Channeling and blocking of energetic charged particles in crystals

The development of channeling and blocking since the foundation of the field was laid by Jens Lindhard in his classical paper in 1965 is discussed, and the question is asked whether this theory has passed the test of time. Have important aspects of the theory been challenged? Where has the theory needed modification or extension? Are there still open questions to be solved? A basic theoretical issue is the applicability of classical mechanics in the description. Lindhard showed that for particles heavy compared with the electron classical orbital pictures may always be applied. However, for electrons and positrons there are strong quantal features like Bragg interference. The quantal description introduced by Lindhard and co-workers has been used as the basis for a comprehensive treatment of the channeling of MeV electrons and positrons and of channeling radiation. At very high energies, GeV and TeV, the motion becomes classical, due to the relativistic increase of the field seen by the particles in the reference frame following their longitudinal motion. Channeling radiation in this regime is still an active field of research. For channeling and blocking of ions, the concept of statistical equilibrium plays a central part in Lindhard's theory. The application of this concept has been confronted with two important challenges, the first based on computer simulations and the second on experiments on the transmission of heavy ions through thin crystals. In both cases the challenges have been met and new insight has been gained but there are still problems to be solved. The channeling and blocking of ions have found very many applications, and a few problems and opportunities worth pursuing are suggested.