Sub-picosecond lifetime measurements by gamma ray induced Doppler broadening

The gamma ray induced Doppler broadening (GRID)-technique is reviewed. This experimental method, relying on ultra-high resolution gamma ray spectroscopy, involves the direct measurement of the Doppler broadening of gamma rays emitted when nuclei decay in-flight following recoil induced by preceding gamma rays. The technique permits the deduction of absolute transition rates in atomic nuclei formed after neutron capture and can therefore contribute to obtaining crucial information for sensitive tests of nuclear models. The method can be used for the study of lifetimes below a few picoseconds, but also, inversely, allows the investigation of interatomic collisions at very low energies. Thus one can probe the repulsive interatomic potential in solids. The characteristic features of the GRID technique are discussed and illustrated by some selected examples. Different approaches to model the slowing-down process at low energies are presented.