Čerenkov radiation from a finite trajectory of electrons

Abstract

Properties of radiation emitted by relativistic electrons traveling through a limited trajectory of length L in helium and nitrogen gas at various pressures are experimentally investigated in the visible and the millimeter-wave regions. The radiation was generated by short bunches of electrons from a linear accelerator. In the long-wavelength region the \ifmmode \check{C}\else \v{C}\fi{}erenkov threshold was blotted out, and the observed radiation showed characteristics of transition radiation even if the \ifmmode \check{C}\else \v{C}\fi{}erenkov criterion \ensuremath{\beta}n>1 was fulfilled. An alternative criterion \ensuremath{\beta}(n-\ensuremath{\lambda}/L)>1 is proposed on the basis of the consideration of the formation zone.