Absolute validation of nuclear resonance fluorescence models and cross sections in $^{238}$U and $^{27}$Al.

We performed absolute cross section validation experiments for three nuclear resonance fluorescence (NRF) transitions near 2.2 MeV in $^{27}$Al and $^{238}$U using the measurements of transmission NRF count rates in Vavrek et al., PNAS 115 (17), 2018. Absolute NRF photon count rates observed in data and predicted by extensive modeling agree to within ${sim}20%$ in the $^{238}$U NRF transitions and $9%$ in $^{27}$Al, for an average $14%$ discrepancy across the entire study. Such agreement provides approximate but absolute validation of the literature NRF cross section parameters and good predictive capability for the design and analysis of future NRF experiments. Moreover, the close agreement in $^{27}$Al affirms the validity of using $^{27}$Al as a normalization target in past experiments. Inverting the analysis, new values of absolute level widths (for both $^{238}$U and $^{27}$Al) and branching ratios (for $^{238}$U) are also obtained.