Abstract

It was found that the room temperature (cold) Fe opacity was lower than experimental solar Fe opacity in the short-wavelength quasi-continuum region, which contradicts our conventional understanding of atomic theory. A caveat is that prior high-quality experimental cold Fe opacity measurements yielded uncertainties of 10%.  Room temperature Fe opacity is determined here using transmission measurements of Fe foils at three characteristic line energies in the soft X-ray range (950-2100 eV). Transmission is determined with overall errors of  ≤1%. This was achieved through simultaneous measurements of attenuated and unattenuated spectra using an X-ray source and Bragg crystal spectrometer. The required areal density is independently measured using two different techniques: Rutherford Backscattering Spectroscopy and a recently-developed technique using calibrated cold absorption in the 3-17 keV range. The present opacities are in reasonable agreement with current opacity databases, except at the highest energy, where it differs by up to 10%. More measurements are required to draw definitive conclusions about this disagreement with current opacity models.