# A Novel Method for Spectrometry Based on Imaging Systems
## 3 Results and Discussions
### Spectrometry Proof of the N/I Parameter  
The reliability of this parameter has been verified by dividing \(N/I\) distributions into four regions corresponding to a different layer of the crystal (i.e., depth) and by observing the resulting spectra for each region. In Figure 10, the spectrum obtained for the first division is reported. The first region corresponds to the shallowest layer of the crystal (\(N/I\) parameter ranges from 25 to 35). In fact, as expected from the \(N/I\) distributions, only the 31 keV energy peak is clearly visible along with the background contribution given by the electronic noise (65% of the total number of events in the spectrum are related to 31 keV emission). A finer selection starting from the upper limit of 31 keV \(N/I\) distribution (i.e., from 30 to 35), inside the first region, allows to investigate events that occurred in the surface proximity. As shown in Figure 11, this selection has strongly reduced source events enhancing the electronic noise component below 20 keV (25% for 31 keV vs. 75% for noise). Hence, a proper selection in the range identified by the higher \(N/I\) parameters in the distribution allows to considerably reduce the electronic noise component in the pulse height spectrum. The capability offered by this parameter is crucial when low energy gamma emissions have to be properly measured. In Figure 12, the spectra obtained for the following three division is reported. Figure 9: \(N/I\) parameter distributions for 31, 81, 122 and 356 keV. It is remarkable to notice how all the energy peaks visible in the spectra change in their shape as the next depth is selected. In Table 2, the mean free paths for the gamma emissions involved in this study are presented. \begin{table}
\begin{tabular}{c c} \hline
**Energy (keV)** & \(\lambda\) (mm) \\ \hline
31 & 0.2 \\
81 & 2.1 \\
122 & 5.4 \\
356 & 21 \\ \hline \end{tabular}
\end{table} 
Table 2: Mean free path for gamma emissions in CRY018 spectrum. Figure 11: Spectrum resulting from a finer selection of events starting from the upper limit of the 31 keV \(N/I\) parameter distribution, contained in the first depth region (\(N/I\) form 30 to 35). Figure 10: Spectrum resulting from the selection of events in the first depth region identified from the \(N/I\) parameter distributions (\(N/I\) from 25 to 35). The 31 keV gamma emission inside CRY018 has a mean free path of \(\approx\)0.2 mm and it is expected to interact in close proximity of the crystal surface. It is well-recognisable in the first spectrum, corresponding to the top layer (in this discussion the so called second depth region) of the crystal (Figure 12a), and it rapidly disappears as spectra at higher depths are observed (Figure 12b,c). The ratio of the 31 keV events for the three depth regions over those in the total spectrum resulted, respectively, in 72%, 1%, <1%. The 81 keV gamma emission interacts more likely at \(\approx\)2.1 mm but it is expected to be present along the entire thickness of the crystal (respectively for the three regions 67%, 26%, 7%). Similar considerations apply to 122 keV, whose mean free path is around 5.4 mm (respectively for the three regions 41%, 40%, 19%). Despite this, the high sensitivity of the \(N/I\) parameter allows to observe the differences in energy peaks formation due to their different interaction probability inside the crystal. In fact, the highest contribution to the 81 keV peak occurs in the second depth region (Figure 12a), corresponding to an intermediate crystal thickness, and gradually drops in the next depth regions whereas 122 keV peak still greatly contributes (Figure 12b,c). The 356 keV mean free path is considerably longer than the crystal size and its interaction probability inside the entire thickness is expected to be small and nearly constant. In Figure 12a, a zoom around 356 keV gamma emission is shown for a better visualisation.