# Performance Evaluation of an Imaging Radiation Portal Monitor System
## 2 Materials and Methods  
### Detection System  
Radiation portal monitors are installed at nuclear facilities, air and seaports, border crossings, cargo terminals, etc. to screen persons, vehicles, and other objects in order to detect and prevent the trafficking of radioactive materials. Basic systems are sensitive only to gamma-emitting radioactive sources, while the detection of plutonium and other types of SNM also warrants detection of neutrons. Currently, the most commonly used neutron detectors in nuclear safeguards and security systems, such as RPMs, are \({}^{3}\)He thermal neutron gaseous proportional counters [4, 5]. Nevertheless, there is a rising interest in replacing \({}^{3}\)He detectors with other technologies driven by the global shortage of \({}^{3}\)He [6]. Besides replacing \({}^{3}\)He with other high-neutron-absorption cross-section materials for thermal neutrons like boron and lithium, there is also an increasing focus on fast-neutron detection using organic scintillators, high-pressure \({}^{4}\)He systems [7], etc. Organic scintillators have been found to be competitive alternatives in terms of intrinsic detection efficiency compared with with the \"gold standard\" \({}^{3}\)He-based systems [8]. Furthermore, while thermal or epithermal neutrons typically have scattered multiple times in surrounding materials on their way from the source, fast neutrons preserve some direct information about the initial emission process and its location. Thanks to their generally excellent timing properties this leads to key advantages for organic scintillator-based detection systems for background suppression and selectivity by using fast time correlations between the detected particles. The phase I RPM prototype system developed at KTH consists of eight organic scintillator detector cells, placed in two vertical pillars (see Figure 1). The current mechanical support structure is adapted to the ANSI N42.35-2016 industry standard [9] and is suitable for different package (or conveyor) and pedestrian RPMs, but it can easily be modified to different scenarios and applications. The mechanical structure can support up to 50 detector cells, 20 horizontally oriented in each pillar and 10 detector cells at the top of the structure. Organic scintillators were chosen as detection medium because they are sensitive to both gamma rays and fast neutrons with high efficiency and are available with pulse-shape discrimination (PSD) capabilities for distinguishing between the two types of particles. In addition, organic scintillators typically have excellent timing properties, of the order of 1 ns or better (FWHM), depending on the particle energies, detector geometry, and the properties of the sensors used for reading out the scintillation light. In the current configuration of the KTH Phase I RPM prototype system, detector cells of cylindrical shape with 127 mm diameter by 127 mm length were mounted in a zig-zag pattern in the mechanical support structure. This configuration is rather compact and was adapted for use in radioactive waste scanning applications. It therefore conforms most closely to the package/conveyor RPM category specified in the ANSI N42.35-2016 standard document. Each detector cell contains approximately 1.6 l of EJ-309 liquid organic scintillator [10] and it is optically coupled to a Hamamatsu R1250 photomultiplier tube (PMT). The PMTs are powered by two four-channel CAEN DT5533 high-voltage power supplies and their anode pulses were collected and digitized using an eight-channel CAEN DT5730 digitizer board. The digitizer board has a 2-Vpp dynamic range, 14-bit resolution, 500 MHz sampling rate, and features firmware programmable PSD capabilities for distinguishing between gamma rays and neutrons using field programmable gate arrays (FPGA).