The two gamma-ray spectra you obtain look like those in the figure on the opposite page. The gamma rays from the irradiated paint taken from the victim’s clothing indicate the presence of the common elements sodium, potassium, and copper, but gold, lanthanum, and europium are also conspicuously present. The gamma rays from the other sample also reveal sodium, potassium, and gold but in rather different proportions. More striking is the absence of copper and the two rare earths, and the presence of manganese and arsenic, which were not indicated in the first sample. The paint samples definitely do not match. Therefore, you inform the detective that his suspect is innocent after all. You’ve solved your problem, but he still has his. Perhaps the same technique will provide positive proof when he finds the real culprit. [Illustration: Graph: “Counts/10 min per 3.3 KeV channel (arbitrary scale)” _vs_ “Channel number”] Element Channel number .122 MeV Eu (Europium) 35 .328 MeV La (Lanthanum) 95 .344 MeV Eu 96 .412 MeV Au (Gold) 110 .486 MeV La 120 .511 MeV Cu (Copper) 145 .815 MeV La 240 .837 MeV Eu 245 .961 MeV Eu 290 1.37 MeV Na (Sodium) 410 1.53 MeV K (Potassium) 455 1.60 MeV La 475 (channel numbers estimated) [Illustration: Graph: “Counts/10 min per 3.3 KeV channel (arbitrary scale)” _vs_ “Channel number”] Element Channel number 0.412 MeV Au (Gold) 110 0.511 MeV Na 145 0.559 MeV As (Arsenic) 160 0.657 MeV As 195 0.847 MeV Mn (Manganese) 250 1.21 MeV As 370 1.37 MeV Na (Sodium) 410 1.53 MeV K (Potassium) 455 (channel numbers estimated) Gamma-ray spectra of two samples of paint. These two spectra are obviously different and, therefore, could not have come from the same source. SUMMING UP: WHAT LIES AHEAD These five situations are intended to show why neutron activation analysis is used, when it can be applied, and how it works.