Download Scintillation Crystal Arrays and Assemblies

Scintillation
Crystal Arrays
and Assemblies
About Saint-Gobain
Crystals and Detectors
Saint-Gobain Facts –
• Established in 1665.
• The first major project was the production of the mirrors for the famous Hall of
Mirrors in Versailles Palace.
• Today the Saint-Gobain group is listed
among the 100 largest industrial groups
worldwide.
• Produces 30 billion glass bottles each
year.
• Supplies half of Europe’s cars with glass.
Saint-Gobain is a global leader in the manufacture and development of
engineered materials such as glass, insulation, reinforcements, containers,
building materials, ceramics and plastics. The formation of the Crystals and
Detectors Division reinforces Saint-Gobain’s commitment to the development of radiation detection and measurement products. The Division
employs over 700 people at plants and sales offices throughout the world.
The Scintillation Products business of the
Division is a combination of companies that
have been prominent in crystal growth or in
radiation detection and measurement.
Notable names include: Bicron, Harshaw
and Crismatec (inorganic and organic
scintillators and detectors); Gamma
Laboratories and TGM Detectors (gas-filled
radiation detectors); Bicron and NE
Technology (instruments).
Being a part of Saint-Gobain brings us the
long-term industrial strategy and investment benefits of such a dynamic group.
There is a coherence centered on materials,
applied to increasingly diversified needs.
Saint-Gobain encourages research and
development and the expansion of relevant
technologies and their applications.
The Scintillation Products Group manufactures nuclear and X-ray radiation detectors .
The products cover a broad range of
materials and configurations from large
plates of NaI(Tl) crystal to sub-millimeter
BGO pixels. Specific applications for the
crystals and detectors include medical
imaging, oil well logging, airport luggage
scanners, density gauging and high energy
physics particle research.
Linear arrays are used in CT scanning
(medical/security/industrial), line scanning
(security/industrial) and many research
applications. Two-dimensional arrays
provide imaging capability in CT scanning,
nuclear medical applications (e.g. SPECT/
PET), portal imaging, X-ray flash radiography, non-destructive testing and various
research applications.
We have strong technical expertise to
understand your application. Our technical
and manufacturing facilities are geared to
provide either prototypes or “one-of-a kind”
specials as well as volume production.
Beenham, UK
Tokyo, Japan
•
Washougal, WA
•
Sidcup, UK
Beijing, China
Houston, TX
Bangalore, India
Wermelskirchen,
Germany
Solon, OH
St.-Pierres-lesNemours, France
Newbury, OH
Gieres, France
CdWO4 –
The high light output and low afterglow of CdWO4
makes it ideal for use with silicon photodiodes in
detectors for medical and industrial CT scanners.
CdWO4 has very good radiation resistance; and its
temperature dependence is small in the 0 to 60oC
range. Multislice CT is an example of a recent
application for CdWO4 arrays.
CsI(Tl) –
CsI(Tl) has the highest light output of these
scintillators, and its emission matches well with
silicon photodiodes. However, its long afterglow
limits its use to applications for which intervals
between sampling are long or some residual signal
can be tolerated. CsI(Tl) is a rugged, malleable
material that can be easily fabricated into a
variety of geometries. It is slightly hygroscopic.
CsI(Tl) is regularly fabricated into both linear and
2-dimensional (2D) arrays with pixel sizes as small
as 300 microns square.
BGO –
BGO has a fast decay time and low afterglow in
addition to high nuclear absorption. These
properties make it useful in dual energy detectors
and in high energy physics imaging and PET
applications. However, its temperature dependence
is large in the 0 to 60oC range.
BGO is a hard material which can be precision
machined into long, thin pieces for 2D arrays. We
have fabricated a 320,000 pixel 2D BGO array with
pixels 0.85mm x 0.85mm x 20mm.
NaI(Tl) –
NaI(Tl) has a very high luminescence (scintillation)
efficiency and exhibits no significant self-absorption
of the scintillation light. Pixellated sodium iodide
arrays can be used with position-sensitive photomultiplier tubes to produce small field of view and
organ-specific detectors with excellent performance
and maximum active area. The arrays are designed
to particular OEM requirements.
We have produced arrays up to 15 cm by 20 cm in
size and with pixels as small as one mm square with
minimal spacing (as low as 0.2 mm) required
between the pixels.
Other Materials –
SGCD fabricates arrays from a variety of other
scintillation materials including LSO and ceramics.
General Description
Materials for Array Applications
As noted in the table below, the optimal material choice involves a combination of
factors including the application, the photo readout device to be used, etc. For
example, if the application calls for rapid detection of radiation pulses, then the
decay time and afterglow drive the choice (BGO, LSO, NaI(Tl), CdWO4). If high efficiency and low cost are paramount, then NaI(Tl) with a PMT readout or CsI(Tl) with
a photodiode readout are the first detectors to consider. The physical properties of
each material must be considered as well. For example, the natural cleavage plane
in CdWO4 restricts the minimum size of 2D pixels. Saint-Gobain’s technical experts
can help guide your decision.
Properties of the Materials
General Properties
CdWO4
Density, g/cm3
8.0
Solubility in H2O, g/100g@25oC
0.5
Hygroscopic
no
Relative Light Output [CsI(Tl) = 100%] *
25
Relative Rad Hardness
High
Wavelength of Maximum Emission
475
Index of Refraction
2.2 -2.3
Primary Decay Time
1.4µs
Afterglow
0.005%
CsI(Tl)
BGO
NaI(Tl)
LSO
4.51
85.5
slightly
100
Medium
565
1.8
1µs
0.5-5%
7.13
–
no
14
High
480
2.15
300ns
0.005%
3.67
185
yes
34
Medium
415
1.85
250ns
0.3-5%
7.40
–
no
26
High
420
1.82
40ns
<0.1%
* Dependent on material quality and photodiode used
PSPMT Readout –
Photodiode Readout –
Pixellated crystals direct photons onto the
readout device with less light spread. The
use of a Position Sensitive Photomultiplier
Tube (PSPMT) will improve spatial resolution
and reduce image distortion. Each sensitive
area of the PSPMT provides spatial X and Y
coordinates for the exposed pixel. Spatial
resolution depends on the crystal and septa
size. Energy resolution is improved by the
more efficient PSPMT and more effective
collecting of the crystal’s scintillation light.
Photodiodes offer some advantages over
PMT’s for certain applications. Because
CsI(Tl) has most of its emission above
500nm , the material is well suited for a
photodiode readout. For certain crystals,
the shape of the emission spectrum is a
function of the temperature. Therefore,
the response for crystals with photodiode
readout can be different.
NaI(Tl) and BGO materials lend themselves
well to the use of a PSPMT due to the emission wavelength of less than 480nm.
Photodiodes are available in a variety of
sizes. When choosing a photodiode, the size
of the diode should be such that a maximum amount of the scintillation light can
be detected by the diode.
2
Designing an Array
General Parameters
There are choices of scintillator materials and separator/reflectors to optimize
performance to a specific application. The listing of parameters addresses the
elements that must be considered in the design of a linear or 2D array.
Array Design Parameters –
• Material: Type of scintillation crystal or
material desired.
• Pixel or Element Size: The “X” and “Y”
dimensions of each scintillator pixel.
• Separator Type and Thickness: The type
of reflector between the crystal pixels
and its overall thickness, “G.” Note: this
may be a composite or laminate of
white reflector and metal materials.
• Pitch: This is the distance between the
center of one element to the center of
an adjacent element, “X” + “G” or
“Y” + “G.” Note: In 2D arrays with
rectangular pixels, the pitches in the
“X” and “Y” directions will be different.
The table below shows the materials and the associated pixel sizes that are
producible today. This list is still evolving. For example, only a few years ago, the
table would have shown a minimum 2D pixel size: (1) NaI(Tl) at 7.0mm – seven
times today’s value, and (2) BGO at 0.6mm – twice today’s value. The pixel sizes are
controlled primarily by mechanical properties of the crystals, e.g. hardness, cleavage, ease of machining. For example, CdWO4 has a cleavage plane in one crystallographic direction. For that reason, it is not possible, with current techniques, to
achieve 0.3mm2 pixels because of fractures along the cleavage planes that occur
during cutting and grinding in manufacture. However, 0.3 x 1.0mm2 pixels can be
produced.
Minimum Discrete Pixel Sizes Available in Crystal Scintillators
Material
Cadmium Tungstate – CdWO4
Cesium Iodide (Thallium) – CsI(Tl)
Bismuth Germanate – BGO
Lutetium Oxyorthosilicate – LSO
Calcium Fluoride (Europium) – CaF2(Eu)
Sodium Iodide (Thallium) – NaI(Tl)
Minimum Pixel Sizes *
Linear (mm)
2D (mm)
0.3
0.3
0.3
0.8
0.5
1.0
1.0
0.3
0.3
0.8
0.7
1.0
Comments
Cleavage Plane
Min. Untested
* Available as of this date; smaller pixel sizes may be developed.
• Radiation Thickness: This is the “Z”
dimension and specifies the thickness
of the array in the direction of the
incoming radiation.
• Back Reflector Thickness: Usually a
white reflector is applied to the
radiation entrance side of the array to
reflect the light back into the pixel so it
can be directed to the light sensor.
• Material adjacent to the end pixels or
elements: The end crystals may need a
special reflector thickness or other
treatment, e.g., to keep a constant
pitch from array to array if they will be
joined together in use.
Dimensions to consider in the
design of a linear array
Dimensions to consider in the
design of a 2D array
All dimensions, including array length and width and pixel size and length, may be defined by
the application.
3
Separator/Reflector Type and Thickness –
The geometry of the pixel, the thickness of the
reflector, the scintillator material used and other
factors influence the reflectivity obtained in each
array design. Array reflector materials are listed in
the order of decreasing reflectivity. The reflectivity
numbers are presented as a guide only. The first
two separator materials listed (white powder and
Teflon sheet) are not practical for most of the small
pixel arrays discussed here – they cannot provide
the bonding properties required. However, they are
useful in some encapsulated units. Once mixed with
epoxy, the white powder provides the diffuse
reflectivity required to channel the scintillation light
to the exit surface and the adhesive properties for a
mechanically stable array.
Metal or metallized separators prevent optical
crosstalk between the pixels while maintaining
minimum gap “G” thicknesses. However, the metal
surfaces, even polished, do not provide the best
reflection of the scintillation light to the exit
surface. This is where composites are useful. They
combine the reflective properties of the white
materials with the “zero” optical crosstalk of solid
BC-490 Plastic Scintillator Casting Resin –
metals or films. Metal separators can serve another
BC-490 is a partially polymerized plastic
function: to absorb the radiation that is incident on
scintillator that can be cured to full hardness
the separator area before it strikes the light sensor
by the end user. The scintillator thus formed
and produces noise. Nuclear dense materials like
is clear, with scintillation and mechanical
Lead, Tungsten, and Tantalum are used. Also
properties similar to those of our general
available are white epoxies where the reflector
purpose plastic scintillators. It is most
particle fillers are more nuclear dense than TiO2 or
frequently used in applications that require
Al2O3. However, in practice, their effectiveness is
other materials to be imbedded in the
limited to low energies, up to 60keV.
scintillator, and those that require unique
shapes to be cast, often in special holders.
Designing an Array
Specific Parameters
Proprietary processes developed by SGCD ensure superior light output and
improved pixel-to-pixel uniformity. The reflector material we use between the
pixels provides outstanding reflection as well as excellent protection against
optical crosstalk.
Separator Types and Thicknesses in Order of Decreasing Reflectivity
Material
White Powder (e.g. TiO2, MgO) **
Teflon Sheet **
White Reflector Paint
White Plastic
White Epoxy
Composites ***
Aluminum/Epoxy
Metals (Pb, Ta) / Epoxy
*
**
***
Approx. Relative
Reflectivity *
Thickness Range
1.0 mm and up
0.15mm - 0.50 mm
0.04mm - 0.10mm
0.05 mm and up
0.10mm - 0.75mm
0.10mm and up
0.05mm - 0-.1mm
0.05mm and up
100%
98%
96%
95%
94%
94%
75%
65%
These are guidelines only and are based on optimum, not minimum, thickness.
Values will vary with pixel geometry, surface finish and other specific design parameters.
These are used as reflector materials in large scintillation crystal packaging.
Composite separators are clear epoxy-paint-clear epoxy, white epoxy-metal-white epoxy.
BC-490 is supplied in complete kits with
detailed instructions. Each kit contains three
parts: partially polymerized scintillator resin,
catalyst and catalyst solvent.
A green-emitting version, BC-490G, is also
available.
Injection Molded Plastic Scintillators –
Injection molded scintillator made from a polyvinyltoluene (PVT) base is intended for applications in
which a large number of identical pieces are
required. This material offers a cost-effective
alternative to traditional cast sheets.
The use of PVT as the base plastic leads to an
Material
K-edge
intrinsic light yield that is 15 to
20% greater than
(keV)
moldings made from polystyrene.
Sizes up to 300 x 300 mm can be produced in
CdWO
thicknesses ranging from 3 mm
to 504 mm. 69.5
CsI
33.2
This scintillator has a formulation similar to BC-404
BGO
90.5
(Pilot B) which is well-suited for use with green
NaI
wavelength shifters. Other formulations
are 33.2
63.3
available on request. To obtainLSO
a detailed quotation,
contact your SGCD representative with sizes,
quantities, and specific application requirements.
The chart below shows the thickness of material, in mm, required to absorb 95% of the
noted X-ray energies. We can determine the optimum thickness for your application. (Note
that the absorption changes significantly at the K-edge.)
40keV
(mm)
60keV
(mm)
80keV
(mm)
100keV
(mm)
300keV
(mm)
600keV
(mm)
0.33
0.9
0.5
0.9
12.6
35.9
0.29
0.8
1.8
3.3
36.5
79.3
0.38
1.1
2.2
1.1
13.3
37.8
0.43
1.3
2.7
4.9
49.2
99.2
0.55
1.6
0.7
1.3
16.0
40.9
4
Applications for
Detector Arrays
Examples of applications –
Baggage Scanning
Shown to the right are some commonly recognized
applications in medical and security markets. There
are other imaging applications that are emerging in
nuclear medicine, industrial imaging (nondestructive testing), portal imaging, flash radiography and research and space applications.
CT Imaging
Cargo Scanning
Chart of Applications well-suited for SGCD Array Materials
Material
Gamma
Cameras
CdWO4
CsI(Tl)
Positron Emission
Tomography
X
X
X
X
LSO
X
X
Line
Scanning
Industrial
Imaging
Flash
Radiography
Digital
Radiography
X
BGO
NaI(Tl)
5
CT
Imaging
X
X
X
X
X
X
X
X
Purification and Growth –
By processing our raw materials in-house, we can
control performance, especially light output, and
quality consistency.
Capabilities
Manufacturing Control –
Arrays and detectors are assembled complete with
photodiode and reflector in clean rooms, in dry
boxes or dry rooms, as required, in our facility.
Precision machining is done in a technologically
advanced fabrication shop.
Our experience with array technology is extensive. Below are examples of some of
the configurations we have provided customers.
Performance Testing –
Example of our precision
Array and detector performance can be tested
during and following assembly, and we maintain
advanced and specialized test equipment to
simulate actual use.
pixel alignment technology
Material: LSO
Manufacturing Support –
Through years of various application experience, we
can help you with the selection of crystal and spacer
materials and reflectors. Custom engineering and
design assistance are also available.
Scintillator panel
measuring 30cm x 40cm
with 0.3mm pixels,
X-ray thickness 10mm
Material: CsI(Tl)
9
Example of a
50X magnification
container scanner array
of a linear array
Close-up of a CdWO4 2D array
1mm pixel BGO
with white reflector technology
2D array
6
Scintillation Products
Scintillation Crystal Arrays
and Assemblies
®
USA
Saint-Gobain Crystals & Detectors
12345 Kinsman Road
Newbury, OH 44065
Tel: (440) 564-2251
Fax: (440) 564-8047
Europe
Saint-Gobain Cristaux & Detecteurs
104 Route de Larchant
BP 521
77794 Nemours Cedex, France
Tel: 33 (1) 64 45 10 10
Fax: 33 (1) 64 45 10 01
Japan
Saint-Gobain Crystals & Detectors KK
3-7, Kojimachi, Chiyoda-ku,
Tokyo 102-0083 Japan
Tel: 81 (0) 3 3263 0559
Fax: 81 (0) 3 5212 2196
China
Saint-Gobain China Investment Co., Ltd.
24-05 CITIC Building
19 Jianguomenwai Ave.
Beijing 100004 China
Tel: 86 (0) 10 6513 0311
Fax: 86 (0) 10 6512 9843
www.detectors.saint-gobain.com
The data presented in this brochure are believed to be correct but are not
guaranteed to be so. Nothing herein shall be construed as suggesting the
use of our product in violation of any laws, regulations, or rights of third
parties. User should evaluate suitability and safety of product for user’s
application. We cannot assume liability for results that user obtains with
our products since conditions of use are not under our control.
Bicron is a registered trademark of Saint-Gobain Ceramics & Plastics.
©2002 Saint-Gobain Ceramics & Plastics, Inc., All rights reserved
(05-02)