LIBS Analysis for Identifying Sr Compounds in Urban Nuclear Terror Scene

Rapid and precise identification of characteristics of threats is significant in nuclear terror scenes for prompt response. To protect inspectors and collect evidence on-site, Laser-induced breakdown spectroscopy (LIBS) has been explored at remote distance. In addition, among potential materials of radiological explosive device (RDD) suggested by the Argonne National Laboratory, 90Sr and 210Po does not emit gamma ray during their decay. To supplement conventional techniques typically used to detect gamma-emitting radionuclides, LIBS can be used in real-time to detect non-gamma emitting radionuclides with possibility to measure chemical impurities in the samples as signatures in only a few seconds. In this paper, LIBS was applied to detect Sr in various chemical forms on the surfaces of common materials in urban environments. It was assumed that Sr in RDD was dispersed and attached on solid materials including Al alloys, concretes, and soil. The chemical forms, SrF2, SrTiO3, SrCO3, were selected by analyzing pathways of terrorists to obtain these materials as they could be from reprocessing plants and radioisotope thermoelectric generators. These chemical compounds were dissolved into highly purified water and the solutions were dropped on the surface of urban simulated materials. For each matrix (3 Sr compounds for 3 urban simulated materials), limit of detection (LOD) for each matrix was calculated at 707.0 nm. Multiple measurements for each matrix up to 100 times were conducted to reduce the errors of LOD. While Sr compounds has the lowest LOD in Al, approximately ~10.0 µg/cm2 , LOD is about 100.0~150.0 in concrete and 20.0~90.0 in soil. It was found that different compounds had a similar LOD in the same urban material. In addition, different Sr compounds release different characteristic continuous spectrums that may be later used to identify the element of Sr as well as the chemical form of Sr by using LIBS. This preliminary may support that LIBS will be used on site to quickly investigate terror sites for identifying terror materials, distinguishing chemical forms of the materials and obtaining chemical evidence in real time.