The protection of first responders from radioactive contamination that may result from a radiological accident is of great importance and complexity. The complexity of detecting and quantifying such radioactive contamination is very high when it comes to the dispersion of alpha-emitting radionuclides in the environment. This work presents the development of an un-manned aerial monitoring system that integrates an unmanned aircraft system (UAS) and a novel optical alpha radioluminescence detection system to scan and obtain information about the alpha-emitting radionuclides contaminated area on a 3D map. This manuscript describes a flexible hardware and software architecture for optical detection of alpha-emitting sources. The UAS is equipped with 4G/LTE air-ground communications and onboard computation to geolocate and send to the ground control station (GCS) the measured data in real-time and thus react to any anomaly detected during the mission. A GCS has also been developed for the correct visualization of the radioactive samples in a 3D map. This article shows the results obtained from the calibration of the detector in the laboratory as well as from the different flights performed on UV LEDs and a 100 MBq 241Am source to demonstrate the feasibility of the system in a realistic scenario.