subsequently be detected by the detector. The result is a peak structure with approximately the energy of the incident photon minus the energy of the Compton edge. Single escape and double escape peaks For incident photon energies E larger than two times the rest mass of the electron (1.022 MeV), pair production can occur. The resulting positron annihilates with one of the surrounding electrons, typically producing two photons with 511 keV. In a real detector (i.e. a detector of finite size) it is possible that after the annihilation: Both photons deposit their energy in the detector. One of the two photons escapes the detector and only one of the photons deposits its energy in the detector, resulting in a peak with E − 511 keV, the single escape peak. Both photons escape the detector, resulting in a peak with E − 2 × 511 keV, the double escape peak. The above Am-Be-source spectrum shows an example of single and double escape peak in a real measurement. Calibration and background radiation If a gamma spectrometer is used for identifying samples of unknown composition, its energy scale must be calibrated first. Calibration is performed by using the peaks of a known source, such as caesium-137 or cobalt-60. Because the channel number is proportional to energy, the channel scale can then be converted to an energy scale. If the size of the detector crystal is known, one can also perform an intensity calibration, so that not only the energies but also the intensities of an unknown source—or the amount of a certain isotope in the source—can be determined. Because some radioactivity is present everywhere (i.e., background radiation), the spectrum should be analyzed when no source is present. The background radiation must then be subtracted from the actual measurement. Lead absorbers can be placed around the measurement apparatus to reduce background radiation. See also Alpha-particle spectroscopy Gamma probe Gamma ray spectrometer Isomeric shift Liquid scintillation counting Mass spectrometry Mössbauer spectroscopy Perturbed angular correlation Pandemonium effect Total absorption spectroscopy Scintillation counter X-ray spectroscopy Works cited Gilmore G, Hemingway J. Practical Gamma-Ray Spectrometry. John Wiley & Sons, Chichester: 1995, . Knoll G, Radiation Detection and Measurement. John Wiley & Sons, Inc. NY:2000, . Nucleonica Wiki. Gamma Spectrum Generator. Accessed 8 October 2008. References External links Amateur gamma spectrometry of a chunk of a black mold picked in Minamisoma, close to the Fukushima Dai-ichi nuclear plant. Japan.