A proposed semi-empirical method for 235U mass calibration of the active-well neutron coincidence counter

In this work, a proposed semi-empirical method for 235U mass calibration of the active-well neutron coincidence counter (AWCC) is presented. Samples of low enriched uranium oxide powder in form of cylinders were used to investigate and verify the validity of the method. The calibration process was carried out via two main steps. First, different experimental setup configurations (for the detector and samples) were modeled with the Monte Carlo multi-particle transport code MCNP-4B to calculate, numerically, fission rates in the nuclear material (NM) samples for each configuration. Second, the absolute efficiency of the detector for the real coincidence neutrons was measured using a standard 252Cf point source. The measured efficiency was combined with the calculated fission rates to estimate the real coincidence count rate of the detector due to NM for each setup configuration. A semi-empirical calibration curve relates the 235U mass content in each modeled setup configuration with its corresponding estimated count rate was constructed. The calibration curve was checked against experimental data to verify the proposed method. 235U masses estimated from the calibration curve were found to be in agreement with the measured values within the error limits. An accuracy of better than 7.2% was achieved with an average precision of about 4%.