Legal Definitions - neutron-activation analysis

Neutron-activation analysis (NAA) is a highly sensitive scientific technique used in legal contexts to identify and quantify the elemental composition of physical evidence. It works by exposing a small sample of material to a beam of neutrons, typically within a nuclear reactor. This bombardment causes the atoms in the sample to become temporarily radioactive and emit gamma rays. Scientists then measure the unique energy and intensity of these gamma rays, which act like a "fingerprint" to identify the specific elements present in the sample and their precise quantities.

This technique is particularly valuable in forensic investigations for analyzing trace evidence:

• Consider a situation where a small paint chip from a suspect's vehicle is found at the scene of a hit-and-run accident. Investigators can use NAA to analyze this tiny paint chip. By bombarding it with neutrons, they can determine the exact elemental makeup of the paint, including trace elements that might be unique to a particular manufacturer, model, or even a specific batch. This detailed elemental "signature" can then be compared to paint samples from a suspect's vehicle, providing strong evidence to link the suspect to the crime scene. This illustrates NAA's ability to identify the precise composition of physical evidence to establish connections.

• In a case involving a break-in, a suspect's shoe might have a small amount of soil adhering to it. Meanwhile, soil samples are collected from the entry point of the crime scene. NAA can be employed to analyze both the soil from the shoe and the crime scene soil. Even if the soils appear similar to the naked eye, NAA can reveal the precise concentrations of various elements (like iron, aluminum, silicon, or rare earth elements) in each sample. If the elemental profiles from the suspect's shoe soil and the crime scene soil are a close match, it provides compelling evidence that the suspect was present at the location. This demonstrates how NAA helps in identifying and comparing the elemental composition of trace evidence to place an individual at a specific location.