Chloride, bromide and iodide scintillators with europium doping

ACa 1-y Eu y I 3 where A=K, Rb and Cs

Referring to FIG. 1 , there is shown a PRIOR ART exemplary system potassium iodide and calcium iodide showing a mol concentration versus temperature plot. Halide salts generally rapidly absorb moisture and preferably are maintained dry so as not to pre-assume a crystalline form with water. As will be explained herein, the purification process and handling of these halide salts is best performed under protective atmosphere to obtain pure, anhydrous salts, for example, using a zone refining technique in furnaces, melt-filtering or other known techniques where the material may be sealed in a quartz ampoule either under vacuum or nitrogen or argon gas.

FIG. 2 shows similar plots for rubidium and cesium, respectively, and calcium chloride. RbI is presently not available in pure, anhydrous form and must be purified and rendered as anhydrous as possible using known techniques. First referring to FIG. 1 , KCaI 3 is a congruently melting compound with a melting point at 524° C.

The PRIOR ART table 1 below provides the details for each crystal formed for each of potassium, rubidium and cesium and Z eff calculated using a known formula:

From the table 1 summary above, all three crystals exhibit an ortho-rhombic crystal structure. Accurate black and white line drawings of the respective crystals with Eu 3% are depicted in FIG. 8 . The melting temperature varies from 524° C. for potassium to 686° C. for Cesium. There is no phase transition exhibited for either potassium or cesium. A phase transition at 469° C. or approximately 470° C. was noted for rubidium. The Z eff did not vary much, from 50.0 to 52.6. The density in grams per cubic centimeter was between 3.8 and 4.1 approximately.

The vertical gradient freeze and a modified Bridgman technique depicted in FIG. 3 may be used to form the crystals whose characteristics are described in table 1. The method may also be used with or as a substitute for a vertical gradient freeze (VGF) method. A Czochralski technique or combined Czochralski/Bridgman technique may be used in the alternative to grow scintillator crystals. As explained above, halide salts are moisture sensitive. One step in the process is to synthesize the compound using high purity anhydrous starting halides such as CsI, KI, RbI, EuI 2 and CaI 2 . In the case of RbI, the salt was dried and purified in a glove box using known techniques. It is recommended that the dry salt raw materials be handled in a glove box prior to movement to the ampoule of FIG. 3 . A typical ampoule is a vacuum-scaled quartz ampoule at 10 −6 vacuum pressure. A Mellen Electro-Dynamic Gradient (EDG) furnace with a translation motor may be utilized to generate heat. Synthesized material may be further purified via filtering through a frit followed by several runs of zone refining. Growth parameters are suggested as follows: a temperature gradient may be between 25-75° C. per inch of crystal growth; a translation rate may be one millimeter per hour (range of 0.5 to 2 mm per hour); a typical cool down rate may be 5° C. per hour. Crystal growth results are shown in FIG. 8 against rulers showing length of growth.

In addition to growth in single crystalline form, the scintillator compounds discussed herein may be prepared as polycrystalline powders by mixing fine grain powder components in stoichiometric ratios and sintering at a temperature somewhat below the melting point. Furthermore, these compositions may be synthesized in polycrystalline ceramic form by hot isostatic pressing of fine grained powders.

Referring now to FIG. 4 , there is shown a graph for CsCaI 3 and KCaI 3 with excitation at 285, 290 nm wavelength (in dashed line) and emission at 460 nm wavelength for each crystal (in solid line) shown as the respective peaks. Intensity levels are shown in the vertical axis as normalized intensity. A Flourolog3 lifetime spectrofluorometer (horiba Jobin Yvon) was used in the measurements. Emissions and excitation bands are characteristic of Eu 2+ 5d to 4f luminescence. The Ca 2+ ion provides a substitution site for Eu 2+ doping. The characteristic data confirms incorporation of Eu 2+ into the lattice of each crystal scintillator in the stable divalent state as a substitute for calcium. Moreover, Eu 2+ doping is confirmed to demonstrate excellent luminosity and high spectral energy resolution at normal room temperatures.

Referring to FIG. 5 , there are shown graphs for X-Ray radioluminescence spectra for each crystal: CsCaI 3 :Eu, KCaI 3 :Eu, and RbCaI 3 :Eu where the emission peak maxima are at 450, 470 and 470 nm respectively and normalized intensity shown as the vertical axis as before. Radioluminescence spectra were measured at room temperature under continuous irradiation from an X-ray generator (35 kV and 0.1 mA). Again, the emission bands are characteristic of Eu 2+ 5d-4f luminescence. The emission is suitable for use with conventional photodetectors known in the art such as photomultiplier tubes (PMT's), for example, a Photonis XP2020Q PMT and fast timing electronics.

Referring now to FIG. 6 , there is shown 137 Cs gamma-ray energy spectra for the CsCaI 3 :Eu 3% and KCaI 3 :Eu 3% crystals. Light output measurements were carried out on samples covered in mineral oil and directly coupled to a photomultiplier tube (PMT) and covered with Teflon tape. A Hamamatsu H3177-50 PMT may be used for the absolute light measurements. Gamma-ray energy spectra were recorded using a 137 Cs source with a 6 μsec shaping time. Both the CsCaI 3 :Eu 3% (generally upper, side-wise square) and KCaI 3 :Eu 3% (generally lower, triangle pointing down) scintillator crystal spectra exhibit the position of a 662 keV gamma-ray photopeak at a much higher channel number than a benchmark bismuth germanate (BGO) crystal with its photopeak at channel 100 , indicating much higher light output. While different Eu doping was used between 1 and 10 mol %, an optimal Eu 2+ activator concentration was found to be 3 mol %.

The photo peaks in FIG. 6 were fitted with Gaussian functions to determine the centroid of the peak and the energy resolution. The integral quantum efficiency of the PMT according to the emission spectrum of the scintillators was used to estimate the light output in photons per unit gamma ray energy. All photon (light) output, temperature measurements, energy resolutions, wavelengths, durations, densities and the like in the tables are to be considered approximate measurements. Table 2 below shows the absolute light output and energy resolution for the particular samples used for the measurements compared to a NaI:Tl reference sample. The energy resolution at 662 keV was determined from the full width at half maximum (FWHM) of the 662 keV photopeak.

As can be seen from the table 2, the RbCaI 3 :Eu 3% sample did not provide a resolved gamma-ray photo peak. On the other hand, the KCaI 3 :Eu 3% crystal greatly exceeded the benchmark NaI:Tl crystal at a light yield of approximately 60,000 photons per Me V compared with approximately 38,000 for NaI:Tl while the CsCaI 3 :Eu 3% crystal matched, if not, exceeded the NaI:Tl light yield. As for energy resolution, both the KCaI 3 :Eu 3% and CsCaI 3 :Eu 3% crystals exhibited a very acceptable range between 5% (for KCaI 3 :Eu) and 7% for CsCaI 3 :Eu at 662 keV.

Scintillation decay is shown in FIGS. 7A through 7C for each crystal: where FIG. 7A represents KCaI 3 :Eu 1%; FIG. 7B represents RbCaI 3 :Eu 1% and FIG. 7C represents CsCaI 3 :Eu 1% scintillation decay results in counts over time. Scintillation decay was measured using a time-correlated single photon counting technique and using a 137 Cs 662 keV gamma-ray source. Scintillation decay constants obtained from fitting the curves with exponential functions are shown in legends. A primary decay around one μsecond is characteristic of Eu 2+ 5d to 4f luminescence.

Referring now to FIG. 8A , there are shown black and white line drawings prepared from photographs of a KCaI 3 :Eu 3% crystal compared with a metric ruler. The lengthwise ruler indicates that the potassium crystal was grown to a length of over six centimeters and measured approximately 0.5 centimeters in width. Referring now to FIG. 8B , there are shown black and white line drawings prepared from photographs of a CsCaI 3 :Eu 3% crystal compared with a ruler. The lengthwise ruler in inches indicates that the potassium crystal was grown to a length of over two inches and measured approximately 0.5 centimeters in width. Referring now to FIG. 8C , there is shown a black and white line drawing prepared from a photograph of a RbCaI 3 :Eu 3% crystal.

The following table 3 provides a summary of results for CsCaI 3 :Eu 3% and KCaI 3 :Eu 3% crystals compared with those of a benchmark NaI:Tl crystal:

In summary for this example, it is demonstrated that practical crystal growth comparable to NaI:Tl may be obtained at reasonable cost (involving congruent melting and acceptable melting point). Since the crystals are hygroscopic, hermetic packaging may be used with conventional photodetectors such as photo-multiplier tubes such as a Photonis XP2020Q PMT and fast timing electronics. X-ray, gamma-ray, and optical excitation have been demonstrated. With improved crystal quality (removal of raw material impurity and improved processing), both light output and energy resolution are expected to improve beyond their current levels.

CsA 1-y Eu y X 3 (where A=Ca, Sr, Ba, or a Combination thereof and X═Cl, Br or I or a Combination thereof) with Divalent Europium Doping where 0≦y≦1

Now, single crystal inorganic crystal scintillators will be described of the formula CsAX 3 will be described with divalent Europium doping investigated as above between 1 and 10 mol % where A is calcium (Ca), strontium (Sr) or barium (Ba) or a combination thereof and X is a halide selected from chlorine, bromine or iodine. In particular, it will be shown that a crystal of CsSrCl 3 :Eu 10% exhibits a light yield on the order of 46,000 photons per MeV and a scintillation decay at 2.6 μseconds and provides excellent gamma-ray and X-ray detection characteristics. The A 2+ lattice site provides a substitutional site for the Eu 2+ doping.

Referring to FIGS. 9A to 9C , FIG. 9A shows a PRIOR ART phase diagram of the CsCl—CaCl 2 system whereby CsCl is shown at left and CaCl 2 is shown at right between 0 and 100% mol concentration while temperature is depicted along the left vertical axis between 200° C. and 800° C.; FIG. 9B is a similar PRIOR ART phase diagram of the CsCl—SrCl 2 system whereby CsCl is shown at left and SrCl 2 is shown at right; and FIG. 9C is a PRIOR ART phase diagram of the SrBr 2 —CsBr system whereby SrBr 2 is shown at left and CsBr is shown at right between 0 and 100% mol concentration while temperature is depicted along the left vertical axis between 200° C. and 800° C. Divalent europium was used primarily for doping as follows: r(Eu 2+ )=1.20 A (CN=7); r(Sr 2+ )=1.21 A and r(Ca 2+ )=1.06 A. Perovskite type ABX 3 compounds were obtained by congruently melting and with practical crystal growth according to the methods discussed above and below.

Table 4, in part PRIOR ART, below provides a summary of the crystal growth for each scintillator crystal:

A method of crystal growth has already been described above with respect to a discussion of FIG. 3 . Bridgman crystal growth may comprise a gathering of anhydrous 99.99% pure raw materials. The materials may be further purified by vacuum drying and melt filtering. As described above, a quartz ampoule may be employed having hot and cold zones whereby crystals may be grown under vacuum seal. A Mellen Electro-Dynamic Gradient (EDG) furnace with a translation motor may be utilized to generate heat. The crystal growth parameters may be the same as described above: temperature gradient at 75° C./inch, translation rate at one millimeter per inch and a cool down rate of 5° C. per hour (exemplary). The scintillator crystals were grown with spontaneous orientation. The ternary halide scintillators may be synthesized using vertical gradient freeze (VGF), Bridgman and melt synthesis and other techniques as suggested above.

FIG. 10 is a graph over time of moisture intake of Cs 2 LiYCl 6 :Ce and NaI as benchmarks and the following crystals: CsSrCl 3 :Eu 10%; CsSrBr 3 :Eu 10% and CsCaCl 3 :Eu 10% where the latter two crystals exhibit little or no moisture intake. CsSrBr 3 :Eu 10% and CsCaCl 3 :Eu 10% both exhibit practically flat moisture intake over a period of 250 minutes (over four hours). Moreover, CsSrCl 3 exhibits greatly improved moisture intake at 2% over four hours compared with NaI at over 6.5%. The samples were measured in a closed box with controlled environment at room temperature. All samples showed significantly lower moisture sensitivity compared to NaI and CLYC scintillators.

FIG. 11 represents photoluminescence spectra for each of the CsCaCl 3 : 10%, CsSrCl 3 :Eu 10% and CsSrBr 3 :Eu 10% scintillator crystals where dashed lines represent wide excitation bands and solid lines represent emission bands. The emission and excitation bands are characteristic of Eu 2+ 5d to 4f luminescence. Incorporation of Eu 2+ is thus confirmed into the lattice structures in the stable divalent state of the crystals. Photoluminescence excitation and emission spectra may be measured with a Horiba Fluorolog 3 spectrofluorometer utilizing a Xe lamp excitation source and scanning monochromators. The following table 5 provides a summary of excitation (EXC) bands and emission (EM) bands:

FIG. 12 is a graph of normalized intensity versus wavelength in nanometers of X-ray excitation of the CsCaCl 3 : 10%, CsSrCl 3 :Eu 10% and CsSrBr 3 :Eu 10% scintillator crystals. Radioluminescence spectra were measured at room temperature under continuous irradiation from an X-ray generator (1 mA, 35 kV). An Acton monochromator may be used to resolve the spectrum as a function of wavelength. The graph demonstrates successful results of efficient radioluminescence under X-ray excitation. The emission bands are characteristic of Eu 2+ 5d-4f luminescence. The emission bands were at a wavelength suitable for capture using conventional photo detectors such as photo multiplier tubes (PMT's) such as a Photonis XP2020Q PMT and fast timing electronics. The measured emission peaks are as follows: for CsCaCl 3 :Eu 10%, the peak was at 450 nm; for CsSrCl 3 :Eu 10%, the peak was at 437 nm; and for CsSrBr 3 :Eu 10%, the peak was at 443 nm.

FIG. 13A to 13C represent graphs of scintillation decay over time for each crystal whereby FIG. 13A represents CsCaCl 3 :Eu 10%; FIG. 13B represents CsSrCl 3 :Eu 10% and FIG. 13C represents CsSrBr 3 :Eu 10%. Scintillation time profiles were recorded using a 137 Cs 662 keV gamma-ray source. Scintillation decay constants obtained from fitting the curves with exponential functions are shown in legends. The primary decay around 2-4 μseconds is characteristic of Eu 2+ 5d to 4f luminescence. The measured scintillation decay times were measured as follows: for CsCaCl 3 :Eu 10%, the decay time was at 4.1 μseconds; for CsSrCl 3 :Eu 10%, the decay time was at 2.6 μseconds (the fastest); and for CsSrBr 3 :Eu 10%, the decay time was at 3.5 μseconds.

FIG. 14A to 14C represent graphs of 137 Cs gamma-ray emission spectra for each crystal whereby FIG. 14A represents CsCaCl 3 :Eu 10%; FIG. 14B represents CsSrCl 3 :Eu and FIG. 14C represents CsSrBr 3 :Eu normalized to benchmark bismuth germanate (BGO) with its photopeak at channel 100 . Efficient scintillation characteristics were exhibited under excitation by ionizing radiation with respect to all three crystals. The following table 6 provides light yield and energy resolution for each crystal compared with NaI:Tl as a benchmark:

As can be seen by the results, CsSrCl 3 :Eu 10% demonstrates improved light yield over known NaI:Tl.

FIG. 15A to 15B are graphs demonstrating light output proportionality whereby FIG. 15A is a graph of light output per keV versus energy in keV for CsSrBr 3 :Eu and FIG. 15B is a similar graph for CsCaCl 3 :Eu. The importance of FIG. 15 is that good proportionality contributes to good energy resolution as these graphs demonstrate good proportionality for light output per keV over a wide range of energy level from 10 to 1000 keV.

FIG. 16A to 16C are black and white line drawings made from photographs of each respective crystal: CsSrCl 3 :Eu 10%; CsSrBr 3 :Eu 10% and CsCaCl 3 :Eu 10%. CsSrCl 3 :Eu 10% was grown to be about two inches long; however, some cracking may be seen, possibly due to phase transitions. The crystal was about one centimeter in cross-section. CsSrBr 3 :Eu 10% was gown to be about two centimeters in length and was relatively clear, colorless and crack-free. Its circular cross-section was just less than 2 centimeters in diameter. CsCaCl 3 :Eu 10% was grown to be over three centimeters in length. It too was relatively clear, colorless and crack-free. Its circular cross-section measured about 11/16 inch in diameter.

Table 7 provides a summary table for these crystals as follows compared with benchmark NaI:Tl:

The table demonstrates that promising results have been obtained for new, inorganic, single crystals as indicated above when compared with NaL: TI including good transparency, low to no hygroscopicity, good light output and excellent proportionality. It is expected that with greater effort at purification of raw material and optimizing growth parameters that even better results will be achieved.

A combination crystal is now described comprising CsSr 1-x Ba x I 3 :Eu 1% and 3%. Crystals were grown at x=0.03, 0.06, 0.09, 0.14 and 0.24. All of these combination crystals operated as a scintillator. X-ray radioluminescence exhibited a peak at approximately 448 nm wavelength. The emission bands are characteristic of Eu 2+ 5d-4f luminescence. The emission is at a wavelength that is suitable for use with conventional photo detectors such as photo multiplier tubes (PMT's). Ultraviolet/visual excitation and emission were measured with an emission peak at 446 nm. The maximum light output was measured under 662 keV excitation when x was at 3% and Eu mol 1% and measured at approximately 28,000 photons per MeV with 1.9 μsecond decay time, the light output decreased with increasing x such that when x was 24%, the light output approximately 21,000 photons per MeV and the scinitillation decay time approximately 1.5 μseconds.

While various aspects of the present invention have been described above, it should be understood that they have been presented by way of example and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the present invention. Thus, the present invention should not be limited by any of the above described exemplary aspects, but should be defined only in accordance with the following claims and their equivalents.

In addition, it should be understood that the figures in the attachments, which highlight the structure, methodology, functionality and advantages of the present invention, are presented for example purposes only. The present invention is sufficiently flexible and configurable, such that it may be implemented in ways other than that shown in the accompanying figures.

Further, the purpose of the foregoing Abstract is to enable the U.S. Patent and Trademark Office and the public generally and especially the scientists, engineers and practitioners in the relevant art(s) who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of this technical disclosure. The Abstract is not intended to be limiting as to the scope of the present invention in any way.