## 1 Introduction  
The role of scintillators in medical-imaging systems using either X-rays or gamma rays is crucial [1]. An appropriate choice of scintillator should reduce the radiation dose received by the patient without reducing the quality of the obtained image and therefore its diagnostic value, as guided by the ALARA principle [2]. In addition, due to the variety of imaging modalities, as well as the constantly evolving imaging techniques, new scintillators with improved properties are required [3, 4]. Today, hybrid SPECT/CT and PET/CT systems are used in medical imaging. A suitable scintillator with good response over the whole range of energies employed in these systems would enable the use of a common detector, with obvious advantages in economic and operational terms. A result of the research activity in this direction was the discovery in 2001 of LaBr\({}_{3}\):Ce crystal, which combines excellent characteristics, such as a high light-yield, very good energy resolution and short decay-time [5]. The light yield of LaBr\({}_{3}\):Ce for \(\gamma\)-ray absorption at 662 KeV (\({}^{137}\)Cs), has been calculated from 60,000 photons/MeV to approximately 81,000 photons/MeV, depending on the measurement conditions, the Ce concentration and the crystal size [5, 6, 7, 8, 9, 10, 11, 12, 13]. Energy resolution for the 662 KeV full-energy-absorption peak has been measured at between 2.8% and 3.9%, and is considered the best value measured for a crystal scintillator [5, 6, 8, 10, 11, 12, 13, 14, 15, 16]. LaBr\({}_{3}\):Ce is a fast scintillator with a decay-time of between 35 ns and 25 ns, which drops to 15 ns with the increase in the concentration of Ce ions [5; 6; 7; 8; 9; 11; 17]. The optical emission spectrum of LaBr\({}_{3}\):Ce at room temperature presents an emission band of between 325 nm and 425 nm, with peaks at 360 nm and 380 nm. These peaks are due to Ce\({}^{3+}\) ions, and appear in all crystal spectra, regardless of the Ce\({}^{3+}\) concentration [6; 7; 9; 12; 18]. LaBr\({}_{3}\) crystals have a UCl\({}_{3}\)-type lattice in the P63/m space group, with a lattice constant of 0.6196 and anisotropic properties [6; 7; 10; 18]. The density is 5.2 g/cm\({}^{3}\), the melting point 1116 \({}^{\circ}\)C, and the effective atomic number 46.9 [16; 18; 19]. The disadvantages of LaBr\({}_{3}\):Ce are its hygroscopicity and its intrinsic radioactivity, due to the presence of a very small fraction of the natural radioisotope \({}^{138}\)La. However, the latter is important only for certain applications, such as low-radioactivity measurements [15; 19; 20], and is not considered a major drawback in medical imaging, where higher exposures are used. Based on the above characteristics, the crystal can be used in several applications, such as nuclear physics, medical imaging, oil exploration, X-ray diffraction, etc. In the field of medical imaging, the suitability of the crystal has been studied for use in SPECT nuclear-medicine-imaging systems, and especially in PET scanners based on time-of-flight (TOF) [8; 16; 17; 20; 21]. The aim of this study is to experimentally investigate the response of LaBr\({}_{3}\):Ce, when excited with X-rays in tube voltages from 50 kVp to 150 kVp. The response was examined via (a) the absolute efficiency (AE), describing the light output-power per incident exposure rate, (b) the spectral-matching factor (SMF) and the overall efficiency (EE), investigating the suitability of various photon sensors attached to LaBr\({}_{3}\):Ce, (c) the radiation-absorption properties of the crystal, and d) the signal-to-noise-ratio-squared (SNR)2 transfer properties of the scintillator in the zero spatial-frequency, as described by the detective quantum efficiency (DQE(0)) [22]. DQE(0) can act as a complementary efficiency index for AE, since it also incorporates noise transfer. Generally, a high DQE(0) value asserts a high SNR transfer and a resulting image of higher quality.