Utilization of MeV Van de Graaff Electrons to Produce Characteristic X-Rays for Precision Measurements

Electrons from a 4 MeV Van de Graaff accelerator are used to produce intense sources of characteristic X-rays from a sampling of mid-to-high Z elements. Thick solid targets and high pressure gas samples are bombarded by up to 50 microamperes of 2.5 MeV electrons. The X-rays are viewed in the backward direction to reduce the background resulting from bremsstrahlung radiation. Wavelengths and widths of the X-rays are measured with a precision two-axis flat crystal spectrometer. The spectrometer is equipped with angle interferometers having an accuracy of a few tenths of a milliarc second. The lattice spacing of the crystals has been measured in terms of the wavelength of visible light by means of simultaneous X-ray and optical interferometry and is consistent with the scale used in theoretical calculations. The accuracy of the new wavelength measurements is limited to 3 to 5 ppm by the intrinsic line width of the X-rays. Comparison of the new more reliable wavelength values with relativistic theoretical calculations has resulted in revisions of the theoretical calculations. The large discrepancies (~ 10 eV) which existed in 1979 have been significantly reduced (~ 2 eV). Larger discrepancies for Z > 92 suggest that additional precision measurements need to be made in this region.