## 3 Results and Discussion  
After the crystal growth, rods with \(\sim\)20 mm length and \(\sim\)5 mm \(\varphi\) were obtained. They were cut and polished for the evaluation of optical and scintillation characteristics. Figure 1 shows the appearance of the prepared crystals. The length and thickness were \(\sim\)3 mm and \(\sim\)1 mm. All the crystals were transparent, and appeared pale blue as the dopant concentration increased. Some of the remained parts of as-grown rods after the single crystal growth were crushed, and the crashed powders were used for the XRD measurement. The obtained patterns and a reference from the Crystallography Open Database (COD: 9009627) are indicated in Figure 2. All the patterns matched that of the reference. Impurity phases such as CaO, WO\({}_{2}\), and WO\({}_{3}\) were not confirmed. The diffuse transmission spectra of the crystals are shown in Figure 3. The transmission spectra at 850-900 nm were not described because noise due to a detector switching from a PMT to an InGaAs-based detector was dominant in the range. The transmittance was -70-90% in the range from 330 to 1600 nm, although signals at 900-1000 nm were especially unstable owing to noise from a detector switching. Absorption was observed at 270 nm, originating from the transition from the 2p states of O\({}^{2-}\) to the 5d states of W\({}^{6+}\) in (WO\({}_{4}\))\({}^{2-}\)[32; 33]. Additionally, some absorption bands were confirmed at 350, 520, 590, 750, and 800 nm. They originated from the transitions from the \({}^{4}\)I\({}_{9/2}\) level to the \({}^{2}\)I\({}_{11/2}\), \({}^{2}\)G\({}_{9/2}\), \({}^{2}\)G\({}_{7/2}\), \({}^{4}\)S\({}_{3/2}\), and \({}^{4}\)F\({}_{5/2}\) levels of Nd\({}^{3+}\)[34]. The excitation and emission contour maps of the 1% Nd-doped CaWO\({}_{4}\) crystal are exhibited in Figure 4a,b as a representative. No significant change was observed in the spectral shapes of the other two crystals. The vertical axis presents the excitation wavelength, and the horizontal axis presents the emission wavelength. A broad emission band was confirmed at 350-500 nm under excitation wavelength at 250-300 nm. It would be derived from the host (CaWO\({}_{4}\)) [35]. In the NIR range, some emissions were confirmed at 900, 1060, and 1300 nm, and they were ascribed to the typical emissions derived from the 4f-4f transitions of Nd\({}^{3+}\)[34; 35]. The quantum yields (\(QY\)) of the crystals accumulated 750-1500 nm under excitation at 590 nm of 0.1, 0.5, and 1% doped samples were 70.7, 79.5, and 61.2%, respectively. Figure 1: Photograph of Nd-doped CaWO\({}_{4}\). Figure 2: XRD patterns of Nd-doped CaWO\({}_{4}\) with reference. The PL decay curves were measured to confirm the origin of the emissions. Figure 5a exhibits the PL decay curves monitored at 420 nm. The excitation wavelength was selected to 280 nm. The decay curves were composed of the sum of two exponential functions. The fast components were attributed to the Instrumental response function (IRF) due to the excitation source. The slow components with decay times of \(\sim\)7-9 \(\upmu\)s would be derived from the charge transfer transition in \((\text{WO}_{4})^{2-}\) anion complex [36]. The decay curves monitored at 880 nm are exhibited in Figure 5b. The excitation wavelength was set to 575-625 nm. The curves were agreed with an exponential function with decay times of \(\sim\)160 \(\upmu\)s. The decay times were consistent with the reasonable values as the \({}^{4}\)F\({}_{3/2}\)-\({}^{4}\)I\({}_{9/2}\) transition of Nd\({}^{3+}\) was reported in other phosphors doped with Nd [37; 38]. Figure 4: PL excitation and emission contour maps of 1% Nd-doped CaWO\({}_{4}\) monitored (a) 195–950 nm and (b) 895–1675 nm. Figure 3: Diffuse transmission spectra of Nd-doped CaWO\({}_{4}\). Figure 6a,b show the X-ray-induced scintillation spectra of the crystals when monitored at 300-600 nm and 700-1500 nm, respectively. A broad emission with the peak top at 420 nm was observed at 300-600 nm.