performed with due regard for preservation of evidence and perpetuation of the chain-of-custody. The collection of traditional forensic evidence on radioactively contaminated materials must also be performed in a manner consistent with good radiological safety practice.
The variety of traditional forensic evidence, as well as the methods of collection and evaluation, is almost limitless. For example, evidence such as tissue, hair, fingerprints, and shoeprints can often associate a specific individual with a specific place or object. The analysis of fibers, pollen, or chemical substances found at the incident scene can provide information about motives or transportation routes. Documentary evidence provides useful information not only in the content of the communication itself, but also in the incidental details of its creation (paper, ink, film type, extraneous noises, and accents). Similar to collection of radioactive evidence, the international community has agreed upon a sequence for traditional evidence collection. In Table 1, the collection of fingerprint and environmentally sensitive samples, e.g., gunshot or high explosive residues or DNA bearing samples, must occur within the first 24 hours after sample receipt. Fingerprint evidence should be collected by non-destructive means first (laser and photographic methods), then by dusting and lifting. The chemical analysis of other evidence by techniques, such as gas chromatography/mass spectrometry, may occur up to two months after the recovery of evidence.
Case development is very much a deductive process (see Figure 1). The nuclear forensic expert develops a hypothesis or set of hypotheses based upon the results to that point. This hypothesis suggests additional signatures, which either might or must be present if the hypothesis is true. The expert then devises tests to verify the presence or absence of the signatures. Access to other experts around the world, to forensics knowledge bases, and to archived sample libraries are important tools that allow the nuclear forensics expert to formulate the hypothesis and the method to test it. If these tests show that the signature is absent, then the nuclear forensic scientist must abandon or adjust his hypothesis to fit the new results. If the tests show that the signature is present, then either a unique interpretation has been achieved or additional tests must be devised to exclude the other possible scenarios. At the beginning of the nuclear forensics process, the results from the radioactive materials analysis and traditional forensic analysis will most likely be consistent with many scenarios. As the process continues and new results prove inconsistent with those scenarios, certain scenarios are excluded. In the optimum case, only a single scenario will eventually prove consistent with all results.