We investigated a method for monitoring a typical large storage vault for unauthorized removal of plutonium. The method is based on the assumption that .the neutron field in a vault produced by a particular geometric configuration of bulk plutonium remains constant in time and space as long as the configuration is undisturbed. To observe such a neutron field, we installed an array of 25 neutron detectors in the ceiling of a plutonium storage vault at Argonne National Laboratory West. Each neutron detector provided an independent spatial measurement of the vauit neutron field. Data collected by• each detector were processed to determine whether statistically significant changes had occurred in the neutron field. Continuous observation experiments •measured the long-term stability of the system. Removal experiments were performed in which known quantities of plutonium were removed from the vault. Both types of experiments demonstrated that the neutron monitoring system can detect removal or addition of bulk plutonium (11% 240 Pu) whose mass is as small as 0.04% of the total inventory.

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