Detection of energetic neutral atoms

The techniques for the detection of energetic neutral atoms (ENAs) currently in use and those developed for future application in space research are reviewed. Various sources of ENAs are currently the target of intense investigations. Typically, ENAs are produced from plasma ions by charge-exchange with a neutral background gas. In addition, the neutral interstellar gas penetrating the heliosphere is also an important source of ENAs. Since ENAs travel virtually unperturbed for very long distances they can be used for remote sensing of space plasma populations, for objects ranging from planetary magnetosphere at all scales to the quite distant heliospheric termination shock. The ENA sources and their respective energy ranges and fluxes are discussed briefly. The energy range of ENAs accessible to direct observation spans from about 10 eV to more than 1 MeV. On the high-energy side, the energy limit for ENAs is given by experimental limitations, but there are also good scientific reasons why ENA fluxes should be negligible at these energies. At the low-energy side, the limit is given by the available instrumentation. Several fundamentally different experimental techniques are necessary to cover such a large energy range. Moreover, not just the mere detection of the ENAs is desired but also the measurement of their arrival direction, possibly in two dimensions, is needed for many applications.