Characterisation of the neutron field at the ANSTO instrument calibration facility

Abstract

ANSTO’s Instrument Calibration Facility (ICF) provides calibration services to radiation monitors, used in radiation protection applications. The facility has a large calibration room that accommodates the neutron source rig and the monitor table, which is remotely controlled. The room also hosts the gamma calibration services. Determination of the free field (direct) and scattered components of neutron field in a calibration room was essential to obtain an accurate response of the neutron monitor under testing. The free field fluence response and the fractional room return scatter, caused by the interaction of neutron fluence with the room structure, were determined. The fluence response was 1.210x10-4 μSv/h per n/m2; and the neutron field has a fractional room scatter of 0.044 at 1 m and increases linearly versus square of distance. The standard calibration methods, described by ISO-10647, IAEA-TR285, NCRP-112 and NPL-RS(EXT)5, were utilized in this characterisation and gave comparable results. The shadow-shield (truncated cone) were found more suitable to describe the neutron field compared with the other methods e.g. the polynomial fitting, semi-empirical due to the fact of the size, shape of the ICF room and source/monitor positions. Nevertheless; all methods resulted in good response curves with correlation coefficients of fitting greater than 0.97. The shadow shield consisted of two stacked conical sections. The first section was made from iron of 200mm height and the second section was a hollow and made from aluminium of 350mm height. The hollow section was then filled with neutron-moderating/absorbing materials i.e. water solution of LiBr 24% w/w. A performance test was conducted on the shield and gave a very satisfactory result e.g. the readings of fluence response to the free field neutron did follow the inverse square law with correlation >=0.999. It is worth noticing that at the completion of this characterisation and report, the calibration results with the Physikalische-Technische Bundesanstalt (PTB) in Germany became available. As a result, the neutron characterisation at ICF calibration room did agree with the BTP calibration within five percent. Consequently, the neutron field in ICF rig calibration room is now traceable to BTP standard laboratory in Germany. Also, this agreement confirms the integrity of the current neutron source e.g. anisotropy stability, which should save substantial cost and efforts in replacing the source or sending it overseas for re-certification.