Transcript ORNL Research on Radiation Detection Materials for

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
Wilhelm Conrad Röntgen
Barium Platinocyanide purchased in
the late 1800’s by Sidney Rowland,
King’s College, London
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.
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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
4,100
17
1-inch round, 2mm thick/
$4,739 for 6.2-inch square,
2mm thick plate)
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Transparent Polycrystalline Ceramic Scintillators
Glass Scintillators
Why would we want these?
 Single crystal growth is a time-consuming, expensive, and ratelimiting 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:
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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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Oak Ridge National Laboratory-Transparent
Polycrystalline Scintillators
Why is it important to develop methods for forming transparent ceramic scintillators of non-cubic materials?
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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)
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Project Objectives and Key Research
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
Lu2O3: 5%Eu2O3 Transparent Ceramics
 Scintillator Characterization
Hot pressed at 1520 ºC, 321 kg/cm2 for
2 hrs
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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
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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Photograph of a Lu2O3:Eu
ceramic excited by a 30kV
continuous X-ray source.
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Photographs of a transparent
Lu2O3:Eu ceramic (~1mm thick)
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
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 backlight is used in this photograph.
Transmission electron
microscopy (TEM) image of
LSO:Ce powder from Nichia
Corporation
Scanning electron microscopy
(SEM) image of LSO:Ce powder
from Nichia Corporation.
•Annealed in water vapor at 1050°C/32h
•Annealed in air at 1150°C/32h
Particle size distribution of the Nichia LSO:Ce
powder used to make the LSO ceramic.
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X-ray diffraction Θ-2Θ scan for the
LSO:Ce ceramic with the standard
powder diffraction pattern.
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X-ray pole figure for LSO:Ce
ceramic showing no evidence for
texturing in the ceramic
microstructure.
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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
Phosphate Glass Scintillators
IR Phosphors
Linda Lewis
Phosphate
Glass
Modeling
David Singh
Gamma, X-Ray, and Neutron Scintillators
Lynn Boatner and John Neal
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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.
High Durability Lead Scandium Phosphate Glasses
 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.
582
100
1057
PbScPbOx+Nd+Ce glass
80
Intensity
 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.
525
60
40
20
excitation m1057nm
emission x583nm
745
355
472
683
895
1327
0
200 400 600 800 1000 1200 1400 1600 1800
Wavelength, nm
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 Phosphate
Silicate
Germanate
Arsenate
…
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Activation
Ce,Pr,Nd,Eu,Tb,Yb
Co-doping
Structure
Phosphate glass only
Phosphate chain length
Post-synthesis Treatment
Time
Temperature
Atmosphere
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Phosphate Glass - Gamma and X-ray Scintillators
Ce3+ valence can be a
challenging problem - partially
solved for silicate glasses, needs
to be solved for phosphate and
other glasses.
UV Excitation
LuPbPO4:Eu
ScPbPO4:Eu
Retort for variations of synthesis routes
for introducing Ce3+ and maintaining it
in the trivalent state.
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Energy Spectra of Ce Doped
Ca-Na Phosphate Glasses
137Cs
1μCi γ source
662 keV γ photons
0.5 μs shaping time
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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 And
Gd Co-doping in Li-Ca Glass
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Key General Objectives and Goals
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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.
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