# State-of-the-Art Mobile Radiation Detection Systems for Different Scenarios
## 4 Mobile Radiation Detection Systems
### Combination of Neutron and Gamma Detection Systems  
According to Section 2.2, the use of neutron detection together with gamma spectrometry is often considered and used for security-related scenarios, as the illicit trafficking of SNM and radioactive sources and materials. Neutron detection systems are normally composed by three components: (i) a material to convert neutrons into charged particles or gamma rays, (ii) a sensitive volume to convert the resultant charged particles or gamma rays into an electric signal or light, and (iii) the data acquisition system. Examples of converter materials and their absorption cross section for thermal neutron detection are: \({}^{3}\)He (5300 b), \({}^{6}\)Li (940 b), \({}^{10}\)B (3800 b), and \({}^{156}\)Gd (61000 b). These materials are normally used in proportional counters or scintillators. Semiconductor detectors are expensive and, therefore, usually used in neutron imaging or neutron spectrometry. Most detectors used in nuclear security feature a higher sensitivity to thermal neutrons; therefore, a moderator can be used to slow down (mainly by elastic scattering) fast neutrons to increase the likelihood of their detection [19]. The detection of fast neutrons can be done directly by measuring the energy released by the recoil particles (nuclei) due to the elastic scattering of neutrons. The scattering material normally has a significant hydrogen content for higher energy transfer due to neutron elastic scattering. Examples of fast detectors are: stilbene, organic liquid scintillators with good pulse shape discrimination (PSD), PSD plastic scintillator (cheaper but PSD is worse than stilbene), PSD liquid scintillator (relatively inexpensive), and gas scintillator (e.g., \({}^{4}\)He). Unlike thermal neutrons, the detection of fast neutrons allows determination of the direction of the neutrons (source localization and neutron imaging) and the source type (e.g., AmBe, fission, shielded) [19]. In order to have compact, lightweight, and lower power consumption detectors, dual-mode sensors can also be used to detect simultaneously neutrons and gamma-rays. The discrimination of either type of radiation is normally made by means of PSD. Some examples of dual-mode detectors are: organic liquid scintillators (e.g., NE-213, EJ-309), \(\rm{Cs_{2}LiYCl_{6}Ce^{3+}}\) (CLYC) scintillator (energy resolution 4.5-5%) [19]. The detection of neutrons in CLYC sensor is based on the \({}^{6}\)Li reaction. Typical gamma energy resolution is less than 5% at 662 keV. Other compact dual-mode scintillators are: NaI(Tl+Li) [101], \(\rm{Cs_{2}LiLaCl_{6}(Ce)}\) (CLLC) with an energy resolution of 3.4% at 662 keV, and \(\rm{Cs_{2}LiLaBr_{6}(Ce)}\) (CLLB) with an energy resolution of 2.9% at 662 keV. Despite the fact that CLLB has better energy resolution, it features a worse PSD capability compared to CLYC or CLLC. Both CLLC and CLLB crystals present self-activity due to the presence of La-138 [19]. A compact CLYC detector was integrated in a multi-rotor for radiation safety and security applications (Figure 14a) [200, 201]. The UAV can be used for monitoring, mapping, and source localization purposes (e.g., using the maximum likelihood estimation algorithm). [PERSON] and [PERSON] [201] used the robot operating system (ROS) to implement a \"plug and fly\" concept and perform the data analysis and fusion functions on board. These data are timestamped and georeferenced to be sent to the user in real time. Since security scenarios are normally characterized by weak sources or shielded sources, ground-based platforms, such as cars or trucks, are essential to carry large detection systems (\(\sim\)1 m\({}^{2}\)) [202]. However, larger detectors also measure more background events and are more sensitive to background fluctuations [202, 203]. [PERSON] et al.