Abstract: A system and a method for the remote monitoring of an irradiated salt mass within a decay enclosure is provided. The system and the method determine a mass ratio of a first radioisotope relative to a second radioisotope, the second radioisotope having a significantly shorter half-life than the first radioisotope. In addition, the second radioisotope includes a shorter half-life than an effective half-life of the decay enclosure, and the first radioisotope includes a longer half-life than the effective half-life of the decay enclosure. The mass ratio quickly decreases outside of a target range after material diversion, while remaining below the target range for several years. As a consequence, the isotopic mass ratio presents a rapid and enduring indicator of inventory change, which is of extreme importance in detecting a diversion of the irradiated salt mass, which remains a potential proliferation target.