Downhole deployable tools for measuring tracer concentrations are disclosed. According to one embodiment, an apparatus comprises a support cable configured to allow the apparatus to be lowered into and raised from a wellbore. A housing is attached to the support cable. The housing includes a detector window on the exterior of the housing. A detector system within the housing includes a detector that measures tracer concentrations. The detector is operably connected to the detector window to direct energy or particles from the detector window to the detector.