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Description
Slovak Institute of Metrology (SMU) in cooperation with Slovak University of Technology in Bratislava developed a primary standard of dosimetry quantities for neutron radiation, which will ensure the realization of the emission rate unit at the sufficient metrology level. The key component is the manganese sulphate bath, which is used by many metrology institutes around the world. Utilization of manganese activation offers elegant technique, how the neutron source emission rates can be determined, but this method includes several parameters, which must be properly determined to achieve final uncertainty acceptable at the metrology level. The method implemented at SMU covers in-vessel measurement, where the detector is placed in the center of the bath using dry channel. Geometry calibration must be performed to transfer detector response to an activity of the solution. Two methods are compared in this paper; solution with known activity of fluorine, prepared in the cyclotron, and MnSO4 irradiated in LVR-15 reactor were used. Inherent part of the method is determination of the conversion factor, which turns measured activity of the solution to neutron source emission rate. This conversion factor can only be determined computationally. The Monte Carlo transport code MCNP6 was used, where various relevant geometric models were considered in the iterative modeling process. Material composition of the solution was derived from precise density measurements for the final MnSO4 concentration. The super-sequence SAMPLER module of the SCALE6 system was utilized to assess the uncertainties in computed responses used for conversion factor calculation. All above mentioned methods and approaches are summarized in the paper and validated against experimental measurements with certified neutron sources. All determined neutron emission rates fell in the uncertainties margins. Based on achieved results, it can be concluded that the manganese sulphate bath was successfully implemented in the national standard and is fully operable.