BEYOND THE CONTINUUM POTENTIAL APPROXIMATION: CHANNELING RADIATION FROM NONIDEAL CRYSTALS.

摘要

Channeling radiation (CR) is a novel form of X-ray and ;Several experimental and theoretical examples are cited to illustrate the successes and failures of the continuum potential approximation (CPA.) A detailed comparison of theoretical calculations with observations of planar channeling radiation from silicon demonstrates that the CPA makes accurate predictions of the centroids and intensities of CR peaks. The excellent correspondence between theory and observation makes it possible to refine the accepted value for the thermal vibration amplitude of the nuclei in crystalline silicon.;However, since the CPA neglects the statistical fluctuations of the crystal potential, it is inherently incapable of explaining the scattering of channeled particles. An analysis of CR spectra reveals that the coherence lengths of channeled electrons with energies between 17 and 54 megaelectron volts are a few tenths of a micron, whereas the occupation lengths of electrons and positrons are two orders of magnitude greater.;In order to explore the effects of natural or man-made periodicities of the crystal potential in the direction of relativistic motion upon channeled particles, the quantum theory of channeling and channeling radiation is extended beyond the CPA. New techniques for solving the wave equation for channeled particles in periodic three-dimensional potentials are introduced and used to make detailed calculations of two new types of radiation--"longitudinal channeling radiation" (LCR) and "superlattice channeling radiation" (SCR). Attention is given to the experimental conditions under which these new types of radiation are likely to be observed, and possible applications as material probes and as X- or ;In order to make a quantum-electrodynamical analysis tractable, it is customary to average the actual crystalline potential over statistical fluctuations and over variations in the direction of relativistic motion. The resultant idealized periodic one-dimensional potential (for planar channeling) or two-dimensional potential (for axial channeling) is called the "continuum potential.".