A gamma-ray spectrometer (GRS) is an instrument for measuring the distribution (or spectrum—see figure) of the intensity of gamma radiation versus the energy of each photon. The study and analysis of gamma-ray spectra for scientific and technical use is called gamma spectroscopy, and gamma-ray spectrometers are the instruments which observe and collect such data. Because the energy of each photon of EM radiation is proportional to its frequency, gamma rays have sufficient energy that they are typically observed by counting individual photons. Gamma-ray spectroscopy Atomic nuclei have an energy-level structure somewhat analogous to the energy levels of atoms, so that they may emit (or absorb) photons of particular energies, much as atoms do, but at energies that are thousands to millions of times higher than those typically studied in optical spectroscopy. (Note that the short-wavelength high-energy end, of the atomic spectroscopy energy range (few eV to few hundred keV), generally termed X-rays, overlaps somewhat with the low end of the nuclear gamma-ray range (~10 MeV to ~10 keV) so that the terminology used to distinguish X-rays from gamma rays can be arbitrary or ambiguous in the overlap region.) As with atoms, the particular energy levels of nuclei are characteristic of each species, so that the photon energies of the gamma rays emitted, which correspond to the energy differences of the nuclei, can be used to identify particular elements and isotopes. Distinguishing between gamma-rays of slightly different energy is an important consideration in the analysis of complex spectra, and the ability of a GRS to do so is characterized by the instrument's spectral resolution, or the accuracy with which the energy of each photon is measured. Semi-conductor detectors, based on cooled germanium or silicon detecting elements, have been invaluable for such applications. Because the energy level spectrum of nuclei typically dies out above about 10 MeV, gamma-ray instruments looking to still higher energies generally observe only continuum spectra, so that the moderate spectral resolution of scintillation (often sodium iodide (NaI) or caesium iodide, (CsI) spectrometers), often suffices for such applications. Astronomical spectrometers A number of investigations have been performed to observe the gamma-ray spectra of the Sun and other astronomical sources, both galactic and extra-galactic. The Gamma-Ray Imaging Spectrometer, the Hard X-ray/Low-Energy Gamma-ray experiment (A-4) on HEAO 1, the Burst and Transient Spectrometry Experiment (BATSE) and the OSSI (Oriented Scintillation Spectrometer Experiment) on CGRO, the C1 germanium (Ge) gamma-ray instrument on