Improvement of detection limit in differential die-away analysis system for nuclear non-proliferation and nuclear security

In the fields of safeguards, nuclear non-proliferation, and nuclear security, non-destructive analysis (NDA) techniques useful for highly radioactive nuclear materials (NMs) are not established yet because there are so many technical difficulties to measure the amount of the highly radioactive NMs. In order to determine the composition and quantification of mixed NMs, a new NDA system with a pulsed neutron source has been developing in the Japan Atomic Energy Agency (JAEA). In the NDA system, a differential die-away analysis (DDA) technique is used to quantify the amount of fissile materials. The detection limit of fissile materials in DDA system is determined by the signal to noise ratio in fast prompt neutron counting. A method to reduce the noise signal by using neutron absorber (B4C rubber) sheets mounted on the inner entire surface in the measurement space is proposed. The effect of the sheets on the reduction of noise signal in the fast prompt neutron counting was investigated in both experimental test and simulation. The experimental results show that it is possible to detect a nuclear fissile material ( ²³⁹ Pu) of as low as 1 mg in a vial bottle when the absorber sheets with a thickness of 3 mm is used. The detection limit could be improved from ²³⁹ Pu mass of 10 mg to 1 mg by using the absorber rubber sheets with a thickness of 3 mm as the simulation results suggested. This paper also presents comparison between experimental data and simulation results.