# A Novel Method for Spectrometry Based on Imaging Systems
## 2 Materials and Methods
### Methods  
The CRY018 surface has been simultaneously irradiated by using 2.5 mm \({}^{57}\)Co and \({}^{133}\)Ba collimated sources in correspondence of an anode positioned near the center of the crystal surface. Both radionuclides are point sources absorbed in an ion-exchange bead approximately 1 mm in diameter. The bead is then fixed inside a solid plastic capsule 25 mm in diameter and 3 mm thick as shown in Figure 3. These sources are produced by \"Eckert & Ziegler Nuclitec GmbH\" and supplied with a proper certificate of calibration (BDR8152 and CTR8152, respectively, for \({}^{133}\)Ba and \({}^{57}\)Co product codes) and are characterized by the following activities: \({}^{133}\)Ba 2.51 MBq and \({}^{57}\)Co 1.96 MBq [22]. In Table 1, X-ray and \(\gamma\) emissions of the radioactive sources along with their branching ratio are reported. Figure 3: Schematic view of the radioactive housing [22]. Figure 2: On the top part, a picture of the whole electronic system: four analog readout boards, a docking board and the FPGA based control board. On the bottom part, a block diagram representing the board configuration [21]. The radioactive sources were directly arranged on the detector surface and housed inside a lead collimator (from the top, respectively, \({}^{133}\)Ba and \({}^{57}\)Co) of 1.25 mm radius and 39.45 mm length. The collimated geometry defines the most accurate solid angle resulting in a geometric efficiency of \(2.5\times 10^{-4}\) and in this case, defines a count rate of about 630 counts/s and 490 counts/s, respectively, for \({}^{133}\)Ba and \({}^{57}\)Co sources, corresponding to a maximum dead time of 5%. A schematic view of the experimental and measurement setup is shown in Figure 4.  
The double source irradiation allowed to investigate the DoI estimator parameter reliability as a function of the energy (\(\gamma\) emissions ranging from 31 keV to 356 keV). The choice of a monolithic scintillation crystal with a full reflective treatment assures a good spectrometry allowing the identification of the different full energy peaks generated by the source gamma emissions. The DoI estimator parameter has been evaluated starting from the scintillation light distribution corresponding to each gamma emission energy. Thus, an energy selection on the spectrum is needed. Since the position-sensitive nature of the proposed system, the region of interest (ROI) method has to be taken into account and properly applied. In fact, the pulse height spectrum related to the spot irradiation shows the full energy peak but also a background component: the contribution of the photons that undergoes Compton scattering before reaching the detector, the diffuse contribution due to the lead collimator presence and the electronic noise. These additional factors impair peaks identification, introducing an error in the determination of the activity, which is related to the area below the peak. The ROI method allows to overcome this effect. It is possible to consider \begin{table}
\begin{tabular}{c c c} \hline \hline
**Radionuclide** & **Energy (keV)** & **BR (\%)** \\ \hline  & 30.6 & 33.9 \\  & 31.0 & 62.2 \\  & 35.0 & 20.7 \\  & 53.2 & 2.1 \\  & 79.6 & 2.7 \\ \({}^{133}\)Ba & 80.9 & 32.9 \\  & 160.6 & 0.6 \\  & 223.2 & 0.5 \\  & 276.4 & 7.2 \\  & 302.9 & 18.3 \\  & 356.0 & 62.1 \\  & 383.9 & 8.9 \\ \hline  & 14.4 & 9.2 \\ \({}^{57}\)Co & 122.1 & 85.6 \\  & 136.5 & 10.7 \\ \hline \end{tabular}
\end{table} 
Table 1: \({}^{133}\)Ba and \({}^{57}\)Co X-ray and \(\gamma\) emissions along with their branching ratio. In red energies corresponding to higher intensities are highlighted. Figure 4: Schematic view of the experimental setup. just a selection on the image corresponding to the source in order to obtain a new pulse height spectrum in which the full energy peak is enhanced with respect to the background component. As a consequence, energy resolution performance can be improved. First, the ROI method has been employed.