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X-ray parametric down-conversion is a fundamental nonlinear effect thatpromises a wide range of possible applications. Nevertheless, it has beenscarcely investigated and unequivocal evidence of the effect is still missingfor certain spectral regions. In particular, this is the case fordown-conversion into pairs of x-ray and optical photons, which would openpathways to characterise and image valence electron-density fluctuations. Inthis work, we present a systematic approach to scan the parameter space,wherein we expect to identify the effect based on its characteristic signature.Within the resolution of the experimental setup we cannot establish anyevidence for the nonlinear effect. Instead, we trace the measured signals backto elastic scattering contributions, thereby challenging the interpretation ofprevious studies on the effect of x-ray parametric down conversion producingphotons in the visible spectral range. As a benchmark for future investigationsof x-ray parametric down-conversion - both experimental and theoretical - weextract an upper bound for the effect's conversion efficiency of $10^{-11}$within the resolution of our setup.

X-ray parametric down-conversion: Challenging previous findings on the basis of improved experimental methods