This paper details the system and methods developed to enable autonomous nuclear radiation source localization and mapping using aerial robots in GPS-denied environments. A Thallium-doped Cesium Iodide (CsI(Tl)) scintillator and a Silicon Photomultiplier are combined with custom-built electronics for counting and spectroscopy, and the provided radiation measurements are pose-annotated using visual-inertial localization enabling autonomous operation in GPS-denied environments. Provided this capability, a strategy for radioactive source localization, as well as active source search path planning was developed. The proposed method is motivated and accounts for the limited endurance of the vehicle, which entails a very small amount of dwell points, and the fact that GPS-denied localization implies varying uncertainty of the robot's position estimate. The complete system is evaluated in multiple experimental studies using a small aerial robot and a Cesium-137 radiation source. As shown, accurate radioactive source localization is achieved, enabling efficient radiation mapping of indoor GPS-denied environments.