Abstract: A method and apparatus for identifying an article and detecting its unauthorized movement comprises disposing a predetermined amount of radioactive isotopes on or within the article. The plurality of radioactive isotopes has a characteristic gamma spectrum unique to that plurality. Continuous monitoring is used to obtain a signal to detect unauthorized movement of the article. For identification purposes, means is provided for resolving the unique gamma spectrum into isotopic abundances. The plurality of radioactive isotopes is disposed on the article in an amount which preferably does not exceed an activity level of about one microcurie. Additionally, the half-life of the plurality is matched to the period of time needed to protect a particular article.