Bridgman growth of large SrI2:Eu2+ single crystals: A high-performance scintillator for radiation detection applications

Author’s Accepted Manuscript

Bridgman Growth of Large SrI2:Eu2 þ Single Crystals: A High-performance Scintillator for Radiation Detection Applications L.A. Boatner, J.O. Ramey, J.A. Kolopus, R. Hawrami, W.M. Higgins, E. van Loef, J. Glodo, K.S. Shah, Pijush Bhattacharya, Eugene Tupitsyn, Michael Groza, Arnold Burger, N.J. Cherepy, S.A. Payne www.elsevier.com/locate/jcrysgro

PII: DOI: Reference:

S0022-0248(13)00082-1 http://dx.doi.org/10.1016/j.jcrysgro.2013.01.035 CRYS21401

To appear in:

Journal of Crystal Growth

Cite this article as: L.A. Boatner, J.O. Ramey, J.A. Kolopus, R. Hawrami, W.M. Higgins, E. van Loef, J. Glodo, K.S. Shah, Pijush Bhattacharya, Eugene Tupitsyn, Michael Groza, Arnold Burger, N.J. Cherepy and S.A. Payne, Bridgman Growth of Large SrI2:Eu2 þ Single Crystals: A High-performance Scintillator for Radiation Detection Applications, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2013.01.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Bridgman Growth of Large SrI2:Eu2+ Single Crystals: A High-performance

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Scintillator for Radiation Detection Applications

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L. A. Boatner, J. O. Ramey, and J. A. Kolopus

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Center for Radiation Detection Materials and Systems

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Materials Science and Technology Division

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Oak Ridge National Laboratory

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Oak Ridge, Tennessee 37831

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R. Hawrami, W. M. Higgins, E. van Loef, J. Glodo, and K. S. Shah

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Radiation Monitoring Devices, Inc.

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Watertown, Massachusetts 02472

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Pijush Bhattacharya, Eugene Tupitsyn, Michael Groza, and Arnold Burger

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Department of Life and Physical Sciences

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Fisk University, Nashville, Tennessee 37208

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N. J. Cherepy and S. A. Payne

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Lawrence Livermore National Laboratory

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Livermore, California 94550

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Corresponding author:

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L. A. Boatner

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[email protected] Oak Ridge National Laboratory

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1 Bethel Valley Road

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Oak Ridge, TN 37831

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Phone: (865) 574-5492

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FAX: (865) 574-4814

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ABSTRACT:

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Single-crystal strontium iodide (SrI2) doped with relatively high levels (e.g., 3 - 6

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%) of Eu2+ exhibits characteristics that make this material superior, in a number

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of respects, to other scintillators that are currently used for radiation detection.

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Specifically, SrI2:Eu2+ has a light yield that is significantly higher than LaBr3:Ce3+

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-a currently employed commercial high-performance scintillator. Additionally,

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SrI2:Eu2+ is characterized by an energy resolution as high as 2.6% at the 137Cs

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gamma-ray energy of 662 keV, and there is no radioactive component in SrI2:Eu2+

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- unlike LaBr3:Ce3+ that contains 138La. The Ce3+-doped LaBr3 decay time is,

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however, faster (30 nsec) than the 1.2 Psec decay time of SrI2:Eu2+. Due to the

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relatively low melting point of strontium iodide (~515 oC), crystal growth can be

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carried out in quartz crucibles by the vertical Bridgman technique. Materials-

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processing and crystal-growth techniques that are specific to the Bridgman

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growth of europium-doped strontium iodide scintillators are described here.

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These techniques include the use of a porous quartz frit to physically filter the

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molten salt from a quartz antechamber into the Bridgman growth crucible and

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the use of a “bent” or “bulb” grain selector design to suppress multiple grain

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growth. Single crystals of SrI2:Eu2+ scintillators with good optical quality and

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scintillation characteristics have been grown in sizes up to 5.0 cm in diameter by

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applying these techniques. Other aspects of the SrI2:Eu2+ crystal-growth methods

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and of the still unresolved crystal-growth issues are described here.

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Keywords:

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A2 Bridgman technique

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B1 Halides

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B2 Scintillator materials

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A2 Growth from melt

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Boatner et al

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Introduction The scintillation properties of SrI2:Eu2+ were first discovered by Robert

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Hofstadter and described in his U.S. Patent [1] that was issued in 1968. The

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crystals used by Hofstadter were grown using low concentrations of europium,

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and the highest light yield reported in this patent was only 93% of that of NaI(Tl)

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– i.e., a value of ~35,300 photons/MeV. This reported modest performance

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probably accounts for the lack of any further significant interest in or continued

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development of SrI2:Eu2+ as a scintillator until recently. More recent

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investigations [2-11] have shown that when strontium iodide is doped with

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relatively high levels (e.g., 3 to 6%) of divalent europium, single crystals of this

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material exhibit outstanding scintillator characteristics – including light-yield

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values of 90,000 photons/MeV or greater and an energy resolution of ~2.6%. The

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maximum doping level employed by Hofstadter in his original studies of

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SrI2:Eu2+ was 1.6%- but whether or not this factor alone accounts for the lower

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light yields reported in the 1968 patent versus the much higher values

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determined in more recent investigations is not known. Other factors involving

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the material synthesis and single-crystal quality could also potentially account

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for differences in the earlier (Hofstadter) and more recent light-yield and energy

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resolution values.

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When SrI2 is doped with Eu2+ at levels of 3 to 6%, its characteristics are, in

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fact, superior in several ways to commercial scintillators that are currently

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widely used for gamma ray detection. Specifically, the europium-doped

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strontium iodide representative light yield of 90,000 photons/MeV or higher is

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significantly larger than the value of 74,000 photons/MeV reported for the high-

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performance scintillator, cerium-doped lanthanum tribromide (LaBr3:Ce3+) [12].

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Additionally, SrI2:Eu2+ exhibits comparable proportionality (i.e., a

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nonproportionality of 2.20% for strontium iodide versus 2.24% for Ce-doped

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lanthanum tribromide). This better nonproportionality and high light yield

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translate into the excellent value of 2.6%, as noted above, for the SrI2:Eu2+ (3%Eu)

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energy resolution - and unlike, LaBr3:Ce3+, there is no radioactive component

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(i.e., 138La) in SrI2:Eu2+. While the strontium iodide decay time is significantly

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slower than that of LaBr3:Ce3+ (i.e., 1.2 microseconds for SrI2:Eu2+ versus 30 nsec

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for LaBr3:Ce3+), this scintillator is still sufficiently fast for many important

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applications including gamma ray spectroscopy.

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Due to the relatively low melting point of strontium iodide (~515 oC), this

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material is amenable to single-crystal growth in quartz crucibles by means of the

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vertical Bridgman technique. Additionally, unlike many of the higher-melting-

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point halide scintillator materials, there is a low level of interaction between the

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molten strontium iodide salt and the quartz Bridgman crucible. Single crystals of

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this scintillator with good optical quality and excellent scintillation properties

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have now been grown in sizes up to 5.0 cm in diameter by using the Bridgman

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technique. The present work focuses on the materials-processing and crystal-

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growth techniques that are specific to the Bridgman growth of europium-doped

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strontium iodide scintillators and on the quality and performance characteristics

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of the resulting crystals. These processing and growth techniques include the

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use of a porous quartz frit to physically filter the molten salt from a quartz

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antechamber into the Bridgman growth crucible and the use of a “bent” grain

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selector (similar to that utilized for nucleation suppression in the growth of high-

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performance alloy turbine blades) or a “bulb” grain selector to suppress multiple

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grain propagation. The molten-salt filtration technique is utilized to remove

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physically insoluble particles (e.g., hydrates or oxyhalides) from the melt as it

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fills the quartz Bridgman crucible. In the absence of this filtration step, the

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insoluble particles adversely affect the initial crystal nucleation and subsequent

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solidification process. Additional SrI2:Eu2+ purification methods have included

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initial zone refining of the components. An initial directional solidification

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Bridgman growth process is also employed, and the resulting solidified boule is

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then melted, re-filtered and used as the starting material for a subsequent

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Bridgman growth.

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Experimental

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Strontium iodide is an orthorhombic material (space group Pbca, No. 61)

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with lattice constants: a = 15.268Å, b = 8.2351Å, and c = 7.896Å. The SrI2 crystal

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structure is illustrated in Fig. 1a and the packing arrangement of the Sr-Iodine

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coordination units are illustrated in Fig. 1b. In both ambient pressure forms of

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SrI2 [Pnma (No. 62) and Pbca (No. 61) (The Pnma form is shown in Fig. [1])], the Sr

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coordination polyhedron is a mono-capped trigonal prism (CN = 7), with the Sr

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atom displaced towards the capping atom. The mean Sr-I bond length in both

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forms is 3.35 Angstroms. Also, in each form, the two unique I atoms are in a

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rough tetrahedral and a flattened pyramidal coordination with the near-neighbor

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Sr atoms. The Pbca form (4.59 g/cm3) is slightly more dense as compared to the

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Pnma form (4.46 g/cm3). The material is highly hygroscopic. Several SrI2

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melting points of up to 538oC are given in the literature, but the literature value

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of 515oC appears to be the most accurate based on our experience obtained with

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the present crystal-growth procedures.

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Single crystal growth was carried out using starting SrI2 and EuI2

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primarily supplied by Sigma Aldrich, Inc. Strontium iodide with 5-9’s, 4-9’s or

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3-9’s % purity (with respect to metals) was employed, and the nominal purity of

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the EuI2 component was 99.9%. Quartz ampoules with internal diameters of 1.9,

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2.5, and 5.0 cm were used, and these ampoules were heated under vacuum to red

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heat prior to introducing the crystal-growth charge. Bridgman crystal growth

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was carried out in either two-zone or single-zone furnaces – with the preferred

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approach being growth using a two-zone Trans-Temp furnace (5.0 or 10 cm ID)

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where the growth process could be visually monitored. In the case of growth

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using a two-zone furnace, a relatively wide range of upper and lower zone

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temperatures was investigated. For growth using the 5.0 cm ID two-zone

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TransTemp furnace, the upper zone temperatures ranged from 555 to 575oC

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while the lower zone temperatures varied from 125 to 350oC. For growth in the

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single-zone (Mellen, Inc.) furnace, the maximum temperature was set at 560oC.

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The crystal growth rate was nominally in the range of 0.8 to 0.4 mm/h –

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although other slower and faster growth rates were evaluated as well.

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The growth of single crystals of Eu-doped SrI2 has been carried out at the

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Oak Ridge National Lab., RMD, Inc., and Fisk University followed by extensive

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scintillator characterization studies and device fabrication investigations

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performed at the Lawrence Livermore National Laboratory. Accordingly, while

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there is clearly some variation in the crystal-growth procedures among the

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different laboratories, the following is a generic general description of the

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representative crystal-growth process. This description incorporates the use of a

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method of filtration of the molten salt in order to physically remove insoluble

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hydrated, oxyhalide, and other precipitates. The mixed SrI2 plus EuI2 starting

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material, either in powder or bead form, is initially loaded into the upper

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chamber of a dual-chamber quartz Bridgman ampoule assembly like that

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illustrated in Fig. 2. This process is carried out in a dry box. As shown in Fig. 2,

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the upper chamber of the quartz assembly is separated from the lower Bridgman

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growth crucible by a porous quartz “frit” filter. Below this frit filter, the quartz

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tube is “necked-down” for purposes of sealing off the lower Bridgman ampoule

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under vacuum once the filtration of the molten salt is complete. As shown in the

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figure, a “bulb type” grain selector is incorporated in the assembly for the

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purpose of inhibiting the propagation of multiple grains into the larger diameter

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portion of the Bridgman crucible. An enlarged view of the section incorporating

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the porous quartz filter as fused into the outer quartz tube is shown in Fig. 3.

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Two different frit filters have been employed – a medium porosity filter with

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pore sizes of 4-90 microns or a fine porosity filter with pore sizes of 4-5 microns.

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An interlock system is employed to transfer the loaded quartz assembly from the

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dry box to the vacuum drying system with no exposure of the charge to the

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atmosphere. Once the starting material is dried by heating under vacuum

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(preferably with a system that incorporates a liquid-nitrogen-filled cold trap to

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remove condensable vapors), the growth charge is heated to a temperature that

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is just above the melting point. The molten-salt is then filtered through the

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quartz frit so that it flows into the lower Bridgman ampoule. When the medium

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porosity filter is used and the initial material quality is good, the molten salt

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generally flows on its own through the filter. In the case of the fine porosity filter,

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however, insoluble particulates can begin to clog the filter, and it is frequently

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necessary to apply a slightly increased pressure of high-purity argon above the

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filter in order to complete the filtration process. The unfiltered material

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remaining on the frit is generally black in color, and by analysis using X-ray

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powder diffraction, it was found to contain primarily various forms of hydrated

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SrI2.

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Following the molten-salt filtration step, the lower Bridgman growth

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ampoule is sealed off under vacuum and attached to a quartz rod for subsequent

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lowering through the vertical furnace. As noted previously, both single- and

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two-zone furnaces were employed. In order to further purify the material, an

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initial Bridgman growth was carried out – usually at a relatively fast directional

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solidification rate. Following a second molten-salt filtration step, the loaded

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Bridgman crucible was then placed in the vertical furnace for an additional

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growth pass at a growth rate lying in the range of 0.8 to 0.4 mm/h.

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A 5.0 cm-long single crystal of SrI2:Eu2+ grown at RMD, Inc. is illustrated

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in Fig. 4 for: the un-encapsulated but polished state, the packaged or “canned”

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condition, and exhibiting luminescence under UV excitation after hermetic

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packaging. The pulse-height spectrum obtained for this large, high-quality (3.0%

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Eu) specimen using excitation by 662 keV gamma rays emitted by 137Cs yields an

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energy resolution of 3.2%. The inset in Fig. 4 also shows a pulse-height spectrum

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that was obtained for this scintillator when it was coupled to the compact, hand-

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held instrument shown in the figure inset. Further details regarding the

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instrumentation for large SrI2:Eu2+ crystals are described in [8]. The Cs-137

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gamma ray pulse-height spectrum that was obtained using a small single crystal

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grown at Fisk University is shown in Fig. 5. This sample exhibited an

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outstanding scintillator energy resolution at 662 keV of 2.6% - a value that

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matches the best resolution that has been obtained to date with any scintillator

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operating at room temperature.

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Figure 6 shows a polished, un-encapsulated SrI2:Eu2+ (3.0 % Eu) single

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crystal grown at ORNL that is 2.5 cm in diameter by 2.5 cm in length. The

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associated Cs-137 gamma-ray pulse-height spectrum exhibits an energy

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resolution of 3.9% for this material. This crystal was grown by a single Bridgman

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pass through a single-zone furnace after drying and filtration of the starting

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material. Accordingly, it should be emphasized that success in the growth of

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high quality, un-cracked Eu-doped SrI2 single crystals has, in our experience,

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been highly dependent on the quality of the starting SrI2 or EuI2 components

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received from the supplier - and that significant variability in this quality has, in

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fact, been found from one synthesis batch to another. With some material

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synthesis lots, good un-cracked crystals were obtained simply following the

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initial single fast Bridgman pass - as illustrated by the example of such a crystal

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shown in Fig.6 that was produced by only a single pass after drying and frit

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filtration. For other synthesis lots, the multiple filtering and pre-growth

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purification protocols described above were necessary to obtain good quality

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single crystals. For other synthesis lots, no amount of filtering, pre-final-growth

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purification, etc. yielded good un-cracked single crystals. These issues will be

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discussed further in the Discussion and Summary section of this paper.

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Single crystals of SrI2:Eu2+ that were 5.0 cm in diameter have been

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successfully grown at both ORNL and RMD, Inc. In the growth of these

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relatively large crystals carried out at ORNL, a quartz crucible of the type

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illustrated in Fig. 7 was used. The quartz frit filter is visible in the upper left-

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hand portion of Fig. 7, and the entire assembly is being held by the reduced

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“neck” region where the crucible containing the dried and frit-filtered growth

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charge is ultimately sealed off under vacuum. The “bent” grain selector with an

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incorporated “bulb” configuration (as shown in Fig. 7) was used to prevent the

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propagation of dual or even multiple grains from the nucleation region into the

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main chamber of the Bridgman crucible. After sealing under vacuum, the quartz

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Bridgman crucible was attached to a quartz “lowering” rod and positioned in a

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two-zone, 10 cm ID TransTemp furnace. A 5.0 cm diameter SrI2:Eu2+ crystal

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grown using the crucible and molten salt filtration methods described above is

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shown in Fig. 8 under combined room and UV illumination.

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Discussion and Summary The methods and apparatus described here have been successfully

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applied to the growth of single crystals of the high-performance scintillator

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SrI2:Eu2+ with crystal diameters up to 5.0 cm. Light yields of 90,000

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photons/MeV (and higher in some cases) have been obtained along with an

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energy resolution as high as 2.6% for 662 keV gamma rays from 137Cs. This is

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apparently currently the “limiting value” for the room temperature energy

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resolution of a scintillator in general. Naturally, achieving further decreases in

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this value of energy resolution remains a current and important goal of

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scintillator research and development. However, energy resolution values of

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around ~3% are, in fact, now routinely obtained with standard analog readout

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for 1 in3-size SrI2:Eu2+. It should be noted that the methods described here have

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also previously been applied to the growth of Eu-doped mixed alkaline-earth

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iodide scintillators [13], and that they are, in fact, applicable to the growth of a

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wide range of iodide and other halide scintillators.

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As noted previously, the ability to grow high-quality, un-cracked single

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crystals of SrI2:Eu2+ is very strongly dependent on the quality of the starting

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components. Since this is a binary material incorporating both SrI2 and EuI2,

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both of these components need to be of high starting quality – otherwise severe

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cracking of the crystals generally occurs. A number of recent and continuing

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investigations have been undertaken to try to identify the root cause of this

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starting-material-related cracking or to ameliorate the cracking of SrI2:Eu2+

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crystals that can occur on cool down - and that takes place usually well below the

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solidification point of the material. These investigations have included attempts

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to relate impurities in the starting material to cracking effects and attempts to

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strengthen the material through either isovalent or aliovalent alloying. To date

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none of the investigations or approaches has provided a definitive determination

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of the actual cause of crystal cracking manifested in some starting material, nor

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has it been possible, in our experience, to prevent such cracking of defective

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material through compositional variations, slow cooling, annealing, etc.

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Extensions of these studies are currently ongoing in order to achieve highly

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reliable and reproducible growth of large un-cracked single crystals of SrI2:Eu2+.

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Acknowledgements

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This work was supported by the US Department of Homeland Security,

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Domestic Nuclear Detection Office, under competitively awarded IAA

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HSHQDC-09-x-00208/P00002. This work was performed under the auspices of

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the U.S. DOE. Oak Ridge National Laboratory is managed for the U.S. DOE by

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UT-Battelle under contract DE-AC05—00OR22725. Lawrence Livermore is

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managed for the U.S. DOE under contract DE-AC52-07NA27344. The authors are

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indebted to Bryan Chakoumakos for his assistance in the preparation of Fig. 1

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and to Dariusz Wisniewski and John Neal for contributions to the early stages of

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this effort.

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[1]. R. Hofstadter, Europium-activated Strontium Iodide Scintillators, U.S.

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Patent No. 3,373,279 (1968).

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Choong, William W. Moses, Stephen A. Payne, Kanai Shah, Steven Sheets,

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Hawrami, "Evaluation of large volume SrI2(Eu) scintillator detectors," Nuclear

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[12]. J. T. M. de Haas and P. Dorenbos, “Advances in yield calibration of

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[13]. John S. Neal, Lynn A. Boatner, Joanne O. Ramey, Dariusz Wisniewski, James

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Eu2+-Doped Mixed Alkaline-Earth Iodide Scintillator Crystals,” Nuclear

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Instruments and Methods in Physics Research A 643 (2011) 75-78.

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FIGURE CAPTIONS:

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Fig.1. (Left), A ball-and-stick model of the crystal structure of orthorhombic SrI2

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(space group Pbca, No. 61; lattice constants: a = 15.268Å, b = 8.2351Å, and c =

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7.896Å) is shown.

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spheres are iodine ions. (Right), Packing of the polyhedral Sr coordination units

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is shown where the Sr ions are located in the center of the polyhedra and the

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iodine atoms represented by small spheres are at the vertices.

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Fig.2. A dual-chamber quartz assembly is shown that incorporates an upper

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chamber (top right) into which the starting composition is placed. A quartz frit

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filter is fused into the bottom portion of the upper chamber adjacent to the

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necked-down region that is ultimately used for sealing-off the Bridgman crucible

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(lower left). The assembly is connected to a vacuum system for heated drying of

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the charge - after which the material is heated to just above the melting point,

The large spheres represent strontium while the smaller

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and the molten salt is then filtered through the frit into the Bridgman crucible

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and sealed under vacuum. This process removes unwanted insoluble phases that

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are present in the molten starting material. A “bulb-type” grain selector, as

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shown in the lower left hand portion of the figure, is used to suppress multiple

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grain propagation.

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Fig.3.

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incorporating the frit filter is shown. The quartz frit is carefully fused around its

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outer edge to the surrounding quartz tube of the upper chamber of the assembly.

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Fig. 4. (Bottom, left), A 5.0 cm-long single crystal of SrI2:Eu2+ (3% Eu) grown at

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RMD, Inc. is shown after removal from the Bridgman crucible and subsequent

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polishing. (Bottom, center), The crystal is shown after packaging or “canning.”

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(Bottom, right), A top view of the packaged crystal is shown in combined room

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and UV light. (Top), The pulse-height spectrum and a fit to the experimental

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data are shown for excitation using 662 keV gamma rays. The top inset shows a

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compact, hand-held device along with a corresponding pulse height spectrum

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obtained using the subject crystal.

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Fig. 5. A pulse-height spectrum and fit to the data are shown for a small single

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crystal of SrI2:Eu2+ (3% Eu) grown at Fisk University are shown. The energy

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resolution for this specimen was 2.6%.

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Fig. 6. Side, oblique, and end views are shown for a 2.5cm X 2.5 cm cylindrical

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crystal grown at ORNL. This crystal was grown with zone-refined EuI2 using

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only a single pass through a single-zone Bridgman furnace. The associated pulse

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height spectrum for a 137Cs excitation source is also shown.

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Fig. 7. A quartz crucible assembly (6.3 cm OD) is shown with the quartz frit filter

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visible in the upper left-hand portion of the Fig. The entire assembly is being

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hand held by the “neck” region that is used to seal off the crucible containing the

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frit-filtered growth charge under vacuum. A “bent” grain selector with an

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incorporated “bulb” configuration is employed to prevent the propagation of

An enlarged, detailed view of the portion of the quartz assembly

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dual grains into the main chamber of the Bridgman crucible from the nucleation

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region in the tip of the grain selector.

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Fig. 8. A 6.3 cm diameter single crystal of SrI2:Eu2+ (3% Eu) grown at ORNL is

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shown under both room light and UV excitation. Similar 6.3 cm diameter single

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crystals of SrI2:Eu2+ (3% Eu) have also been grown at RMD, Inc.

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Bridgman Growth of Large SrI2:Eu2+ Single Crystals: A High-performance

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Scintillator for Radiation Detection Applications

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L. A. Boatner, J. O. Ramey, and J. A. Kolopus

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Center for Radiation Detection Materials and Systems

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Materials Science and Technology Division

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Oak Ridge National Laboratory

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Oak Ridge, Tennessee 37831

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R. Hawrami, W. M. Higgins, E. van Loef, J. Glodo, and K. S. Shah

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Radiation Monitoring Devices, Inc.

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Watertown, Massachusetts 02472

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Pijush Bhattacharya, Eugene Tupitsyn, Michael Groza, and Arnold Burger

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Department of Life and Physical Sciences

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Fisk University, Nashville, Tennessee 37208

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N. J. Cherepy and S. A. Payne

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Lawrence Livermore National Laboratory

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Livermore, California 94550

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428

Corresponding author:

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L. A. Boatner

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[email protected]

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Oak Ridge National Laboratory

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1 Bethel Valley Road

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Oak Ridge, TN 37831

434

Phone: (865) 574-5492

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FAX: (865) 574-4814

436 437 438

ABSTRACT:

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Single-crystal strontium iodide (SrI2) doped with relatively high levels (e.g., 3 - 6

440

%) of Eu2+ exhibits characteristics that make this material superior, in a number

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of respects, to other scintillators that are currently used for radiation detection.

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Specifically, SrI2:Eu2+ has a light yield that is significantly higher than LaBr3:Ce3+

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-a currently employed commercial high-performance scintillator. Additionally,

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SrI2:Eu2+ is characterized by an energy resolution as high as 2.6% at the 137Cs

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gamma-ray energy of 662 keV, and there is no radioactive component in SrI2:Eu2+

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- unlike LaBr3:Ce3+ that contains 138La. The Ce3+-doped LaBr3 decay time is,

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however, faster (30 nsec) than the 1.2 Psec decay time of SrI2:Eu2+. Due to the

448

relatively low melting point of strontium iodide (~515 oC), crystal growth can be

449

carried out in quartz crucibles by the vertical Bridgman technique. Materials-

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processing and crystal-growth techniques that are specific to the Bridgman

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growth of europium-doped strontium iodide scintillators are described here.

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These techniques include the use of a porous quartz frit to physically filter the

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molten salt from a quartz antechamber into the Bridgman growth crucible and

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the use of a “bent” or “bulb” grain selector design to suppress multiple grain

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growth. Single crystals of SrI2:Eu2+ scintillators with good optical quality and

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scintillation characteristics have been grown in sizes up to 5.0 cm in diameter by

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applying these techniques. Other aspects of the SrI2:Eu2+ crystal-growth methods

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and of the still unresolved crystal-growth issues are described here.

459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490

Bridgman Growth of Large SrI2:Eu2+ Single Crystals: A High-performance Scintillator for Radiation Detection Applications L. A. Boatner, J. O. Ramey, and J. A. Kolopus Center for Radiation Detection Materials and Systems Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831 R. Hawrami, W. M. Higgins, E. van Loef, J. Glodo, and K. S. Shah Radiation Monitoring Devices, Inc. Watertown, Massachusetts 02472 Pijush Bhattacharya, Eugene Tupitsyn, Michael Groza, and Arnold Burger Department of Life and Physical Sciences Fisk University, Nashville, Tennessee 37208 N. J. Cherepy and S. A. Payne Lawrence Livermore National Laboratory Livermore, California 94550

HIGHLIGHTS: x x x x x

Large single-crystal Eu-doped strontium iodide scintillators grown. Filtration of the molten salt with a quartz frit used to remove particles. Bent, bulb-type grain selectors used to eliminate spurious grains. New Bridgman growth methods applicable to a wide range of halides. Scintillator energy resolution of 3% routinely obtained.

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