Construction of Thermal Neutron Calibration Fields Using a Graphite Pile and Americium-Beryllium Neutron Sources at KAERI

Thermal neutron calibration fields are under preparation using a graphite pile and eight americium-beryllium neutron sources at the Korea Atomic Energy Research Institute (KAERI). Eight Am-Be sources, of [approximately]37 GBq each, are located in a 1.5- × 1.5- × 1.5-m³ graphite pile that has four neutron source mounting geometries to make different intensities at the reference irradiation positions. At this time two kinds of neutron calibration fields are categorized according to the position of the neutron sources in the graphite pile. These neutron fields were simulated by using the MCNPX code and quantified experimentally by using the Bonner sphere spectrometry system of KAERI. The neutrons of a low energy below the Cd cutoff energy of 0.5 eV were 68.6 and 95.9% of the total neutron fluence, respectively. The ambient dose equivalent rates H*(10) were 30.6 and 167 Svh^-1 , and the personal dose equivalent rates H_p(10) were 31.7 and 174 Svh^-1 . These can be used to determine the response of thermal neutron measuring devices.