Warhead Monitoring by Environmental Activation Products Detection

Through a series of measurements with a high purity germanium detector, we have established that the past presence of neutron emitting material can be detected by the decay of activation products in aluminum containers, tungsten shielding, and concrete floors even several days after last exposure. The time since last exposure can be estimated by the gamma-ray detection rate. These findings may lead to interesting new concept of operations (CONOPS) in the detection of illicit special nuclear material (SNM) or the verification of the absence (or presence) of SNM containing objects in facilities and/or transit even after the material has been removed. In the work presented here, we measure the products of neutron activation in materials that may be commonly found in environments near SNM, whether in storage or transport. Neutrons emitted by an SNM containing item interact with and activate certain specific isotopes that may be present nearby. Through a combination of inelastic reactions, these isotopes are transformed into metastable products (having half-lives on the order of 10s of minutes to 10s of hours) imprinting a record of the presence of SNM in the recent past.As the metastable isotopes decay, they emit gamma-rays with energies characteristic of the material type. The decay gamma rays can then be detected hours or days after the SNM has been removed, verifying its past presence even though the SNM may now be quite some distance away. Further, the relative detection rates of these characteristic decay gamma-rays can be used to estimate the time since last neutron exposure (i.e. when the SNM was moved).This detection method may find application in which the presence or absence of SNM containing items must be verified. For example, containers and other objects within a facility, such as the floor itself may be used to confirm declarations that treaty accountable items have not been stored in empty containers or storage areas within some period of time. Additionally, the detection of activated shielding material such as tungsten may be an indication that some effort is being taken to hide the presence of SNM containing items.