A downhole tool includes a radiation generator configured to output radiation using electrical power received from a power supply. A first portion of the radiation is emitted into a surrounding sub-surface formation. The downhole tool also includes a radiation detector coupled proximate the radiation generator. The radiation detector includes a micromesh gaseous detector, and the radiation detector is configured to output a measurement signal based at least in part on interaction between a second portion of the radiation output by the radiation generator and the radiation detector. Additionally, the downhole tool includes a control system communicatively coupled to the radiation generator and the radiation detector. The control system is configured to determine measured characteristics of the radiation output from the radiation generator based at least in part on the measurement signal and to control operation of the radiation generator based at least in part on the measured characteristics.