Characteristics of the 252Cf neutron fields in the irradiation facility of the PTB

The determination of neutron fluence or dose equivalent in practice requires that calibrated instruments be available. For the calibration, well-defined neutron fields are necessary. This paper demonstrates by a simple model how the neutron field available at the irradiation facility with radionuclide neutron sources of the Physikalisch-Technische Bundesanstalt (PTB) is formed. Contributions are made directly by the source, by neutrons backscattered from the walls and by neutrons inscattered by the air and the components of the facility. In the case of the D2O-moderated 252Cf source, several contributions to the neutron energy spectrum to be expected are discussed. It is shown that both the integral value and the spectral distribution of the contribution from the walls depend strongly on the materials of which the walls are made. For special configurations, a dependency of the wall contribution on the distance between the point of interest and the source is found. (author)