Experiments with relativistic electrons producing tunable X-rays from Cu crystals

INTRODUCTION Energy tunable Parametric X-rays (PXR) in the range of a few KeV to tens of KeV have been experimentally realized at the Rensselaer LINAC [1, 2]. PXR makes use of the interaction of relativistic electrons with periodic structures like those found in crystal. This phenomenon can be modeled as the diffraction of the “virtual photon” field associated with the relativistic electron as it traverses the crystal target [3]. Practical engineering applications of PXR have been challenged by crystal heating at higher electron beam current of greater than a few A. Two LINAC facilities have demonstrated PXR imaging [4, 5]. In both of these studies, electron beam currents were increased to A-levels, and both the Si and LiF crystal targets cracked under thermal stress. As an alternative, metallic crystals have been considered as PXR targets because of their thermal characteristics and their relatively high electric susceptibility, which is a factor in determining PXR production In previous experiments at the Rensselaer LINAC, 60 MeV electrons produced 12.0 keV and 13.6 keV PXR, respectively, from the 1-mm thick Cu222 and the W222 crystallographic planes in a Bragg geometry [7]. These experiments were proof of principle only and failed to minimize target crystal thickness for maximum PXR production. The excess crystal thickness produced excessive Bremsstrahlung, target florescence, and electron scattering. This work reports experimental PXR results using an optimized Cu thickness and makes qualitative comparison’s of these results with earlier experiments.