The two spectra are virtually identical except that the suspect sample has one obviously different peak in channel 157 and a somewhat smaller peak in channel 183. Referring to an energy calibration curve for the pulse height analyzer, you find that these channels correspond to 0.559 and 0.657 MeV respectively. A search of a table of nuclides, arranged by gamma-ray energy, reveals that this combination is emitted by arsenic-76, which would be the activation product for arsenic. Other data also indicate that for arsenic there should be a number of smaller peaks, including some corresponding to energies of 1.216, 1.228, 0.624, and 1.441 MeV. A closer look at the spectrum of the suspect sample reveals that these are also present. Finally, noting that the half-life of arsenic-76 is approximately 27 hours, you wait a day and count the sample again in the same position as the previous count. A decrease in the heights of the 0.559 and 0.657 MeV peaks, by a little less than half in 24 hours, confirms that arsenic is the unusual element in this sample. It may not be the only impurity causing the peculiar behavior of this semiconductor, but it does seem a likely candidate. [Illustration: Graph: “Counts in 20 minutes per 3.8 KeV channel” _vs_ “Channel Number”] Element Counts Channel Number Gold 1300 125 Sodium 900 140 Antimony 650 150 Antimony 550 160 Scandium 220 240 Iron 150 300 Scandium 60 310 Cobalt 110 320 Iron 45 350 Cobalt 50 360 Sodium 900 370 Sodium 140 425 (values estimated from the graph) The gamma-ray spectrum obtained after activation of a sample of “pure” silicon having “ordinary” properties of this type of semiconductor. Only very small quantities of various trace impurities are indicated. [Illustration: Graph: “Counts in 20 minutes per 3.8 KeV channel” _vs_ “Channel number”] Element Counts Channel Number Gold Sodium 0.559 Arsenic-75 1500 150 Antimony 0.624 Arsenic-75 190 165 0.657 Arsenic-75 135 175 Scandium Iron Scandium Cobalt 1.216 Arsenic-75 100 330 1.228 Arsenic-75 60 335 Iron Cobalt Sodium 1.44 Arsenic-75 25 380 Sodium (distinct values estimated from graph)