Determination of the multiplication of leakage neutrons and the fissile material mass in deeply subcritical systems

Abstract

The previously developed analysis of measurements of neutron multiplication in deeply subcritical states of multiplying media by the α-Rossi method (method of double neutron-neutron coincidences) is supplemented by a new analysis of triple coincidences. To obtain the neutron multiplication and the effective neutron multiplication coefficient, it is no longer necessary to know the intensity of the source of spontaneous fissions - it is determined from experiments. This makes it possible to determine simultaneously the mass and enrichment of a fissile substance placed in a container. Measurements are performed and analyzed using uranium and plutonium blocks and fast-neutron detectors as well as AWCC (active well coincidence counters) neutron detectors. Previous measurements performed using the coincidence counters are analyzed. It is shown that without calibration of the 240 Pu and 239 Pu mass by measurement of counting rate of single neutrons as well as double and triple coincidences in ordinary experiments the error (1σ) in determining the mass fraction of 240 Pu in plutonium is 0.3-0.8% and the error for all plutonium is 3-12%.