Absolute X-ray emission cross section measurements of Fe K transitions

We have measured the absolute X-ray emission cross sections of K-shell transitions in highly charged L- and K-shell Fe ions using the LLNL EBIT-I electron beam ion trap and the NASA GSFC EBIT Calorimeter Spectrometer (ECS). The cross sections are determined by using the ECS to simultaneously record the spectrum of the bound-bound K-shell transitions and the emission from radiative recombination from trapped Fe ions. The measured spectrum is then brought to an absolute scale by normalizing the measured flux in the radiative recombination features to their theoretical cross sections, which are well known. Once the spectrum is brought to an absolute scale, the cross sections of the K-shell transitions are determined. These measurements are made possible by the ECS, which consists of a 32 channel array, with 14 channels optimized for detecting high energy photons (hν > 10 keV) and 18 channels optimized for detecting low energy photons (hν < 10 keV). The ECS has a large collection area, relatively high energy resolution, and a large bandpass; all properties necessary for this measurement technique to be successful. These data will be used to benchmark cross sections in the atomic reference data bases underlying the plasma modeling codes used to analyze astrophysical spectra, especially those measured by the Soft X-ray Spectrometer calorimeter instrument recently launched on the Hitomi X-ray Observatory.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and supported by NASA grants to LLNL and NASA/GSFC and by ESA under contract No. 4000114313/15/NL/CB.