CESIUM AND SODIUM-CONTAINING SCINTILLATOR COMPOSITIONS

Abstract

The present invention relates to scintillator compositions and related devices and methods. The scintillator compositions may include, for example, a scintillation compound and a dopant, the scintillation compound having the formula x.sub.1-x.sub.2-x.sub.3-x.sub.4 and x.sub.1 is Cs; x.sub.2 is Na; x.sub.3 is La, Gd, or Lu; and x.sub.4 is Br or I. In certain embodiments, the scintillator composition can include a single dopant or mixture of dopants.

Claims

1. A scintillator composition, comprising a scintillation compound and a dopant, the scintillation compound having the formula, Cs.sub.2NaMX.sub.6, wherein: M is La, Gd, or Lu; and, X is Br or I.

2. The scintillator composition of claim 1, wherein the scintillator composition is a fast scintillator.

3. The scintillator composition of claim 2, wherein the scintillator composition comprises a fast decay-time constant is less than about 60 ns.

4. The scintillator composition of claim 1, wherein the scintillator composition comprises a light output of greater than about 10,000 Photons/MeV.

5. The scintillator composition of claim 1, wherein M is Gd.

6. The scintillator composition of claim 1, wherein the dopant comprises Ce, Lu, La, Eu, Pr, Sm, Sr, Tl, Cl, F, or I.

7. The scintillator composition of claim 6, wherein the dopant is present at less than about 20% by molar weight.

8. The scintillator composition of claim 1, wherein the dopant is Ce and present at equal to or less than about 5% by molar weight.

9. The scintillator composition of claim 1, wherein the scintillation composition comprises Cs.sub.2NaGdI.sub.6:Ce, Cs.sub.2NaLaI.sub.6:Ce, or Cs.sub.2NaLuI.sub.6:Ce.

10. A radiation detection device, comprising the scintillator composition of claim 1

11. The device of claim 10, wherein the photodetector assembly comprises a photomultiplier tube, a photodiode, a PIN detector, charge-coupled device, or an avalanche detector.

12. The device of claim 10, further comprising a radiation source.

13. The device of claim 10, further comprising a computer system coupled to the photodetector assembly so that the computer outputs image data in response to detected radiation.

14. The device of claim 13, wherein the computer comprises instructions for constructing an image from detected radiation.

15. The device of claim 10, wherein the device is configured to detect x-rays, gamma-rays, neutron emissions, or a combination thereof.

16. A gamma-ray and neutron detection device, comprising: a scintillator composition of claim 1; a photodetector assembly optically coupled to the scintillator; and electronics configured for performing pulse-shape analysis to differentiate gamma-ray detections from neutron detections.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1A through 1D depict .sup.137Cs spectra collected with Cs.sub.2NaGdI.sub.6:2% Ce (FIG. 1A), Cs.sub.2NaLaBr.sub.6:0.2% Ce (FIG. 1B), Cs.sub.2NaLaI.sub.6:5% Ce (FIG. 1C), and Cs.sub.2NaLuI.sub.6:1% Ce (FIG. 1D) scintillator compositions coupled to a PMT. The energy resolutions of a 662 keV peak for Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaBr.sub.6:0.2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce are 7.6%, 11%, 5.25%, and 8.5% (FWHM), respectively. FIGS. 1A, 1C, and 1D further depict .sup.137Cs spectra for BGO coupled to a PMT.

[0018] FIG. 2A through 2D depict optical emission spectra for Cs.sub.2NaGdI.sub.6:Ce (FIG. 2A), Cs.sub.2NaLaBr.sub.6:Ce (FIG. 2B), Cs.sub.2NaLaI.sub.6:Ce (FIG. 2C), and Cs.sub.2NaLuI.sub.6:Ce (FIG. 2D) scintillator compositions upon exposure to X-rays. FIGS. 2A and 2D show optical emission spectra for 1% and 2% Ce with Cs.sub.2NaGdI.sub.6:Ce and Cs.sub.2NaLuI.sub.6:Ce, respectively.

[0019] FIG. 3A through 3C depict time profiles for Cs.sub.2NaGdI.sub.6:2% Ce (FIG. 3A), Cs.sub.2NaLaI.sub.6:5% Ce (FIG. 2B), and Cs.sub.2NaLuI.sub.6:1% Ce (FIG. 2C) exposed to gamma rays. Risetimes (τ.sub.r) for Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce were 0.85 ns, 4 ns, and 0.85 ns, respectively, in certain embodiments. Principal decay times (τ.sub.d1) for Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce were 55 ns, 50 ns, and 35 ns, respectively.

[0020] FIG. 4A through 4C illustrate non-proportionality for Cs.sub.2NaGdI.sub.6:2% Ce (FIG. 4A), Cs.sub.2NaLaI.sub.6:5% Ce (FIG. 4B), and Cs.sub.2NaLuI.sub.6:1% Ce (FIG. 4C) scintillator compositions. The figure shows light output of the scintillator compositions measured under excitation from isotopes such as .sup.241Am (60 keV γ-rays), .sup.57Co (122 keV γ-rays), .sup.22Na (511 keV and 1275 keV γ-rays), and .sup.137Cs (662 keV γ-rays).

[0021] FIG. 5 is a conceptual diagram of a detector assembly of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] This invention will be better understood with resort to the following definitions:

[0023] A. Rise time, in reference to a scintillation crystal material, shall mean the speed with which its light output grows once a gamma-ray has been stopped in the crystal. The contribution of this characteristic of a scintillator combined with the decay time contribute to a timing resolution.

[0024] B. A Fast timing scintillator (or fast scintillator) typically requires a timing resolution of about 500 ps or less. For certain PET applications (e.g., time-of-flight (TOF)), the fast scintillator should be capable of localizing an annihilation event as originating from within about a 30 cm distance, i.e., from within a human being scanned.

[0025] C. Timing accuracy or resolution, usually defined by the full width half maximum (FWHM) of the time of arrival differences from a point source of annihilation gamma-rays. Because of a number of factors, there is a spread of measured values of times of arrival, even when they are all equal. Usually they distribute along a bell-shaped or Gaussian curve. The FWHM is the width of the curve at a height that is half of the value of the curve at its peak.

[0026] D. Light Output shall mean the number of light photons produced per unit energy deposited by the detected gamma-ray, typically the number of light photons/MeV.

[0027] E. Stopping power or attenuation shall mean the range of the incoming X-ray or gamma-ray in the scintillation crystal material. The attenuation length, in this case, is the length of crystal material needed to reduce the incoming beam flux to 1/e.sup.−.

[0028] F. Proportionality of response (or linearity). For some applications (such as CT scanning) it is desirable that the light output be substantially proportional to the deposited energy. For applications such as spectroscopy, non-proportionality of response is an important parameter. In a typical scintillator, the number of light photons produced per MeV of incoming gamma-ray energy is not constant. Rather, it varies with the energy of the stopped gamma-ray. This has two deleterious effects. The first is that the energy scale is not linear, but it is possible to calibrate for the effect. The second is that it degrades energy resolution. To see how this occurs, consider a scintillator that produces 300 photons at 150 keV, 160 photons at 100 keV and 60 photons at 50 keV. From statistics alone, the energy resolution at 150 keV should be the variability in 300 photons, which is 5.8%, or 8.7 keV. If every detected event deposited 150 keV in one step this would be the case. On the other hand, if, as it occurs, an event deposited 100 keV in a first interaction and then another 50 keV in a second interaction, the number of photons produced would not be 300 on the average, but 160+60=220 photons, for a difference of 80 photons or 27%. In multiple detections, the peak would broaden well beyond the theoretical 8.7 keV. The smaller the non-proportionality the smaller this broadening and the closer the actual energy resolution approaches the theoretical limit.

[0029] The scintillation compositions of the present invention will respond by emitting light after detecting charged particles, high energy photons, and for some embodiments, neutrons, thereby providing useful scintillation properties. The scintillation compound has the formula, x.sub.1-x.sub.2-x.sub.3-x.sub.4, and can include Cs as x.sub.1, Na as x.sub.2, La, Gd, or Lu as x.sub.3, and a halide such as Br or I as x.sub.4. Gd has a large neutron cross-section. In certain embodiments, a dopant as specified in the specification and claims, can be added to the scintillator composition. In certain embodiments, the scintillation compound elements exist in atomic ratios of 2:1:1:6 with a dopant, such as Cs.sub.2NaGdI.sub.6:2% Ce.

[0030] The scintillator compositions of the invention are particularly useful, for example, for spectroscopy detection of energetic photons (e.g., X-rays, gamma-rays), as well as for neutron emission detection. Notable characteristics for the scintillation compositions of the invention include surprisingly robust light output, high gamma-ray and neutron stopping efficiency (attenuation), fast response, and good non-proportionality. Furthermore, the scintillator compositions can be efficiently and economically produced. Thus, detectors having scintillator compositions described in the present invention are useful in a wide variety of applications, including without limitation nuclear and high energy physics research, medical imaging, diffraction, non-destructive testing, nuclear treaty verification and safeguards, and geological exploration.

[0031] The scintillator composition of the present invention can optionally include a “dopant”. Dopants can affect certain properties, such as physical properties (e.g., brittleness, etc.) as well as scintillation properties (e.g., luminescence, etc.) of the scintillator composition. The dopant can include, for example, cerium (Ce), praseodymium (Pr), lutetium (Lu), lanthanum (La), europium (Eu), samarium (Sm), strontium (Sr), thallium (Tl), chlorine (Cl), fluorine (F), iodine (I), and mixtures of any of the dopants. The amount of dopant present will depend on various factors, such as the application for which the scintillator composition is being used; the desired scintillation properties (e.g., emission properties, timing resolution, etc.); and the type of detection device into which the scintillator is being incorporated. For example, the dopant is typically employed at a level in the range of about 0.1% to about 20%, by molar weight. In certain embodiments, the amount of dopant is in the range of about 0.1% to about 100%, or about 0.1% to about 5.0%, or about 5.0% to about 20%, by molar weight.

[0032] The scintillator compositions of the invention may be prepared in several different forms. In some embodiments, the composition is in a crystalline form (e.g., monocrystalline). Scintillation crystals, such as monocrystalline scintillators, have a greater tendency for transparency than other forms. Scintillators in crystalline form (e.g., scintillation crystals) are often useful for high-energy radiation detectors, e.g., those used for gamma-ray or X-ray detection. However, the composition can include other forms as well, and the selected form may depend, in part, on the intended end use of the scintillator. For example, a scintillator can be in a powder form. It can also be prepared in the form of a ceramic or polycrystalline ceramic. Other forms of scintillation compositions will be recognized and can include, for example, glasses, deposits, vapor deposited films, and the like. It should also be understood that a scintillator composition might contain small amounts of impurities. Also, minor amounts of other materials may be purposefully included in the scintillator compositions to affect the properties of the scintillator compositions.

[0033] Methods for making crystal materials can include those methods described herein and may further include other techniques. Typically, the appropriate reactants are melted at a temperature sufficient to form a congruent, molten composition. The melting temperature will depend on the identity of the reactants themselves (see, e.g., melting points of reactants), but is usually in the range of about 300° C. to about 1350° C. Non-limiting examples of the crystal-growing methods can include certain techniques of the Bridgman-Stockbarger methods; the Czochralski methods, the zone-melting methods (or “floating zone” method), the vertical gradient freeze (VGF) methods, and the temperature gradient methods. See, e.g., Example 1 infra. (see also, e.g., “Luminescent Materials”, by G. Blasse et al, Springer-Verlag (1994) and “Crystal Growth Processes”, by J. C. Brice, Blackie & Son Ltd (1986)).

[0034] In the practice of the present invention, attention is paid to the physical properties of the scintillator material. In particular applications, properties such as hygroscopy (tendency to absorb water), brittleness (tendency to crack), and crumbliness should be minimal.

TABLE-US-00001 TABLE I Properties of Scintillators Light Wavelength Principal Output Of Rise- Decay (Photons/ Density Emission time Time Material MeV) (g/cm.sup.3) (nm) (ns) (ns) NaI(Tl) 38,000 3.67 415 >10 230 CsI(Tl) 52,000 4.51 540 >10 1000 LSO 24,000 7.4 420 <1 40 BGO 8,200 7.13 505 >1 300 BaF.sub.2 10,000~ 4.88 310, slow <0.1 620, slow 2,000 fast 220, fast 0.6, fast GSO 7,600 6.7 430 ~8 60 CdWO.sub.4 15,000 8.0 480 5000 YAP 20,000 5.55 370 <1 26 Cs.sub.2NaLaBr.sub.6:Ce 12,000 3.91 386 30 55 Cs.sub.2NaGdI.sub.6 Ce 26,000 ~4 431 0.85 55 Cs.sub.2NaLaI.sub.6:Ce 49,000 ~4 429 4 50 Cs.sub.2NaLuI.sub.6:Ce 27,000 4.6 428 0.85 35

[0035] Table I provides a listing of certain properties of a number of scintillators. Compared to other commercially available scintillators, including CsI, which is among the scintillation materials with the highest known light output, the scintillator compositions of these inventions produce comparable light output. In addition, they have a fast principal decay-time constant.

[0036] As set forth above, scintillator compositions of the present invention may find use in a wide variety of applications. In one embodiment, for example, the invention is directed to a method for detecting energy radiation (e.g., gamma-rays, X-rays, neutron emissions, and the like) with a scintillation detector including the scintillation composition of the invention.

[0037] FIG. 5 is a diagram of a detector assembly of the present invention. The detector 10 includes a scintillator 12 optically coupled to a light photodetector 14 or imaging device. The detector assembly 10 can include a data analysis, or computer, system 16 to process information from the scintillator 12 and light photodetector 14. In use, the detector 10 detects energetic radiation emitted form a source 18.

[0038] A data analysis, or computer, system thereof can include, for example, a module or system to process information (e.g., radiation detection information) from the detector/photodetectors can also be included in an invention assembly and can include, for example, a wide variety of proprietary or commercially available computers, electronics, or systems having one or more processing structures, a personal computer, mainframe, or the like, with such systems often comprising data processing hardware and/or software configured to implement any one (or combination of) the method steps described herein. Any software will typically comprise machine readable code of programming instructions embodied in a tangible media such as a memory, a digital or optical recording media, optical, electrical, or wireless telemetry signals, or the like, and one or more of these structures may also be used to transmit data and information between components of the system in any of a wide variety of distributed or centralized signal processing architectures.

[0039] The detector assembly typically includes material formed from the scintillator composition described herein (e.g., one or more scintillator crystals). The detector further can include, for example, a light detection assembly including one or more photodetectors. Non-limiting examples of photodetectors include photomultiplier tubes (PMT), photodiodes, CCD sensors, image intensifiers, and the like. Choice of a particular photodetector will depend in part on the type of radiation detector being fabricated and on its intended use of the device. In certain embodiments, the photodetector may be position-sensitive.

[0040] The detector assemblies themselves, which can include the scintillator and the photodetector assembly, can be connected to a variety of tools and devices, as mentioned previously. Non-limiting examples include nuclear weapons monitoring and detection devices, well-logging tools, and imaging devices, such as nuclear medicine devices (e.g., PET). Various technologies for operably coupling or integrating a radiation detector assembly containing a scintillator to a detection device can be utilized in the present invention, including various known techniques.

[0041] The detectors may also be connected to a visualization interface, imaging equipment, or digital imaging equipment (e.g., pixilated flat panel devices). In some embodiments, the scintillator may serve as a component of a screen scintillator. For example, powdered scintillator material could be formed into a relatively flat plate, which is attached to a film, such as photographic film. Energetic radiation, e.g., X-rays, gamma-rays, neutron, originating from a source, would interact with the scintillator and be converted into light photons, which are visualized in the developed film. The film can be replaced by amorphous silicon position-sensitive photodetectors or other position-sensitive detectors, such as avalanche diodes and the like.

[0042] Imaging devices, including medical imaging equipment, such as the PET and SPECT devices, and the like, represent another important application for invention scintillator compositions and radiation detectors. Furthermore, geological exploration devices, such as well-logging devices, were mentioned previously and represent an important application for these radiation detectors. The assembly containing the scintillator usually includes, for example, an optical window at one end of the enclosure-casing. The window permits radiation-induced scintillation light to pass out of the scintillator assembly for measurement by the photon detection assembly or light-sensing device (e.g., photomultiplier tube, etc.), which is coupled to the scintillator assembly. The light-sensing device converts the light photons emitted from the scintillator into electrical pulses that may be shaped and digitized, for example, by the associated electronics. By this general process, gamma-rays can be detected, which in turn provides an analysis of geological formations, such as rock strata surrounding the drilling bore holes.

[0043] In many of the applications of a scintillator composition as set forth above (e.g., nuclear weapons monitoring and detection, imaging, and well-logging and PET technologies), certain characteristics of the scintillator are desirable, including high light output, fast rise time and short decay time, good timing resolution, and suitable physical properties. The present invention is expected to provide scintillator materials that can provide the desired high light output and initial photon intensity characteristics for demanding applications of the technologies. Moreover, the invention scintillator compositions are also expected to simultaneously exhibit the other important properties noted above, e.g., short decay time and good stopping power. Furthermore, the scintillator materials are also expected to be produced efficiently and economically, and also expected to be employed in a variety of other devices which require radiation/signal detection (e.g., gamma-ray, X-ray, neutron emissions, and the like).

[0044] The following examples are intended to illustrate but not limit the invention.

EXAMPLES

Example 1

[0045] The present example provides a method for growing and provides characterization for the scintillator composition crystals. The following examples are offered by way of illustration, not by way of limitation.

Crystal Growth of Cs.sub.2NaLaBr.sub.6, Cs.sub.2NaGdI.sub.6, Cs.sub.2NaLaI.sub.6, and Cs.sub.2NaLuI.sub.6

[0046] In one example, a one zone Bridgman furnace was used for crystal growth. Typical growth rates for the Bridgman process are about 1-6 mm/hour. Growth rates ranging from about 1 mm/day to about 1 cm/hour may be utilized. The range of rates may be extended to improve material quality.

[0047] Cs.sub.2NaLaBr.sub.6, Cs.sub.2NaGdI.sub.6, Cs.sub.2NaLaI.sub.6, and Cs.sub.2NaLuI.sub.6 have a cubic crystal structure. The densities of Cs.sub.2NaLaBr.sub.6, Cs.sub.2NaGdI.sub.6, Cs.sub.2NaLaI.sub.6, and Cs.sub.2NaLuI.sub.6 are between about 3.9 and about 4.6 g/cm.sup.3. The compositions melt congruently at approximately 78, 925, 778, and 1050° C., respectively, and therefore their crystals can be grown using melt based methods such as those described by Bridgman and Czochralski. These melt-based processes are well suited for growth of large volume crystals (Brice, Crystal Growth Processes, Blackie Halsted Press (1986)). The Bridgman method has been used for growing Cs.sub.2NaLaBr.sub.6, Cs.sub.2NaGdI.sub.6, Cs.sub.2NaLaI.sub.6, and Cs.sub.2NaLuI.sub.6. Both the vertical and horizontal orientations of the Bridgman method can be used in producing crystals of the present invention. In certain embodiments, the vertical Bridgman method was used in producing crystals examined and discussed below.

[0048] Cs.sub.2NaLaBr.sub.6: Single crystals of this material were grown by the Bridgman technique in vertical silica ampoules under vacuum. Starting materials were CsBr (Aldrich, anhydrous, 99.9%), NaBr (Aldrich, anhydrous, 99.9%), and LaBr.sub.3 (Aldrich, anhydrous, 99.99+%).

[0049] Cs.sub.2NaGdI.sub.6: Single crystals of this material were grown by the Bridgman technique in vertical silica ampoules under vacuum. Starting materials were CsI (Aldrich, anhydrous, 99.9%), NaI (Aldrich, anhydrous, 99.9%), and GdI.sub.3 (Aldrich, anhydrous, 99.99+%).

[0050] Cs.sub.2NaLaI.sub.6: Single crystals of this material were grown by the Bridgman technique in vertical silica ampoules under vacuum. Starting materials were CsI (Aldrich, anhydrous, 99.9%), NaI (Aldrich, anhydrous, 99.9%), and LaI.sub.3 (Aldrich, anhydrous, 99.99+%).

[0051] Cs.sub.2NaLuI.sub.6: Single crystals of this material were grown by the Bridgman technique in vertical silica ampoules under vacuum. Starting materials were CsI (Aldrich, anhydrous, 99.9%), NaI (Aldrich, anhydrous, 99.9%), and LuI.sub.3 (Aldrich, anhydrous, 99.99+%).

Scintillation Properties of Scintillator Compositions

[0052] Scintillation properties of small Bridgman grown scintillation composition crystals 300 mm.sup.3) have been characterized. This investigation involved measurement of the light output, the emission spectrum, and the scintillation decay time of the crystals. Energy resolution of sample crystals and their proportionality of response were also measured.

1. Light Output and Energy Resolution

[0053] As shown in FIGS. 1 A, B, C, and D, the energy resolution of the 662 keV photopeak recorded with the scintillator compositions has been measured to be in the vicinity of 7.6%, 11%, 5.25%, and 8.5% (FWHM) at room temperature for Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaBr.sub.6:0.2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce, respectively. The light output of scintillator composition crystals was measured by comparing their response to 662 keV γ-rays (.sup.137Cs source) to the response of a BGO scintillator to the same isotope (see FIGS. 1A, C, and D). This measurement involved optical coupling of a scintillator crystal to a photomultiplier tube (with multi-alkali S-20 photocathode), irradiating the scintillator with 662 keV photons, and recording the resulting pulse height spectrum. In order to maximize light collection, the scintillator composition crystal was wrapped in reflective white Teflon tape on all faces (except the one coupled to the PMT). An index matching silicone fluid was also used at the PMT-scintillator interface. A pulse height spectrum was recorded with a scintillator composition crystal. This experiment was then repeated with a BGO scintillator. Comparison of the photopeak position obtained with the scintillator composition for 662 keV photon energy to that with BGO provided estimation of light output for the scintillator composition crystal. FIGS. 1 A, B, C, and D show the pulse height spectra for a scintillator composition under .sup.137Cs irradiation and amplifier shaping time of 4.0 μs. This shaping time is long enough to allow full light collection from both the scintillators. The PMT bias and amplifier gain were the same for both spectra. Based on the recorded photopeak positions for each scintillator composition and BGO, light output of Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaBr.sub.6:0.2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce crystals was estimated to be about 26,500 photons/MeV, 12,000 photons/MeV, 48,500 photons/MeV, and 27,000 photons/MeV, respectively.

2. Emission Spectrum

[0054] Normalized emission spectra for the scintillator compositions are shown in FIGS. 2A through 2D. The scintillator composition samples were excited with radiation from a Philips X-ray tube having a Cu target, with power settings of 40 kVp and 20 mA. The scintillation light was passed through a McPherson monochromator and detected by a photomultiplier tube. The peak emission wavelength for the Cs.sub.2NaGdI.sub.6:Ce, Cs.sub.2NaLaBr.sub.6:Ce, Cs.sub.2NaLaI.sub.6:Ce, and Cs.sub.2NaLuI.sub.6:Ce samples was at approximately 431 nm, 386 nm, 429 nm, and 428 nm, respectively. Peak emission wavelengths in this range are attractive for gamma-ray spectroscopy because they match well with the spectral response of the photomultiplier tubes as well as a new generation of silicon photodiodes.

3. Time Profiles

[0055] FIGS. 3 A, B, and C show the time profiles recorded for Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce samples, respectively. Time profiles of the scintillator compositions have been measured under gamma ray excitation using the delayed coincidence method (Bollinger and Thomas, Rev. Sci. Instr. 32:1044 (1961)). For Cs.sub.2NaGdI.sub.6:2% Ce, the rise time of the scintillation pulse is ˜0.85 ns and the principal decay time is about 55 ns. For Cs.sub.2NaLaI.sub.6:5% Ce, the rise time of the scintillation pulse is ˜4 ns and the principal decay time is about 50 ns. For Cs.sub.2NaLuI.sub.6:1% Ce, the rise time of the scintillation pulse is ˜0.85 ns and the principal decay time is about 35 ns. FIGS. 3A-C also show secondary, slower decay components present in time profiles.

4. Non-Proportionality

[0056] As shown in FIGS. 4 A, B, and C, the non-proportionality of Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce scintillator compositions was evaluated, respectively. Non-proportionality (as a function of energy) in light yield can be one of the important reasons for degradation in energy resolution of established scintillators such as NaI(Tl) and CsI(Tl) (Dorenbos et al., IEEE Trans. Nuc. Sci. 42:2190 (1995)). Light output of the scintillator compositions was measured under excitation from isotopes such as .sup.241Am (60 keV γ-rays), .sup.57Co (122 keV, 136 keV, and 14.4 keV γ-rays), .sup.22Na (511 keV and 1275 keV γ-rays) and .sup.137Cs (662 keV γ-rays). The test crystals were wrapped in Teflon tape and coupled to a PMT. Pulse height measurements were performed using standard NIM equipment with the scintillator exposed to different radioisotopes. The same settings were used for the PMT and pulse processing electronics for each isotope. From the measured peak position and the known γ-ray energy for each isotope, the light output (in photons/MeV) at each γ-ray energy was estimated. The data points were then normalized with respect to the light output value at 662 keV energy and the results (shown in FIGS. 4 A, B, and C) indicated that Cs.sub.2NaGdI.sub.6:2% Ce, Cs.sub.2NaLaI.sub.6:5% Ce, and Cs.sub.2NaLuI.sub.6:1% Ce were very proportional scintillators. Over in the energy range from about 60 to about 1275 keV, the non-proportionality in light yield was less than about 5%, typically between about 2 to 3% (for corresponding values for other established scintillators see, e.g., Guillot-Noel et al., IEEE Trans. Nuc. Sci 46: 1274-1284 (1999)).

[0057] Overall, these measurements indicated that the scintillator compositions as described in the present invention have high light output, fast response and show good qualities in terms of light output, energy resolution, speed and exceptional non-proportionality.

[0058] Although the invention has been described with reference to the above examples, it will be understood that modifications and variations are encompassed within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims along with their full scope of equivalents. All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.