INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA) AND ITS APPLICATION TO FORENSIC INVESTIGATIONS

Instrumental Neutron Activation Analysis (INAA) is a straightforward technique for determining elemental abundances in a wide range of materials. The interaction between a thermal neutron and a nucleus produces a radioactive nuclide that emits characteristic gamma rays. The energy of the emitted gamma rays is used to identify the nuclide and the intensity of the radiation can be used to determine abundance. Solid state detectors are used to sense the emitted gamma rays. Following decay and interference corrections, elemental concentrations are determined. The advantages of INAA are (1) it is a relatively cheap analytical method; (2) the method is non-destructive and the same sample can be used for other measurements; (3) sample size can be very small, often as little as a milligram; (4) detection limits for many elements are in the nanogram range; (5) no chemical preparation is required; and (6) on the order of 40 elements can be measured essentially simultaneously.

There are numerous potential applications of INAA in forensic investigations. The underlying concept is that materials within a class will have different elemental signatures. For example: (1) The elemental composition of maple syrup is determined by the chemical composition of the sap which reflects the underlying soil chemistry, elements introduced during the tapping of the tree and transport to the sugar house, and elements introduced during the boiling down of the sap to produce maple syrup. Maple syrup from different sources can be chemically fingerprinted. (2) Grass chemistry varies as a function of soil chemistry which in turn is related to the chemistry of the underlying bedrock. These variations can be used to identify the geographic location of a grass sample. Similarly mineral exploration geologists have used chemical variations in plants to identify potential ore deposits and geochemists have looked at the relationship between plant chemistry, soil chemistry, and human health. (3) Ceramics consist of mineral components and transition metals that are added for color. For example, Zircopax (essentially zircon) may be added to glazes and this addition leads to a distinctive enrichment in HREE. Co is added for color. Different combinations of minerals and elements in a glaze produce characteristic elemental signatures.