Abstract:
Lanthanide halide alloys have recently enabled scintillating gamma ray spectrometers comparable to room temperature semiconductors (&lt;3% FWHM energy resolutions at 662 keV). However brittle fracture of these materials upon cooling hinders the growth of large volume crystals. Efforts to improve the strength through non-lanthanide alloy substitution, while preserving scintillation, have been demonstrated. Isovalent alloys having nominal compositions of comprising Al, Ga, Sc, Y, and In dopants as well as aliovalent alloys comprising Ca, Sr, Zr, Hf, Zn, and Pb dopants were prepared. All of these alloys exhibit bright fluorescence under UV excitation, with varying shifts in the spectral peaks and intensities relative to pure CeBr 3 . Further, these alloys scintillate when coupled to a photomultiplier tube (PMT) and exposed to  137 Cs gamma rays.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional U.S. Patent Application Ser. No. 60/961,058 originally filed Jul. 17, 2007 entitled “New Scintillator Compositions” and to provisional U.S. Patent Application Ser. No. 60/962,696 originally filed Jul. 30, 2007 entitled “Lanthanide Halide Scintillator Compositions” from which benefit is claimed. 
    
    
     STATEMENT OF GOVERNMENT SUPPORT 
     The United States Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a material composition used in detecting ionizing radiation, particularly gamma-ray and X-ray radiation, and methods for toughening crystals of these materials, as the crystals are grown. More particularly, the present invention relates to improved lanthanide halide compositions that introduce structural perturbations in the crystal lattice to toughen the crystal against formation of critical flaws along cleavage planes of the crystal. 
     Scintillators are materials that emit flashes or pulses of light when they interact with ionizing radiation. Scintillator crystals are widely used in detectors for gamma-rays, X-rays, cosmic rays, and particles characterized by an energy level of greater than about 1 keV. It is possible to make radiation detectors, therefore, by coupling the crystal with some means for detecting the light produced by the crystal when it interacts, or “scintillates,” when exposed to a source of radiation. The photo-detector produces an electrical signal proportional to the intensity of the light pulses received. 
     2. Related Art 
     A number of the scintillator materials in current use possess most of the important properties that define these materials as useful: properties such as high light output, high cross-section, short decay times, and minimum afterglow. Prior art examples used for many years include thallium-activated sodium iodide (NaI(T1)), bismuth germanate (BGO), cerium-doped gadolinium orthosilicate (GSO), and cerium-doped lutetium orthosilicate (LSO). 
     While each of these materials exhibit properties which are suitable for many applications, each also has its shortcomings. Problems such as mediocre light yield, poor physical strength, and the difficulty and expense of producing large, high quality crystals continue to plague the industry. Moreover, non-proportional light yield seriously limits spectroscopic performance of these materials. 
     A number of lanthanide-halide compounds have recently been described as promising scintillators to address the problem of light yield and proportional response. In particular: 
     U.S. Pat. Nos. 6,437,336 and 6,818,896 describe a monoclinic single crystal with a lutetium pyrosilicate structure. The crystal is formed by crystallization from a congruent molten composition of Lu 2(1-x) M 2x Si 2 O 7  where Lu is lutetium or a lutetium-based alloy which also includes one or more of scandium, ytterbium, indium, lanthanum, and gadolinium; where M is cerium or cerium partially substituted with one or more of the elements of the lanthanide family excluding lutetium; and where x is defined by the limiting to level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate structure. The crystals are said to exhibit excellent and reproducible scintillation response to gamma radiation. 
     U.S. Pat. No. 7,060,982 describes a fluoride single crystal for detecting radiation having high luminescence intensity. The fluoride single crystal contains Ce and at least one element (R 1 ) of Lu and Gd, wherein the single crystal is represented by the general formula Ce 1-x R 1   x F 3  where 0.001&lt;x.&lt;0.5. 
     U.S. Pat. No. 7,067,815 describes an inorganic scintillator material, a method for growing a single crystal of the scintillator material. The inorganic scintillator material has the general composition M 1-x Ce x Cl 3 , where M is selected among lanthanides or lanthanide mixtures, preferably among the elements or mixtures of elements of the group consisting of Y, La, Gd, Lu, in particular among the elements or mixtures of elements of the group consisting of La, Gd and Lu; and x is the molar rate of substitution of M with cerium, x being not less than 1 mol % and strictly less than 100 mol %. 
     U.S. Pat. No. 7,067,816 describes an inorganic scintillator material of general composition M 1-x Ce x Br 3 , wherein: M is selected among lanthanides or lanthanide mixtures of the group consisting of La, Gd, Y in particular among lanthanides or lanthanide mixtures of the group consisting of La, Gd; and x is the molar rate of substitution of M with cerium, x being not less that 0.01 mol % and strictly less than 100 mol %. 
     U.S. Pat. No. 7,084,403 describes scintillator materials based on certain types of halide-lanthanide matrix materials are described. In one embodiment, the to matrix material contains a mixture of lanthanide halides, i.e., a solid solution of at least two of the halides, such as lanthanum chloride and lanthanum bromide. In another embodiment, the matrix material is based on lanthanum iodide alone, which must be substantially free of lanthanum oxyiodide. The scintillator materials, which can be in monocrystalline or polycrystalline form, also include an activator for the matrix material, e.g., cerium. 
     U.S. Pat. No. 7,129,494 describes fast scintillator materials capable of resolving the position of an annihilation event within a portion of a human body cross-section. In one embodiment, the scintillator material comprises LaBr 3  doped with cerium. Particular attention is drawn to LaBr 3  doped with a quantity of Ce that is chosen for improving the timing properties, in particular the rise time and resultant timing resolution of the scintillator, and locational capabilities of the scintillator. 
     The foregoing compounds, however, do not address the issue of crystal strength nor do they address the issue of producing large, fracture-resistant or crack-free single crystals. However, the lanthanide halides are generally soft, compliant crystals which would suggest that strength would not be an issue with these crystals. However, while soft, crystals of these materials are also highly anisotropic with a strong preference for slip along on hexagonal prismatic planes. We believe that slip on these prismatic planes results in dislocation pile-ups and critical-sized flaws that intensify the stress leading to cleavage fracture on these same planes. Lanthanide halide scintillators form hexagonal crystals in the uranium (III) chloride prototype structure, which is characterized by 9-fold coordination of to the cation, low c/a ratio, and open channels along the c axis. These materials while relatively “soft” tend to be very brittle, and fracture by cleavage along their prismatic planes, orthogonal to the c or basal plane. Analysis of this structure indicates dislocation formation energies and mobilities on these same prismatic planes are highly favored relative to the more usual basal plane slip; that is, the slip and cleavage planes are identical. This structure is therefore highly susceptible to cracking during crystal growth and subsequent detector fabrication. 
     Growth of lanthanide halide crystals is presently accomplished through various melt-solidification methods, wherein the crucible, or “boat,” of molten material is directionally cooled to nucleate and grow a crystal boule. Failure of the lattice due to critical flaws introduced by plastic strain is most likely during crystal growth or cool-down, due to the resultant longitudinal or radial thermal gradients within the crystal. Furthermore, the problem is exacerbated by anisotropy in the thermal expansion coefficient that is considerably larger in the [10•0] or a directions than in the [00•1] or c direction. Along the c-axis, thermal expansion occurs rather slowly at 7.5×10 −6 ° C. −1 , while expanding far faster along the a-axis at approximately 28.1×10 −6 ° C. −1 . This anisotropy in thermal expansion results in the formation of significant shear stresses on the prismatic [00•1] planes whenever a thermal gradient is applied more than 10° off the c-axis. We believe this anisotropy results in dislocation formation and motion and the formation of critical stress-concentrating flaws on these (weakest) planes of the crystal that subsequently lead to brittle fracture or cleavage. This, in combination with the propensity of these crystals to cleave along their slip planes, has been the chief impediment in achieving the necessary availability of large, cost effective to crystals. 
     Therefore, there remains an unmet need for providing a means for toughening lanthanide halides scintillating crystal compositions. 
     SUMMARY 
     Lanthanide halides form generally “soft,” compliant crystals which would suggest that strength should not be an issue. Nonetheless, cracking is a serious and as yet unsolved problem when attempting to grow lanthanide halide crystals of several inches in length. The prior art compounds do not address the issue of increased crystal strength nor do they address the issue of producing fracture-resistant single crystals. 
     There remains then an unmet need for providing a means for toughening lanthanide halides. To meet this need we have undertaken to identify agents to strengthen the lanthanide halide lattice without negatively impacting the scintillation properties. 
     Therefore, it is an object of this present invention to provide a material composition from which fracture-resistant single crystals may be grown and which is also exhibits useful scintillation characteristics. 
     In one embodiment of this invention a method is provided for inducing a strengthening effect in lanthanide-halide single crystals without compromising the scintillation effect of these materials. 
     In another embodiment of the invention is a chemical composition comprising one or several suitable atomic species outside of the lanthanide series which, when incorporated into the lattice of a lanthanide-halide single crystal, induces sufficient strain the to lattice to reduce slip along on hexagonal prismatic planes within the crystal. 
     In still another embodiment of the invention ions having atomic radii and/or valences that are different from the atomic radius and/or valence of a host lanthanide-halide crystal are incorporated into the crystal. 
     In yet again embodiment of this invention ions having nearly the same atomic radii and/or valence as the host lanthanide-halide crystal are incorporated into the crystal. 
     In still another embodiment of the invention a lanthanide-halide scintillator is “doped” with a salt of an ion selected from any of Groups 2-5 or 12-14 of the new IUPAC Groups of the Periodic Table of the Elements. 
     Still further embodiment of the invention provides a means for enhancing the UV fluorescence response of a lanthanide-halide crystal. 
     Therefore, it is a further object of this invention to provide alloying element(s) for strengthening the lanthanide-halide crystals that do not degrade the crystal scintillation properties. 
     Both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute part of this specification, illustrate several embodiments of the invention, and together with the description serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
         FIG. 1A  shows a schematic representation of a region of a crystal lattice strained by the introduction of an isovalent cation with an ionic radius slightly larger than the surrounding host cations. 
         FIG. 1B  shows a schematic representation of a region of a crystal lattice strained by the introduction of a aliovalent cation having a net positive charge with respect to the surrounding host cations together with an ionic radius slightly larger than the host cations. 
         FIG. 2A  graphically illustrates the effect on the resolved shear stress within CaF 2  crystal lattice with the introduction of small amounts of Y 3+  dopant cations and that only about 250 ppm dopant concentration is necessary to change the shear stress by a factor of 10. 
         FIG. 2B  shows the effect on the resolved shear stress within KBr crystal lattice with the introduction of KCl and illustrating the substantial quantities of substitution necessary for a 10-fold increase in shear stress 
         FIG. 3A  illustrates the cerium (III) bromide crystal “rubble” typically obtained following crystal cool-down. 
         FIG. 3B  shows a photograph of a cerium (III) bromide crystal doped with 500 to ppm of hafnium (IV) cations. 
         FIG. 3C  shows a photograph of a cerium (III) bromide crystal doped with 500 ppm of strontium (II) cations. 
         FIG. 4  shows a graphical comparison of a number of Vickers microhardness tests of doped and undoped lanthanide-halide crystals and illustrating the increase in strength typically obtained by this technique. 
         FIGS. 5A-F  show the emission response of isovalent or aliovalently doped crystals to both UV and the unexpected increase in response seen in most doped crystal formulations. 
         FIG. 6  shows the emission response of isovalent or aliovalently doped crystals to gamma radiation. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Existing scintillation materials typically use only lanthanide and halogen atoms to form an isomorphous crystal designed for high gamma ray cross sections and scintillation light yield. The improved material incorporates cations from outside the lanthanide series which strengthen the material through lattice strains caused by differing atomic radii and/or valence. Our solution to the problem of fracture in large, single crystals of lanthanide scintillators is somewhat counterintuitive because high fracture toughness is generally related to higher plasticity and, as noted earlier, lanthanide-halide crystals are relatively soft and compliant. However, in this specific case we have demonstrated that decreasing the to plasticity of these crystals also improved their fracture toughness by blocking the formation of critical flaws within the strained crystal lattice. Moreover, while investigating various alloying agents we found a serendipitous effect, wherein alloying the scintillation crystal with trivalent cations also enhanced the fluorescence and scintillation light yield of the parent material. 
     Three methods are generally useful for strengthening crystalline materials. These include mechanical work hardening, particle strengthening, and solid-solution hardening (i.e., alloying with impurity atoms). However, while lanthanide-halide crystals are, relatively-speaking, soft and compliant, they exhibit limited plasticity and are too brittle for the first two of these methods to be used successfully. Furthermore, the scintillating properties of these crystals are of primary importance and the introduction of particles into a scintillator can be expected to scatter and/or absorb light, thereby degrading scintillation performance. These strengthening methods are therefore impractical for generating large, high quality scintillation crystals. 
     Therefore, only solid-solution hardening remains as a viable alternative for strengthening lanthanide-halide crystals. Two different approaches to solute substitution are possible. The first comprises substituting an atom of the same valence (isovalent) but having a larger ionic radius than the host lattice is substituted into the crystal lattice. This substitution introduces short range strain in the crystal structure surrounding it as illustrated schematically in  FIG. 1A . The second comprises substituting an atom having a different valence (aliovalent) but having a similar ionic radius as the host lattice. This second to substitution introduces longer range strain in the crystal structure as illustrated schematically in  FIG. 1B . The technique has long been known to harden metals by forming alloys through the substitution of host atoms for impurity atoms having atomic radii or valences which differ from the surrounding host solute material. However, forming alloys of dissimilar members of the lanthanide series is unlikely to provide much strengthening due to the similarity in size of the ionic radii of the members of the series. Instead, alloys using non-lanthanide cations as the impurity species would be expected to induce a strengthening effect within a host crystal since these species exhibit large differences in ionic radii from that of the members of the lanthanide series. 
     Both approaches strain the underlying crystal lattice but the effect is most marked in aliovalent substitution in that only several hundred parts-per-million (ppm) are necessary to dramatically alter the minimum stress required to initiate slip on a given slip plane within a crystal (known as the critical resolved shear stress, or “CRSS”). In particular, it is known that the IUPAC Group 2, 4, 12, and 14 cations have a marked effect on the mechanical properties of ionic crystals doped with these materials, on a per-mole basis. For example, ppm level impurities can have a dramatic effect on dislocations and plasticity in alkali halide crystals (Nadgornyi, E, “Dislocation Dynamics and Mechanical Properties of Crystals,”  Progress in Materials Science,  1988, v.31: pp. 1-530). 
     By comparison, to achieve the same effect using isovalent substitution one would need to introduce tens of percent of the substituted constituent into the crystal lattice. A comparison of the two effects is shown graphically in  FIGS. 2A and 2B , wherein roughly 250 ppm of Y 3+  substituted into CaF 2  increases the critical resolved shear stress (CRSS) by to an order of magnitude (M. N. Sinha and P. S. Nicholson,  Journal of Materials Science,  1977, v.12: pp. 1451-1462) while it is necessary to add about 20 mol % KBr to a KCl crystal lattice in order to improve the alloy CRSS by a similar amount (T. Kataoka and T. Yamada,  Japanese Journal of Applied Physics,  1977, v.16: pp. 1119-1126). 
     Several isovalent and aliovalent dopants were screened as strengthening agents in cerium bromide (Ce(III)Br 3 ) scintillators. Each alloy was screened on basic properties such as presence of scintillation and emission spectra in order to determine whether the presence of the strengthening agents in any way effected the scintillation in CeBr 3 . A detailed description of this investigation and its results are provided below. 
     EXAMPLES 
     Sample Preparation 
     Initial tests were performed with small 400-600 mg samples of each alloy as well as a control sample of pure CeBr 3  in order to test for degrading effects on crystal fluorescence. Twelve (12) crystals were grown using cerium bromide doped with various cations and their fluorescence spectra measured. The test samples were prepared by weighing out the necessary masses of each component in bead or powder form in a nitrogen glove box. Actual sample weights are given in T ABLE  1 below. The powders were then placed in 5 in. long, ¼″ OD quartz tubes each having had one of its ends previously flame sealed. After loading the material into the tubes the open end of each tube was attached to a vacuum system with an Ultra-Torr fitting, evacuated to less than 1×10 −3  Torr, and then flame sealed approximately 1-2″ above the level of the powders. Each of the sealed ampoule and sample was then individually placed upright into a bench top tube furnace and rapidly heated to 950° C.: just above the highest melting point of the constituent compounds. The samples were held at temperature for 12 hours to allow time for the alloy constituents to mix and homogenize, after which each was cooled to room temperature at a rate of about 1° C./min. The finished samples were noted to be very polycrystalline. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Component masses of alloy samples. 
               
             
          
           
               
                   
                   
                 Cation 
                 Mass of 
                 Mass of 
                 Molar Fraction 
               
               
                   
                   
                 Substitution 
                 CeBr 3   
                 Dopant 
                 Dopant 
               
               
                 Sample 
                 Dopant 
                 State 
                 (mg) 
                 (mg) 
                 (%) 
               
               
                   
               
             
          
           
               
                 1 
                 AlBr 3   
                 Isovalent 
                 363.2 
                 41.1 
                 13.9 
               
               
                 2 
                 GaBr 3   
                 Isovalent 
                 348.5 
                 26.7 
                 8.6 
               
               
                 3 
                 ScBr 3   
                 Isovalent 
                 352.6 
                 33.7 
                 11.3 
               
               
                 4 
                 YBr 3   
                 Isovalent 
                 345.0 
                 82.6 
                 21.7 
               
               
                 5 
                 InBr 3   
                 Isovalent 
                 427.5 
                 40.6 
                 9.2 
               
               
                 6 
                 CaBr 2   
                 Aliovalent 
                 534.4 
                 3.8 
                 1.3 
               
               
                 7 
                 SrBr 2   
                 Aliovalent 
                 620.7 
                 5.4 
                 1.3 
               
               
                 8 
                 ZnBr 2   
                 Aliovalent 
                 440.7 
                 2.9 
                 1.1 
               
               
                 9 
                 CdBr 2   
                 Aliovalent 
                 434.4 
                 2.1 
                 0.7 
               
               
                 10 
                 PbBr 2   
                 Aliovalent 
                 452.5 
                 3.9 
                 0.9 
               
               
                 11 
                 ZrBr 4   
                 Aliovalent 
                 589.0 
                 4.7 
                 0.7 
               
               
                 12 
                 HfBr 4   
                 Aliovalent 
                 470.1 
                 7.5 
                 1.2 
               
               
                   
               
             
          
         
       
     
     Some differences were found in the fluorescence performance between the control sample and the prepared alloys listed in T ABLE  1 as will be discussed later. Preliminary gamma ray spectroscopy results indicate no degradation in energy resolution is incurred by doping. 
     Because some impurity species can “quench” scintillation we wished to introduce as little impurity elements to the host lattice as possible. The preferred embodiment of the invention, therefore, uses cerium bromide alloyed with (aliovalent) divalent and tetravalent cations. The formulations listed below were prepared by the method described above except that the samples were prepared from ingot-sized quantities of powders and grown via the horizontal Bridgman method in an electrodynamic gradient furnace. The dopants used included CaBr 2 , SrBr 2 , BaBr 2 , ZrBr 4 , HfBr 4 , ZnBr 2 , CdBr 2 , and PbBr 2 . All ingots were prepared under vacuum or in an inert atmosphere. 
     More than 25 ingots were grown in this way. Each ingot was nominally doped with one of the several identified dopants at a concentration of 250 ppm, 500 ppm, or 1000 ppm. Preliminary ingots were grown in 25 mm ID ampoules, but recent upsizing results indicate the growth method scales easily to 34 mm ID ampoules. Single D-shaped crystals over 200 mm in length and 34 mm in diameter have been achieved. Nearly every ingot exhibited a single-crystal cross-section at the tail. Samples from each alloy were harvested from the as-grown crystals by either cleaving or cutting with a diamond wire saw. 
     The formulations that were prepared are listed below in Tables 2A and 2B. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2A 
               
             
             
               
                   
               
               
                 Component masses of alloy samples having divalent cation dopants. 
               
             
          
           
               
                   
                   
                 Mass of 
                 Mass of 
                 Molar Fraction 
               
               
                   
                   
                 CaBr 3   
                 Dopant 
                 of Dopant 
               
               
                 Sample 
                 Dopant 
                 (g) 
                 (mg) 
                 (10 −6 ) 
               
               
                   
               
             
          
           
               
                 1 
                 CaBr 2   
                 150.005 
                 44.0 
                 557 
               
               
                 3 
                 SrBr 2   
                 149.900 
                 49.0 
                 501 
               
               
                 6 
                 ZnBr 2   
                 139.951 
                 45.0 
                 542 
               
               
                 7 
                 CdBr 2   
                 140.016 
                 51.4 
                 512 
               
               
                 8 
                 PbBr 2   
                 139.720 
                 74.9 
                 554 
               
               
                 9 
                 SrBr 2   
                 140.317 
                 104.7 
                 1144 
               
               
                 12 
                 ZnBr 2   
                 140.008 
                 91.2 
                 1098 
               
               
                 13 
                 CdBr 2   
                 140.065 
                 101.4 
                 1009 
               
               
                 14 
                 CaBr 2   
                 140.007 
                 21.3 
                 289 
               
               
                 16 
                 BaBr 2   
                 140.099 
                 55.3 
                 504 
               
               
                 17 
                 SrBr 2   
                 140.001 
                 32.9 
                 361 
               
               
                 20 
                 ZnBr 2   
                 140.024 
                 28.8 
                 347 
               
               
                 21 
                 CdBr 2   
                 140.003 
                 53.2 
                 530 
               
               
                 22 
                 BaBr 2   
                 140.017 
                 90.2 
                 823 
               
               
                 23 
                 CaBr 2   
                 140.026 
                 68.8 
                 933 
               
               
                 24 
                 SrBr 2   
                 500.0 
                 163.0 
                 500 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2B 
               
             
             
               
                   
               
               
                 Component masses of alloy samples having tetravalent cation dopants. 
               
             
          
           
               
                   
                   
                 Mass of 
                 Mass of 
                 Molar Fraction 
               
               
                   
                   
                 CaBr 3   
                 Dopant 
                 of Dopant 
               
               
                 Sample 
                 Dopant 
                 (g) 
                 (mg) 
                 (10 −6 ) 
               
               
                   
               
             
          
           
               
                 18 
                 ZrBr 4   
                 140.031 
                 39.1 
                 258 
               
               
                 19 
                 HfBr 4   
                 140.014 
                 49.4 
                 269 
               
               
                 4 
                 ZrBr 4   
                 149.900 
                 94.1 
                 580 
               
               
                 5 
                 HfBr 4   
                 139.964 
                 105.2 
                 573 
               
               
                 10 
                 ZrBr 4   
                 140.033 
                 156.4 
                 1032 
               
               
                 11 
                 HfBr 4   
                 140.084 
                 171.5 
                 933 
               
               
                   
               
             
          
         
       
     
     Hafnium and strontium doped ingots, shown in  FIGS. 3B and 3C  respectively, exhibited less cracking than their undoped counterparts, shown in  FIG. 3A , indicating an increase in fracture strength. Illustrative fracture toughness measurements, estimated from Vickers microhardness tests, are shown in  FIG. 4  and in T ABLE  3 below, suggest at least an average 25% to 60% increase in strength over undoped lanthanide-halide crystals. In particular, undoped CeBr 3  crystals exhibited a fracture toughness of about 0.179 MPa·√{square root over (m)} to while CeBr 3  crystals doped with 1000 ppm of Hf exhibited a fracture toughness of 0.285 MPa·√{square root over (m)}, i.e., or about a 60% increase in fracture strength. Moreover, Vickers microindentation hardness tests performed on a number of the crystals demonstrated that achieving up to a doubling in fracture toughness over similar undoped crystals was possible. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Fracture strength as a % of CeBr 3  fracture strength 
               
             
          
           
               
                 CeBr 3  + Zn(II) 
                 CeBr 3  + Cd(II) 
                 CeBr 3  + Hf(IV) 
               
               
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
             
          
           
               
                 1.0 
                 2.46 
                 2.47 
               
               
                 1.2 
                 1.9 
                 1.82 
               
               
                 1.22 
                 1.89 
                 1.54 
               
               
                 1.27 
                 1.83 
                 1.44 
               
               
                 1.33 
                 1.66 
                 1.4 
               
               
                 1.4 
                 1.39 
                 1.22 
               
               
                 1.58 
                 1.19 
                 1.18 
               
               
                 1.65 
                 1.15 
                 1.14 
               
               
                   
                 1.13 
                 1.1 
               
               
                   
                   
                 1.01 
               
               
                   
                   
                 0.94 
               
               
                   
                   
                 0.85 
               
               
                 1.33 ± 0.2 
                 1.62 ± 0.42 
                 1.34 ± 0.43 
               
               
                   
               
             
          
         
       
     
     Serendipitously, the emission response of the isovalent and the aliovalently doped crystals was found to be enhanced when each was exposed to both UV radiation as shown in  FIGS. 5A-5F , and to gamma radiation, shown in  FIG. 6 . That is, the applicants have discovered a class of scintillation materials which show enhanced light output above a baseline standard. It is significant that with 1% of Cd −2  ions introduced into CeBr 3 , the fluorescence spectrum was not substantially altered; indicating that at the anticipated ppm to levels required for strengthening, the scintillation properties of the parent crystal will be preserved. Other agents believed to be equally effective are the Group 2 elements up to barium, i.e., Be, Mg, Ca, Sr, and Ba; Group 3 elements Sc, Y, and La; Group 12 elements Zn, Cd, and Hg; and the Group 13 elements through indium B, Al, Ga, and In. 
     The addition of certain dopants, therefore, for the purpose of strengthening the crystal lattice of lanthanide halide scintillators has been shown to be effective at providing relatively large rugged single scintillator crystals with superior scintillation performance. 
     Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the disclosures herein are exemplary only and that various other alternatives, adaptations, and modifications may be made within the scope of to the present invention. Accordingly, the present invention is not limited to the specific embodiments as illustrated herein, but is only limited by the following claims. 
     Finally, to the extent necessary to understand or complete the disclosure of the present invention, all publications, patents, and patent applications mentioned herein are expressly incorporated by reference therein to the same extent as though each were individually so incorporated.