A system for a downhole tool includes an X-ray generator that outputs X-ray radiation into a surrounding sub-surface formation using electrical power received from a power supply and a detector device adjacent the X-ray generator. The detector device includes a first detector with a first radiation sensitivity that outputs a first measurement signal based on interaction between a first portion of the X-ray radiation and the first detector. The detector device also includes a second detector with a second radiation sensitivity adjacent the first detector, where the second detector outputs a second measurement signal based on interaction between a second portion of the X-ray radiation and the second detector. Further, the detector device includes a control system communicatively coupled to the X-ray generator and the detector device, where the control system determines characteristics of the X-ray radiation output from the X-ray generator based at least in part on a radiation model that describes expected relationship between the characteristics of the X-ray radiation and doses indicated by the first measurement signal and the second measurement signal.