory predictions; the results of an automatic data adjustment which made use of the measurements are also presented.