Neutron activation analysis (NAA) is one of the most sensitive analytical techniques for many elements. A major breakthrough came in 1964 when NAA was applied to the quantitative detection of antimony and barium in FDR.130 (Antimony is the most valuable elemental indicator for FDR, because lead and barium are more common in occupational and environmental surveys.)
The method is based on the fact that when a sample is irradiated in a nuclear reactor for a specific length of time, atoms of some elements absorb neutrons. Nuclei that acquire an excess electron have a large excess of energy that is often released in the form of gamma rays. Nuclei with added neutrons are called radionuclides.
On placing the irradiated samples into a radio counter system capable of detecting and recording specific radiations it is possible to identify and quantify the elements of interest. If the elements emit gamma rays, the energy of the emissions and the decay lifetimes provide qualitative identification of the elements. The number of gamma rays per unit time versus the energy of the gamma rays is directly proportional to the amount of the element in the sample.131-133
NAA is an excellent analytical tool which has been used successfully for the detection of barium and antimony in FDR. It has been applied to the detection of FDR on suspects, the identification of bullet holes in a variety of target materials, and range of fire estimations. Copper and mercury in FDR have also been determined.134
NAA suffers from several major disadvantages for routine operation by most forensic laboratories:
1. Availability of and access to a nuclear reactor;
2. High equipment costs and lack of trained staff;
3. Slow throughput of samples due to time required for irradiation, cooling, and radiochemical separation;
4. Poor detection limit for lead (~10 ^g), a very important element in FDR work.
Much development work has been done on the NAA technique for FDR detection, but the inherent disadvantages of the technique led to a search for other more suitable quantitative methods. Despite its disadvantages, NAA did much to increase our knowledge of quantitative aspects of the deposition and subsequent behavior of FDR.
Many alternative techniques, both qualitative and quantitative, have been investigated either for screening purposes or as primary methods. Such techniques include atomic absorption spectrophotometry, molecular luminescence, electron spin resonance spectrometry, X-ray analysis methods, and electro analytical methods. Flameless atomic absorption spectrophotometry (FAAS) is the technique that has almost completely replaced NAA.
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