## 4 Discussion  
The shape of the AE curve of LaBr\({}_{3}\):Ce is a combination of the X-ray absorption and optical-photon production and escape from the crystal. At lower energies, the increased QDE enables a high optical-photon production. As the X-ray energy increases, and while QDE remains approximately 1, the number of optical photons generated increases, in proportion to the light yield; thus, the AE values are also measured as increasing. For X-ray tube voltages above 120 kVp, where the QDE drops below 1, the percentage of the X-rays absorbed is reduced, which in turn reduces the energy offered for optical-photons production. This concludes with a smaller slope, resembling a plateau, in the AE-value increase. If the QDE value was 1 for these energies, the rate of AE increase for X-ray tube voltages above 120 kVp might be similar to the increase rate between 50 kVp and 110 kVp. It is worth commenting that the use of the LaBr\({}_{3}\):Ce scintillator can give an equivalent result to the rest of the crystals regarding the signal, but with a significant reduction in radiation and therefore in the dose received by the examinee. In effect, compared to the BGO and LSO crystals used in hybrid SPECT/CT and PET/CT imaging systems, LaBr\({}_{3}\):Ce will give the same output but, as calculated at 140 kVp, with 6.2% and 29.5% of the radiation, respectively. In addition, the reduction in emitted radiation means a reduction in the current-time mAs, and therefore a reduction in the operational cost of the X-ray tube, without affecting the quality of the examination. Although LaBr\({}_{3}\):Ce is hygroscopic, other scintillators that were used in the past for radiation-imaging purposes, such as CsI:Na and NaI, were also hygroscopic. This suggests that the technology for managing this type of scintillators is applicable. The high light-yield of the LaBr\({}_{3}\):Ce crystalline scintillator, compared to other scintillators, can lead to radiation-dose savings for the examinees, which is of importance in radiation imaging. The LaBr\({}_{3}\):Ce crystal's higher efficiency when compared to the BGO and LSO crystals across the whole energy range, as well as its exceptional performance at high energies, demonstrate its potential for use in medical-imaging applications, especially in nuclear medicine. Figure 19: DQE(0) of the LaBr\({}_{3}\):Ce and LaCl\({}_{3}\):Ce single crystals. Concerning the SMF values, when the LaBr\({}_{3}\):Ce crystal was compared to the LaCl\({}_{3}\):Ce crystal, a halide scintillator of similar characteristics, it presented almost the same values when coupled with various photocathodes, but considerably better compatibility with silicon photomultipliers, CCDs and CMOS, as the SMF values of LaCl\({}_{3}\):Ce range from 0.62 to 0.67 for silicon photomultipliers, from 0.002 to 0.22 for CCDs, and from 0.0 to 0.28 for CMOS [27]. The DQE values are a function of the radiation-absorption performance of the scintillator crystal as described by QDE, and the escaping optical-photon signal fluctuation. Both these parameters are embedded in Equations (7) and (13). At lower X-ray energies, where QDE is approximately 1 and AE is low, DQE(0) presents its lowest values. As the X-ray energy increases up to approximately 120 kVp, QDE is still almost 1, but the optical-photon production is increased, leading to a corresponding increase in DQE(0) values. At higher energies however, the drop in QDE as shown in Figure 5, combined with a tendency to AE saturation, as shown in Figure 6, leads to somewhat lower DQE(0) values. In every case, LaBr\({}_{3}\):Ce is superior to LaCl\({}_{3}\):Ce, mainly due to its better radiation to optical conversion characteristics. It may be deduced that the LaBr\({}_{3}\):Ce scintillator under study performs better, both in terms of ionizing radiation to optical-photon-emission efficiency and in terms of SNR transfer at X-ray tube-voltages over 110 kVp.