Designand Synthesis ofTb3+-dopedKGWCrystal forthe Ex ploration ofScintillationPerformancewithX-ray Imaging①

XU Hui XU Wei-Wei XU Liu-Wei WANG Zhi-Lin WANG Shuai-Hua② SU Qing ZHANG Bi-Sheng ZENG Qing-You WU Shao-Fan

a (College of Chemistry, Fuzhou University, Fuzhou350108, China)

b(Key Laboratory of Optoelectronic Materials Chemistry and Physics, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou350002, China)

c (Fujian Communications Planning & Design Institute Company, Fuzhou350004, China)

d(Fujian Luhai Engineering Investigation & Designing Company, Fuzhou350000, China)

ABSTRACT X-ray scintillators have been widely used in many fields owing to their strong penetrating ability,including security inspection, medical imaging, nuclear cameras, high energy physics and so forth. To explore new scintillation materials, we designed and synthesized KGW:Tb bulk scintillation crystals with monoclinic phase structure. We explored the fluorescence performance of KGW:Tb by testing the photoluminescence spectrum and fluorescence decay curve and observed strong green light emission in the visible light range at room temperature.Moreover, our crystal has a high stability under X-ray irradiation, a good sensitivity response and an appreciable light output (3424 ph/MeV) that is approximately twenty times more than PbWO4. Moreover, we used a 0.98 mm thickness polished crystal sheet for X-ray imaging applications and observed excellent results. Therefore, KGW:Tb crystal may be an important direction for X-ray scintillation detection.

Keywords: KGW, X-ray scintillators, light yield, high stability, X-ray imaging;

1 INTRODUCTION

It is well known that rare earth (RE) ions have underfilled 4felectron shells, which can get very sharp characteristic emission spectrum after being excited by light[1,2]. For example, Eu3+can emit red characteristic peak located at 617 nm and Tb3+can obtain green characteristic peak at 542 nm under the excitation of ultraviolet light[3-6]. Therefore, RE elements with excellent optical properties are widely used in electronics, military, new materials and other fields.Moreover, luminescent materials doped with trivalent rare earth ions, such as oxides, fluorides, sulfide, phosphates and vanadates, have been widely used in solid-state lasers,biological imaging, information storage and color displays,etc[7-11]. Furthermore, the oxide crystals have become the preferred matrix material for rare earth ion doping due to their smooth surface, performing excellent chemical and physical stability and high conductivity. In the past few decades, researchers have paid much attention to RE3+doped oxide bulk crystals grown by the Czochralski and Bridgman method[12-16]. On one hand, due to their excellent luminous efficiency and adjustable wavelength emission range, rare earth doped oxides have been widely used in laser materials,up-conversion (UC) luminescence and white light-emitting diode (W-LED)[17-22]. On the other hand, RE3+doped oxide materials with high effective atomic numbers have been used as important scintillators for scintillation detection, such as LSO:Ce (Lu2SiO5:Ce), LYSO:Ce, (LuYSiO5:Ce) and GAGG:Ce (Gd3(Al,Ga)5O12)[23-26], which show good performance under X-ray irradiation. Although different scintillation detection applications need different requirements for inorganic scintillation single crystal, the basic demands for all scintillators are high effective atomic number, fast decay time, good stability, high light yield and high transparency, etc[27]. Therefore, in order to meet the ever-increasing performance requirements of scintillation detectors, the exploration of RE3+doped oxide materials with high scintillation characteristics still has important scientific value and practical significance.

In recent years, rare earth-doped KLn(WO4)2(Ln = Y, Lu and Gd) crystals have not only been widely reported as high-efficiency up-conversion luminescent materials due to their excellent structure, optical properties and lower phonon energy, but also studied in depth as a laser material due to the wide emission wavelength range and high efficiency of the ultrashort pulse laser output[28-33]. Kasprowicz et al.[32]have reported adjustable multicolor RE-doped KGd(WO4)2(KGW) phosphors synthesized by the modified Pechini method. Esrom Kifle and his colleagues[29]have studied Ho3+and Tm3+co-doped KLu(WO4)2crystals and obtained a maximum total output power of 448 mW at ～1942 nm(Tm3+) and 2059 nm (Ho3+) and got the slope efficiency (M= 40.6%). Hence, KLn(WO4)2crystal is considered to be one of the excellent host materials, which is attributed to its excellent fluorescence emission, high stability and large-scale preparation. In addition, Gd3+has been widely used as a matrix of scintillation materials in previous studies[34]. For these reasons, we creatively use KGW as the matrix of scintillation crystal materials, and different rare earth ions are used to promote the scintillation performance.To the best of our knowledge, Tb3+has great potential in X-ray applications owing to the relative matching relationship between the green light emission range and the sensitivity of CCD (charge coupled device). For instance,Tb3+-doped glasses, glass ceramics and crystals have all been reported as scintillating materials[35-38]. Based on the above reasons, it is worth trying to systematically study Tb3+-doped KGW crystals whose scintillation performance has not been reported.

In this article, we designed and prepared the KGW:Tb crystal based on the above ideas and synthesized it by the top-seeded solution growth method (TSSG). The KGW:Tb crystal has excellent stability (melting point 1020 ℃), strong green photoluminescence (The strongest emission wavelength is 542 nm) and microsecond fluorescence lifetime (490μs). More interestingly, we obtained the highest RL (radiant luminescence) intensity of KGW:Tb within the allowable range of the instrument to be 6612 cps (The dose rate under this condition is 42.29 mGy/s by measuring the RL intensity under the same voltage and different currents).Surprisingly, the relationship between RL intensity at different dose rates is almost completely consistent with the linear regression equation ofy= 155.4x– 49.8, where the slope of the equation is much greater than BaF2and PbWO4.At the same time, we also studied the stability of the crystal under ray irradiation, which shows very excellent performance. In particular, we used the object to perform X-ray imaging detection and the internal structure of the object is very clear. Therefore, the synthesis and characterization of KGW:Tb scintillation bulk crystals are of great significance for the exploration of new materials for scintillation detection.

2 EXPERIMENTAL

2. 1 Materials and instruments

In this work, Gd2O3(99.99%) and Tb4O7(99.99%) are supplied by Shanghai Diyang Chemical Co., Ltd. All other experimental materials are analytical grades from the Sinopharm Reagent Network. Powder X-ray diffraction(PXRD) of KGW:Tb is evaluated at room temperature using MiniFlex 600. The infrared absorption spectrum and the UV diffuse reflectance spectroscopy are obtained byin-situinfrared-Nicolet 6700 and the UV-Vis-NIR spectrophotometer Lambda950. The TG-DTA curve requires the STA449-F3 integrated thermal analyzer. The photoluminescence spectrum (PL) and fluorescence decay curve of the crystal are determined by FLS 1000 (fluorescence spectrophotometer) with 500W Xenon lamp. The X-ray source (MagProTUB0014) is used to test the RL spectrum of microcrystals. A digital camera for photography (Canon EOS 5D Mark IV with EF 24-70mm f/2.8L II USM lens) is used to obtain X-ray images.

2. 2 Synthesis

Generally, the flux method is used to grow the crystal at a temperature below the phase transition point because of the high temperature of the melting point (℃) and phase transition point (1031 ℃) of KGW crystal. We use K2W2O7as a fluxing agent due to its advantages of low melting point,low melt viscosity and weak volatilization, which are more suitable for crystal growth compared to K2WO4. Firstly, we separately synthesized KGW:Tb and K2W2O7polycrystalline raw materials according to the stoichiometric ratio. Secondly,we mix it with the ratio of KGW(Tb):K2W2O7= 1:4,Gd2O3:Tb2O3= 95:5 and put it into a Ø65 mm × 50 mm platinum crucible. Then, the molten salt furnace is heated to about 50 ℃ higher than the saturation point of KGW:Tb and kept at a constant temperature for 24 hours to make the melt evenly mixed. Finally, the KGW:Tb crystal is grown by the TSSG method after determining the saturation point by the multiple crystal method. The crystal growth process is as follows: the temperature is kept at the saturation temperature of KGW:Tb for 48 hours after the seed crystal of KGW is introduced into the melt and then the temperature is lowered according to 1～5 ℃/day. At the same time, the seed crystal is rotated alternately forward and backward at a speed of 10～20 r/min. The seed crystal is pulled upward at a speed of 0.1～2 mm/day during the growth process. The crystal is lifted off the liquid surface and dropped to room temperature at a rate of 10～50 ℃/h after the crystal growth process is completed.

3 RESULTS AND DISCUSSION

3. 1 Structure and characterization

The KGW:Tb crystal belongs to monoclinic crystal system,C2/cpoint group. As we can see the unit cell structure of KGW:Tb from Fig.1a includes a tetragonal anti-prism structure formed by eight-coordinated Gd and O,a curved icosahedron formed by twelve-coordinated K and O and a deformed octahedral structure formed by eight-coordinated W and O. Through structural analysis, the Tb3+introduced in the crystal will replace part of the Gd3+site. All polyhedra in the structure are strongly connected by sharing sides and corners. In order to compare the structure of Tb3+-doped KGW crystal with the intrinsic KGW crystal,we investigated their PXRD (Fig.1b). It reflects that the distribution and relative intensity of all diffraction peaks of the sample can be well matched with the standard card. This confirms that the lattice constant is not obviously affected by Tb3+doping. Based on molten-salt growth, a large Tb3+-doped KGW crystal was successfully obtained by the control of growth conditions. As shown in Fig.1c, it is a photo of a KGW:Tb crystal with a length of 5.5 cm, a width of 2.7 cm and a thickness of 1.4 cm under natural light. The disadvantage of bulk crystals may be due to the rapid cooling rate, leading to the rapid crystal growth, in which white precipitates appear inside the crystal. In addition, there are a large number of white inclusions on the left part of the crystal. On one hand, it may be due to the large concentration gradient that appears along the crystal surface,which is affected by hydrodynamics during the rotation of the crystal, especially in the angle between the two crystal faces and the edge area of the crystal. The flux impurities are more likely to accumulate here due to the slowing of the convection of the melt. On the other hand, owing to the narrow metastable zone of KGd(WO4)2and the high flux content, there may be a large number of network structures containing K and W ions, acting as a shield to hinder the diffusion of the melt. We know that the melt has a considerable viscosity which makes the solute diffusion difficult and leads to the generation of component overcooling. Fig.1d exhibits the TG-DTA curve of KGW crystals. There are two endothermic peaks on the DTA curve in the figure, which shows that the melting point of KGW:Tb crystal is 1072 °C and the phase transition (transition from tetragonal crystal system to monoclinic crystal system)temperature is 1020 °C. Compared with the undoped KGW crystal (The melting point is 1087 °C and phase transition temperature is 1031 °C), the melting point and phase transition temperature of Tb3+doped crystals are slightly lower caused by the substitution of doped rare earth ions for Gd3+caused by misalignment of the lattice. The difference between the DTA curve of the crystal and the crystal doped with other rare earth ions in the literature may be due to the different radii of the doped ions.

Fig.2a shows the infrared spectrum of KGW:Tb. As we all know, the stretching vibration frequency range of WO4is 900～750 cm-1and the bending vibration frequency range is 420～300 cm-1. The infrared absorption peaks of KGW:Tb at 923, 889, 835, 771, 745 and 637 cm-1are the manifestation of the stretching vibration of the WO4atomic group. The infrared absorption peak at 435 cm-1reflects the bending vibration of the WO4atomic group. In Fig.2b, we use the Kubelka-Munk function to convert the band gap value of the sample from the ultraviolet diffuse reflection data by the double tangent method. The band gap value is the intersection of the tangents of the absorption edge and the absorption platform, which is 3.85 eV. The PL(photoluminescence) spectrum obtained by excitation at 310 nm in Fig.2c reflects the characteristic peak of Tb ions. As shown in the illustration, there is a broad excitation peak at 310 nm and a sharp excitation peak at 488 nm in the excitation spectrum of KGW:Tb. The strongest green luminescence peak is observed at 542 nm which is derived from the5D4→7F5energy level transition of Tb3+. As we can see, all emission peaks are concentrated in the visible light region. In addition, two relatively weaker emission peaks around 582 and 619 nm can be found in Tb ion doping KGW crystal, which are attributed to the transition of5D4→7F4and5D4→7F3energy level transitions. It can be observed that the emission peaks centered at 486 nm can be ascribed to the5D4→7F6levels transition of Tb ions. The fluorescence decay curve of KGW:Tb at room temperature is shown in Fig.2d. The fluorescence lifetime is measured under the conditions of 310 nm excitation wavelength and 542 nm emission wavelength (energy level5D4→7F5). We use a single exponential curve to complete the calculation of the fluorescence decay time through the following functional equation:Fit=A+B1exp(-t/T1), whereFit,BandTrepresent the instantaneous intensity, the weight of process and the lifetime of process, respectively. And we receive the fluorescence lifetime of our sample calculated to be 490μs,which is shorter than that for most materials.

Fig.1. (a) Unit cell structure diagram of KGW:Tb crystal, (b) X-ray diffraction of KGW:Tb crystal and JCPD card.(c) Crystal image of KGW:Tb under natural light. (d) TG-DTA curve of KGW:Tb crystal

Fig.2. (a) Infrared absorption spectrum in the range of 1300～400 nm of KGW:Tb crystal. (b) UV-vis-NIR optical absorption spectrum of KGW:Tb crystal (converted from diffuse reflection data by Kubelka-Munk function). (c) Photoluminescence spectrum of KGW:Tb crystal excited at 310 nm. The inset is the excitation spectrum at 542 nm. (d) Fluorescence decay time curve of KGW:Tb crystal (λex = 310 nm, λem = 542 nm)

3. 2 Properties of scintillators and X-ray imaging

In Fig.3a, the radiant luminescence intensity of KGW:Tb changes with the current within the measurement range of the radiation source (the current increases and the signal becomes stronger). It is worth mentioning that the strongest peak of the sample at 542 nm is 6612 cps, which is nearly double that of BaF2(3058 cps) and much higher than the value for PbWO4(900 cps). We use the transmissive method to collect data with an integrating sphere and calculate them through the following formula with LYSO:Ce as a reference:

We obtain the light output value of KGW:Tb to be 3424 ph/MeV, which is more than half of BaF2(6500 ph/MeV)and more than twenty times of PdWO4(150 ph/MeV).Therefore, the crystal has a strong scintillation light yield and is expected to be a candidate material for imaging the detection applications. Further, we measured the changes in the RL intensity of the three scintillators (KGW:Tb, BaF2,PbWO4) in the range of 8.19～42.29 mGy/s, as shown in Fig.3b. Generally, the higher the sensitivity in scintillation materials, the better the imaging quality of the scintillation detector and the less radiation damage to the patient. We found that KGW:Tb has higher response sensitivity to radiation dose compared with BaF2which has been used in high energy physics. In addition, we obtain the linear regression equation of KGW by linear fitting:y= 155.4x‒49.8 (BaF2:y= 18.7x+ 120.9; PbWO4:y= 75x‒ 164.4),which is applicable to each measurement point. It is well known that the radiation dose rate of the X-ray source was calibrated by using an ion chamber dosimeter. The RL intensity is quite stable within 60 minutes by exposing to X-rays (The dose rate is 42.29 mGy/s), as shown in Fig.3c.There is a noticeably elevated point at 30 minutes which may be caused by the instability of the X-ray source. This material not only shows excellent stability under radiation,but also has non-hygroscopicity in the air, which overcomes the hygroscopicity of traditional scintillation materials, such as CsI:Tl, NaI:Tl, and so on. The relationship between the absorption coefficient and photon energy for Si, PbWO4,LYSO:Ce, CsI:Tl, BaF2and KGW:Tb is depicted in Fig.3d,which is obtained from photon cross section database. It can be found that the absorption coefficient of KGW:Tb is higher than that of the commercial scintillation crystals like CsI:Tl and LYSO:Ce, which is consistent withZeff(PbWO4(68.36) ＞ KGW(56.41) ＞ CsI(54.02) ＞ LYSO(51.6)＞ BaF2(45.81)).

Fig.3. (a) Changes of X-ray intensity under different currents of KGW:Tb crystal. (b) Linear relationship between RL intensity and different dose rates for KGW:Tb, PbWO4 and BaF2 scintillator. (c) Radiation stability of KGW:Tb scintillator at 42.29 mGy/s dose rate. (d) Relationship between the absorption coefficient and the energy of photon for Si, KGW:Tb, PbWO4, LYSO:Ce, CsI:Tl and BaF2 scintillators

In fact, we polished both sides of the crystal of KGW:Tb to the thickness of 0.98 mm after cutting it for X-ray imaging (Fig.4a). Due to the limitation of crystal quality, the length of the imaging lens we produced is about 2.5 cm and the width is about 1.9 cm. We use our scintillation crystal to indirectly image the object in Fig.4b under 50 kV, 150μA X-ray irradiation, and finally get the photo of Fig.4c.Interestingly, we clearly observe the coils inside the plastic case in the photo, which proves that our sample is very effective in X-ray imaging. This further illustrates the great potential of our KGW:Tb crystal for scintillation detection applications. We will continue to explore the synthesis of new scintillating materials in the future.

Fig.4. (a) Crystal photo for imaging of KGW:Tb. (b) Physical image for X-ray imaging.(c) Object imaging photos under X-ray irradiation (50 kV, 150 μA)

4 CONCLUSION

In conclusion, the consistency of the diffraction peaks in XRD pattern with the standard card confirms that we successfully synthesized KGW:Tb scintillation crystals with a size of 5.5cm × 2.7cm × 1.5cm by the TSSG method,which has good thermal stability (Melting point is 1020 °C)and excellent air stability. This crystal has a wide band gap of 3.85 eV and strong green fluorescence emission in the visible light range. Moreover, we can receive the KGW:Tb crystal with high effective atomic number under X-ray irradiation, which has excellent stability, good sensitivity and considerable light output. Especially, the RL intensity of KGW:Tb is nearly twice that of commercial scintillation crystal BaF2. Our materials have shown excellent imaging results in X-ray applications. The research results fully confirm the truth that our Tb-doped KGW crystal can be used as an effective X-ray detection material.