Editing Gamma-ray spectrometer (section)

==Gamma-ray spectroscopy==
[[File:Scintillation counter as a spectrometer.jpg|right|thumb|Laboratory equipment for determination of γ-radiation spectrum with a scintillation counter. The output from the scintillation counter goes to a Multichannel Analyser which processes and formats the data.]]
Atomic [[atomic nucleus|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 [[Spectroscopy|optical spectroscopy]].
(Note that photons in the short-wavelength high-energy end of the atomic spectroscopy energy range (few [[Electronvolt|eV]] to few hundred [[keV]]), generally termed [[X-ray]]s, 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 (physics)|scintillation]] (often sodium iodide (NaI) or caesium iodide, (CsI) spectrometers), often suffices for such applications.