Long counter and its application for the calibration of the neutron irradiators

The Korea Research Institute of Standards and Science (KRISS) has constructed a new long counter that can be disassembled in parts and reassembled. This counter can be easily transported and used as a transfer standard instrument for neutron fluence measurements. The response function and the effective centre of the counter are investigated by calculating neutron energies from thermal to 30 MeV using MCNPX. By carrying out measurements using a 252Cf source in the KRISS irradiation room, the accuracy of the evaluated effective centre position is confirmed. The 'distance variation method' is adopted to eliminate the effect of in-scatter neutrons. This method is effective and used for the experimental determination of the effective centre. The neutron emission rates determined by the neutron fluence measurements using the long counter developed are compared with those measured by a manganese sulphate bath, and show good agreement within 3 %. A novel long counter, KRISS-LC, was constructed which can be single-handily operated and transported without the use of a chain block or a crane. The response and effective centre position of the counter were determined to a high degree of precision by performing MCNPX calculations. The effective centre position for the 252Cf source was measured in the KRISS irradiation room. The same method can be used to experimentally determine the effective centre positions of counters in small-sized experimental halls. Using the KRISS-LC, fluence measurements for 241Am-Be and 252Cf sources were performed and the neutron emission rate from the measured fluence rate was evaluated. The emission rate evaluated by fluence measurement agreed with its certified value within the uncertainties. As for 252Cf sources, the discrepancies are ∼1.5-2.0 times standard deviation, which are a bit large but statistically acceptable discrepancy, especially for the purpose of the calibration of neutron protection devices. The direct measurement of neutron fluence of neutron irradiators using the KRISS-LC can be used for irradiator calibration. (authors)