X-Ray generator regulation with high energy tail windows

The techniques and device provided herein relate to regulating a source generator in X-ray based measurement for downhole applications. A source stream of photons is produced, via a generator of an X-ray system of a logging tool. A direct channel allows for the passage of a stream of photons, where a high energy filter filters a low energy part of the stream of photons. The resultant stream is measured by a reference detector to identify a high energy peak in a spectrum measurement derived based upon the resultant photon stream. From there, a normalized difference between a plurality of windows of the high energy peak is determined and subsequent output of the generator is based upon the normalized difference.