Material Processing Methods
-----In conjunction
with our research into the chemical composition, morphology
and performance of advanced scintillators and related
materials is our development and refinement of a wide
variety of fabrication and treatment methods that are
essential to and/or enhance the properties of our scintillators
and related or other materials.
-----Our
growing array of material processing methods currently
includes:
Microcolumnar film scintillator fabrication
produces films of scintillator materials that channel
and conserve light within densely packed, highly uniform
microcolumns. The needle-like microcolumns are individual
and highly parallel, each typically from 250 nm to 2
microns in diameter, to as much as 10 microns in diameter,
and number into the millions per square centimeter.
Such films up to 1 cm thick have been grown. Films are
typically 10 to 500 microns to 3 mm thick and may be
from under 1 cm2 to over 19 x 19 inches (48x48
cm2) in area (or over 19" in diameter).
The optical property of total internal reflection
within the microcolumns yields high spatial resolution,
enhanced contrast resolution and near-maximum light
yield without the loss and "smearing" of light inherent
in single-crystal amorphous scintillators. These scintillators
are produced via vapor deposition techniques on a variety
of opaque, translucent and transparent substrates (glasses,
metals, ceramics, graphite, etc.) in high-vacuum evaporator
chambers at RMD, some of which were developed by RMD
to meet specific material growth requirements.
Crystal scintillator fabrication of CsI:Tl,Eu
and CsI:Tl,Sm is performed using the Bridgman technique,
producing crystals typically up to 2" in diameter.
Crystalline film scintillator fabrication produces
uniform, non-columnar, amorphous, monolithic slabs of
crystalline scintillator material, up to 7 mm or more
in thickness on large area substrates, for imaging applications
such as high-magnification radionuclide imaging and
for those applications that do not require high intrinsic
resolution. These scintillators are usually produced
by vapor deposition, and may be made in very large formats.
Hybrid scintillator fabrication combines multiple
scintillator, converter and other materials to provide
sensors uniquely suited to unusual application requirements.
Pixelated crystalline films (solid slabs) and microcolumnar
films using our laser ablation techniques are truly
engineered scintillators that offer the advantages of
high spatial resolution imaging to unusual and/or demanding
applications. The ability to actually separately focus
individual fine, uniform crystalline scintillator pixels
onto one or more points or regions in space (in front
of or behind the scintillator) with constant or variable
pitch overcomes the parallax problem heretofore encountered
in applications such as pinhole imaging. This pixel
structures, in a manner similar to that of microcolumnar
films, thereby overcomes the sensor limitations imposed
by the traditional trade-off between scintillator resolution
and sensitivity, and allows improved light collection
efficiency. Typical pixelations include thin films with
10 micron wide grooves and 30 to 250 micron pixel pitch,
and thick films with 30 micron wide grooves and 100
to 500 micron pixel pitch. Our economical, well-controlled,
uniform pixelation techniques eliminate the expensive
and complex sawing and assembly often used to fabricate
scintillators.
Neutron detection applications further benefit from
our ability to surround each crystalline or microcolumnar
scintillator pixel individually with converter
materials such as gadolinium or boron compounds (such
as Gd2O3) that otherwise cannot
or should not be incorporated into the crystal lattice,
and with reflective, absorptive and protective coatings
to maximally conserve light or eliminate crosstalk.
Scintillator injection techniques
allow us to inject scintillator materials (such as CsI-based,
NaI-based and plastic scintillators) into fine capillary
arrays and honeycomb structures made of various materials
and of virtually any size or form for high resolution,
high efficiency gamma-ray imaging and neutron detection.
Such capillaries may be parallel, with uniform diameter,
or may be focused on a point or region in space in front
of or behind the scintillator structure, with uniform
or varying diameter. Injected structures of thickness
ranging from 1 mm to 1 cm can be fabricated to enhance
absorption efficiency. The capillary/honeycomb structure
eliminates optical crosstalk between neighboring pixels,
thereby resulting in high spatial resolution, regardless
of sensor thickness. Capillary diameters may vary from
25 microns to over 2 mm, depending on the resolution
requirement and the material to be injected. This process
is suitable for alkali halide scintillators and plastic
scintillators.
Applied slurry scintillator films
typically serve the needs of cost-sensitive applications
and/or those requiring large-area scintillators that
are lens-coupled to photosensors, where spatial resolution
and high material thickness is not a critical factor.
Such sensors include GOS/Gadox (terbium-doped gadolinium
oxysulfide, Gd²O²S:Tb) or other powdered materials
that are applied in suspension as thick liquids to medium-
to large-area substrates (often aluminum plates), and
then allowed to dry.
Pre- and post-fabrication processing
and treatment operations allow us to enhance scintillator
performance and robustness and, in certain cases, achieve
fundamental performance requirements in scintillators
fabricated by deposition and other means.
Substrate treatments before deposition (typically
in the cases of microcolumnar and crystalline film
deposition) help to conserve scintillation light (by
the application of reflective coatings), reduce unwanted
light and, thus reduce blur (which is also reduced
by the application of absorptive coatings), increase
sensitivity to thermal neutrons (by the application
of a converter material) or promote the adherence
of scintillator material to a substrate or even inhibit/prevent
adherence of scintillator material to specific substrate
areas.
Re-doping after scintillator
fabrication enhances scintillation light yield.
Passive functional coatings (typically
reflective) before fabrication (typically to substrates)
or after fabrication (typically to the tops of scintillators)
improve light conservation, internal reflection and
emission/transmission, and help to control light propagation.
Active functional coatings before
or after fabrication participate in the scintillation
process and promote, enhance or otherwise actively
affect or participate in light emission, as in the
case of neutron detection, where a converter coating
(such as a gadolinium-, boron- or lithium-based material)
on the substrate and/or the top and other areas of
the scintillator provides the mechanism for converting
thermal neutron interactions in the coating layer(s)
into prompt gamma rays that then interact detectably
in the primary scintillator.
Protective coatings after deposition
provide hermetic moisture barriers (for hygroscopic
materials) and firm to hard outer coats for the physical
protection of scintillators and even other coatings.
Scintillator substrate design and
selection depends upon the needs of the intended
application, and can include opaque and transparent
substrates of graphite, glass, fiberoptics, alumina
(or other ceramics), aluminum and other materials, in
virtually any size, shape and thickness. Opaque substrates
are typically selected when the photosensor will be
positioned on the scintillator side of the substrate,
while photosensors are typically positioned on the substrate
side of a sensor using a transparent or fiberoptic substrate.
In fact, RMD can supply freestanding scintillators with
no substrate, or with a thin, flexible, conformal film
(such as Parylene) as the substrate. While most substrates
are passive, RMD's methods are also used to deposit
or bond scintillators directly onto CCD and CMOS sensors.
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