Parametric X-ray radiation as a conversion of virtual to real quanta

The emission of parametric X-ray radiation (PXR) by a relativistic charged particle moving in a crystal rectilinearly is considered in terms of a conversion of virtual quanta accompanying the particle to real quanta. Some recent experimental data on the PXR properties are analyzed and the asymmetry of PXR reflection is considered. Simple analytical expressions for conversion factors are obtained from experimentally confirmed data. The properties of the conversion factors for the parametric X-ray radiation are discussed.