In this paper, a thermal control design for a high-resolution gamma-ray computed tomography detector is presented. It accounts for the generation of heat produced by active electronic components as well as heat transfer from external heat sources. The development and implementation of this feature were motivated by stringent requirements for measurement accuracy at thermal hydraulic test facilities, where ambient thermal conditions are constantly changing. As a first step, the thermal behavior of the existing tomography detector was analyzed, critical components were identified, and different approaches for heat removal were tested. Eventually, an improved thermal detector design was elaborated and a controlled active cooling system was implemented. Performance tests proved its effectiveness and accuracy improvement.