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Novel Materials for Radiation Detection:Transparent Ceramics and Glass Scintillators

Lynn A. Boatner

Oak Ridge National Laboratory

Center for Radiation Detection Materials & Systems

2008 AAAS Annual Meeting

February 16, 2008

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Scintillators: A Little History

Barium platinocyanide is considered to be the first radiation detector. The scintillation from a screen of platinocyanide alerted Wilhelm Röntgen to the presence of some strange radiation emanating from a gas discharge tube he was using to study cathode rays. Since Röntgen did not know what these “rays” were, he named them x-rays.

Wilhelm Conrad Röntgen Barium Platinocyanide purchased in the late 1800’s by Sidney Rowland,

King’s College, London

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Scintillators: A Little History

This radiographic image was formed by placing the barium platinocyanide on a photographic film. The reason for this effect is that the state of separations chemistry in the 1800’s was poor and the barium platinocyanide was contaminated by radium.

G. Brandes found that sufficiently energetic x-rays produced a uniform blue-grey glow that seemed to originate within the eye itself.

Friedrich Giesel (Curie) “saw” radiation from radium “to obtain this effect, one places the box containing the radium in front of the closed eye or against the temple" and "one can attribute this phenomenon to a phosphorescence in the middle of the eye under the action of the invisible rays of radium" (Curie 1900, 1903). In the United States, the Colorado physician George Stover5 was among the first investigate radiation phosphenes (the proper name for visual sensations induced by radiation within the eye): "Sitting in a perfectly dark room and closing the eyes, if the tube of radium is brought close to the eyelids a sensation of light is distinctly perceived, which disappears on removal of the tube...”

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State of the Art Scintillators

Material Light Yield (photons/MeV)

Resolution @ 662keV (%)

NaI(Tl) 38,000 5.5

BGO 8,200 9.0

LaBr3(Ce) 70,000 2.8

LSO(Ce) 39,000 7.9

BC-408 Plastic 10,600 -

GS-20 Li Glass ($2930 for 1-inch round, 2mm thick/ $4,739 for 6.2-inch square, 2mm thick plate)

4,100 17

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Transparent Polycrystalline Ceramic Scintillators

Glass Scintillators

Why would we want these?

Single crystal growth is a time-consuming, expensive, and rate-limiting process.

Transparent polycrystalline ceramic scintillators and glass scintillators offer an alternative approach to scintillator synthesis that eliminates single crystal growth.

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Oak Ridge National Laboratory-Transparent Polycrystalline Scintillators

Conceptual Overview:

• The Realization of an Entirely New Paradigm for the Fabrication of Inorganic Scintillators — Specifically, an approach that is applicable to non-cubic as well as cubic materials––

• Through the development of versatile methods for producing large, optically transparent, high-performance (resolution, light yield, decay time,) inorganic scintillators without the necessity of growing large single crystals.

• Accomplish this goal by applying the concept of developing highly crystallographically oriented (highly textured) ceramic microstructures1,2 ––

• So that the material looks like a single crystal from the crystallographic point of view, and therefore, light scattering due to the effects of birefringence of the randomly oriented grains in a conventional ceramic is obviated.

1. L. A. Boatner, J. L. Boldú, and M. M. Abraham, “Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: I, Concepts and Theory,” J. Am. Ceram. Soc. 73, (8) 2333–2344 (1990).

2. J. L. Boldú, L. A. Boatner, and M. M. Abraham, “Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: II, Formation and Properties of Textured MgO,” J. Am. Ceram. Soc. 73, (8) 2345–2359 (1990)

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Why is it important to develop methods for forming transparent ceramic scintillators of non-cubic materials?

• The current scintillators with the highest energy resolution (LaBr3:Ce at ~2.6%) or very high light yield (LuI3:Ce at 100,00 photons/Mev) are not cubic materials.

• There are a lot more non-cubic materials than there are cubic materials.

• The technology for forming transparent ceramics of cubic materials is already well developed for a number of materials (e.g. YAG and related materials) as a result of Japanese research on polycrystalline laser rods.

• The claims that transparent ceramics can only be made with cubic materials is WRONG!

Transparent HAp sintered body fabricated by PECS. (2.0 cm O.D., 1 mm thick) Ca10(PO4)6(OH)2 A hexagonal (NOT CUBIC) crystal

XRD patterns of the sintered HAp body measured on the sections parallel (a) and perpendicular (b) to the pressure direction.

Transparent polycrystalline ceramic prepared by developing a high degree of texturing.

Y. Watanabe, et all, J. Am. Ceram. Soc., 88 [1] 243-5 (2005)

Oak Ridge National Laboratory-Transparent Polycrystalline Scintillators

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Project Objectives:

Develop New Densification Techniques for Producing Optically Transparent, Highly Textured Inorganic Scintillators––of both non-cubic and cubic materials

At reduced cost

At increased production rates

Eliminate single crystal growth

Minimize fabrication steps –near-net shape ceramics

Maintain scintillator performance

Key Research:

Hot pressing and annealing

Vacuum sintering

Precursor Development

Post Sintering Processing

Scintillator Characterization

Lu2O3: 5%Eu2O3 Transparent Ceramics

Hot pressed at 1520 ºC, 321 kg/cm2 for 2 hrs

Project Objectives and Key Research

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Photograph of a Lu2O3:Eu ceramic excited by a 30kV continuous X-ray source.

Lu2O3:Eu

Synthesis and Post Synthesis Treatment

•Lu2O3 and Eu2O3 (5 wt. %) powders combined physically

•Powder heated in vacuum to dry

•Hot pressed at 1530°C with 262 kg/cm2 of pressure

•Annealed with flowing oxygen for 72 hours at 1050°C

Photographs of a transparent Lu2O3:Eu ceramic (~1mm thick)

Photograph of a Lu2O3:Eu ceramic before (right) and after (left) annealing in an oxygen atmosphere. Hot pressing technique tends

to draw oxygen out of the host lattice, creating a dark color in the densified body.

This coloration can be removed by annealing in an O2.

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LSO:Ce

Synthesis and Post-synthesis Treatment

•High quality LSO:Ce powder produced by Nichia Corporation (Japan) used

•Powder heated in vacuum to dry

•Hot pressed at 1400°C with 337 kg/cm2 of pressure for 2 hours

•Annealed in vacuum at 1050°C/108h

•Annealed in water vapor at 1050°C/32h

•Annealed in air at 1150°C/32h

Photograph of a LSO:Ce ceramic before (left) and after (right) annealing in vacuum

Photograph of an LSO:Ce ceramic (0.6 mm thick). Note that no back-light is used in this photograph.

Scanning electron microscopy (SEM) image of LSO:Ce powder from Nichia Corporation.

Transmission electron microscopy (TEM) image of LSO:Ce powder from Nichia Corporation

Particle size distribution of the Nichia LSO:Ce powder used to make the LSO ceramic.

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X-ray pole figure for LSO:Ce ceramic showing no evidence for texturing in the ceramic microstructure.X-ray diffraction Θ-2Θ scan for the

LSO:Ce ceramic with the standard powder diffraction pattern.

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Scintillating pulse shape of a LSO:Ce polycrystalline ceramic excited by 662 KeV gamma photons. The solid line represents single- and three-exponential (+ noise) fits to the experimental data . The decay time constants and contribution of faster components in comparison to the decay time of about 42 ns generally accepted for single crystal LSO.

Energy spectra (for 662 keV excitation photons) of the LSO:Ce refernce crystal (the light yield for this crystal was ~30,000 photons/MeV) and the LSO:Ce ceramic at various post-sintering annealing stages. Symbol “A” denotes a ceramic with a 2 mm thickness after annealing in vacuum, “A1” denotes a 0.7 mm thick piece of the former ceramic after additional annealing in water vapor, and “A1a” the same after additional annealing in air.

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LaBr3:Ce

Synthesis and Post-synthesis Treatment

• LaBr3 and CeBr3 (2 wt. %) powders combined manually

•Powder heated at 350°C/24 in vacuum to dry

•Hot pressed under vacuum at 780°C with 388kg/cm2 of pressure for 3 hours.

•Annealed in vacuum at 650°C/24h

Photograph of a translucent LaBr3:Ce ceramic scintillator (-0.7 mm thick). Illumination circle is about 2.5 cm in diameter.

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Energy spectra of a BGO reference crystal and the LaBr3:Ce (2%) ceramic for several excitation energies.

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Novel Cerium-Activated Novel Cerium-Activated Phosphate Glass ScintillatorsPhosphate Glass Scintillators

IR PhosphorsLinda Lewis

Gamma, X-Ray, and Neutron ScintillatorsLynn Boatner and John Neal

ModelingDavid SinghPhosphate

Glass

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Research Goals & Objectives

Research Goals: To exploit the chemical flexibility, optical properties, and the unusual structural features and variability of the ORNL phosphate glasses and other glass systems in order to develop a new glass scintillator with significantly improved radiation-detection (i.e., light yield) characteristics.

Applications: Glass scintillators can be easily and economically fabricated in the form of large structures (e.g., as large area plates, tubes, rods, or bars) or pulled into optical fiber structures with wave-guiding properties.

As a result of their thermal, mechanical and chemical durability, glass scintillators are ideal for use as radiation detectors in devices that have to operate in the field under a variety of frequently adverse conditions.

High Durability Lead Scandium Phosphate Glasses

200 400 600 800 1000 1200 1400 1600 1800

0

20

40

60

80

100

895

1057

1327

355

472

525

582

683

745Intensity

Wavelength, nm

excitation m1057nm emission x583nm

PbScPbOx+Nd+Ce glass

Nd-doped phosphate glasses known to be effective phosphors when excited by IR

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Glass Scintillators: How Can We Improve Their Performance?

Glass Scintillator Parameter Space

Composition(Glass-forming space)Cladding Phosphate

Lead PhosphateSilicate

GermanateArsenate

…

ActivationCe,Pr,Nd,Eu,Tb,Yb

Co-doping

Post-synthesis TreatmentTime

TemperatureAtmosphere

StructurePhosphate glass only

Phosphate chain length

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Phosphate Glass - Gamma and X-ray Scintillators

Retort for variations of synthesis routes for introducing Ce3+ and maintaining it

in the trivalent state.

UV ExcitationLuPbPO4:Eu ScPbPO4:Eu

Ce3+ valence can be a challenging problem - partially solved for silicate glasses, needs to be solved for phosphate and other glasses.

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137Cs 1μCi γ source

662 keV γ photons

0.5 μs shaping time

Energy Spectra of Ce Doped Ca-Na Phosphate Glasses

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Variation Of Compton Edge Position as a

Function

of Ce Concentration in Ca-Na Phosphate Glass

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Comparison of Energy Spectra

137Cs 1μCi γ source

662 keV γ photons

0.5 μs shaping time

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Effects of Na Substitution By Li AndGd Co-doping in Li-Ca Glass

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Key General Objectives and Goals

• Develop new glass scintillator systems through the exploration of compositional variations in phosphate, silicate, germanate, arsenate and other glass-forming systems.

• Explore the effects of post-synthesis treatments (i.e., thermochemical processing) on glass scintillator properties.

• Investigate the performance of activators other than cerium in various glass host systems.

• Investigate energy transfer processes in glasses as a function of the material’s structure (amorphous versus crystalline materials) and electronic properties.

– Variable temperature studies– Identification of luminescence centers, impurities, defects, etc– Relative position of luminescence center levels in the bandgap of the host – Idendtify mechanisms that delay the transfer of energy to luminescence centers

• Apply this increased understanding to the synthesis of glass scintillators with improved performance characteristics.