HEAO 3, and the Ge gamma-ray spectrometer (SPI) on the ESA INTEGRAL mission are examples of cosmic spectrometers, while the GRS on the SMM and the imaging Ge spectrometer on the RHESSI satellite have been devoted to solar observations. Planetary gamma-ray spectrometers Gamma-ray spectrometers have been widely used for the elemental and isotopic analysis of bodies in the Solar System, especially the Moon and Mars. These surfaces are subjected to a continual bombardment of high-energy cosmic rays, which excite nuclei in them to emit characteristic gamma-rays which can be detected from orbit. Thus an orbiting instrument can in principle map the surface distribution of the elements for an entire planet. Examples include the mapping of 20 elements observed in the exploration of Mars, Eros and the Moon. They are usually associated with neutron detectors that can look for water and ice in the soil by measuring neutrons. They are able to measure the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon. Knowing what elements are at or near the surface will give detailed information about how planetary bodies have changed over time. To determine the elemental makeup of the Martian surface, the Mars Odyssey used a gamma-ray spectrometer and two neutron detectors. GRS instruments supply data on the distribution and abundance of chemical elements, much as the Lunar Prospector mission did on the Moon. In this case, the chemical element thorium was mapped, with higher concentrations shown in yellow/orange/red in the left-hand side image shown on the right. How a GRS works Some constructions of scintillation counters can be used as gamma-ray spectrometers. The gamma photon energy is discerned from the intensity of the flash of the scintillator, a number of low-energy photons produced by the single high-energy one. Another approach relies on using Germanium detectors - a crystal of hyperpure germanium that produces pulses proportional to the captured photon energy; while more sensitive, it has to be cooled to a low temperature, requiring a bulky cryogenic apparatus. Handheld and many laboratory gamma spectrometers are therefore the scintillator kind, mostly with thallium-doped sodium iodide, thallium-doped caesium iodide, or, more recently, cerium doped lanthanum bromide. Spectrometers for space missions conversely tend to be of the germanium kind. When exposed to cosmic rays (charged particles from space thought to possibly originate in supernova and active galactic nuclei),