WO2015007229A1

WO2015007229A1 - Ultrabright csi:tl scintillators with reduced afterglow: fabrication and application

Info

Publication number: WO2015007229A1
Application number: PCT/CN2014/082456
Authority: WO
Inventors: Yuntao Wu; Guohao REN; Xiaofeng Chen; Huanying LI; Shangke PAN
Original assignee: Shanghai Institute Of Ceramics, Chinese Academy Of Sciences; Research And Development Center Of Shanghai Institute Of Ceramics
Priority date: 2013-07-19
Filing date: 2014-07-18
Publication date: 2015-01-22
Also published as: FR3008823A1; FR3008823B1

Abstract

The present invention provides the methods to fabricate thallium doped cesium iodide (CsI:Tl) scintillators with high light yield and reduced afterglow,the scintillators prepared according to the disclosed method,and their applications in radiation detection.

Description

Ultrabright Csi:Tl Scintillators With Reduced Afterglow:

Fabrication and Application

FIELD OF THE INVENTION

The present invention relates to the methods to fabricate thallium doped cesium iodide (CsLTl) scintillators with high light yield and reduced afterglow, the scintillators prepared according to the disclosed method, and their applications in radiation detection.

BACKGROUND

Scintillating materials have been applied for more than a century, and they played a crucial role in the discovery of X-rays, β-particles, and a-particles and today in the quest for the Higgs boson. Thallium-doped cesium iodide (CsLTl) was introduced in 1951 as one of the first single crystal scintillators. ^[1] It shows high light yield 66,000 photons/Me V, scintillation response dominated by 800-1000 ns decay time and an emission wavelength peaking at 550 nm matching well with the semiconductor photodetector sensitivity range. ^[2] Furthermore, it has medium density (4.53 g/cm ) and higher effective atomic number (Z_eff = 54) and low melting temperature (621°C) , which make it easy to grow bulk single crystal and micro- columnar poly crystalline films. Because of its low cost, CsLTl materials have been widely used for X-ray digital radiography, ^[3] gamma ray spectroscopy, homeland security and nuclear medicine applications.

The common types of CsLTl crystals include bulk crystal, micro-columnar poly crystalline films and fibers. The bulk crystals are usually grown from melt by Bridgman method or Czochralski method, but the micro-columnar films are prepared by thermal depositing, electron beam depositing or ion beam sputtering. Both Csl and Til combine together and deposit perpendicular on the substrate and grow along a special crystallographic orientation in a thin column. This columnar structure is beneficial for the transmission of scintillation light along the longitudinal direction and prevents its transversal transmission, so that the space resolution of the screens can be improved greatly. Up to now, the largest size scintillation film with size of 47 47cm composed of CsI(Tl) micro-column is manufactured by Radiation Monitoring Division In of USA . In addition, the single crystal fibers of CsLTl with diameter of less than 3μιη have been produced by micro-pulling down technology. By means of suitable assembling with reflective layers, these fibers can be arranged into two dimension arrays used as scintillation screens.

However, no matter what forms of CsLTl scintillator takes, its strong afterglow problem, which is the phenomenon that luminescence can still be observed long time after the excitation pulse ( G.Blass, Luminescence Materials), has not been suppressed effectively. The afterglow will cause pulse pileup in high count-rate applications, ghosting in X-ray computer tomography (X-CT) and image blurring in high-speed X-ray imaging, so that become a bottleneck to impede the development of high resolution radiography .^[4'^5] Thus, the way to suppress the afterglow in CsLTl has been searched intensively in last two decades.

As reported in earlier literature, co-doping by an appropriate ions is found to be an effective method to suppress the afterglow in scintillators and phosphors as has been shown e.g. in Gd₂O₂S-based or (Y,Gd)₂O₃-based phosphors and optical ceramics, see Ref. 2 and refs. therein. Afterglow in Lu₂SiO₅:Ce (LSO:Ce) scintillator has been soon recognized as a serious limitation ^[6] and the Ca²⁺ codoping is found efficient for its suppression ^[7'^8]. Positive role of Yb²⁺ ions in the afterglow suppression of LSO:Ce is found as well, decreasing it by more than two orders of magnitude, however, at serious expense of the light output. ^[9] In CsLTl scintillators, the afterglow level is effectively reduced by codoping Eu²⁺ or Sm²⁺ ions.^[1(M4] However, co-doping with these ions seriously deteriorated the light yield in both cases. Recently, Totsuka et al. claimed that using the Bi³⁺ codoping the afterglow of CsLTl can be less than 0.1% after 10 ms without strong decrease of the radioluminescence efficiency under low energy X-ray (less than 30 keV) excitation. ^[15] However, in the attempt to verify this result, we found that the light yield and energy resolution of Bi-codoped CsLTl crystals became much worse even for the lowest Bi concentration of about 0.005 mol% (in the melt). The possible reasons for the deterioration are the redshift of absorption edge after Bi³⁺ codoping and the induced absorption bands related to Bi³⁺ ions.

Therefore, despite of the success in the afterglow suppression in CsLTl crystals the codoping strategies mentioned above have simultaneously deteriorated the other important scintillation characteristics such as light yield and energy resolution which points to the complex character of scintillation mechanism. That is because impurities (doped ions) may introduce energy levels in the band gap of the host crystal which interfere with the charge carrier migration and relaxation processes. Thus, the search for new method including new codopants which can effectively diminish the delayed radiative recombination (afterglow) at Tl⁺ centres and not deleteriously affect other scintillation properties is extremely important for its application in the field of highspeed imaging and X-ray computer tomography.

References

W. Van Sciver, R. Hofstadter, Scintillations in thallium-activated Cal₂ and Csl, Phys. Rev. 1951, 84, 1062.

M. Nikl, Scintillation detectors for X-rays, Meas. Sci. Technol. 2006, 17, R37.

B.K. Cha, J.H. Bae, C.H. Lee, S.H. Chang, G. Cho, The sensitivity and spatial resolution dependence on the micro structures of CsLTl scintillation layer for X-ray imaging detectors, Nucl. Instr. Meth. Phys. Res. A 2011, 633, s297.

J.H. Siewerdsen, D.A. Jaffray, A ghost story: spatial-temporal response characteristics of an indirect-detection flat-panel imager, Med. Phys. 1999, 26, 1624.

S.C. Thacker, B. Singh, V. Gaysinskiy, E.E. Ovechkina, S.R. Miller, C. Brecher, V.V. Nagarkar, Low-afterglow CsLTl microcolumnar films for small animal highspeed microCT, Nucl. Instr. Meth. Phys. Res. A 2009, 604, 89.

P. Dorenbos, Afterglow and thermoluminescence properties of Lu₂SiO₅:Ce scintillation crystals, J. Phys.: Condens. Matter 1994, 6, 4167. K. Yang, C. L. Melcher, P. D. Rack, and L. A. Eriksson, Effects of calcium codoping on charge traps in LSO:Ce crysatls, IEEE Trans. Nucl. Sci. 2009, 56, 2960.

J.J. Zhu, M. Gu, X.L. Liu, B. Liu, S.M. Huang, C. Ni, First-principles study on stability of Li, Na and Ca in Lu₂SiO₅, J. Lumin. 2013, 139, 1.

N.G. Starzhinskiy, O. Sidletskiy, G. Tamulaitis, K.A. Katrunov, I.M. Zenya, Y.V. Malyukin, O.V. Viagin, A.A. Masalov, LA. Rybalko, Improving of LSO(Ce) scintillator properties by co-doping, IEEE Trans. Nucl. Sci. 2013, 60(2), 1427.

C. Brecher, A. Lempicki, S.R. Miller, J. Glodo, E.E. Ovechkina, V. Gaysinskiy, V.V. Nagarkar, R.H. Bartram, Suppression of afterglow in CsLTl by codoping with Eu²⁺-I: Experimental, Nucl. Instru. Meth. Phys. Res. A 2006, 558, 450.

R.H. Bartram, L.A. Kappers, D.S. Hamilton, A. Lempicki, C. Brecher, J. Glodo, V.Gaysinskiy, E.E. Ovechkina, Suppression of afterglow in CsLTl by codoping with Eu²⁺-II: Theoretical model, Nucl. Instru. Meth. Phys. Res. A 2006, 558, 458.

V.V. Nagarkar, C. Brecher, E.E. Ovechkina, V. Gaysinskiy, S.R. Miller, S. Thacker, A. Lempicki, R.H. Bartram, Scintillation properties of CsLTl crystals codoped with Sm²⁺, IEEE Trans. Nucl. Sci. 2008, 55(3), 1270.

L.A. Kappers, R.H. Bartram, D.S. Hamilton, A. Lempicki, C. Brecher, V. Gaysinskiy, E.E. Ovechkina, S. Thacker, V.V. Nagarkar, A tunneling model for afterglow suppression in CsLTLSm scintillation materials, Rad. Meas. 2010, 45, 426.

Brecher et al. Patent No.: US7,759,645 Bl.

D. Totsuka, T. Yanagida, Y. Fujimoto, Y. Yokota, F. Moretti, A. Vedda, A. Yoshikawa, Afterglow suppression by codoping with Bi in CsLTl crystal scintillator, Appl. Phys. Exp. 2012, 5, 052601.

SUMMARY

This invention provides a novel CsLTl scintillator by using codopants, which could enhance light yield and suppress afterglow.

This invention also provides the chemical formula of the codoped CsLTl scintillators, namely (Cs_1-x-yTl_xRE_y)(I_1-xX_2y), where RE is Yb 2+ or Sm 2+ , X is one of the halogen ions F , C Br or I , 0<x<0.05 for Tf , 0<y<0.05 for Yb²⁺ and 0<y<0.001for Sm²⁺ ₀

This invention provides the chemical formula of the codoped CsLTl scintillators, namely (Cs_1-x-yTl_xRE_y)(I_1-yX_3y), where RE is Yb³⁺, X is one of the halogen ions CK Br or I, 0<x<0.05 , 0<y<0.05 _o

The optimal doping and codoping concentration are 0<x<0.001 for Tl ions and 0<y<0.001 for Yb or Sm, respectively o

The afterglow level after X-ray excitation in codoped CsLTl scintillators can be loared at least one magnitude than codopant-free ones after 50 millisecond.

By codoping Yb 2+ or Sm 2+ ions, besides the improvement of afterglow properties, the light yield and energy resolution are also improved further in comparison to the codopant-free ones.

Thus, this improved CsLTl scintillators could be widely used in X-ray medical imaging and other radiation detection techniques.

This invention also provides the crystal growth methods for forming the codoped CsLTl single crystals, including vertical Bridgman, Czochralski or Kyropoulos methods.

One embodiment of crystal growth method is Bridgman method. The high-purity Csl, Til, and YbX₂, YbX₃ or SmX₂ (X=F, CI, Br, I) according to the stoichiometric ratio are loaded in the fused silica crucible, which is coated with a thin carbon film and then heated in vacuum to eliminate the residual humidity. The crucibles are maintained at a temperature 100°C above the melting point of cesium iodide for 24 hours to ensure homogeneity of the melt. Then, crucibles are passed through an optimal temperature gradient with a speed of 0.1-5 mm/h.

Other embodiment of crystal growth method is Czochralski method. The high- purity Csl, Til, and YbX₂, YbX₃ or SmX₂ (X=F, CI, Br, I) according to the stoichiometric ratio are loaded in a platinum or iridium crucible. Crystals are grown in protection atmosphere like Argon and Nitrogen, and with 1.0-10 mm/h pulling rate and 5.0-50 rpm rotating rate. Other embodiment of crystal growth method is Kyropoulos method. The high- purity Csl, Til, and YbX₂, YbX₃ or SmX₂ (X=F, CI, Br, I) according to the stoichiometric ratio are loaded in an iridium or platinum crucible. Crystals are grown in protection atmosphere like Argon and Nitrogen, without rotating and pulling.

This invention also provides the method to fabricate the codoped CsLTl thin films. One embodiment is thermal evaporation method. The substrate could be glasses or silicon crystals. The high-purity Csl, Til, and YbX₂, YbX₃ or SmX₂ (X=F, CI, Br, I) according to the stoichiometric ratio are loaded in the evaporation boat. Variety of samples is prepared for several different deposition parameters. The evaporation pressure is 10^" 2 -10^" 5 Torr and substrate temperature is 30-250 °C. Another embodiment is ion beam sputtering method.

This invention also provides the potential application of codoped CsLTl scintillators in the field of X-ray high-speed imaging and computer tomography, either in the form of single crystals or thin films.

Brief description of the drawings

FIG. l shows the pictures of polished codoped CsLTl single crystals with the size of 1 inch in diameter and 1 inch in length with different Yb concentration: non(a), 0.005%mol (b), 0.05%mol(c), 0.5%mol(d).

FIG.2 shows the afterglow profiles of CsI:Tl⁺,Yb²⁺ scintillators after X-ray pulse excitation. ST3 represents CsI:Tl⁺, ST1 and ST4 represent CsI:Ti⁺,0.05mol%Yb²⁺ and CsI:Tf,0.005mol%Yb²⁺ respectively.

FIG.3 The light output as a function of integration time of Yb 2+ -free and Yb 2+ - codoped CsLTl single crystals. ST3 represents CsI:Tl⁺, ST1 and ST4 represent CsI:Tl⁺, 0.05mol%Yb²⁺ and CsI:Tl⁺,0.005mol%Yb²⁺ respectively.

+ 2+

FIG.4 shows the pulse height spectra of optimized CsLTl ,Yb crystals coupled with Hamamatsu R1306 PMT under 22 Na excitation. Spectra start at channel no. 354, net photopeak position is marked in the figure, and solid red line is the Gaussian approximation of photopeak. The "net peak" equals the measured peak after subtracting the ADC pedestal signal. The equation, energy resolution (E.R.) = FWHM / Channel of "net peak", is used to determine the energy resolution. ST3 represents CsLTf, ST1 and ST4 represent CsI:Tl⁺,0.05mol%Yb²⁺ and CsLTf ,0.005mol%Yb²⁺ respectively.

FIG.5 shows the pulsed height spectra of CsI:Tl⁺,Yb³⁺ crystals coupled with

Hamamatsu R878-WT1734 series under 137 Cs gamma ray excitation. The CsLTl standard sample is used to calculate the light yield. The equation, energy resolution (E.R.) = Full width at half maximum (FWHM) / Channel number, is used to determine the energy resolution. ST3 represents CsI:Tl⁺, ATI and AT2 represent CsLTf ,0.0 lmol%Yb³⁺ and CsI:Tl⁺,0.1mol%Yb³⁺ respectively.

FIG.6 shows the afterglow profiles of Sm 2+ -free and 0.005mol%Sm 2+ codoped CsLTl crystals after pulsed X-ray excited.

FIG.7 shows the pulsed height spectra of CsLTf ,Sm²⁺ crystals coupled with

Hamamatsu R878-WT1734 series under 137 Cs gamma ray excitation. The CsLTl standard sample is used to calculate the light yield. The equation, energy resolution (E.R.) = Full width at half maximum (FWHM) / Channel number, is used to determine the energy resolution.

FIG.8 shows the picture of CsLTf, Yb²⁺ thin film on glass subtract.

FIG.9 shows the afterglow profiles of Yb-free and Yb-codoped CsLTl thin films after pulsed X-ray excited

DETAILED DESCRIPTION

The Yb²⁺, Yb³⁺ or Sm²⁺ ions codoped CsLTl single crystals are grown by vertical Bridgman, Czochralski or Kyropoulos methods. The obtained crystals are transparent and colorless (see Fig.1). Because for different crystal growth methods the effects of certain codopant on the scintillation properties of CsLTl are the same, we show the results of vertical Bridgman-grown single crystals as representative. Fig. 2 shows the afterglow profiles of Yb²⁺-free and Yb²⁺-codoped CsLTl single crystals after the X-ray pulse excitation. It is found that the Yb-doped crystal with highest light yield value exhibits the lowest afterglow level of about 0.035% at 80 ms while the Yb-free sample shows of about 1.14% at 80ms.

Pulse height spectra of optimized CsLTl , Yb²⁺ crystals under ²²Na (511 keV) excitation are measured. The light output as a function of integration time is measured and shown in FIG.3. When integration time is 4μ8, the light output for ST3, ST1 and ST4 is 3350, 4241, and 4510 p.e./MeV, respectively. Considering the emission weighted quantum efficiency (EWQE) of R1306 PMT at 550 nm, the emission

+ 2+

maximum of CsLTl ,Yb and light collection efficiency (LCE), it is estimated that the light yield of the best Yb-codoped crystal can reach 90,000±6000 photons/MeV. The light yield of SIC CsI(Tl)-STl sample is 85,000±5000 photons/Me V, which is still much higher than that of the Yb-free one which is 67,000±4800 photons/Me V, a typical value for commercial CsLTl single crystal. On the basis of the Bartram- Lempicki model LR=10⁶/(pxE_g), the number of photons per unit of absorbed energy (MeV) can be roughly estimated, where E_g is the band gap of Csl and equals to 6.2 eV, the value of β is 1.5-1.8 for ionic halide compounds. Thus, the theoretical light yield LR for CsLTl should be within 89,600-107,500 photons/MeV. It is evident that the

+ 2+

light yield of the optimized CsLTl ,Yb approaches its theoretical value. The pulse

+ 2+ 22

height spectra of optimized CsLTl ,Yb crystals under Na excitation are presented in Fig. 4. The FWHM energy resolutions obtained for 511 keV γ-rays from the 22 Na source are 9.2%, 8.1% and 7.9% for ST3, ST1 and ST4, respectively.

The CsLTl⁺,Yb²⁺ single crystal grown by this invention has so far exhibited an ultra-high light yield value of 90,000±6000 photons/Me V, energy resolution 7.9%@511 keV and suppressed afterglow level down to 0.035% at 80 ms. Simultaneous improvement of afterglow level, light yield and energy resolution in Yb- codoped CsLTl scintillator compared to standard CsLTl one is considered as a breakthrough in the optimization of the scintillator and paves the way for its application in the X-ray fast imaging applications. However, as for the Yb³⁺ codoped CsLTl single crystals, their pulse height spectra under 137 Cs irradiation are plotted in Fig. 5. It indicates that the light yield still maintain the same level after Yb³⁺ codoping, but its afterglow level is not suppressed to lower level.

The afterglow profiles of Sm-free and Sm-codoped CsLTl crystals are shown in Fig. 6. The afterglow level of the Sm²⁺ codoped ones decreases by about three orders of magnitude in comparison with Sm -free one after 50 ms. Comparing with afterglow suppression effect of other codopants like Eu and Bi ions, the performance of Sm codopant is the best as it lowers the afterglow level far below 0.01%@50ms in comparison with 0.03%@50ms for Eu²⁺ and ~0.03%@50ms for Bi³⁺ codopant. Surely, apart from the positive influence of afterglow suppression, in fact, the effects of ultralow-concentration Sm 2+ on light yield and energy resolution is also worthy of attention. Because the light yield and energy resolution in CsLTl single crystals is deteriorated after 0.05mol%-0.5mol% Sm codoping (see Brecher et al. Patent No.: US7,759,645 Bl). Pulse height spectra of ultralow-concentration Sm

27

codoped CsLTl crystals under Cs (662 keV) excitation are plotted in Fig. 7. Considering the light yield of CsLTl standard sample about 67,000±4800 photons/MeV and the relative channel number, it is estimated that the 0.005mol%

2+

Sm codoped one can reach 7 l,700±6000pho tons/Me V, much better than 65,000 photons/Me V, the value reported for commercial CsLTl. Thus, simultaneous improvement of afterglow level, light yield and energy resolution in ultralow concentration Sm 2+ -codoped CsLTl scintillator compared to standard CsLTl one is considered as a breakthrough in the optimization of the scintillator and paves the way for its application in the X-ray fast imaging applications.

Besides the single crystals, we also fabricated the codoped CsLTl thin films by vapor deposition method. The CsLTl + ,Yb 2+ thin film with about 0.1 mm thickness is presented in Fig. 8. Fig. 9 shows that the afterglow of CsLTl thin film could be loared down by about one order of magnitude after Yb codoping, similar with its positive effect in single crystals. EXAMPLES

Below with reference to specific embodiments, further illustrate the present invention. It should be understood that these embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention.

This invention is to explore an effective codopant which not only suppress the afterglow but also enhance the light yield and energy resolution of CsLTl scintillators. This invention adopted Til as fixed dopant and YbX₂, or SmX₂ or YbX₃ (X=F, CI, Br or I) as the second dopant, or codopant. The formula of final composition could be (Cs_1-x-yTl_xRE_y)(I_1-yX_2y) (RE=Sm, Yb ; X=F , CI, Br, or I), or (Csi__x__yTl_xYb_y)(Ii_ _yX_3y) (X=F , CI , Br , or I) Among them , 0<x<0.05 , better choice is 0<x<0.05;0<y<0.05 , better choice is 0<y<0.001„

The purity of the raw materials used in the embodiments are not less than 99.99% .

Example 1: (Cso.9985Tlo.001Ybo.0005) (I1.0005) single crystal growth by vertical Bridgman method:

(1) Firstly, weighting Csl 518.5 g, Til 0.6625 g, Ybl₂ 0.4268 g according to the stoichiometric ratio of (Cso.9985Tlo.001Ybo.0005) (I1.0005) , with the condition that purity of Csl and Til is 99.99% , and the purity of Ybl₂ is 99.999% ;

(2) The material is loaded in a quartz ampoule crucible with diameter of < 40mm, mixing, using acetylene welding nozzle seal, the crucible is placed in a ceramic tube in the primer, and then the ampoules are placed on the driving platform;

(3) Heating the material to a molten state, until completely melted;

(4) The lowering rate of quartz crucible is 1 mm/ hour;

(5) The resulting ingot is cut, grinded and polished into 1 inch in diameter and 1 inch in length crystal, which is colorless, transparent and without inclusion (sees Figure 1).

As can be seen in Fig. 2, the afterglow intensity@80ms of CsLTl sample codoped with Yb 2+ ions is reduced by about a factor of three comparing with Yb 2+ - free one. Meanwhile, the light yield and energy resolution properties are improved to 90,000ph./MeV and 7.9%(¾662keV, derived from the data show in Fig.3 and Fig.4

Example 2: (Cso.9985Tlo.001Ybo.0005) (Io.9995Bro.001) single crystal growth by vertical Bridgman method:

(1) Firstly, weighting Csl 518.5 g , Til 0.6625 g, YbBr₂ 0.3328 g according to the stoichiometric ratio of (Cso.9985Tlo.001Ybo.0005) (Io.9995Bro.001) , with the condition that purity of Csl and Til is 99.99% , and the purity of YbBr₂ is 99.999%;

(2) The material will be loaded in a fused silica crucible of < 40mm, which inner wall is deposited with a thin carbon film, mixing, using acetylene welding nozzle seal, the crucible is placed in a ceramic tube in the primer, and then the tubes are placed on the motor driving platform;

(3) Heating the material to a molten state, until completely melted insulation materials 8 hours;

(4) The lowering rate of quartz crucible is 0.6 mm/ hour;

(5) The resulting ingot is cut, grinded and polished into 1 inch in diameter and 1 inch in length crystal, which is colorless, transparent and without inclusion.

Its afterglow, light yield and energy resolution properties are improved simultaneously, similar with the results shown in Fig.2, Fig.3 and Fig.4.

Example 3: (Cso.998Tlo.001Ybo.001) (I0.999CI0.002) single crystal growth by Bridgman method:

(1) Firstly, weighting Csl 518.6 g , T1I0.6626 g, YbCl₂ 0.4879 g according to the stoichiometric ratio of (Cso.99gTlo.001Ybo.001) (I0.999CI0.002) , with the condition that purity of Csl and Til is 99.99% , and the purity of YbCl₂ is 99.999%;

(3) Heating the material to a molten state, until completely melted insulation materials 8 hours; (4) The lowering rate of quartz crucible is 0.6 mm/ hour;

Example 4: (Cso.993Tlo.002Ybo.005) (I0.995F0.01) single crystal growth by Bridgman method:

(1) Firstly, weighting Csl 516.0 g , Til 1.325 g, YbF₂ 2.1104 g according to the stoichiometric ratio of (Cso.993Tlo.002Ybo.005) (I0.995F0.01) , with the condition that purity of Csl and Til is 99.99% , and the purity of YbF₂ is 99.999%;

(2) The material will be loaded in a fused silica crucible of < 40mm, mixing, using acetylene welding nozzle seal, the crucible is placed in a ceramic tube in the primer, and then the tubes are placed on the motor driving platform;

(4) The lowering rate of quartz crucible is 1 mm/ hour;

Example 5: (Cso.99gTlo.001Ybo.001) (I1.002) single crystal growth by Bridgman method:

(1) Firstly, weighting Csl 518.6 g, Til 1.253 g, Ybl₃ 1.1075 g according to the stoichiometric ratio of (Cs₀.99gTl₀.ooiYbo.ooi) (I1.002) , with the condition that purity of Csl and Til is 99.99% , and the purity of Ybl₃ is 99.999%;

(2) The material will be loaded in a fused silica crucible of < 40mm, mixing, using acetylene welding nozzle seal, the crucible is placed in a ceramic tube in the primer, and then the tubes are placed on the motor driving platform; (3) Heating the material to a molten state, until completely melted insulation materials 8 hours;

(4) The lowering rate of quartz crucible is 0.6 mm/ hour;

Fig.5 indicates that the light yield and energy resolution properties of CsLTl codoped with Yb³⁺ are improved, despite its afterglow cannot be suppressed.

Example 6: (Cso.9985Tlo.001Ybo.0005) (I1.0005) single crystal grown by Czochralski method :

(1) Firstly, weighting Csl 518.5 g, Til 0.6625 g, Ybl₂ 0.4268 g according to the stoichiometric ratio of (Cso.9985Tlo.001Ybo.0005) (I1.0005) , with the condition that purity of Csl and Til is 99.99% , and the purity of Ybl₂ is 99.999%;

(2) Drying under vacuum at 300 °C after mixing raw materials;

(3) Loading material ingot to Ir crucible of Φ60*50 mm ;

(4) Protection atmosphere is high-purity Ar, together with appropriate thermal insulation structure. (5) Temperature gradient established on the basis of the rotation speed (10 rpm) and pulling speed (2 mm/h), after seeding, shrinking neck, shouldering, isometric process, and ending process to obtain the desired size of the crystal, and finally pulled away from the crystal surface ;

(6) Cool down to room temperature for a total of 20 hours to obtain a single crystal ;

(7) The resulting ingot is cut, grinded and polished into 1 inch in diameter and 1 inch in length crystal, which is colorless, transparent and without inclusion.

Example 7: (Cso.9985Tlo.001Ybo.0005) (I1.0005) single crystal growth by Kyropoulos method : (1) Firstly, weighting Csl 518.5 g, Til 0.6625 g, Ybl₂ 0.4268 g according to the stoichiometric ratio of (Cso.9985Tlo.001Ybo.0005) (I1.0005) , with the condition that purity of Csl and Til is 99.99% , and the purity of Ybl₂ is 99.999%;

(2) Drying under vacuum at 300 °C after mixing raw materials;

(3) Loading material ingot to Ir crucible of Φ60*50 mm ;

(4) Protection atmosphere is high-purity Ar, together with appropriate thermal insulation structure. (5) Appropriate temperature gradient without rotation and pulling, then seeding, shrinking neck, shouldering, isometric process, and ending process to obtain the desired size of the crystal, and finally pulled away from the crystal surface ;

(6) Cool down to room temperature for a total of 20 hours to obtain a single crystal;

Deposition equipment consists of the coating chamber, the working frame, vacuum system and electrical control. Pure aluminum coating chamber bombardment with an electrode poared by leakage transformer; deposition fittings for fixing the pallet; vacuum system consists of mechanical pumps, diffusion pumps, high vacuum valves, and other components; electrical control provides power and control for the bombardment, evaporation electrode, deposition and safety protection device.

Example 8: (Cso_.999Tlo_.001) (I) thin film preparation by thermal evaporation method

(1) High purity 259.55 g Csl and 0.331 g Til are thoroughly mixed, then loaded into the molybdenum metal evaporation boat fixed at electrode;

(2) Glass substrate is heated to 200^°Cby resistance heating, and lower the pressure in thermal evaporation chamber down to 10^" torr;

(3) More than 10 minutes ion bombardment, along with the rotation of the substrate holder; (4) Stop evaporation until the thickness of thin film reached 100 μιη, then cool the temperature to 250^°C , preserve heat at this temperature about 25 minutes, then cool to room temperature;

(5) Maintain the original pressure of approximately 8 hours and then removing the substrate.

Example 9: (Cso.9985Tlo.001Ybo.0005XI1.0005) thin film preparation by thermal evaporation method

(1) High purity 259.25 g Csl, 0.33125 g Til, 0.2134 g Ybl₂ are mixed and then loaded into the molybdenum metal evaporation boat fixed at electrode;

(3) More than 10 minutes ion bombardment, along with the rotation of the substrate holder;

(4) Stop evaporation until the thickness of thin film reached 100 μιη, then cool the temperature to 250^°C , preserve heat at this temperature about 25 minutes, then cool to room temperature;

Resulting thin films are in white and translucent, shown in Fig.8. As can be seen in Fig.9, the afterglow level of CsLTl ,Yb thin film (samples of Example 9) is clearly lower than that of the thin film without Yb ion codoping (samples of Comparative Example 8).

Finally, it is necessary to illustrate in this description: The above embodiments are only used for the technical solution of the present invention will be described in more detail, which cannot be construed as limiting the scope of the present invention, those skilled in the art from the foregoing the present invention is made of some unessential belong to improve and adjust the scope of the present invention.

Claims

What is claimed is:

1 A codoped CsLTl scintillator, which exhibits enhanced light yield and suppressed afterglow.

2. The codoped CsLTl scintillator of claim 1, comprising the chemical formula

(Cs_1-x-yTl_xRE_y)(I_1-xX_2y), where RE is Yb 2+ or Sm 2+ , X is one of the halogen ions selected from F, CI, Br or I; 0<x<0.05 for Tl⁺, 0<y<0.05 for Yb²⁺ and 0<y<0.001 for

Sm²⁺ ₀

3. The codoped CsLTl scintillator of claim 1, comprising the chemical formula (Cs_1-x-yTl_xRE_y)(I_1-yX_3y), where RE is Yb³⁺, X is one of the halogen ions selected from F, CI, Br or I, 0<x<0.05 , 0<y<0.05„

4. The codoped CsLTl scintillator of any of the claim 2-3, wherein the values for x and y are 0<x<0.001, and 0<y<0.001.

5. The codoped CsLTl scintillator of any of the claim 2-3, wherein the afterglow level after X-ray excitation in codoped CsLTl scintillators is lowered at least one magnitude than the codopant-free ones after 50 millisecond.

6. The codoped CsLTl scintillator of claim 2, wherein by codoping Yb²⁺ or Sm²⁺ ions, besides the improvement of afterglow properties, the light yield and energy resolution of said scintillator is also further improved in comparison to the codopant- free ones.

7. The codoped CsLTl scintillator of claim 6, wherein the CsLTl scintillator is capable to be widely used in X-ray medical imaging, radiography, security inspection and other radiation detection techniques.

8. A crystal growth method for forming the codoped CsLTl single crystals, including vertical Bridgman, Czochralski or Kyropoulos methods.

9. The crystal growth method of Claim 8, wherein the crystal growth method is Bridgman method, comprising:

1) . loading the high-purity Csl, Til, and YbX₂, YbX₃ or SmX₂ in the fused silica crucible according to the stoichiometric ratio, where X is selected from F, CI, Br and I, then heating the crucibles undervacuum;

2) maitaining the crucibles at a temperature 100°C above the melting point of cesium iodide for 24 hours;

3) passing the crucibles through a temperature gradient with a speed of 0.1-5 mm/h.

10. The crystal growth method of Claim 8, wherein the crystal growth method is Czochralski method, comprising:

1) loading the high-purity Csl, Til, and YbX₂, YbX or SmX₂ in an iridium or platinum crucible according to the stoichiometric ratio, where X is selected from F, CI, Br or I;

2) growing the crystal in protection atmosphere like Argon and Nitrogen, using pulling rate at 1-10 mm/h and rotating rate at 5-50 rpm.

11. The crystal growth method of Claim 8, wherein the crystal growth method is Kyropoulos method, comprising: 1) loading the high-purity Csl, Til, and YbX₂, YbX₃ or SmX₂ in an iridium or platinum crucible according to the stoichiometric ratio,where X is selected from F, CI, Br or I ;

2) growing the crystal in protection atmosphere like Argon and Nitrogen, without rotating and pulling.

12. A method for fabricating the codoped CsLTl thin films, wherein the method is thermal evaporation method or ion beam sputtering method, comprising:

1) loading the high-purity Csl, Til, and YbX₂, YbX or SmX₂ in the evaporation boat according to the stoichiometric ratio, to form a mixture, where X is selected from F, CI, Br or I ;

2) depositing the mixture formed in 1) on a substrate, wherein the evaporation pressure is 10^" - 10^" Torr and the substrate temperature is30 - 250 °C.

13. The method of Claim 13, wherein the substrate is glass or silicon wafers.

14. Use of the CsLTl scintillator according to anyone of Claims 1-3 and 6-7 in the field of X-ray high-speed imaging, radiography and computer tomography, either in the form of single crystals or thin films.