Establishment of a simple neutron calibration field using a moderated 252Cf source. Pt. 2

The paper describes the development and experimental standardization of neutron fields simply arranged for detector calibrations used for radiation control and environmental measurement. These fields are the following: (1) bare 252Cf fission field, (2) iron-moderated 252Cf field, (3) carbon-moderated 252Cf field, and (4) polyethylene-moderated 252Cf field. These fields are most suitable for calibrating the detectors used in and around nuclear and radiation facilities, since the fields are designed to simulate the typical neutron fields in and around the facilities. The direct neutron components of these fields have been standarized by the following two methods: (1) calculation by the ANISN code, and (2) measurements with and without a shadow shield by detectors standardized in the national standard field at the Electrotechnical Laboratory (ETL). The neutron emission rates of the 252Cf source have been calibrated also at ETL. We have standardized only direct components because of their independence of room size and peripheral structures. The standardized values are energy spectra and dose equivalent rates of the direct neutron components; the accuracies have also been evaluated to be 20% below 100 keV, 15% at 1 MeV, and 50% above 5 MeV. These fields including room scattered components have also been characterized especially to calibrate neutron detectors having high sensitivity to low energy room scattered neutrons, because of large errors caused by shadow shield subtraction. (orig.)