# Performance Evaluation of an Imaging Radiation Portal Monitor System
## 5 Discussion  
Rapid and efficient detection of nuclear and other radioactive materials using radiation portal monitors constitutes one of the critical links in the global efforts against nuclear terrorism and the proliferation of nuclear weapons. Additionally, the possibility of rapid, precise, and automatic location of SNM detected at security checkpoints would tremendously speed up the response time and reduce or remove the need for on-site personnel trained in radiation detection. It would also enable potentially critical time savings for nuclear emergency responders and facilitate the work of safeguards inspectors. It has been demonstrated that by adding the capabilities to measure detailed correlations in time and space between gamma rays and fast neutrons from nuclear fission in an RPM system based on an array of organic scintillators, an increased sensitivity and selectivity for the notoriously elusive SNM, such as plutonium, can be achieved while maintaining FAR at a very low level. Using the NGET imaging modality, such a detection system is then also capable of rapidly and precisely locating SNM within the field of view. Based on these principles, an imaging RPM prototype system suitable for pedestrian and package (or conveyor) screening has been designed and tested at KTH Royal Institute of Technology. The novel imaging method has superior performance in terms of spatial resolution and detection efficiency [2] compared with neutron scatter imaging (see, e.g., Ref. [17]). The spatial resolution from around 2 up to a few cm obtained in this work is remarkable considering the several times larger dimensions of the detector cells used in the current version of the Phase I KTH RPM prototype system. The results also indicate a robustness of the NGET imaging technique against the presence of moderate amounts of shielding  
Figure 5: Spatial resolution obtained from event-by-event NGET image reconstruction of the \({}^{252}\)Cf source. The results were obtained for 1800 different 10 s measurements. The three panels show projections of the three-dimensional spatial distributions on the \(x\), \(y\), and \(z\) axes of the most probable source position determined in each measurement. Results are given for the bare source (black line) and inside PE1000 and lead shielding (blue and red line, respectively). The \({}^{252}\)Cf source was kept fixed in the position (\(x\), \(y\), \(z\)) = (0, 0, 52) cm during the measurements.  
\begin{table}
\begin{tabular}{c c c c} \hline  & \(\sigma_{x}\) & \(\sigma_{y}\) & \(\sigma_{z}\) \\ \hline bare \({}^{252}\)Cf & 2.09 \(\pm\) 0.04 & 2.16 \(\pm\) 0.04 & 2.89 \(\pm\) 0.05 \\ \hline \({}^{252}\)Cf + PE1000 & 3.20 \(\pm\) 0.07 & 2.65 \(\pm\) 0.05 & 3.97 \(\pm\) 0.08 \\ \hline \({}^{252}\)Cf + lead & 4.8 \(\pm\) 0.2 & 4.76 \(\pm\) 0.09 & 6.2 \(\pm\) 0.2 \\ \hline \end{tabular}
\end{table}
Table 5: Spatial resolution (\(\sigma_{xyz}\)) obtained from 1800 10-second measurements of the 1.25 \(\upmu\)Ci \({}^{252}\)Cf source.  
materials of different types. Future developments of the imaging RPM concept will include comprehensive studies of the localization capabilities for sources placed inside a variety of shielding materials and matrices, as well as further optimization of the image reconstruction of multiple and spatially distributed sources.