Characterization of neutron fields from bare and heavy water moderated 252Cf spontaneous fission source using Bonner Sphere Spectrometer

• We use Bonner sphere spectrometer to quantify neutron fields from bare and heavy water moderated 252Cf source.
• We use shadow cone subtraction method, proposed by ISO-8529, to distinguish between direct and scattered neutron spectra.
• We use MAXED and STAY'SL codes to unfold neutron spectra.
• We use Monte Carlo code to simulate heavy water moderated 252Cf fields.

In this work a calibrated Bonner Sphere Spectrometer (BSS), together with ISO shadow cones, was used to quantify the total and scattered components of bare and heavy water moderated 252Cf neutron fields. All measurements were performed with a BSS that was calibrated at the National Physical Laboratory (NPL), Teddington, UK, which is a global primary standard laboratory and world-leading facility for neutron metrology and neutron instruments calibration. The fields were characterized for source-spectrometer distances of 80, 100, 150 and 200 cm; and at heights of 103 and 200 cm from the facility floor. As expected, the scattered contribution was greatest at the farthest distance from the source and closer to the floor. Hence, at a distance of 200 cm and a height of 103 cm, the scatter added to the direct field up to 162% of the total neutron fluence and up to 61% of the ambient dose equivalent, while at the same distance and height of 200 cm above the floor, these values were up to 146% and 52%, respectively. In the case of heavy water moderated 252Cf neutron fields, a shadow cone subtraction technique could not be implemented, however Monte Carlo simulations were utilized in order to differentiate between the direct and scatter components of the neutron fields. In this case, at a source–detector distance of 200 cm and a height of 103 cm, the scatter added to the direct field up to 148% of the total neutron fluence and up to 45% of the ambient dose equivalent, while at the same distance and a height of 200 cm above the floor, these values were up to 134% and 42%, respectively.