# Improving a PGNAA Technique to Detect Heavy Metals in Solid Samples
## 2 Experimental  
### Optimization  
The PGNAA setup used for the NIS measurements is well-described in Ref. [20]. In a nutshell, the setup consists of the Genie 16 portable neutron generator, which produces 2.5 MeV neutrons in a deuterium-deuterium (DD) reaction with a maximum intensity of \(4.7\times 10^{7}\) n/s. As suggested by the manufacturer, the dc deuteron beam is operated at 70 keV, and its current is set at 50 uA [26]. The gamma rays were observed using a CeBr\({}_{3}\) detector. The signals from the detector were finally sent to a multi-channel analyzer coupled to a PC. The data were recorded, monitored, and analyzed using ScintiVission software.  
In the previous work [20], the sample container was placed upright and in contact with the neutron target plane, and the detector faced the container such that its longitudinal axis was along the target plane of the neutron generator, and it was also touching the container. Hence, neutrons that traveled through the sample would hit the detector vertically at zero angle. Despite the fact that this increased the probability of the NIS reaction and the amount of observed gamma-ray lines, it also substantially activated the detector's elements since the alignment allowed neutrons to travel through the detector directly after leaving the sample. The sample volume inside the container was large enough to cover the front surface of the detector and the side surface of the generator tube surrounding the target plane to limit direct contact.  
As illustrated in Figure 1, the new optimization began with an empty sample to capture the background spectrum, which reflects neutron activation of the detector's material. The neutrons were then delayed by sandwiching a thin (1.0 cm thick) moderator between the empty sample and the detector, which treats the spectrum as a background moderator. The background spectrum showed a significant drop, by over 25%. The system was then re-aligned, as shown in Figure 2, with just the neutron-generating tube tilted by 30\({}^{\circ}\) to reduce the number of neutrons penetrating the sample and arriving at the detector. Finally, two tests were run, one without the moderator (background-angle) and the other with the moderator in place (background-angle-moderator). Both cases demonstrated further reductions in the background, with just minor differences between them.  
A pure titanium sample replaced the empty container in Figure 2 to explore the effects on the signal-to-noise ratio, and prompt gamma-ray spectra were obtained for each circumstance described above. Table 1 summarizes the study's findings, which reveal a nearly 100 percent increase in net counts when the neutron generator tube is tilted by 30 degrees and a moderator is introduced between the sample and the detector.  
The thickness of the moderator and the orientation of the generating tube were also put to the test. The background was reduced, and the signal-to-noise ratio was improved by increasing the tube's angle. However, this required a lengthier period of measurement, almost a few hours, to obtain accurate statistics. Increasing the thickness of the moderator to more than 1.0 cm, on the other hand, lowers the net count rates. Considering the relatively small flux of Genie 16 in comparison with strong neutron sources such as Am-Be source, \({}^{256}\)Cf source, or neutron accelerator, which could be at least two orders of magnitudes greater, in addition to the lifetime of the deuterium target for Genie 16, the most optimum and appropriate situation for our PGNAA setup that provided the maximum net counts was chosen to perform several NIS reactions, which are discussed in the next section.