# Performance Evaluation of an Imaging Radiation Portal Monitor System
## 3 RPM Detection Performance Results
### Radiation Source Tests  
The RPM prototype system was tested for its response to the passage of standard radioactive sources \({}^{252}\)Cf, \({}^{133}\)Ba, and \({}^{137}\)Cs. The sigma multiplier factor \(N\)[16] and the false-negative probability, \(p_{N}\), (i.e., the probability that the passage of the source was not detected) were determined for a one-second interrogation time period. This means that the source moves through the RPM at a speed of 1.2 m/s, covering the distance from \(-\)0.6 m to  
\begin{table}
\begin{tabular}{c c} \hline \multicolumn{2}{c}{**Counts/s**} \\ \hline Single \(\gamma\)-rays & 5189.0 \(\pm\) 0.2 \\ \hline Single neutrons & 1.000 \(\pm\) 0.002 \\ \hline \(\gamma\)–neutron coinc. & 0.0030 \(\pm\) 0.0002 \\ \hline \(\gamma\)–\(\gamma\) coinc. & 63.00 \(\pm\) 0.02 \\ \hline Neutron-neutron coinc. & 0.0021 \(\pm\) 0.0001 \\ \hline \end{tabular}
\end{table}
Table 1: Measured background count rates without detector shielding. The energy thresholds for gamma rays and neutrons were 20 keV and around 500 keV, respectively.  
\begin{table}
\begin{tabular}{c c c} \hline \multicolumn{2}{c}{**Detection Mode**} & \multicolumn{1}{c}{**Threshold (Counts/s)**} & \multicolumn{1}{c}{**Threshold (\(\sigma_{\text{B}}\))**} \\ \hline Single \(\gamma\)-rays & 5477 & 4 \\ \hline Single neutrons & 5 & 4 \\ \hline \(\gamma\)-neutron coinc. & 1 & 18 \\ \hline \(\gamma\)-\(\gamma\) coinc. & 95 & 4 \\ \hline Neutron-neutron coinc. & 1 & 21 \\ \hline \end{tabular}
\end{table}
Table 2: Adopted alarm trigger thresholds, \(\gtrsim\)4\(\sigma\) above mean background count rates.  
+0.6 m with respect to the plane through the center of the RPM assembly, as prescribed by the ANSI N42.35-2016 standard [9]. The ANSI N42.35-2016 standard document describes a series of measurements using eight different standard radiation sources, each with its specified activity. We here present the results obtained with two different gamma-ray sources (\({}^{133}\)Ba and \({}^{137}\)Cs) and one neutron source (\({}^{252}\)Cf). These results are deemed to be representative of the overall performance, especially since the detection threshold of the KTH Imaging RPM prototype system for gamma rays (20 keV) is significantly below the threshold of 40 keV required by the ANSI N42.35-2016 standard.  
The passing of each source through the RPM was simulated by means of a series of static measurements covering a range of distances along a line parallel with the x-axis through the center of the RPM, see Figure 1 and Tables 3 and 4. The sigma multiplier factor is defined by the condition that the sum of the mean number of background counts \(<\)C\({}_{\rm B}\)\(>\) and the product of its standard deviation \(\sigma_{\rm B}\), and the sigma multiplier factor \(N\), equals the mean counts measured with the source present during a given interrogation time:  
\[<C_{\rm B}>+N\times\sigma_{\rm B}\equiv<C_{\rm S+B}> \tag{1}\]  
The individual detectors were gain matched using standard calibration sources, using the internal conversion K X-ray photopeak and Compton edge for \({}^{137}\)Cs (32 keV and 478 keV, respectively), and the 59.5 keV photopeak of \({}^{241}\)Am.