NEUTRON-SPECTRUM MEASUREMENTS IN URANIUM-WATER AND URANIUM-MONOISOPROPYL DIPHENYL LATTICES

Previous neutron spectrum determinations in heterogeneous U--H/sub 2/O systems yielded valuable information concerning the distribution of the neutrons in the fuel and the moderator, without however, clarifying the theoretically important neutron spectrum changes. An attempt was therefore made to collect detailed data on the spatial and energy distribution of the neutrons for the purpose of determinlng the laws governing the thermalization process. The spectra were determined in U--H/sub 2/O and U- monoisopropyl diphenyl systems by means of a chopper and a time-of-flight analyzer, using the water-cooled, water- moderated VVR reactor for the experiments. Results obtained showed that the neutron spectrum in both the U and the moderator differs from the Maxwellian distribution, exhibiting a sudden change at the boundary while changing only slightly within the U or the moderator. A simple, semi-empirical function was derived for correlating the thermal neutron fluxes in these materials. The spectrum became harder in the Umonoisopropyl diphenyl lattice, as compared to the U--H/sub 2/O system; in the 0.005 to 0.25 ev region the average neutron velocities obtained were 3.13 x 10/sub 5/ cm/sec for the organic and 3.0 x 10/sub 5/ cm/sec for water while in the U the corresponding values were 3.62 and 3.16more » x 10/sub 5/ cm/sec, respectively. This hardening was due to the increased absorption with respect to the moderator at the expense of the thermal utilization. (TTT)« less