# State-of-the-Art Mobile Radiation Detection Systems for Different Scenarios
## 4 Mobile Radiation Detection Systems
### Gamma Imaging
#### 4.3.1 Compton Cameras  
[PERSON] et al. [182; 183] developed a SiPM-based Compton telescope for safety and security (SCoTSS) to be used by nuclear emergency response and security teams. The SCoTSS was the first Compton gamma imager based on solid scintillators coupled to SiPM, performing as a directional survey spectrometer and a source localization equipment. It consists of pixelized layers of crystal scintillators made by two scatter layers of CsI(Tl) scintillating crystals coupled to SiPMs and a absorber layer of NaI(Tl) scintillating crystals coupled to PMTs. The small and lightweight SiPMs allow to reduce the scattering of gamma-rays in the dead material. Since this camera is modular, it can be used as a handheld or backpack instrument (single module with 4 \(\times\) 4 array of crystals) or installed in a mobile platform (e.g., truck) using the 3 \(\times\) 3 module configuration. This latter configuration can achieve a localization precision better than 2\({}^{\circ}\) for an integration time of 3 s using a 1 mCi Cs-137 source at a 10 m distance. This performance anticipates their use for imaging in motion. For field measurements, it will be necessary to account for the control gain variation of SiPM due to their temperature dependence. Figure 10: Basis of gamma cameras operation: (**a**) Pinhole camera with a single hole. Reproduced, with permission, from Reference [22]; copyright Elsevier (2013). (**b**) Compton camera. Reproduced, with permission, from Reference [22]; copyright Elsevier (2013). (**c**) Coded-aperture camera. Incident radiation (gamma-rays or neutrons) pass through a coded-aperture mask creating an image in a position sensitive detector. Reproduced, with permission, from Reference [21]; published by MDPI, 2019, under the terms of the [[https://creativecommons.org/licenses/by/4.0/CC](https://creativecommons.org/licenses/by/4.0/CC)]([https://creativecommons.org/licenses/by/4.0/CC](https://creativecommons.org/licenses/by/4.0/CC)) BY 4.0 license. A prototype Compton camera carried by an unmanned helicopter (Figure 11a) for localizing radionuclides was developed by [PERSON] et al. (2016) [178]. The gamma camera was composed by two arrays of Ce:Gd\({}_{3}\)(Al,Ga)\({}_{5}\)O\({}_{12}\) (Ce:GAGG) crystals coupled to SiPMs (scatter array) and to APDs (absorber array). This work was followed by [PERSON] et al. [184], who tested an improved version of the gamma camera nearby the FDNPP site. The choice of Ce:GAGG crystals was mainly due to their low cost (compared to NaI and LaBr\({}_{3}\)), sufficient energy resolution to discriminate the \({}^{137}\)Cs and \({}^{134}\)Cs peaks (between 600-700 keV), high light yield, and good Compton efficiency. Compton images were obtained while the helicopter was hovering, e.g., during 5 min at a height of 10 m was enough to obtain the gamma-ray image of a 400 m\({}^{2}\) area with a dose rate between 5-12 \(\mu\)Sv/h. The fusion technique of the 3D scene with the contamination distribution is amongst future improvements needed. The application of this system for the fast evaluation of decontamination efforts, hotspots search, and assessment of the distribution and mobility of \({}^{137}\)Cs over large areas of difficult access in Fukushima (e.g., forests, riverbeds, residential areas) can be anticipated. The first aerial demonstration of a Compton camera carried by a multi-rotor was achieved by [PERSON] et al. (2017) [185]. The compact Compton camera also made with Ce:GAGG scintillating crystals (1.9 kg) has the following advantages: high density, high light yield, non-deliquescence, and no self-activity. This imaging method allowed faster (factor of 10) surveys compared to foot-based, measurements in places of difficult access, and better spatial resolution compared to unmanned helicopters. The most important limitation is the short flight time (10-20 m). A significant impact of the altitude accuracy in the resultant images was also reported. The use of multi-rotors UAV to carry lightweight Compton cameras was also developed by [PERSON] et al.