Establishment of simple neutron reference calibrator field using moderated Cf-252 source

This paper describes the development and standardization of the neutron fields simply manufactured 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 standardized 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 the energy spectrum of direct component because of its independency to room size and peripheral structures; the accuracies have also been evaluated to be 20% below 100 keV, 15% at 1 MeV, and 50% above 5 MeV. The fields including room scattered components have also been characterized especially to calibrate the neutron detectors having high sensitivity to low energy room scattered neutrons; because large errors are included in the evaluation of low energy neutron components of the direct component field obtainable by shadow shield subtraction. These two kinds of fields can be situated on the secondary standard fields traceable directly to the primary national standard fields in ETL. (author)