Passive Radiological Inspection of Shipping Containers at Seaports using UAV-based Detectors with the Method of Spectral Comparison Ratios

Radiation detectors mounted on unmanned aerial vehicles (UAVs) can be utilised to screen freight passing through a seaport for radioactive material that could be used in radiological dispersal devices (RDDs) or improvised nuclear explosive devices. Payload and battery life restrictions limit the size of the sensor package that can be deployed on a UAV and as a result a low-cost system is likely to deploy small and lightweight detectors that produce count-starved spectra. The method of spectral comparison ratios (SCRs) is a well-studied approach to processing such data and is used in this work to process measurements obtained with a 3.8×3.8×2.5 cm CsI(Tl) scintillator mounted on a UAV. An aerial survey of an outdoor facility housing several shipping containers is carried out to obtain a "benign source population" which characterises variation and features in background radiation at the site. The benign source population is then used in combination with laboratory measurements to determine optimum bins for the detection of
¹³⁷
Cs and
⁶⁰
Co - two commercially available sources that could be used in an RDD. To account for the shielding effect of shipping container cargo on concealed sources, measurements of
¹³⁷
Cs and
⁶⁰
Co with different thicknesses of attenuating material are obtained. It is shown that the optimal bins required to detect these isotopes using the SCR method are dependent the amount of shielding present. With the optimal bins determined the SCR method is then used to process a combination of experimental data from sources concealed in shipping containers and modelled data, where source profiles are artificially injected into measurements.