Scattered neutron reference fields produced by radionuclide sources

Realistic fields similar to those occurring in practice can be approximately realised using the ISO-recommended radionuclide sources if the neutrons scattered by the concrete walls of a medium sized irradiation room are used. At the PTB, such fields were established at the irradiation facility with radionuclide reference neutron sources. In order to determine the spectral fluence rate at the point of test as the basis of calibrations, the different spectra were calculated using the MCNP code, and measured by means of a Bonner sphere spectrometer. The measured count rates were unfolded using different codes. The total fluence rate can be determined with a relative standard uncertainty of about 4%. The relative standard uncertainty of the ambient dose equivalent is found to be about 5%.