First experimental assessment of the Z opacity sample evolution using time-resolved spectroscopy with a gated hybrid CMOS detector.*

The discrepancy between opacity models and laboratory experiments injects uncertainty into stellar interior models. The model/data discrepancy in Fe opacity measurements at high temperature (T \textgreater 180 eV) and high electron density 3x10$^{\mathrm{22}}$ cm$^{\mathrm{-3}}$ [Bailey et al, \textit{Nature} (2015), Nagayama et al. \textit{PRL} (2019)] have yet to be resolved. Systematic errors from unresolved temporal gradients are one possible hypothesis, despite evidence that such errors are unimportant. Past data recorded on x-ray film provide spectral measurements over a time determined by the backlighter time history, but direct sequential time-resolved measurements didn't exist; until now. The novel hCMOS Ultra-fast X-ray Imager (UXI) camera developed at Sandia National Laboratories and implemented in the opacity spectrometers allows such tests for the first time. Mg K-shell absorption was recorded to measure the opacity sample evolution. These measurements enable further evaluation of possible temporal gradient effects, test simulation predictions, and to optimize future opacity experiment designs.