High Light Yield Fast Scintillator

Abstract

The invention concerns a material comprising a compound of formula Pr(1-x-y)LnyCexX3 wherein—Ln is chosen from the elements or mixtures of at least two elements, of the group: La, Nd, Pm, Sm, Eu, Gd, Y, —X is chosen from the halides or mixtures of at least two halides, of the group: Cl, Br, I, —x is above 0.0005 and is lower than 1, —y is from 0 to less than 1 and—x+y) is less than 1, and its use as scintillation detector, for example in PET scanner with time of flight apabilities.

Description

The invention pertains to new scintillating materials, in particular in the form of single crystals, a process to manufacture them as single crystals, and their use to detect and/or identify X-ray and/or gamma-ray radiations.

Scintillation materials are largely used for detection of gamma-rays, X-rays, cosmic rays and particles or electromagnetic waves of lower energies down to 1 keV or lower, below designated generically as “radiation”.

The scintillation mechanisms rely on a number of physical principles which essentially convert the high energy of the incoming photons or particles into light which is within or reasonably close to the visible range, so it can be detected by standard photo-detectors. Of particular interest are single crystal forms of scintillators, i.e. pieces that are at the scale of use constituted of one (at most a few) crystals. A single crystal (monocrystal) configuration allows easier extraction of the emitted light over thick dimensions thanks to the lack of internal diffusion through grain boundaries, heterogeneties and other defects that tend to be present in polycrystalline materials. A crystalline structure (in the atomic sense) is required as it determines the scintillation mechanisms: a glassy, amorphous state of matter is likely to yield different scintillation properties. The extracted light is then collected with various devices well known to the man of the art, like photomultiplier tubes, photodiodes etc. Another configuration is to still retain the crystalline structure of the material, and use it in powder form, either packed or sintered or mixed with a binder in a way that still allows light extraction. Usually, those configurations are too opaque when more than a few millimeters thick, a thickness which may not be sufficient to stop enough incoming particles or photons. Overall, whenever possible and cost effective, single crystals are preferred.

Radiation detection is of major interest in a host of applications in nuclear medicine, fundamental physics, industrial gauging, baggage scanners, oil well logging etc. In those applications, it is often desirable to discriminate at a high counting rate radiations that may also reach the detector and a scintillation detector should be able to produce different luminescence signals depending on the type of radiation (see G. F. Knoll, Radiation Detection and Measurements (Wiley, New York, 2000)).

Several criteria are needed to constitute a good X-ray or gamma-ray detector.

In a way well know to the man in the field, an energy spectrum of the scintillator under incoming radiation is drawn, whereby events are represented on a histogram (with energies on the x-axis, the number of counts on the y-axis). In the acquisition protocol, “channels” are defined to collect the signal within a particular energy range.

Good (low) energy resolution is necessary for good energy peak identification of the incoming radiation. Energy resolution is usually determined for a given detector at a given energy as the full width at half maximum of the peak considered on an energy spectrum, divided by the energy at the centroid of the peak (see G. F Knoll, “Radiation Detection and Measurement”, John Wiley and Sons, Inc, 2^nd edition, p 114).

Another very important parameter is the decay time constant, which is described in particular by W. W Moses (Nucl. Instr and Meth. A336 (1993) 253). Fast decay times allow fast analyses. In general, the time spectrum of the collected signal from a scintillator under radiation can be fitted by a sum of exponentials characterized each by a decay time constant. The quality of a scintillator is determined essentially by the contribution of the fastest emission component. This is the number we report further in the text.

A family of known scintillator crystals widely used is of the thallium-doped sodium iodide Tl:NaI type. This scintillating material, discovered in 1948 by Robert Hofstadter and which forms the basis of modern scintillators, still remains the predominant material in this field in spite of almost 60 years of research on other materials. However these crystals have a scintillation decay which is not very fast.

One family of scintillator crystals that has undergone considerable development is of the bismuth germanate (BGO) type. The crystals of the BGO family have high decay time constants, which limit the use of these crystals to low count rates.

Recently, scintillating materials have been disclosed by O. Guillot-Noel et al. (“Optical and scintillation properties of cerium-doped LaCl₃, LuBr₃ and LuCl₃” in Journal of Luminescence 85 (1999) 21-35). This article describes the scintillation properties of cerium-doped compounds such as LaCl₃ doped with 0.57 mol % Ce; LuBr₃ doped with 0.021 mol %, 0.46% mol % and 0.76 mol % Ce; LuCl₃ doped with 0.45 mol % Ce. These scintillating materials have quite useful energy resolutions, of the order of 7%, and decay time constants of the fast scintillation component that are fairly low, especially between 25 and 50 ns. However the intensity of the fast component of these materials is low, especially of the order of 1000 to 2000 photons per MeV, which means that they cannot be used as a component of a high-performance detector.

The present invention pertains to a new material containing praseodymium halide and cerium halide showing a remarkably low decay time (often written τ). The material of the invention comprises a compound of formula Pr_(1-x-y)Ln_yCe_xX₃, wherein

• • Ln is chosen from La, Nd, Pm, Sm, Eu, Gd, Y or a mixture of at least two of them (La, Nd, Pm, Sm, Eu, Gd, Y),
  • x is above 0.0005 and lower than 1;
  • x is preferably above 0.005;
  • x is preferably less than 0.9 (=less than 90 mol %) and preferably less than 0.4;
  • X is chosen from the halides Cl, Br, I or mixtures of at least two halides of the group: Cl, Br, I,
  • y can be 0 and goes from 0 to less than 1 and preferably up to 0.9 and is preferably less than (1-x)/2,
  • (x+y) is less than 1.

Preferably, if X is I (Iodine) or a mixture of halides containing 50 mol % or more of the halide I, Ln is chosen from the group: La, Nd, or a mixture of both. Preferably, if X is containing less than 50 mol % of the halide I (Iodine), Ln is chosen from the group: La, Nd, Pm, Sm, Eu, Gd, Y or a mixture of at least two of them.

Present invention concerns more in particular the material consisting essentially of the compound of formula Pr_(1-x-y)Ln_yCe_xX₃ and more particularly the material consisting of the compound of formula Pr_(1-x-y)Ln_yCe_xX₃.

The compound Pr_(1-x)Ce_xX₃ (wherein x and X have the same signification than explained above, and y is 0) is more particularly concerned. In particular, the compound Pr_(1-x)Ce_xBr₃ (wherein x has the same signification than explained above, and y is 0) is preferred.

The value x is the molar level of substitution of Pr by cerium, subsequently called “cerium content”. The value x is above 0.0005, but it can also be said that x is above 0.05 mol %. Both wordings are equivalent. As an example, if x is 0.05 and y is 0, the concerned compound can be written: PrBr₃:5% Ce. Such wording is well used by the man skilled in the art.

The value y is the molar level of substitution of Pr by Ln.

An inorganic scintillating material according to the invention substantially consists of Pr_(1-x)Ce_xX₃ and may also comprise impurities usual in the technical field of the invention. In general, the usual impurities are impurities coming from the raw material whose content is in particular less than 1 mol %, or even less than 0.1 mol % (case of solubilized impurities). In case of unwanted phases, the volume percentage of these unwanted phases is especially less than 1%.

The scintillating material of the invention can be manufactured under the form of a monocrystal. In this case, its volume is generally at least 10 mm³. The material can also be used in the form of a powder (polycrystalline), possibly either packed or sintered or mixed with a binder.

The compounds in the examples were grown as single crystals by the vertical Bridgman technique. Because the compounds are hygroscopic the experiments were performed on samples sealed in quartz ampoules. Other known techniques to the man in the field of crystal growths could be used such as Czochralski or Kyropoulos techniques, provided the material is reasonably protected from reaction with water and oxygen. The single crystal samples in the examples were about 10 mm³ in volume.

The properties of PrBr₃:Ce³⁺ are given in Table 1 below, along with those of the scintillators already known in the field of gamma-ray detection.

TABLE 1
Comparative properties of PrBr3:Ce3+ and other
gamma-ray scintillators.
	LY	R	τ		λem
Compound	(ph/MeV)	(%)	(ns)	density	(nm)
PrBr3:5% Ce	16000	5.5	10	5.3	370
PrBr3:20% Ce	21000	6.9	10	5.3	390
LaCl3:10% Ce	45000	4	28	3.9	350
LaBr3:5% Ce	60000	2.8	16	5.3	380

The following abbreviations were used in table 1:

• • LY=Light yield
  • R=energy resolution
  • τ=decay time
  • λ_em=wavelength of the main peak of light emission
  • 5% Ce means that x=0.05 in the generic formula which is in fact Pr_0.95Ce_0.05Br₃.

Scintillation decay time spectra of PrBr₃: 5% Ce³⁺ and PrBr₃: 20% Ce³⁺ recorded at room temperature under 137Cs γ-ray excitation using single-photon-counting techniques are shown in FIG. 1. On this figure, the Scintillation decay curves at room temperature of (a) PrBr₃: 5% Ce³⁺ and (b) PrBr₃: 20% Ce³⁺ can be seen. These spectra were recorded using a single-photon counting technique. Solid lines through the data are single exponential fits. For comparison, the scintillation decay spectrum of LaBr₃: 5% Ce³⁺ is also plotted.

The fast component reported for the 5 mol % cerium-doped PrBr₃ represents 90% of the emitted light, a very high number that increases the advantage of the scintillator material over other gamma-ray detectors with respect to its fast component.

FIG. 2 shows pulse height spectra of radiation from a ¹³⁷Cs source recorded with (a) PrBr₃: 5% Ce³⁺ and (b) PrBr₃: 20% Ce³⁺ for 662 keV γ-rays.

It is known to the man skilled in the art that the scintillation properties of a crystal are tightly related to its structure (known as “space group”). It is also known to the man skilled in the art that crystals having the same space group can easily mix and form a crystalline solid solution. The space group of PrCl₃ and of PrBr₃ is P6₃/m. This is the same space group as for LaX₃, CeX₃, NdX₃, PmX₃, SmX₃, EuX₃ and GdX₃, with X: Cl or Br or a mixture of both. The space group of PrI₃ is Cmcm. This is the same space group as for LaI₃, CeI₃, NdI₃.

It is shown in particular that materials of the invention constitute interesting gamma detectors, less bright than the LaBr₃:Ce known in the field of gamma detection, with a poorer energy resolution but much faster decay. It is worth adding that the energy resolution figures given for the samples of the invention do not constitute a fundamental limit for the claimed compositions. It is well known to the man of the art that energy resolution can be improved with crystals of good crystallinity and homogeneity. Homogeneity in Ce contents, low inclusion and defect levels are preferred. Such crystals can be obtained for instance in well controlled furnaces, with the adequate choice of thermal conditions, of thermal gradients at the solid/liquid interface and of the growth rate feedback loop, as is well know to the man of the art to obtain homogeneous crystals.

Such very fast decay is of particular interest for high count rate applications, where the number of incoming photons to be detected is extremely high, above 100 kilocounts per second (kcps), or even above 1 Mcps. This is of particular interest but not exclusively in spectroscopic analyses (where a sample is intentionally bombarded by a high flux of X-rays), or in PET scanners (Positron Tomography Emission). PET scanners with Time of Flight capabilities, which require extremely fast timing properties, could advantageously use the product of the invention.

Thus, present invention concerns also a scintillation detector (scintillator) containing the material of the invention.

Present invention is also described in the following, presented as chapters 1 to 5:

1. INTRODUCTION

Ce³⁺ doped lanthanide trihalides scintillator (REX₃: Ce³⁺ with RE=La, Gd, Lu, Y and X=F, Cl, Br, I) were investigated in recent years in the search for the better scintillators. LaCl₃: Ce³⁺, LaBr₃: Ce³⁺ and LuI₃: Ce³⁺ have excellent scintillation properties including high light yield, good energy resolution, and fast decay time [1-4]. Despite these achievements, there is still an endeavor for searching a better scintillator.

Beside Ce³⁺ there are also efforts to introduce Pr³⁺ as an activator in compound [5]. Theoretically, the 5d→4f emission in Pr³⁺ should be a factor 2 to 3 faster than in Ce³⁺ due to the higher energy 5d→4f emission of Pr³⁺ as compared to Ce³⁺ [6]. Unfortunately, development of LaBr₃: Pr³⁺ as a new fast scintillator was not successful. It does not show the anticipated 5d→4f emission; instead slow 4f→4f emission is observed. The charge transfer from the valence band to Pr³⁺ occurs at energy below the energy of 5d→4f emission, and this quenches Ce³⁺ emission [7].

In this work, we report on the scintillation properties of PrBr₃: Ce³⁺. Its light yields of 16,000 and 21,000 photons/MeV are four times less than that of LaBr₃: Ce³⁺. On the other hand, the 5.6 ns decay time of Ce³⁺ emission at RT of PrBr₃: Ce³⁺ is three times faster than that of LaBr₃: Ce³⁺. Spectroscopy of PrBr₃: Ce³⁺ is also presented.

2. EXPERIMENTAL PROCEDURES

Relatively small (5×3×2 mm³) single crystals of PrBr₃: 5% Ce³⁺ and PrBr₃: 20% Ce³⁺ were cut from the original crystal boules grown from PrBr₃ and CeBr₃ powder by the Bridgman technique. Starting materials were prepared from Pr₂O₃ (Heraeus, 99.999%), NH₄Br (Merck, >99.9%), and HBr (47% Merck, proanalysis) by the ammonium halide method [8,9]. PrBr₃ crystallizes in the UCl₃ type structure with space group P6₃/m (no. 176) [10]. Based on its structure and lattice parameters, PrBr₃ has a calculated density ρ_r of 5.27 g/cm³ and an effective atomic number Z_eff of 48.27. Details on experimental setups can be found in Reference [4].

3. RESULTS AND DISCUSSION

The X-ray excited emission spectra of PrBr₃: 5% Ce³⁺ and PrBr₃: 20% Ce³⁺ recorded at RT are shown in FIG. 3. Each spectrum has been corrected for the transmission of the system and normalized in such a way that the integral over all wavelengths is equal to the absolute light yield in photons/MeV as obtained from pulse height spectra recorded with shaping time of 10 μs.

The spectra are dominated by two main overlapping bands peaked at 365 and 395 nm due to Ce³⁺ emission. The location of this Ce³⁺ emission is similar to that of LaBr₃: Ce³⁺ [11]. When the Ce³⁺ concentration increases, the average of Ce³⁺ emission is slightly shifted to longer wavelengths and the intensity of Ce³⁺ band increases. The intensity of 4f²→4f² Pr³⁺ lines located between 500 and 750 nm is much weaker compared to that of Ce³⁺ emission. These lines in the PrBr₃: 5% Ce³⁺ emission spectrum overlap with those in the PrBr₃: 20% Ce³⁺ emission spectrum.

FIG. 4 shows pulse height spectra of radiation from a ¹³⁷Cs source recorded with PrBr₃: 5% Ce³⁺ and PrBr₃: 20% Ce³⁺. The satellite peaks accompanying the photopeak at lower energy due to the escape of characteristics Kα, K_β X-rays of Pr are not observed in the spectra.

Light yield and energy resolutions derived from pulse height spectra measured with a Hamamatsu R1791 PMT are presented in Table 1. The highest light yield was obtained for PrBr₃: 20% Ce³⁺.

TABLE 1
Light yield and energy resolution derived from pulse height spectra of
PrBr3: 5% Ce3+ and PrBr3: 20% Ce3+ under 662 keV
γ-ray excitation measured with a Hamamatsu R1791 PMT using
shaping time of 0.5, 3, and 10 μs.
	Light yield
	(103 photons/MeV)	Energy
Compound	0.5 μs	3 μs	10 μs	Resolution R (%)
PrBr3:	14.3 ± 1.4	15.2 ± 1.5	15.6 ± 1.5	5.5 ± 0.5
5% Ce3+
PrBr3:	19.1 ± 1.9	20.3 ± 2.0	20.7 ± 2.0	6.9 ± 0.7
20% Ce3+

Scintillation decay time spectra of PrBr₃: 5% Ce³⁺ and PrBr₃: 20% Ce³⁺ recorded at room temperature under ¹³⁷Cs γ-ray excitation using single-photon-counting techniques are shown in FIG. 5. Slow components in the decay curves were not observed. The decay time spectra of PrBr₃: 5% Ce³⁺ and PrBr₃: 20% Ce³⁺ were fitted with a single exponential decay of 10±1 ns. This decay time is faster than the scintillation decay time of LaBr₃: 5% Ce³⁺ (˜16 ns) previously reported by Bizarri et al. [12].

The excitation and emission spectra of PrBr₃: 5% Ce³⁺ are shown in FIG. 6. The excitation spectrum monitoring Ce³⁺ emission of 380 nm recorded at 10 K shows several bands. The five bands at 277, 288, 299, 312, and 328 nm are attributed to Ce³+4f→5d transitions. These bands have the same position with those of LaBr₃: Ce³⁺ [7]. The low intensity band at 240 nm was previously observed in LaBr₃: Pr³⁺ and assigned to charge transfer (CT) band from Br⁻ to Pr³⁺ [7]. The emission spectrum excited at CT band of Pr³⁺ at 240 nm exhibits 5d→4f Ce³⁺ [²F_5/2,²F_7/2] doublet emission and Pr³⁺ 4f²→4f² transition lines. The presence of Ce³⁺ doublet emission when excited at the CT band of Pr³⁺ shows that the energy transfer to Ce³⁺ involves Pr²⁺ as an intermediate state. The Pr³⁺ 4f²→4f² transition lines originate from the ³P₀ state. The absence of transitions from the ³P₁ state is probably caused by energy exchange with neighboring Pr³⁺ ion [13].

FIG. 7 shows the decay time spectra of Ce³⁺ emission at 366 nm of PrBr₃: 5% Ce³⁺ upon excitation in the CT band of Pr³⁺ at 250 nm and excitation of the 4f→5d transition of Ce³⁺ at 300 nm recorded at 10 K and RT.

The decay time spectra were fitted with a single exponential. The decay times of Ce³⁺ emission excited at 300 nm recorded at 10 K and RT, respectively, are 10.0±1.0 and 5.6±0.6 ns (see FIG. 7a). This shows that Ce³⁺ emission quenches at RT. The decay times of Ce³⁺ emission optically excited via the CT band of Pr³⁺ at 250 nm recorded at 10 K and RT, respectively, are 11.1±1.1 and 6.0±0.6 ns (see FIG. 7b). These decay times are similar with the characteristic of the lifetime of the excited state of Ce³⁺ and therefore the aforementioned energy transfer to Ce³⁺ is very fast (˜1 ns). It corresponds with the around 90% contribution of light yield within 0.5 μs to the total light yield. However, the decay time of Ce³⁺ emission is 4-5 ns faster than the scintillation decay time (see FIG. 5). The scintillation process probably explains this difference.

FIG. 3: X-ray excited emission spectra recorded at RT of PrBr3: 5% Ce3+ and PrBr3: 20% Ce3+. The y-axis has been calibrated using light yields derived from pulse height spectra. The spectra ranged from 500 to 750 nm are enlarged by factor of 20 from their actual intensity.

FIG. 4: Pulse height spectra of (a) PrBr3: 5% Ce³⁺ and (b) PrBr3: 20% Ce³⁺ for 662 keV γ-rays. Spectra were measured with a Hamamatsu R1791 PMT.

FIG. 5: Scintillation decay curves at room temperature of (a) PrBr3: 5% Ce3+ and (b) PrBr3: 20% Ce3+. These spectra were recorded using a single-photon counting technique. Solid lines through the data are single exponential fits. For comparison, the scintillation decay spectrum of LaBr3: 5% Ce³⁺ taken from Reference [12] is also plotted.

FIG. 6: (a) The excitation spectrum of PrBr3: 5% Ce3+ monitoring 380 nm emission at 10 K and (b) the emission spectrum on a logarithmic scale of PrBr3: 5% Ce3+ excited at 240 nm at RT. Five gaussian fit in (a) is used to determine five Ce3+4f→5d excitation bands.

FIG. 7: Decay time spectra on logarithmic scale of 366 nm Ce3+ emission of PrBr3: 5% Ce3+ excited via (a) 4f→5d transition of Ce3+ at 300 nm and (b) CT band of Pr3+ at 250 nm recorded at (i) 10 K and (ii) RT. Solid lines drawn through the data are the single exponential fits.

4. CONCLUSION

We have introduced a new scintillator, PrBr₃:Ce³⁺ for γ-ray detection. The lower light yield (˜16,000-21,000 photons/MeV) and the faster scintillation decay time (˜10 ns) than those of LaBr₃: Ce³⁺ were recorded. The fast energy transfer is proposed and the decay times of Ce³⁺ emission recorded at 10 K and RT show that Ce³⁺ emission quenches at RT.

5. REFERENCES

• [1] O. Guillot-Noël, J. T. M de Haas, P. Dorenbos, C. W. E. van Eijk, K. W. Kramer, H. U. Güdel, Jounal of Luminescence 85 (1999) 21.
• [2] E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. W. Krämer, H. U. Güdel, Appl. Phys. Lett. 79 (2001) 1573.
• [3] K. S. Shah, J. Glodo, M. Klugerman, W. Higgins, T. Gupta, P. Wong, W. W. Moses, S. E. Derenzo, M. J. Weber, P. Dorenbos, IEEE Trans. Nucl. Sci. 51 (2004) 2302.
• [4] M. D. Birowosuto, P. Dorenbos, C. W. E. van Eijk, K. W. Krämer, H. U. Güdel, IEEE Trans. Nucl. Sci. 52 (2005) 1114.
• [5] P. Dorenbos, R. Visser, C. W. E. van Eijk, N. M. Khaidukov, M. V. Korzhik, IEEE Trans. Nucl. Sci. 40 (1993) 388.
• [6] P. Dorenbos, Nucl. Instr. Meth. Phys. Res. A 486 (2002) 208.
• [7] P. Dorenbos, E. V. D. van Loef, A. J. P. Vink, E. van der Kolk, C. W. E. van Eijk, K. W. Krämer, H. U. Güdel, W. M. Higgins, K. S. Shah, Journal of Luminescence xx (200x) in publication.
• [8]J. B. Reed, B. S. Hopkins, L. F. Audrieth, Inorg. Synth. 1 (1936) 28.
• [9] G. Meyer, Inorg. Synth. 25 (1989) 146
• [10] B. Schmid, B. Hälg, A. Furrer, W. Urland, R. Kremer, J. Appl. Phys. 61 (1987) 3426
• [11] E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. W. Krämer, H. U. Güdel, Nucl. Instr. Meth. Phys. Res. A 486 (2002) 254
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Claims

1. Material comprising a compound of formula Pr(1-x-y)LnyCexX3, wherein Ln is chosen from the elements or mixtures of at least two elements, of the group: La, Nd, Pm, Sm, Eu, Gd, Y,X is chosen from the halides or mixtures of at least two halides, of the group: Cl, Br, I,x is above 0.0005 and is lower than 1,y is from 0 to less than 1 and(x+y) is less than 1.

2. Material according to claim 1, wherein y is less than (1−x)/2.

3. Material according to claim 1, wherein x is above 0.005.

4. Material according to claim 1, wherein x is less than 0.9.

5. Material according to claim 4, wherein x is less than 0.4.

6. Material according to claim 4, wherein y is less than 0.9.

7. Material according to claim 6, wherein it consists essentially of the compound of formula Pr(1-x-y)LnyCexX3.

8. Material according to claim 7, wherein it consist of the compound of formula Pr(1-x-y)LnyCexX3.

9. Material according to claim 8, wherein y is 0.

10. Material according to either of preceding claims claim 9, wherein X is Br.

11. Material according to claim 1, wherein X is I (Iodine) or a mixture of halides containing 50 mol % or more of the halide I and Ln is chosen from the group: La, Nd, or a mixture of both.

12. Material according to claim 1, wherein X is containing less than 50 mol % of I (Iodine).

13. Material according to claim 12, wherein it is in the form of a monocrystal.

14. Material according to claim 13, wherein the volume of the monocrystal is at least 10 mm3.

15. Material according to claim 1, wherein it is in the form of a powder.

16. Material according to claim 15, wherein it is either packed or sintered or mixed with a binder.

17. Scintillation detector containing the material of claim 16.

18. PET scanner comprising a scintillation detector of claim 17.

19. PET scanner of claim 18, wherein it is with Time of Flight capabilities.

20. The method of using a material according to claim 1 in gamma and/or X-ray and/or cosmic rays detection.

21. The method of claim 20, with count rates of more than 100 kcps.