The Neutron Leakage Spectra Through the Steel Top Plates of Two Heavy-Water-Moderated Research Reactors

The neutron leakage spectra above the steel top plates of the DIDO and PLATO reactors at Harwell have been measured using a high resolution spectrometry system based on spherical hydrogen-filled proportional counters. Supporting measurements were made with a multisphere spectrometer to determine the fraction of dose equivalent due to neutrons having energies below the low energy cut-off (10 keV) of the hydrogen counters. The two spectra were found to be similar, with nearly all of the neutrons having energies of less than 700 keV and with most of the dose equivalent arising from neutrons having energies between 100-700 keV. The dose equivalent responses of several neutron dosemeters and a survey instrument to these spectra have been calculated. The neptunium dosemeter measures 53-59%, NTA photographic film 14-21%, CR-39 68-88%, and the Harwell type 95/0949 survey meter indicates 154-203% of the true dose equivalent of the leakage spectra.