What will you need? A source of neutrons to activate the material and a gamma-ray spectrometer to measure the radiation from the material afterwards. This spectrometer detects and measures gamma rays and sorts them according to their energy. You find that your friend down the hall, who is a nuclear physicist, has a gamma-ray spectrometer that incorporates a lithium-drifted germanium crystal as a detector and a pulse height analyzer. The germanium detector is a device that senses the gamma rays that enter it and gives electrical signals related to the energy of the gamma rays. It was invented only a few years ago and has a very fine resolution. That is, it can easily “pick out” gamma rays that are only slightly different in energy. For example, for gamma rays with energies of approximately 1 MeV (million electron volts), it is not unusual to distinguish between gamma rays that differ by only 2 or 3 tenths of a percent. The pulse height analyzer is an electronic device that sorts the electrical pulses from the detector according to their energy. [Illustration: Gamma-ray detector] A lithium-drifted germanium-crystal gamma-ray detector. The large container is a reservoir of liquid nitrogen that keeps the detector cooled to a temperature of -196° Centigrade (321° below zero, Fahrenheit). The lead brick shield keeps out most of the gamma rays that come from naturally radioactive materials in the room. The plastic slots hold cards upon which the samples are mounted for counting. Sometimes the detector is arranged vertically and samples are placed on shelves above it. [Illustration: Gamma-ray detector] What about the neutrons for the irradiation? Although there isn’t a suitable nuclear reactor[9] in your city, there is one at a university only an hour away by jet. Since it may take a few hours to get the sample to the counter after irradiation, you won’t be able to look for short-lived activation products, i.e., those with half-lives of up to an hour. However, this will exclude only a few elements from detection. [Illustration: Pulse-height analyzer] A pulse-height analyzer used for gamma-ray spectrometry. A gamma-ray spectrum is displayed on the television screen. Data is printed out automatically on the electric typewriter and also may be plotted as a graph on the paper to the left. In other systems, data may be coded onto punched paper tape as well. Such tape may be “read” by a computer that can be programmed to use the data to calculate what radioactive isotopes are present and their quantities. Now you are ready to begin the analysis. This will be a qualitative analysis since you are merely looking for a significantly different element in that silicon crystal. How much of it is present is only of secondary interest. Therefore, if you find anything different, you will rely on an approximate calculation to tell you “how much”.