Reference thermal neutron field at KRISS for calibration of neutron detectors

A reference thermal neutron field has been established at the Korea Research Institute of Standards and Science (KRISS) by using a 241Am-Be neutron source and a high-purity graphite pile constructed by stacking graphite blocks. The properties of the graphite blocks such as impurities, density, and dimensions were studied thoroughly to understand the characteristics of the generated field. The energy spectrum and thermal neutron fractions were simulated with the Monte Carlo N-Particle eXtended code using measured physical parameters. The neutron effective temperature was 308 K, and the fraction of thermal neutrons was approximately 95% at the reference position of the thermal neutron field. The thermal neutron fluence rate was determined by adopting the Westcott convention method based on neutron activation analysis using a gold foil. The Westcott fluence rate for thermal neutron at the reference position was (2326.7 ± 8.4) cm−2s−1. The true thermal neutron fluence rate at the reference position in the KRISS thermal neutron field was (2700 ± 29) cm−2s−1 (at the reference date of June 30, 2014). The response of a spherical proportional counter with He-3 (SP9 neutron detector) was evaluated in the newly established field as (3.083 ± 0.045) cm2 for the reference calibration condition (a parallel neutron beam with a Maxwellian energy distribution having a most probable energy of 0.025 eV).