# Performance Evaluation of an Imaging Radiation Portal Monitor System
## 4 3D localization of SNM  
As discussed above, the capability to measure fast time correlations between gamma rays and neutrons affords the KTH RPM prototype with an increased sensitivity for detection of small quantities of SNM while maintaining the FAR at a minimum. However, an even more radical consequence of this detection mode is that it enables a simultaneous rapid imaging of SNM in the field of view [2].  
The capabilities to locate a \({}^{252}\)Cf source using the NGET technique have been studied for a bare source and in different shielding conditions. The \({}^{252}\)Cf source, see Section 3.3.1, was oriented with its long axis along z as defined in Figure 1. Shielding was applied by placing the source at the center of plastic and lead cylinders in order to investigate the influence of the shielding on the spatial resolution. The plastic cylinder was manufactured from ultra-high molecular weight polyethylene (PE1000) with 40 mm radial thickness and 83 mm height while the lead cylinder had a 16 mm radial thickness and 105 mm height. Figure 3 shows the results of event-by-event NGET image reconstruction for the bare and shielded 1.25 uCi \({}^{252}\)Cf source described in Section 3.3.1. The panels show probability density distributions based on Bayesian inference projected on the \(y-z\) plane. The data was acquired for three different positions of the source; (\(x\), \(y\), \(z\)) = (0, \(-\)30, 22) cm, (0, 0, 52) cm, and (0, 30, 82) cm, each during a 10 s measurement time. Figure 4 illustrates the resulting 3D-representation of the source inside the plastic cylinder with 2D-projections on the \(x-y\), \(x-z\) and \(y-z\) planes.  
In order to evaluate the spatial resolution of the system the NGET algorithm was applied in 1800 different 10 s measurements. The 1.25 uCi \({}^{252}\)Cf source was placed in the central position (\(x\), \(y\), \(z\)) = (0, 0, 52) cm, without shielding and with PE1000 and lead shielding, respectively. Table 5 specifies spatial resolution (\(\sigma_{xyz}\)) obtained from Figure 5. Figure 5 shows projections on the \(x\), \(y\), and \(z\) axes of the three-dimensional spatial distributions of the most probable source position obtained in each 10 s measurement.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Source Type**} & \multirow{2}{*}{**Activity (kBq)**} & **Sigma Multiplier** & **Probability of** \\  & & **Factor** & **False Negative** \\  & & **N** & \(p_{N}\) \\ \hline \({}^{133}\)Ba lab source & 44 & 8 & \(3\times 10^{-4}\) \\ \hline \({}^{133}\)Ba ANSI N42.35-2016 & 518 & 865 & 0 \\ \hline \({}^{137}\)Cs lab source & 184 & 28 & 0 \\ \hline \({}^{137}\)Cs ANSI N42.35-2016 & 592 & 249 & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Alarm test results for the gamma sources \({}^{133}\)Ba and \({}^{137}\)Cs as well as corresponding values calculated for ANSI N42.35-2016 specified gamma source activities.  
Figure 4: 3D representation of the localization of the 1.25 \(\upmu\)Ci \({}^{252}\)Cf source shielded with the lead cylinder, with projections in the \(xy\), \(xz\), and \(yz\) planes. The source was placed at a position \((x,y,z)\) = (30, \(-\)30, 52) cm. The measurement time used for producing the image was 10 s.  
Figure 3: Source localization results based on Bayesian inference applied to event-by-event data. Each panel corresponds to a 10 s measurement of the 1.25 \(\upmu\)Ci \({}^{252}\)Cf source. The source was placed at \((x,y,z)\) = (0, 0, 52) cm. The image panels show projected probability density distributions in the \(y-z\) plane for the bare source (**left**), inside the PE1000 shielding (**middle**), and inside the lead shielding (**right**). The estimated probability of finding the source per cm\({}^{2}\) pixel is indicated by the color scale on the right. See text for details.