## 3 Results and Discussion
The emission is composed of double peaks derived from the charge transfer transition in (WO\({}_{4}\))\({}^{2-}\) anion complex and some defects such as oxygen and lattice defects [39]. In the NIR range, some sharp emission lines were confirmed at 900, 1060, and 1300 nm. Almost the same spectral features due to Nd\({}^{3+}\) were observed in other materials doped with Nd [40, 41, 42]; hence the emissions would originate from the Nd\({}^{3+}\). The X-ray-induced scintillation decay curves monitored at the range from UV to visible light are exhibited in Figure 7a. They were well agreed with a sum of two exponential functions. The origin of the fast (\(\sim\)1 us) and slow (\(\sim\)5-7 us) components were respectively considered to be the WO\({}_{3}\)-V\({}_{\rm O}\) complex defects and the charge transfer transitions in (WO\({}_{4}\))\({}^{2-}\) anion complex because the confirmed decay times were comparable with the scintillation decay times of CaWO\({}_{4}\), reported in the past study [43]. The slow components became fast as the Nd concentration increased. It presented that the energy transfer from the host to Nd\({}^{3+}\) happened because the emission due to the host partially matched  
Figure 5: PL decay curves of Nd-doped CaWO\({}_{4}\) (**a**) monitored at 420 nm under excitation at 280 nm and (**b**) monitored at 880 nm under excitation at 575–625 nm. Figure 6: X-ray-induced scintillation spectra of Nd-doped CaWO\({}_{4}\) monitored (**a**) 300–600 nm and (**b**) 700–1500 nm. with the excitation bands of Nd\({}^{3+}\), as shown in Figure 4. Figure 7b shows the decay curves monitored at visible-NIR ranges. The obtained curves were well-matched with two exponential functions. The fast components (\(\sim\)5-7 \(\upmu\)s) were derived from the WO\({}_{3}\)-V\({}_{\rm O}\) complex defects according to the decay times since the PMT for the NIR range also had a sensitivity down to \(\sim\)500 nm. The slow components had decay times of \(\sim\)160-220 \(\upmu\)s. They are typical values as Nd\({}^{3+}\) 4f-4f transitions reported in some materials doped with Nd [44,45]. The scintillation decay times of the emissions at 1060 and 1300 nm could not be measured in the study owing to the limitation of sensitivity of the PMT. However, the decay times of the transitions at 1060 and 1300 nm can be respectively estimated to \(\sim\)370 and \(\sim\)580 \(\upmu\)s according to the relationship between decay times and wavelength of scintillation (\(\tau\)\(\sim\)\(\lambda^{22}\), where \(\tau\) means the scintillation decay time and \(\lambda\) means emission wavelength) [46]. In the assumed applications, signal accumulation time is from several seconds to minutes, and the decay times observed here are acceptable enough for these applications. Afterglow curves after 2 ms X-ray exposure of the crystals are exhibited in Figure 8. Afterglow demonstrates a relaxation process of electrons trapped at \(\sim\)25 meV from the bottom of the conduction band by thermal stimulation. The afterglow levels (\(A\)) were calculated as follows: \(A=(I_{20}-I_{\rm bg})/(I_{\rm max}-I_{\rm bg})\). Here, \(I_{20}\), \(I_{\rm max}\), and \(I_{\rm bg}\) were intensity at 20 ms after irradiation of X-ray, intensity during irradiation of X-ray, and intensity before irradiation of X-ray. The \(A\)s of 0.1, 0.5, and 1% Nd-doped crystals were respectively estimated to be 1330, 1080, and 390 ppm. The \(A\)s were much higher than that of CdWO\({}_{4}\) (\(\sim\)10 ppm [31]). However, the value of the 1% Nd-doped crystal was comparable to that of Tl-doped CsI (\(\sim\)300 ppm [47]), one of the representative commercial scintillators for X-ray detection. Relationships between the X-ray irradiated dose rate and the signal intensity in NIR ranges were investigated. In Figure 9, the vertical and horizontal axes present the averaged currents during X-ray exposure subtracted by averaged currents before X-ray exposure (background level) and X-ray exposure dose rate, respectively.