Work

High-precision room temperature Fe opacity measurements at 1000-2000eV photon energies

Year: 2023

Type: article

Abstract: Prior measurements of room temperature (cold) Fe opacity have errors as high as ±10% and a spread in values that exceeds the uncertainties. These data, along with current cold opacity databases, were used for comparison with experimental solar Fe opacity. The solar Fe opacity is expected to be lower than cold Fe opacity in the ∼1500-2000 eV photon energy range. However, results from this comparison contradicted this assumption and prompted an investigation of the precision and accuracy of cold Fe opacity measurements. Cold Fe opacity is determined here with high precision using transmission measurements of Fe foils at three characteristic line energies in the soft X-ray range (1000-2000 eV). The present opacities are determined with .1% overall uncertainties. This is achieved through precise measurements of transmission and areal density which are related to opacity by the Beer-Lambert Law. Transmission is measured with overall individual uncertainties of .1% and is obtained through simultaneous measurements of attenuated and unattenuated spectra using an X-ray source and a Bragg Preprint submitted to High Energy Density Physics March 6, 2023 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 final areal density used in the opacity determination is an average between both methods. The measured opacity at 1188 eV is in agreement with current opacity databases but is higher by around 5-10% at the highest photon energy (1924 eV) and lower by around 2-4% at the lowest photon energy (1012 eV). A caveat is that opacity accuracy is linearly dependent on the areal density. We performed the first direct comparison between the two areal density methods which revealed a 7-11% discrepancy between them, which would impact the opacity accuracy. This result affects the solar Fe opacity measurements as they also rely on the accuracy of these areal density measurement techniques.

Source: High Energy Density Physics

DOI: 10.1016/j.hedp.2023.101064