Ionization energy shift in iridium measured with a lutetium edge filter

Summary form only given. In an ionized atom the Kα and Kβ fluorescence lines have different, and usually slightly higher, energies than the same lines in a cold, un-ionized atom. The increase in fluorescence line energy Δ(hν) that pulsed power discharges can produce in high atomic number atoms is usually much smaller than the line energies hν themselves: typical numbers are hν = 60 keV and Δ(hν) = 10-20 eV, while the resolution obtained so far with a transmission crystal spectrometer developed for the purpose is around 100 eV. Hence any change in the energy of the fluorescence X-rays, from tungsten weakly ionized in the tungsten of the Plasma-Filled Rod Pinch (PFRP), could not be observed. It may be possible to enhance the crystal spectrometer´s resolution with a detector that has smaller pixel sizes and faster time resolution, but not within the available funding. However, a spectrometer´s effective resolution can sometimes be increased by supplementing the primary instrument with an additional element that does provide the necessary resolution albeit with other restrictions such as a narrower energy range. For the X-rays of interest here one possibility is an edge filter. Tungsten´s Kα1 at 59.3128 keV is closest to the edge in thulium, at 59.390 keV, but at Δ=78 eV too far away in energy. The pair that is closest is the Kα₂ line of iridium at 63.2867 keV and the K-edge of lutetium, at 63.314 keV, at Δ=27 eV. How a Lu K-edge filter affects the transmission of the Ir Kα₂ line depends not only on the difference between the nominal edge and line energy, but also on the edge and line shapes. Both are life-time broadened, by tens of eV. The paper discusses the estimate of the sensitivity, a 10 % decrease in line intensity for a 10 eV shift, and any results that may have been obtained to date on an iridium PFRP. If the technique works it could- be used to estimate the ionization on the inside of a NIF hohlraum capsule.