Measurement and analysis of neutron spectrum in spherical pile of thoria

The energy spectrum of neutrons in a pile of thoria was measured from several keV to several MeV by the time-of-flight method with an electron linear accelerator and the results were compared with the theoretical ones predicted with three group constants for thorium, (1) constants of Abagyan et al.'s (ABBN), (2) newly produced constants of JAERI-FAST type (NNM) and (3) constants produced from ENDF/B-III in ORNL (DLC-2D). General agreement between measurements and calculations was seen for all cases. However, (1) the predicted spectra with ABBN and DLC-2D gave less flux by about 30% than the measured in the energy below about 100 keV, (2) similar disagreement was observed only above 10 keV for the spectrum with NNM and (3) the measured spectrum became about 20% lower than all of the predicted above 1 MeV. In parallel with the spectrum measurement, neutron spatial distribution was measured by the activation method and the results showed spherical symmetry around a photoneutron target at the center of the pile. From this fact adequacy of using one-dimensional transport code (ANISN or DTF-IV) with spherical symmetry for analysis could be verified. (auth.)