U.S. patent application number 12/443991 was filed with the patent office on 2010-04-08 for radiation detection device.
Invention is credited to Daniel K. Angell, Thomas K. Hunt.
Application Number | 20100084562 12/443991 |
Document ID | / |
Family ID | 39721773 |
Filed Date | 2010-04-08 |
United States Patent
Application |
20100084562 |
Kind Code |
A1 |
Angell; Daniel K. ; et
al. |
April 8, 2010 |
RADIATION DETECTION DEVICE
Abstract
A radiation detection system for detecting the presence and
location of a radiation source includes an optical fiber bundle
having fibers of different lengths, a radiation sensitive material,
a stimulating source and an optical detector. The stimulating
source stimulates the radiation sensitive material and the
radiation sensitive material releases a light output, while the
light output provides a readout signal for each fiber corresponding
in intensity to the radiation received from the radiation source.
The optical detector receives the readout signal such that the
variations in intensity of the readout signals along the length of
the bundle determine the presence and general location of the
radiation source.
Inventors: |
Angell; Daniel K.; (Allen
Park, MI) ; Hunt; Thomas K.; (Ann Arbor, MI) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
39721773 |
Appl. No.: |
12/443991 |
Filed: |
October 4, 2007 |
PCT Filed: |
October 4, 2007 |
PCT NO: |
PCT/US07/80372 |
371 Date: |
December 8, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60849244 |
Oct 4, 2006 |
|
|
|
60849306 |
Oct 4, 2006 |
|
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Current U.S.
Class: |
250/363.01 ;
250/368; 385/12 |
Current CPC
Class: |
G01T 1/11 20130101; G01T
1/10 20130101 |
Class at
Publication: |
250/363.01 ;
250/368; 385/12 |
International
Class: |
G01T 1/20 20060101
G01T001/20; G02B 6/00 20060101 G02B006/00 |
Claims
1. A radiation detection system for enhancing detection signal to
noise comprising: a first layer having at least one light source
configured to stimulate light emission from a radiation sensitive
energy storage material; a second layer adjoining the first layer,
the second layer comprising at least one radiation sensor having a
first surface and a second surface, the at least one radiation
sensor comprising a radiation sensitive material, wherein the
radiation sensitive material stores energy corresponding to the
radiation sensitive material's received radiation dose when it is
exposed to radiation from a radiation source over a variable period
of time; wherein the at least one light source provides to the
radiation sensitive energy storage material a stimulating light
energy having a first wavelength where upon the radiation sensitive
energy storage material releases at least a portion of the stored
energy in the form of a light output having a second wavelength,
wherein said light output provides a readout signal, wherein the
readout signal is used to determine the energy and intensity of the
radiation from the radiation source; a third layer adjoining the
second layer, the third layer comprising an optical detector
configured to receive the readout signal and measure the intensity
of the light output at the second wavelength, wherein the light
output at the second wavelength corresponds in intensity to the
intensity of the radiation from the radiation source; and an
optical filter positioned between the second and third layers, the
optical filter configured to block the stimulating light energy
provided by the at least one stimulating light source from
interacting directly with the optical detector.
2. The radiation detection system of claim 1, wherein the radiation
sensitive material is an optically stimulated luminescent
material.
3. The radiation detection system of claim 1, wherein the at least
one light source is one of a light emitting diode, a field emission
display, an organic light emitting diode, a light emitting plasma
discharge tube, a laser and a vacuum fluorescent tube.
4. The radiation detection system of claim 1, wherein the first
wavelength is longer than the second wavelength, wherein the first
wavelength is in the infrared wavelength range and wherein the
second wavelength is in the visible wavelength range.
5. The radiation detection system of claim 1, wherein the first
wavelength is in a shorter wavelength range than the second
wavelength, wherein the first wavelength is in the ultraviolet
wavelength range and wherein the second wavelength is in the
visible wavelength range.
6. The radiation detection system of claim 1, wherein the at least
one light source is coupled to the first surface of the at least
one radiation sensor.
7. The radiation detection system of claim 6, wherein the at least
one light source is coupled to the first surface of the at least
one radiation sensor by means of one of a light pipe, a fiber optic
plate, and a tapered fiber bundle.
8. The radiation detection system of claim 6, wherein the optical
detector is coupled to the at least one radiation sensor, wherein
the optical detector comprises an optical sensor element having a
third surface, wherein the third surface is coupled to the second
surface of the at least one radiation sensor.
9. The radiation detection system of claim 8, wherein the optical
sensor element comprises a time integrating sensor, wherein the
time integrating sensor is one of a charge coupled device and a
complementary metal oxide semiconductor
10. The radiation detection of system claim 9, wherein the optical
detector is coupled to the at least one radiation sensor by means
of one of a light pipe, a fiber optic plate, and a tapered fiber
bundle.
11. A radiation detection system for detecting the presence and
location of a radiation source, the radiation detection device
comprising: at least one bundle of optical fibers of different
lengths, each fiber of the at least one bundle having a first end
and second end, wherein the first ends share a common longitudinal
location and the location of the second ends is determined by the
length of each fiber, wherein each fiber comprises a radiation
sensitive material, wherein the radiation sensitive material stores
energy when it receives radiation from the radiation source; at
least one stimulating source for supplying stimulation to the
radiation sensitive material, wherein the radiation sensitive
material releases the stored energy corresponding to the radiation
sensitive material's received radiation dose as a light output when
it is exposed to the stimulating source, the light output providing
a readout signal for each fiber, wherein the readout signal
corresponds in intensity to the radiation received by each fiber
from the radiation source, wherein the total energy of the
radiation dose received by each fiber depends upon the fiber
length; at least one optical detector for receiving the readout
signal, wherein the at least one optical detector is configured to
detect variations in intensity of the readout signals along the
length of the bundle and determine the general location of the
radiation source based on the intensity of the readout signal from
different members of the bundle having different lengths exposed to
the radiation source.
12. The radiation detection system of claim 11, wherein the
radiation sensitive material is an optically stimulated luminescent
material.
13. The radiation detection system of claim 11, wherein the
radiation sensitive material is a thermal luminescent detector
material.
14. The radiation detection system of claim 11, wherein the at
least one stimulating source is coupled to the first ends of the
bundle of optical fibers, wherein the at least one optical detector
detects the light output at said first ends.
15. The radiation detection system of claim 11, wherein the at
least one stimulating source is coupled to the first ends of the
bundle of optical fibers, wherein the at least one optical detector
detects the light output at the second ends.
16. The radiation detection system of claim 11, wherein each fiber
comprises a core having an outer wall and an inner wall surrounding
a transmission channel, wherein the inner wall is coated with a
cladding layer of radiation sensitive material.
17. The radiation detection system of claim 16, wherein the
cladding layer interacts with a fluorescent dye material located
within the core, wherein the fluorescent material absorbs a portion
of the stimulated light output released from the radiation
sensitive material and re-radiates light at a longer wavelength,
wherein the re-radiated light is substantially trapped within each
fiber and transmitted along the fiber to the at least one optical
detector.
18. The radiation detection system of claim 16, wherein the outer
wall is coated with a reflective coating, wherein the reflective
coating optically isolates each fiber and redirects a portion of
the stimulated light output released from the radiation sensitive
material back toward the transmission channel, wherein the
redirected light output is transmitted to the at least one optical
detector.
19. The radiation detection system of claim 11, wherein each fiber
comprises a hollow tube having an outer wall and an inner wall
surrounding a transmission channel, wherein the inner wall is
coated with a layer of radiation sensitive material, wherein the
outer wall is coated with a reflective coating, wherein the
reflective coating optically isolates each fiber and redirects the
stimulated light output released from the radiation sensitive
material back toward the transmission channel, wherein the
redirected light output is transmitted to the at least one optical
detector.
20. The radiation detection system of claim 19, wherein the hollow
tube is comprised of a glass substantially transparent in the
stimulated wavelength region.
21. The radiation detection system of claim 19, wherein the hollow
tube is comprised of a polymer substantially transparent in the
stimulated wavelength region.
22. The radiation detection system of claim 11, wherein each fiber
comprises a radiation sensitive material having particular
sensitivity to specific radiation energies, wherein each fiber
responds selectively to radiation in a specific range of
energies.
23. The radiation detection system of claim 11, wherein the at
least one optical detector detects a readout signal during a data
collection period, wherein the detection device comprises a
controller for controlling the timing of the data collection period
such that the detection of a readout signal having a radiation
energy below a threshold results in extending the data collection
period until the at least one optical detector detects a readout
signal having a radiation energy above a threshold.
24. The radiation detection device of claim 11, wherein the at
least one optical detector comprises an array of individually
readable light detectors arranged such that the individually
readable light detectors are coupled to individual fibers, wherein
the array comprises one of a charge coupled detector and a
complimentary metal oxide semiconductor array.
25. A radiation detection system for detecting the presence and
location of a radiation source, the radiation detection device
comprising: a two-dimensional array of optical fibers arranged in a
generally planar area, wherein the fibers comprise a radiation
sensitive material, wherein the radiation sensitive material stores
energy when it is exposed to radiation from a radiation source; at
least one stimulating source for supplying stimulation to the
radiation sensitive material, wherein the radiation sensitive
material releases the stored energy in the form of a light output
when it is exposed to the stimulating source; at least one optical
detector configured to examine the array of optical fibers that are
optically stimulated and output a signal comprising an image of
areas of the array of optical fibers exposed, wherein areas exposed
to radiation produce a lighter image and areas not exposed to
radiation produce a darker image, wherein the lighter and darker
images combine to produce an image of the radiation source suitable
for the identification of the energy and location of the radiation
source.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application of U.S. provisional patent applications
entitled INTEGRAL SELF-EXCITING OPTICALLY STIMULATED LUMINESCENT
RADIATION DETECTOR, application Ser. No. 60/849,306 and OPTICAL
FIBER-BASED RADIATION DETECTOR ARRAYS, application Ser. No.
60/849,244, both of which were filed on Oct. 4, 2006. The entire
contents of the above applications are incorporated herein by
reference.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention generally relates to a radiation
detection device. More specifically, the invention relates to
optical fiber-based radiation detector arrays for monitoring
nuclear radiation sources.
[0004] 2. Description of Related Art
[0005] There is a growing concern that terrorists, or others, may
attempt to import radioactive or nuclear material which may be used
for the construction of nuclear and/or radiation based weapons.
Because of this concern, these materials need to be either
controlled or monitored. Because of the large number of containers
transported in commerce, it is difficult to thoroughly check each
and every container for the presence of any type of radioactive or
nuclear material.
[0006] Therefore, it is desirable to be able to detect and identify
the presence of nuclear radiation sources within packaging or
containment that does not permit direct visual inspection. It is
desirable to be able to do so with both high detection sensitivity
and the ability to locate such radiation sources within the
containment or packaging when immediate physical access to the
interior of the packaging is not convenient or possible. It is
further desirable to do this simply, rapidly, and without the
necessity of large fixed facilities since, in the case of
intermodal shipping containers, it will be important to detect any
contraband before the container arrives in a destination port. It
is further desirable, in addition to detecting the presence of
nuclear radiating materials, to locate them in the object or
objects being inspected. Such a radiation detection device may also
be useful in Homeland security devices and/or automobile
checkpoints.
[0007] Conventional detectors, which detect nuclear radiation in
the forms of alpha, beta and gamma rays, are typically expensive,
have limited sensitivity, are physically fragile, have limited
life, making them unsuitable for widespread field deployment.
Typical issues involve aspects such as the need to have active
cooling in order to achieve high enough sensitivity with
solid-state detectors, high power requirements supplied at mains
(high) voltages as for traditional photomultiplier tube ("PMT")
devices, and physically stable platforms on which the detectors can
be mounted. Additionally it is, in many cases, necessary to collect
the data and send the detection apparatus to a distant laboratory
so that the read out of the data can be made in a thermally and
light controlled environment not easily achieved `in the field.`
The time delay associated with remote processing can prevent prompt
action on the results of the testing or monitoring, and that
sometimes presents a serious problem.
[0008] Some radiation detectors comprise solid state materials
which when interacting with gamma rays or high energy beta rays,
produce electron-hole ("E-H") pairs internally. In particular types
of material, the subsequent recombination of the E-H pairs produces
light output which has a photon energy characteristic both of the
material and the energy of the radiation which produced the E-H
pairs, and which can be detected and measured. Typical materials
include optically stimulated luminescent ("OSL") materials, such as
carbon doped aluminum oxide (.alpha.-Al.sub.2O.sub.3:C),
Al.sub.2O.sub.3:Cr, Mg, Fe, MgAl.sub.2O.sub.4 spinels,
Mg.sub.2SiO.sub.4:Tb, and natural fluorite, europium doped
flourochlorozirconate glass ceramics, or alkali impurity doped
BaFBr:Eu.sup.+2 and thermal luminescent detector ("TLD") material
types. Various examples of these materials can have ranges of times
and temperatures over which the separated hole-electron pairs are
stable. For these materials, when in their `excited` state, the
recombination can be stimulated to occur rapidly either by raising
their temperature (TLD) so as to speed up migration of the
electrons toward their nearby holes or by exposing them to an
incident light flux (OSL) which can stimulate the recombination and
consequent light emission. Furthermore, there are such materials
having, in the absence of external stimulation, different excited
state relaxation lifetimes and whose emissions upon recombination
are characteristic and identifiable by their emission wavelengths.
Additionally, there exist materials which are sensitive to the
energy of the nuclear radiation and whose emission wavelength also
depends on that energy.
[0009] In view of the above, it is apparent that there exists a
need for an easily packaged, compact radiation detector device
efficient in data readout and operable for long periods on low
power. There further exists a need to be able to detect and
identify the presence and location of nuclear radiation sources
within packaging or containment that does not permit direct visual
inspection. Additionally, this detection must be done simply,
rapidly, and without the necessity of large fixed facilities since,
in the case of intermodal shipping containers it will be important
to detect any contraband before the container arrives in a
destination port. Further, monitoring boundaries of nuclear
facilities or of countries for covert transit of nuclear materials
is also an important need.
SUMMARY
[0010] In satisfying the above need, as well as overcoming the
enumerated drawbacks and other limitations of the related art, the
present invention provides an improved, efficient, easily packaged,
and compact radiation detection device. The present invention
further provides an optical-fiber based radiation detection device
for detection of both the presence and the location of the
radiation source.
[0011] In one embodiment of the present invention, the radiation
detector has a layered structure. The layered structure is created
by deposition of successive layers comprising a light source, a
radiation sensor, and an optical detector. The radiation sensor
comprises a radiation sensitive material, such as an OSL material,
which stores energy when it is exposed to a radiation source. It
should be understood that throughout this summary, and detailed
description of this application, OSL material is interchangeable
with TLD material. The OSL material releases the stored energy in
the form of a light output in a characteristic wavelength range
corresponding to the OSL material and in specific embodiments to
the detected radiation source energy and in intensity to the amount
of radiation it has received, in response to stimulation produced
by activating the light source, such as a light emitting diode
("LED") source, a field emission display ("FED") source, an organic
light emitting diode ("OLED") source, a vacuum tube fluorescent
("VFD") source, a light emitting plasma discharge tube or a laser,
operating at a different wavelength than that of the light output.
The light output provides a readout signal which is used to
determine the energy of the radiation from the radiation source. An
optical detector comprising an optical sensor element receives the
readout signal and measures the intensity of the light output. An
optical band-blocking filter is interposed between the radiation
sensor and the optical detector for blocking the stimulating light
energy provided by the stimulating light source from interacting
with the optical detector.
[0012] In another embodiment, the wavelength of the stimulating
light energy from the light source is in a longer wavelength range
such as the infrared range while the wavelength of the light output
is in a shorter wavelength range such as the visible range.
[0013] In another embodiment, the wavelength of the stimulating
light energy from the light source is in a shorter wavelength range
such as the ultraviolet range while the wavelength of the light
output is in a longer wavelength range such as the visible
range.
[0014] In another embodiment, the radiation sensor is coupled to
the light source at one end and to the optical sensor element of
the optical detector at another end. The radiation sensor is
coupled to the light source and/or the optical detector by means
commonly known in the art, such as through a light pipe, a fiber
optic plate, or a fiber optic bundle. The optical sensor element
comprises a time integrating sensor or array, such as a charge
coupled device ("CCD"), a image intensified CCD, a complementary
metal oxide semiconductor ("CMOS"), a photodiode or a
photomultiplier ("PMT") or a positive-intrinsic-negative "PIN"
photodiode.
[0015] In yet another embodiment of the present invention, the
radiation detection device detects both the presence and the
location of a radiation source. The radiation detection device
comprises one or more bundles of optical fibers of different
lengths, wherein the fibers share a common end position. Each fiber
also comprises a radiation sensitive material, such as an OSL
material or a TLD material. The radiation sensitive material stores
energy when it receives radiation from the radiation source. A
stimulating source located at one end of the bundle stimulates the
radiation sensitive material wherein the radiation sensitive
material releases the stored energy in the form of a light output.
The light output provides a readout signal for each fiber, wherein
the readout signal corresponds in intensity to the radiation
received by each fiber from the radiation source. The intensity of
the radiation received by each fiber depends upon the fiber length.
An optical detector receives the readout signal, detects variations
in intensity of the readout signals from fibers of different
lengths, or even bundles of fibers of different lengths, along the
length of the bundle or bundles and determines the general location
of the radiation source based on the intensity of the readout
signals from the fibers or fiber bundles.
[0016] In another embodiment, the stimulating source is coupled to
the fibers at their common end, wherein the optical detector
detects the light output at the same end.
[0017] In another embodiment, the stimulating source is coupled to
the fibers at their common end, wherein the optical detector
detects the light output at the opposite end.
[0018] In another embodiment, each fiber comprises a core having an
outer wall and an inner wall surrounding a transmission channel,
wherein the inner wall is coated with a cladding layer of radiation
sensitive material. The cladding layer interacts with a fluorescent
dye material located within the core. The fluorescent material
absorbs the stimulated light output released from the radiation
sensitive material and re-radiates light at a longer wavelength,
wherein the re-radiated light is substantially trapped within each
fiber and transmitted to the optical detector. The outer wall is
coated with a reflective coating, wherein the reflective coating
optically isolates each fiber and further redirects the stimulated
light output released from the radiation sensitive material back
toward the transmission channel and the redirected light output is
transmitted to the optical detector.
[0019] In another embodiment, each fiber comprises a hollow tube
having an outer wall and an inner wall surrounding a transmission
channel, wherein the inner wall is coated with a layer of radiation
sensitive material and the outer wall is coated with a reflective
coating. The reflective coating optically isolates each fiber and
redirects the stimulated light output released from the radiation
sensitive material back toward the transmission channel and the
redirected light output is transmitted to the optical detector. The
hollow tube is either comprised of glass or a polymer.
[0020] In another embodiment, each fiber comprises a radiation
sensitive material having a particular sensitivity to specific
radiation energies, wherein each fiber responds selectively to
radiation of particular energy or energies.
[0021] In another embodiment, the radiation detection device
comprises a controller. The optical detector detects a readout
signal during a data collection period and the controller controls
the timing of the data collection period such that the detection of
a readout signal having a radiation energy below a threshold
results in extending the data collection period until the optical
detector detects a readout signal having a radiation energy above a
threshold.
[0022] In another embodiment, the optical detector comprises an
array of individually readable light detectors, such as a CCD or a
CMOS, arranged such that the individually readable light detectors
are coupled to each fiber or fiber bundle.
[0023] In another embodiment, the radiation detection device
detects the presence and the location of a radiation source. The
radiation detection device comprises a two-dimensional array of
optical fibers arranged in a generally planar fashion. The fibers
comprise a radiation sensitive material, such as an OSL or a TLD
material, which stores energy when it is exposed to radiation from
the radiation source. A stimulating source stimulates the radiation
sensitive material, wherein the radiation sensitive material
releases the stored energy in the form of a light output. An
optical detector images the array of fibers. The imaging detects
the areas of the array exposed to radiation, wherein the areas
exposed to radiation emit a light output and the areas not exposed
to radiation appear darker. The light and dark images produce an
image of the radiation source suitable for the identification of
the energy and location of the radiation source.
[0024] Further objects, features and advantages of this invention
will become readily apparent to persons skilled in the art after a
review of the following description, with reference to the drawings
and claims that are appended to and form a part of this
specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a perspective view of a layered structure of one
embodiment of a radiation detection device;
[0026] FIG. 2 is a perspective view of the layered structure of the
radiation detection device;
[0027] FIG. 3 is a perspective view of the layered structure of a
another embodiment of the radiation detection device;
[0028] FIG. 4 is a schematic diagram of another embodiment of the
radiation detection device having an optical fiber bundle;
[0029] FIGS. 5A and 5B are schematic diagrams of end and side views
of the optical fiber bundle of the radiation detection device of
FIG. 4;
[0030] FIG. 6 is a schematic diagram of another embodiment of the
optical fiber bundle of the radiation detection device;
[0031] FIG. 7 is a schematic diagram of an optical fiber for use
with a radiation detection device;
[0032] FIGS. 8a-8c are cross-sections of different embodiments of
an optical fiber; and
[0033] FIG. 9 is a schematic diagram of another embodiment of the
radiation detection device which employs an array of fibers or
tubes.
DETAILED DESCRIPTION
[0034] FIG. 1 illustrates a radiation detection system 10a, The
radiation detection system 10a is a layered structure created by
deposition of successive layers. The radiation detection device 10a
includes a light source 12 acting as the first layer. A radiation
sensor 14 acts as a second layer and includes a radiation sensitive
material 16, such as an OSL material. An optical detector 18 act as
the fourth layer, while an optical filter 20, interposed between
the radiation sensor 14 and the optical detector 18, acts, as the
third layer.
[0035] The optical detector 18 has an optical sensor element 19 for
receiving a signal from the radiation sensor 14. The optical
detector may comprise a collection of discrete sensor elements,
such as a CCD, a image intensified CCD, a CMOS, a PMT or a PIN
photodiode, either in a linear array or in a two-dimensional array
to span and address a corresponding array of radiation sensor
elements. Other types of solid-state optical sensors may also be
used within the scope of this invention. The OSL material can also
be provided in the form of small crystals comprising a base carrier
material doped with one or more crystalline dopant materials whose
optical properties constitute radiation sensitive and subsequently
stimulation capable elements. A layer comprising such small
crystallites may be placed within, or on, an inert binder material
in the place of the thin deposited film described above. Radiation
specific sensitizers may be added to increase the absorption
cross-section or adjust radiation sensitivity of the OSL
material.
[0036] For readout, the radiation sensitive material 16 is exposed
to radiation 21 from a radiation source 22. The radiation sensitive
material 16 stores as energy, radiation 21 received from the
radiation source 22. The light source 12 provides a stimulating
light energy (possibly in the form of a light pulse) in one
wavelength range to the radiation sensitive material 16.
Thereafter, the energy stored in the radiation sensitive material
16 is released in the form of a light output in a different
wavelength range corresponding in intensity to the amount of
radiation it has received. The light output provides a readout
signal which is used to determine the energy of the radiation 21
from the radiation source 22. The stimulating light energy is
prevented from reaching and activating the optical detector 18 by
the optical filter 20 interposed between the radiation sensor 14
and the optical detector 18. While operating the light source 12 in
the infrared is convenient, the critical factor is that the
stimulating light can be blocked from or produced in a wavelength
range such that the optical detector 18 is not excited by the
output of the light source 12.
[0037] The readout signal may include a plurality of readout
events. A single readout event may include of one or multiple
pulses of the light source 12 with or without time-coincident
gating of the optical detector 18. The readout signal may be
scheduled to occur at fixed or programmed intervals. Sensitivity is
greatly improved over conventional scintillation counter approaches
because signal integration is performed directly by the material
itself rather than being delivered as a prompt signal with
accumulation done electronically based on low level light outputs
for individual radiation capture events. The total light output
constituting the readout signal arises from the radiation signal
integrated over the time period between read-outs, rather than
being a prompt signal.
[0038] FIG. 2 illustrates another embodiment of the radiation
detection device 10b having a layered structure and comprising an
array of radiation sensors 14 with a closely coupled array of
optical detectors 18. FIG. 3 illustrates another embodiment of the
radiation detection device 10c which allows the optical sensor
element 19 and the optical detector array 18 to have an
unobstructed view of the radiation whose detection is desired. With
appropriate shielding from the optical filter 20, the stimulating
light is prevented from reaching the optical detector 18.
[0039] Referring to FIGS. 2 and 3, the radiation sensor 14 and the
optical detector 18 are essentially in contact in a package design.
Approximately half of the entire light output, which is generally
emitted uniformly in all directions, is available to the optical
detector 18. The readout signal level reaching the optical detector
18 is thus higher than for an optical detector such as a
photomultiplier tube or discrete photodiode counter whose sensitive
elements are located at greater distances from the detector
element. This means that the requirements, and thus cost, of the
optical signal detection system are lowered.
[0040] The radiation detection devices 10a-10c of FIGS. 1-3 are
exposed to the radiation source 22 for a period of time generally
chosen to meet one of two requirements: 1) the exposure period
corresponds to the potential danger limit if the suspected source
intensity may be high; 2) if the suspected source to be inspected
is not sufficiently intense to present an immediate hazard, the
exposure period is chosen to be sufficiently long to accumulate
adequate data, in the form of radiation induced electron-hole pairs
in the optical detector 18, to allow characterization of the
radiation source 22 for the purposes of identification. The ability
to modify the test exposure/data accumulation period over a very
wide range can be utilized to autonomously adapt the optical
detector 18 to the measurements being taken.
[0041] The timing of the readout events and their procedure,
including the duration of the light pulses, their intensity and
their spacing may be specified and varied as needed by programmable
software and a digital controller incorporated directly into the
integrated detector system. It is also within the scope of this
invention for the digital controller and its associated software to
accept the data from the radiation detection devices 10a-10c as
feedback for active, responsive control of the readout spacing.
Under such adaptive control, upon first entry of the radiation
detection devices 10a-10c into an area with unknown radiation
hazard the readout rate may be set to a short interval, as for
example a few seconds. If the sensed radiation level is low or
below threshold in that short data collection period, the data
collection period would be programmed to lengthen in a series of
stages until exposure for the time period gives adequate data for a
well characterized level. Lengthening the exposure time and thus
the energy expenditure in reading out the data can extend battery
life for portable devices. Further, adaptive timing can, in the
presence of a higher than expected radiation level, shorten the
exposure/data collection time in order to prevent saturation of the
sensing system and allow an accurate reading to be achieved.
Detection of the full-scale reading with a given exposure period
would then trigger a reduction in the exposure time for subsequent
measurements. An alarm indication would be provided as a hazard
warning in such cases as indicated by standard exposure limitation
requirements.
[0042] While physically intimate contact between the basic elements
of the radiation detection devices 10a-10c are generally useful,
many of the same advantages can be obtained within the scope of
this invention by interposing a light pipe, fiber optic face-plate,
or a tapered or non-tapered fiber bundle between the light source
12 and the radiation sensor 14 requiring optical stimulation.
Possible utility may be gained if packaging, in particular designs,
can be enhanced by this means to allow changes in the relative
physical orientation of the light source 12 and the radiation
sensor 14. Additional enhancements may incorporate an array of
small lenses coupled to each detector element or a self-focusing
array to enable improvements in light throughput and/or to minimize
crosstalk between channels. In addition, a tapered fiber optic
faceplate may allow for substantially more independent OSL channels
allowing for enhancements in time domain of the radiation data.
[0043] The integrated design of the radiation detection devices
10a-10c of FIGS. 1-3 is compact, easily packaged, rugged,
inexpensive, efficient in data readout, and can be operated for
long periods on low power. By comparison with conventional systems,
the approach with the radiation sensor 14 essentially in contact
with the optical detector 18, the radiation sensitive material 16
transmits to the optical detector 18 a much larger fraction of the
released light output than is possible with conventional
photomultiplier tube or discrete light detector systems simply due
to the larger mutual solid angle subtended by the sensor and
detector.
[0044] FIG. 4 illustrates, another embodiment of a radiation
detection device 10d, wherein the radiation detection device 10d
detects the presence and the spatial location of a radiation source
22 inside a container 23. The radiation detection device 10d
comprises one or more 24 of optical fibers 26 of different lengths
L1-L6 (L1 being the longest while L6 is the shortest), wherein the
fibers share a common end 30. It should be understood that the
optical fibers 26 may represent more than one individual optical
fiber. To put it another way, the optical fibers 26 may represent a
sub bundle of fibers located within the bundle 24. For simplicity's
sake, this disclosure will refer to the optical fiber or sub
bundles of optical fibers simply as "optical fibers 26." Each of
the optical fibers 26 comprise a radiation sensitive material, such
as an OSL or a TLD material. The radiation sensitive material
stores energy when it receives radiation from the radiation source
22. At least one stimulating source 32 located at the common end 30
of the optical fibers 26 stimulates the radiation sensitive
material wherein the radiation sensitive material releases the
stored energy in the form of a light output. The light output
provides a readout signal for each fiber 26, wherein the readout
signal corresponds in intensity to the radiation received by each
fiber 26 or bundle from the radiation source 22. The intensity of
the radiation received by each fiber 26 or bundle depends upon the
fiber length. At least one optical detector, located at the common
end 30 of the fibers 26, receives the readout signal, detects
variations in intensity of the readout signals from fibers of
different lengths within the bundle 24 and determines the general
location of the radiation source 22 based on the intensity of the
readout signals.
[0045] It is desired not only to detect nuclear radiation in a
container 23, but to locate the source 22 spatially. This may be
accomplished by placing a bundle 24 of fibers 26 having, according
to this invention, sensitized fibers 26 of different lengths
therein, along one or more sides of the container 23. If the
container 23 has within it a localized source 22 of nuclear
radiation, because the fibers 26 within the bundle(s) 24 are chosen
to have different lengths, there will be regions of space along the
length of the bundle for which some fibers 26 will not receive the
full intensity of the nuclear radiation and hence will provide a
reduced or no detection signal. When the fibers 26 are interrogated
by optical or thermal stimulation as described above, examination
of the signals will indicate which fibers 26 received radiation and
hence where the source 22 is located along the bundle 24. If the
detector fiber bundle 24 of this invention is in or on, for
example, an intermodal shipping container, the placement of a
localized source 22 can be identified along the direction of the
fiber bundle 24. Placement of additional fiber bundles 24
orthogonally can serve to locate the source in 2 or 3 dimensions.
Because fiber optic bundles 24 are inexpensive, wide deployment can
be more cost effective than discrete sensors.
[0046] As with other radiation detection devices and data storage
systems, the ability to ensure the reliability of such detectors
can, in principle, be compromised by enemy action as, for example,
by resetting the radiation sensitive materials by exposing them to
sufficient light or heat, thus leading the investigator to conclude
that there was no, or little, nuclear radioactive material present.
To avoid this it is desirable to incorporate a modification
comprising incorporation, in close proximity to one or more of the
sensitized elements, a low intensity, localized, radiation source
which produces, in these same sensitized elements radiation damage
in the form appropriate for reading out by optical or thermal
stimulation, a consistent, minimum signal, below the saturation
level of the sensitized elements and independent of radiation
exposure from external sources. The integrated detector, as a
matter of normal practice, records the elapsed time since official
resetting and thus the signal recorded from the low intensity
source should, in the absence of tampering, have a known minimum
value. If the readout value from these same elements is
significantly smaller than this expected value, an inappropriate
reset has occurred and an alert notification can readily be
processed. Additionally, the actual readout values of the sensor
elements monitoring the standard source contains the information as
to when the reset occurred. In the case of the container 23, this
would, together with a known itinerary of the container 23, locate
the place where it occurred.
[0047] Thus in practice of this invention, it is desirable to
incorporate, with the sensor, a known, small, low level gamma or
beta source that provides a known background data level that is
small enough not to interfere with a signal level that would
indicate a serious problem. The presence of this signal must be
observed or the investigator will conclude that an erasure has been
made and the shipment must be physically inspected.
[0048] A second approach is to incorporate both sensitized sensor
materials and the low intensity source in a "paint" that can be
applied, as one example of an application, to the surface of the
container 23. If the container 23 contains nuclear radioactive
materials, the gamma radiation will interact with the paint to
produce a latent dose measurement which can later be queried by
exposing the paint (containing the now excited electron hole pairs)
to either heat or light of an appropriate wavelength depending on
the choice of detector materials, either OSL or TLD. The queried
material then emits light from such pairs as they recombine and the
amount of light emitted indicates the strength of the gamma source.
If a number of `patches` or `stripes` of this sort are placed on
the container, the approximate location of the source 22 may also
be determined in a manner similar to that of the doped optical
fiber detector system described herein. Portable devices suitable
for querying the data `stored` in such detector `paint` surfaces
can be assembled from components of moderate sensitivity because
the intensity of the stimulated light emission, in consideration of
the rapid read out of time integrated stored data, is higher than
for prompt reading detectors such as solid or liquid state
scintillation or Geiger-Muller counters. Imaging detectors of the
general type used in digital cameras are suitable for the
purpose.
[0049] The excited state half-lives, in the absence of external
stimulation, of the different suitable detector materials of the
OSL and particularly the TLD types useful for the present invention
are known to vary widely. The use of materials of different,
un-stimulated, half life is important as it provides temporal
information as to when the radiation source first interacted with
the paint. Reading out the radiation dose from a long half-life
material gives one the total dose received. Knowing the half life
for the shorter life materials and reading them out identifies when
the exposure occurred and whether it was from the beginning of the
testing period (originating port for example) or at a later time
(and hence a different, but, considering the container itinerary,
definable port). Use of several materials having different time
lines gives enough data to determine the history.
[0050] In one implementation, the subject invention utilizes one or
more bundles of dosimetrically doped optically stimulated
luminescent (OSL) fibers of different lengths which are queried
from one end of the bundle 24 using a pulsed light source 32 which
illuminates the fibers 26 and an imaging detection mechanism
reading the secondary light emission from the fibers 26 either at
the same or at the opposite end of the bundle 24. The detection
system imaging the ends of the fibers 26 records the emission
intensity and/or wavelength from the individual fibers 26 or
bundles 24.
[0051] As can be seen in FIG. 4, the source 22 will most strongly
impinge upon the longer fibers 26 (lengths L1-L4) and to a lesser
extent upon the shorter fibers 26 (lengths L5-L6). The subsequently
detected signal from the latter fibers will thus be of lower
intensity than for the former set.
[0052] Referring to FIGS. 5a-5b, another configuration of the fiber
26 is shown. It is noted that FIG. 5a shows the fibers 26 from a
front view, while FIG. 5b shows the fiber 26 from a side view. As
shown in FIGS. 5a-5b, the fibers 26 in the bundle 24 can be
arranged `in depth` against a container wall 23 or the surface of
another object of interest.
[0053] When the examined object emits nuclear radiation detectable
by the OSL dopant in the fiber bundle 24, the signal is read out by
inserting an optical pulse at the end as shown in FIG. 4. Because
the stimulated light emission is chosen to differ in wavelength
from the incident stimulating light, the detector may be protected
from the stimulating radiation by suitable optical filters 20 known
in the art. Alternatively, the stimulated emission is delayed
relative to the stimulating source pulse 32 to permit the two
signals to be separated by electronic gating of the optical
detector elements 18 without the need for the filter(s) 20. The
stimulation source 32 and the detector may be located at the common
end 30 site of the fibers 26 as indicated in FIG. 4, or at opposite
ends of the fiber 26 as shown in FIG. 6. For fibers 26 in which the
OSL or TLD material is placed as a dopant within the whole body of
the fiber, the concentration of the dopant materials must be chosen
such that attenuation or scattering of the emitted signal radiation
along the length of fiber required to survey the object of interest
due to the dopant presence does not unacceptably deteriorate the
received signal at the optical detector.
[0054] Optical fibers 26, including glassy but more particularly
polymer-based optical fibers 26 may be doped with a wide variety of
optically active materials. In the present invention such fibers 26
are doped with a radiation sensitive material 16, such as an OSL
material, at concentrations such that optical transmission of the
emission spectrum of the radiation sensitive materials 16 down the
length of the fibers 26 is maintained at a known level, sufficient
to allow intensity measurements when the radiation sensitive
material 16 is queried with a light pulse. Because there is some
overlap in the optical emission and absorption spectra of the
radiation sensitive materials 16, transmission attenuation of the
transmitted intensity will occur. The distance over which such
transmission levels must be maintained will depend both on the
length of fiber 26 required to collect data over the extent of the
object(s) to be examined and the concentration of the active
detection chemicals in the fiber 26. For internally doped fibers 26
and in the case of conventional intermodal shipping containers 23
said distance may be limited to 20 to 60 feet for the longest
fibers 26 in the bundle 24 by the choice of radiation sensitive
materials 16 and concentrations. Detailed values for the
attenuation coefficients may be obtained for the fibers 26 of a
bundle 24 by calibration using known radiation sources 22 and these
incorporated into the readout computation on a fiber 26 by fiber 26
basis if desired for higher resolution.
[0055] In specific designs for detection systems employing the
principles of the present invention it may be desirable to utilize
subsets of doped fiber bundles 24 in which the active, sensitive
elements of separate bundles 24 are chosen to respond most
sensitively to the radiation from particular isotopes of interest.
This will allow determination of the type of radiation as well as
its location within a container. If an emphasis on reducing false
alarms due to detection of radiation from known benign sources such
as, by example, ceramic tiles, bananas etc. is desired, one or more
fiber bundles 24 may be doped so as to respond to the specific
radiation energies associated with the benign sources it is desired
to account for. Detection of enhanced signals from such
specifically selected fiber dopants at the same location as found
by the more general detection fibers 26 would allow for selection
against false alarms due to benign sources. Common commercial
imaging sensors such as those used in consumer digital cameras can
easily accommodate separate detection of a large number of
identifiable fibers or subsets of doped fiber bundles, each
directing optical signals to one or more of the pixels of a
two-dimensional CCD or CMOS imaging array.
[0056] The arrangement described above can, with modifications that
will suggest themselves, be used to incorporate and interrogate
radiation sensitive materials 16, such as TLD materials or OSL
materials described here. While TLD materials often have shorter
un-stimulated half-lives than most common OSL materials, this
property can be utilized to establish timing of an exposure as
discussed above. While direct optical stimulation is feasible, it
may under particular circumstances, be preferable to use TLD
materials whose stimulation can also be provided by heating due to
the light provided by LEDs or other light sources.
[0057] Referring back to FIG. 4, the fiber bundles 24 most
convenient for implementing the present invention, consist of
"coherent" fibers such that the physical arrangement of the fibers
26 within the bundle 24 is maintained through and at both ends of
the bundle 24. Such coherent optical fiber bundles 24 are used in
optical borescopes and some other remote inspection devices
requiring image transfer, fiber optic data transmission lines and
other applications. The use of coherent bundles 24 allows the
radiation detection data from each fiber 26 or appropriate group of
fibers 26 to be collected and identified separately. In one
particular case, the fibers 26 will have different lengths and thus
allow, as described above, for the location of the radiation source
22 by noting which fibers 26 were most heavily irradiated and the
locations of their radiation sensitive materials 16. Longitudinal
location of nuclear radiation sources 22 may be done with a single
fiber system. If 2 or 3 dimensional information with respect to a
container is desired, orthogonal fiber systems may be utilized and
the 3-D image resolved in software or electronic hardware.
[0058] While coherent fiber bundles 24 are a most straightforward
way to implement the invention, they are somewhat expensive.
Another approach, in keeping with this invention is to use a
simple, incoherent fiber bundle 24 and calibrate it so that the
correlation between fiber position and the 2-D location at the end
of the bundle 24 can be deduced. In this context, calibration
consists of exposing the ends of the fiber bundle 24 to a matrix of
light sources 12 coupled to the individual fibers 26 and recording
the particular imaging sensor elements 19 to which each fiber 26
corresponds. Since the length of each fiber 26 and its position as
seen by the imaging sensor are then known, analysis of the
radiation source 22 input location may then be made easily with a
simple correlation table. Such tables are known in the art and can
be implemented, for example, using programmable gate arrays.
[0059] Referring to FIG. 7, the OSL (or TLD) material of the fiber
26 is excited by nuclear radiation and enters a trapping state in
which quasi-stable electron hole pairs are created and their
numbers increase over time as the nuclear radiation dose is
integrated over time. When the dose is to be read, the OSL material
is probed with the appropriate optical stimulation source,
preferably in the infrared ("IR") region of the spectrum (0.7-1.1
micron) and emits shorter wave length visible photons corresponding
to the dose received. These photons are radiated approximately
isotropically with a fraction passing through the doped or cladding
layer 42 and eventually interacting with a fluorescent dye molecule
in core 34 of one of the fibers 26, for example, but not by
limitation, rhodamine6G-R6G, Rhodamine-B or Coumarin-8 embedded in
the core 34 material chosen from one or more classes of optically
transparent materials such as a glass or Polymethyl Methacrylate
("PMMA"). In operation, the fluorescent dye molecules absorb the
short wavelength OSL emission radiation and re-radiate lower
energy/longer wavelength light essentially isotropically, with a
significant fraction (approximately 70% for glass fibers in air)
being confined to one of the fibers 26 by total internal reflection
("TIR") and transmitted down one of the fibers 26 with little loss.
By this means it is possible to transmit a significantly larger
fraction of the emitted OSL radiation to the detector 18.
[0060] Referring to FIGS. 8A-8C, a cross section of one of the
fibers 26 is shown. As noted above, the internal doping of the
entire inside of the optical fibers 26 with OSL materials presents
an attenuation mechanism for transmission of the light emitted by
the detecting OSL or TLD materials and this may limit the
practical, useful length of the fiber bundles 24. Referring to FIG.
8A, the fiber includes an optical fiber core 40, surrounded by an
OSL (or TLD) doped cladding layer 42, further surrounded by a
reflective coating 44.
[0061] An alternative method utilizes fibers prepared as in FIG. 8B
in which the transmission channel 41, is essentially clear. A
surrounding OSL (or TLD) cladding layer 43 is doped with the active
sensor material and is made to have an index of refraction lower
than that of the transmission channel 41. An outer coating 45 is a
reflective coating which optically isolates the fiber from its
neighbors and redirects externally directed emission from the OSL
(or TLD) cladding layer 43 back toward the transmission channel 40.
With composite fibers of this type, the radiation detection signal
is obtained in the OSL (or TLD) cladding layer 43 deposited on the
outer wall of the optical fiber but established in optically
intimate optical contact with said fiber during manufacture.
Transmission of the radiation responding emitted signal can proceed
down the clear central core 41 with low attenuation thus permitting
greater useful detector lengths. It may be seen that a composite
fiber containing the OSL (or TLD) material as a doped region
outside of a clear central core as in FIG. 8B, can function as
indicated above for a coated fiber. Stimulated light emission from
the OSL cladding layer 43 will be largely inserted into the clear
fiber and transmitted to the image sensor at its end with little
attenuation beyond that occasioned by frustrated TIR in the OSL
coating itself. Properly designed, this attenuation effect is
minimal. The stimulating light source 32 may be coupled directly to
the clear fiber core 34 and through the established intimate
optical contact, coupled with the OSL cladding layer 43. Decoupling
of the signals in adjacent fibers 26 or fiber bundles 24 may also
be accomplished by the usual methods utilized in the optical fiber
communication industry.
[0062] Alternatively, the fiber, as shown schematically in FIG. 8C,
may comprise a hollow tube 46 of glass or a polymer on the interior
wall 38 of which is disposed a layer of OSL (or TLD) material. The
exterior of the tube 46 may be coated with a reflective layer 47 to
redirect the outward-directed portion of the readout emitted light
back into the tube interior along which it is guided to the optical
sensors 18 as with the other designs described herein. The OSL
cladding layer 49 may be applied to the interior wall 38 of hollow
tube 46 by filling the tube 46 with a slurry solution of the OSL
material and pouring it out or by allowing the excess to drip out,
followed by drying and baking at a temperature suitable to promote
adhesion to the wall. Alternatively, sol-gel processes may be
employed as is known in the art. Approaches to establishing layers
of phosphors on the interior of glass tubes are well known in the
manufacture of fluorescent light bulbs. Such methods are easily
controlled in current practice.
[0063] In the case of the hollow tube 46 design of FIG. 8C, the
light stimulation can be provided by several means, including, but
not by limitation, insertion of an intense light beam into the tube
46 either by flash tube or laser, or by electronic or microwave
excitation of a plasma discharge light source within the tube as
can occur in fluorescent light bulbs. If desired, it is possible to
excite individual tubes selectively by means similar to those used
for plasma television displays. Addressing subsets of fibers for
stimulation allows for independent measurements to be made and use
of the resulting redundancy capability to enhance reliability and
sensitivity.
[0064] Another possible approach to enhancing the signal to noise
ratio of the detector of this invention is to employ within the
fluorescent dye doped fibers, a distributed feedback internal
`grating` comprising a repeated, regular, index of refraction
variation along the length of the fiber in such a manner as to
cause lasing action to take place when sufficient light emitted
from the OSL sensor material impinges upon the fiber following
optical stimulation. When lasing action occurs the coherent light
transmitted down the fiber 26 can be focused by lens or other means
to as small as a diffraction limited `spot` at the detector/imager
whether said detector/imager is of CCD or CMOS type. The resulting
much higher intensity spot provides a much higher signal to
background noise ratio thus simplifying and making less expensive,
the detection process and hardware by reducing the relative effect
of background light.
[0065] The fibers 26 may be coated with, or separated from each
other, by material particularly chosen to attenuate nuclear
radiation in general or of particular energies. Such choices
provide, in the readout of the light emission from a bundle 24, an
indication of the direction of the source 22 as well as its
strength, energy and longitudinal position. Digital processing of
the signal from individual fibers 26 or bundles 24 of fibers of
similar lengths, can be accomplished with techniques well known in
image processing software for adjusting contrast, resolution and
brightness in digital images.
[0066] Referring to FIG. 9, another alternative embodiment employs
an array 48 of fibers 26 or tubes arranged in a near planar
fashion. Each of the fibers/tubes is generally as described in the
previous teachings of this invention and comprises a mechanical
substrate coated on the interior or exterior surface(s) with an OSL
or TLD layer suitable for electronic, light or plasma discharge
stimulation. After the 2-D array 48 of tubes/fibers is exposed to
the nuclear radiation such that the OSL material collects the
radiation data over a period of time as electron-hole pairs are
excited, the reading of said collected data is accomplished by
stimulating the OSL material in all of the array tubes. The
emission of light from the recombination is then examined by
imaging the array 48 surface with a conventional instrument such
as, by example but not by limitation, a CMOS or CCD digital camera
sensor. Irradiated areas 50 of the array 48 which have been exposed
to nuclear radiation will emit light appropriately and those which
have not been so exposed will be observed to be darker. This
produces an image of the source suitable for its identification and
location.
[0067] The use of different OSL sensor materials having particular
sensitivity to nuclear radiation of particular types and energies
may be incorporated into separate tubes such that different tubes
respond selectively to radiation of different energies. The use of
a 2-D imaging sensor permits identification of the specific
responses and thus identification of the nuclear radiation sources
as well as their location. Current technology now exists to prepare
a pixilated array of plasma excitable elements which can then give
higher resolution images of the irradiated region(s). In practice
such arrays 48 are operated by first exposing them to the possible
radiation source(s) for an appropriate time depending on the
expected or relevant source intensity, followed by stimulation of
the entire array 48 and imaging of the resultant light emission.
The resultant image delineates the dose and its intensity as a
function of position thus locating the source. Use of two or more
such arrays 48 arranged orthogonally allows 3-D location of sources
within their area coverage.
[0068] The 2-D arrays 48 described are also sensitive to X-ray
radiation and the storage aspects of the dosimetric fibers/tubes
disclosed permit enhanced signal to noise levels in imaging as
called for in X-Ray examinations. The ability to do this allows
lower dosages for patients in medical service or otherwise
sensitive objects in other diagnostic scenarios.
[0069] As a person skilled in the art will readily appreciate, the
above description is meant as an illustration of implementation of
the principles of this invention. This description is not intended
to limit the scope or application of this invention in that the
invention is susceptible to modification, variation and change,
without departing from spirit of this invention, as defined in the
following claims.
* * * * *