Precision Evaluation of the High-Frequency Limit of the Continuous X-Ray Spectrum Using a Gas Target X-Ray Tube

The major uncertainty in previous measurements of the high-frequency limit of the continuous x-ray spectrum was due to the fine structure associated with the observed cutoff. The development of a high-power mercury vapor target x-ray tube permitted observations to be made under idealized thin-target conditions. The x-ray intensity near the high-frequency limit was comparable to that of a solid target x-ray tube. The x-ray intensity was measured as a function of the voltage applied across the tube, with a double-crystal monochromator adjusted for the 1.537400 kxu wavelength of the Cu Kα₁ line. The absence of fine structure in the observed isochromat, in contrast to most solid targets, greatly increased the accuracy in the determination of the voltage cutoff. Accurate measurements of this voltage yielded a voltage-wavelength conversion factor Vλₛ=(12372.26±23 ppm) xu-volts. Using h/e from non-x-ray data, the x-ray wavelength conversion factor from x units to cm (or angstroms) can be calculated, or conversely h/e evaluated. The effects of a revision in the Cu Kα₁ wavelength, which is now in publication by one of the authors, are also discussed.