Instruments for observations of radioactivities

Radioactivity is characterized by the emission of particles or photons that accompany the nuclear transformations of unstable isotopes. Direct observations of secondary particles (e.g. β^±, or α) are only possible with in-situ measurements in the local environment of the solar system and often the detected particles are not very specific as to their parent nuclei. Transitions between energy levels of radioactive and excited nuclei however produce characteristic X- and γ-ray lines that can be detected from astronomical distances. Only one secondary particle resulting from radioactive decay, the positron, signals its presence in a characteristic γ-ray line: positrons annihilate with their anti-particle (electrons) and convert the pair's rest mass into a line at 511 keV (see Chap. 7). Nuclear energy levels range from 10 s of keV upwards to energies of 10 s of MeV with most important astrophysical lines in the range from about 100 keV to several MeV.