U.S. patent application number 12/372058 was filed with the patent office on 2013-04-25 for compact thermal neutron monitor.
The applicant listed for this patent is Lee GRODZINS. Invention is credited to Lee GRODZINS.
Application Number | 20130099125 12/372058 |
Document ID | / |
Family ID | 48135215 |
Filed Date | 2013-04-25 |
United States Patent
Application |
20130099125 |
Kind Code |
A1 |
GRODZINS; Lee |
April 25, 2013 |
COMPACT THERMAL NEUTRON MONITOR
Abstract
A thermal neutron monitor includes at least one neutron
scintillator sheet interposed between light guides. Scintillation
light emitted in opposite transverse directions is captured by the
light guides and conveyed to a common detector. The sandwiched
geometry of the monitor avoids the need to provide multiple
detectors and permits construction of a relatively inexpensive,
compact monitor.
Inventors: |
GRODZINS; Lee; (Lexington,
MA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
GRODZINS; Lee |
Lexington |
MA |
US |
|
|
Family ID: |
48135215 |
Appl. No.: |
12/372058 |
Filed: |
February 17, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10861332 |
Jun 4, 2004 |
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12372058 |
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60476101 |
Jun 5, 2003 |
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Current U.S.
Class: |
250/362 ;
250/367; 250/368 |
Current CPC
Class: |
G01T 3/06 20130101; G01T
1/2008 20130101 |
Class at
Publication: |
250/362 ;
250/368; 250/367 |
International
Class: |
G01T 3/06 20060101
G01T003/06; G01T 1/20 20060101 G01T001/20 |
Claims
1. A thermal neutron monitor, comprising: at least one thermal
neutron scintillator sheet having opposed first and second major
surfaces, the thermal neutron scintillator sheet emitting
scintillation light in opposite transverse directions responsive to
capture of a thermal neutron; first and second light guides
respectively positioned adjacent to the first and second major
surfaces of the thermal neutron scintillator sheet, wherein each of
the first and second light guides receives a portion of the emitted
scintillation light; and a common detector, optically coupled to
the first and second light guides, for detecting the scintillation
light conveyed thereto through the first and second light guides
and responsively generating a signal representative of the
intensity of the scintillation light.
2. The thermal neutron monitor of claim 1, wherein the at least one
thermal neutron scintillator sheet comprises a plurality of thermal
neutron scintillator sheets, each one of the thermal neutron
scintillator sheets being positioned between a corresponding pair
of light guides.
3. The thermal neutron monitor of claim 1, wherein the first and
second light guides are formed from or doped with a hydrogenous
material to thermalize fast neutrons incident thereon.
4. The thermal neutron monitor of claim 1, wherein the at least one
thermal neutron scintillator sheet and the first and second light
guides are generally planar.
5. The thermal neutron monitor of claim 1, wherein the at least one
thermal neutron scintillator sheet and the first and second light
guides are curved around a common axis to form a generally
cylindrical shape.
6. The apparatus of claim 1, wherein the neutron scintillator
comprises a thermal neutron capturing isotope component and a
scintillation component that scintillates upon exposure of the
capturing isotope to thermal neutrons.
7. The apparatus of claim 6, wherein the capturing isotope is
selected from .sup.6Li, .sup.10B, .sup.113Cd, and .sup.157Gd.
8. The apparatus of claim 6, wherein the neutron scintillator
comprises ZnS.
9. An apparatus for selective radiation detection, comprising: a
plurality of plates of light guides; at least one sheet of neutron
scintillator, sandwiched between two plates of light guides, the
neutron scintillator being fabricated from a material that captures
thermal neutrons and responsively produces optical light; and an
optical detector optically coupled to the neutron scintillator by
the light guides; a controller coupled to the optical detector; a
gamma ray scintillator coupled to the optical detector; and an
X-ray fluorescence analyzer.
10. The apparatus of claim 9, wherein the X-ray fluorescence
analyzer is adapted for independent operation by umbilical cord or
wireless communication.
11. The apparatus of claim 9, wherein the apparatus is adapted to
be handheld.
12. A method for selectively detecting radiation, comprising the
steps of: exposing a sheet of neutron scintillator to a source of
neutron radiation, the neutron scintillator being fabricated from a
material that captures thermal neutrons and responsively produces
optical light; directing scintillation from the neutron
scintillator to an optical detector through a plurality of plates
of light guides adjacent to the neutron scintillator sheet, wherein
the light guides thermalize fast neutrons so that they are captured
by the neutron scintillator producing optical light, and wherein
the light guides comprise a hydrogenous material that thermalizes
fast neutrons; detecting the neutron scintillation by a controller
coupled to the optical detector; exposing a gamma ray scintillator,
coupled to the controller, to gamma rays; directing gamma ray
scintillation to the optical detector; detecting the gamma ray
scintillation; exposing an X ray fluorescence analyzer, coupled to
the controller, to X-ray fluorescence evidencing a radioactive
shielding material that includes a high atomic weight element; and
detecting the X-ray fluorescence by the controller.
13. The method of claim 12, wherein the X-ray fluorescence analyzer
is adapted for independent operation by umbilical cord or wireless
communication.
14. The apparatus of claim 9, wherein: the X-ray fluorescence
analyzer and the controller are part of a main unit; and the
plurality of plates of light guides, the at least one sheet of
neutron scintillator, the optical detector, and the gamma ray
scintillator, are part of a subunit which can be detached from the
main unit and can communicate with the controller by an umbilical
cord or a wireless communication link.
Description
RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S. Patent
Application Ser. No. 10/861,332 entitled "Neutron and Gamma Ray
Monitor", which claims the priority benefit of U.S. Provisional
Application No. 60/476,101, filed on Jun. 5, 2003, the entire
teachings of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] With the rise of terrorism there is a growing need for
effective detectors for radioactive weapons of mass destruction, or
materials used to shield their radiation form detection, e.g., high
atomic weight elements. Three weapons of special concern are
so-called "dirty bombs", uranium-based atomic bombs, and
plutonium-based atomic bombs. For example, dirty bombs include
chemical explosives surrounded by radioactive materials to be
dispersed upon detonation, contaminating the surroundings. Dirty
bombs can be detected by their emitted radiation, gamma and
bremsstrahlung radiation being the most common signatures.
Uranium-based atomic bombs can in principle be identified by the
signature gamma rays of .sup.235U or .sup.238U. The radiation flux
from weapons-grade .sup.235U is low, and therefore excellent
efficiency and good energy resolution is desirable to distinguish
.sup.235U or .sup.238U signature gamma rays from background gamma
rays and from innocent sources. Plutonium-based atomic bombs can be
detected by neutron emission. Neutron emitters are sufficiently
rare that the detection of a neutron source several times above
neutron background levels can be prima facie evidence for the
presence of plutonium.
[0003] The detection of gamma rays and neutrons has a long history
dating from their discoveries. Many topical books and monographs
are available, for example, "Radiation Detection and Measurement,
Third Edition, 1999" by Glenn F. Knoll, Wiley Press", the entire
teachings of which are incorporated herein by reference. Until
recently, radiation detectors were used almost exclusively for
benign commercial or research applications. Gamma ray devices with
good efficiency and energy resolution have been available since
NaI(Tl); the most widely used inorganic scintillator, was
introduced in the late 1940's. There are now a number of inorganic
and organic scintillators, as well as a number of semiconductor
detectors that are commercially available for detecting gamma rays
of low and high energy in configurations adapted for a variety of
applications. Light from the scintillators can be detected by an
optical detector, e.g., photomultipliers, photodiodes, and
charge-coupled devices (CCDs) and the like. However, these
detectors cannot detect gamma ray sources shielded by a sufficient
mass of a high Z material, e.g., lead, tungsten, and the like.
Commercial neutron detectors also became available in the early
1960s. These relatively bulky devices detect thermal neutrons with
gas-proportional counters filled with either BF.sub.3 or .sup.3He.
High energy neutrons can typically be measured by plastic and
liquid scintillators that detect the highly ionizing protons
produced when the energetic neutrons collide elastically with the
hydrogen nuclei. The presence of fast neutrons can also be
determined by thermalizing, or moderating the speed of the neutrons
with a hydrogenous material, and detecting the resulting thermal
neutrons with efficient thermal neutron detectors. Plastic and
liquid scintillator containing lithium or boron are examples of
detectors that employ this method.
SUMMARY OF THE INVENTION
[0004] Existing commercial radiation detectors do not meet existing
radiological weapon detection needs, including selectivity,
efficiency, portability, and detection of the three main types of
radioactive weapons. Further, existing radiation detectors cannot
detect gamma rays from a shielded weapon, for example, a weapon
shielded by lead. Therefore, there is a need for effective
detectors of radioactive weapons of mass destruction, including
shielded weapons.
[0005] According to an illustrative embodiment of the invention, a
compact thermal neutron monitor is provided having at least one
sheet of thermal neutron scintillator material sandwiched between
first and second light guides. Upon impingement of a thermal
neutron, the scintillator sheet emits light in opposite transverse
directions, such that a first portion of the scintillation light is
directed to and coupled into the first light guide, and a second
portion of the scintillation light is directed to and coupled into
the second light guide. The first and second portions of the
scintillation light are conveyed through the respective light
guides to a common detector that responsively generates a signal
representative of the intensity of the received light. In certain
implementations, multiple scintillator sheets may be interleaved
with corresponding light guides, such that some of the light guides
may receive light from two scintillator sheets arranged on opposite
sides of the light guide. By capturing and detecting scintillation
light emitted in both transverse directions, the signal-to-noise
ratio and hence the sensitivity of the thermal neutron monitor is
improved relative to equivalent monitors of conventional design,
wherein only the scintillation light emitted in one direction is
captured and detected. Furthermore, the present design avoids the
complexity and manufacturing cost arising from utilization of
multiple detectors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0007] FIG. 1 depicts an embodiment of selective radiation
detection apparatus 10 equipped to detect gamma rays and
neutrons.
[0008] FIG. 2 depicts optional X-ray fluorescence (XRF) detector 40
coupled to controller 70 for detecting high atomic weight (high Z)
materials 54 that can shield radioactive materials, e.g. gamma ray
source 56.
[0009] FIG. 3 depicts an embodiment of new neutron detector
apparatus 120 employing a configuration of light guides 82 and
thermal neutron scintillator layers 80.
[0010] FIG. 4 depicts the components of an apparatus 130 for
selective detection of neutrons and gamma rays viewed by a single
optical detector 26.
[0011] FIG. 5 depicts an isometric drawing of an embodiment of a
new neutron scintillator/light guide apparatus 150.
[0012] FIG. 6 depicts another embodiment of neutron
scintillator/light guide apparatus 168 where multiple light guide
segments 160 are employed to provide the neutron detector with
directional capability.
[0013] FIG. 7 depicts apparatus 700 in which neutron and gamma
detectors and an--ray fluorescent analyzer are integrated with a
controller into a single, compact unit adapted for handheld
Homeland Security bomb detection.
DETAILED DESCRIPTION OF THE INVENTION
[0014] A description of preferred embodiments of the invention
follows.
[0015] The various embodiments herein relate to methods and an
apparatus for detecting targets, e.g., signatures of radioactive
weapons such as neutrons and gamma rays, and high-Z materials,
e.g., lead, tungsten, and the like, that can shield gamma ray
sources from detection. The various embodiments described here are
examples of many configurations of a "universal", portable,
hand-held, terrorist-threat detector that can identify such
targets. In various embodiments, detection is possible for one or
more targets, such as: gamma rays, e.g., gamma rays characteristic
of specific radioisotopes; neutrons characteristic of plutonium;
and high atomic-weight (high Z) material that can shield
radioactive, e.g., gamma ray sources. In some embodiments, a single
handheld detector is employed to record evidence of these targets
and alert the operator to their presence.
[0016] FIG. 1 depicts an embodiment of selective radiation
detection apparatus 10 equipped to detect gamma rays and neutrons.
Neutron scintillator 14 is coupled to light guide 22 and gamma ray
scintillator 18. Optical detector 26 can be coupled to detect
scintillation from neutron scintillator 14 and gamma ray
scintillator 18. Also, the apparatus can optionally be covered by
moderator 38, which can be a material that thermalizes fast
neutrons. Detector 26 can be coupled through preamplifier 30 to a
controller 70 which can provide data acquisition, control, display
and output. Controller 70 can be easily adapted from electronic
controllers known to the art for handheld radiation detection
instrumentation, for example, the acquisition, control and display
system in a commercial X-ray fluorescent unit (Xli, Niton LLC,
Billerica, Mass.). Typically, apparatus 10 is adapted to be
handheld, e.g., all components can be included in a single compact
unit having a total mass less than about 2.5 kg, or more typically,
less than about 1.5 kg.
[0017] As described herein, a gamma ray detector can be any gamma
ray detector known to the art, for example, a solid state
semiconductor detector, or gamma ray scintillator (e.g., 18) in
combination with an optical detector (e.g., 26). Typically, the
gamma ray detector includes a gamma ray scintillator. Of the
disclosed embodiments where a gamma ray scintillator is described,
other embodiments are contemplated where the gamma ray scintillator
is replaced with a solid state gamma ray detector.
[0018] Neutron scintillator 14 can include a material that
scintillates in response to fast neutrons, thermal neutrons, or a
combination of materials that respond to both types of neutrons. As
used herein, thermal neutrons are neutrons that have kinetic energy
on the order of kT, where k is Boltzman's constant and T is
temperature in Kelvin; fast neutrons are neutrons with kinetic
energy greater that kT, typically much greater, e.g., in the range
of thousands to millions of electron volts. Typically, the material
of neutron scintillator 14 can have excellent efficiency for
detecting thermal neutrons and negligible efficiency for detecting
X-rays or gamma rays. This material can include a thermal
neutron-capturing isotope coupled to a scintillation component that
scintillates upon exposure of the capturing isotope to thermal
neutrons. The capturing isotope can be any thermal neutron
capturing isotope known to the art, for example, .sup.6Li,
.sup.10B, .sup.113Cd, .sup.157Gd, and the like, generally .sup.6Li
or .sup.10B, or more typically .sup.6Li. The scintillation
component can be any component known to to the art to scintillation
in response to the reaction products of thermal neutron capture by
a capturing isotope, for example, the scintillation component can
be ZnS. The material of neutron scintillator 14 can be any
combination of capturing isotope and scintillation component, for
example, a compound including at least one of .sup.6Li, .sup.10B,
.sup.113Cd, or .sup.157Gd combined with ZnS. Typically, the neutron
scintillator is a combination of .sup.6LiF and ZnS. For example, in
various embodiments, neutron scintillator 14 is a commercially
available screen material (Applied Scintillation Technologies,
Harlow, United Kingdom), approximately 0.5 mm thick made from a
mixture of LiF and ZnS. The lithium is isotopically enriched
.sup.6Li, an isotope with a cross section of 940 barns for
capturing a thermal neutron and immediately breaking up into a
helium nucleus .sup.4He and a triton .sup.3H, with a total energy
release of 4.78 MeV. The energetic alphas and tritons can lose
energy in the ZnS causing it to scintillate with the emission of
about 50 optical photons for every kilovolt of energy lost as the
alphas and tritons come to rest. There can thus be a high
probability that each captured neutron produces hundreds of
thousands of optical light quanta.
[0019] Tests of .sup.6LiF/ZnS screens have determined that they are
selective for thermal neutrons over other radiation, e.g. gamma
rays, X-rays, and the like, e.g., these screens have intrinsic
efficiencies of about 50% for detecting thermal neutrons, while
their efficiency for detecting gamma rays can be negligible, e.g.
less than about 10.sup.-8. Selectivity for thermal neutrons versus
gamma rays can reduce the rate of "false alarms" due to relatively
common gamma ray sources (medical isotopes, radioactive sources in
industrial testing equipment, and the like) in favor of valid
alarms due to neutron emitters associated with weapons of mass
destruction. This selectivity for detection of thermal neutrons
versus gamma rays can be expressed as a ratio. In typical
configurations, the thermal neutron to gamma ray selectivity is at
least about 10,000:1, more typically at least about 1,000,000:1,
and in some embodiments, at least about 10,000,000:1.
[0020] Optional neutron moderator 38 can be made of a material that
thermalizes fast neutrons. One skilled in the art will know of many
suitable moderator materials and can select a moderator material,
thickness, and location to maximize neutron detection efficiency
while minimizing any loss in efficiency for detecting gamma rays.
For example, typical neutron moderators are hydrogenous materials
such as water, organic solvents (alcohols, ethers (e.g., diethyl
ether, tetrahydrofuran), ketones (e.g., acetone, methyl ethyl
ketone), alkanes (e.g., hexane, decane), acetonitrile,
N,N'dimethylformamide, dimethyl sulfoxide, benzene, toluene,
xylenes, and the like) oils and waxes (e.g., mineral oil, paraffin,
and the like), organic polymers (e.g., polyalkanes (e.g.,
polyethylene, polypropylene, and the like), polyesters,
polyvinylenes (e.g., polyvinylchloride) polyacrylates (e.g.,
polymethymethacrylate), polystyrenes, polyalkylsiloxanes (e.g.,
poly dimethyl siloxane), and the like), composites or gels of water
or organic solvents with polymers (e.g., water gels of gelatin,
polyacrylic acid, hyaluronic acid, and the like), and many other
such moderators known to the art.
[0021] For example, in some embodiments, moderator 38 can be made
of an organic polymer, e.g., high density polyethylene, and can be
placed over the apparatus 10 to moderate (thermalize) incoming fast
neutrons, so that they can be efficiently captured by neutron
scintillator 14. In other embodiments, moderator 38 can be a
container that holds a suitably thick layer of a liquid moderator
covering apparatus 10, for example, water, organic solvents, water
gels, and the like. In various embodiments, the hydrogen nuclei in
the neutron moderator can be enriched in the .sup.2H isotope, i.e.,
the fraction of .sup.2H in the moderator is above natural abundance
level. In some embodiments, at least about 50%, more typically at
least about 90%, or preferably at least about 95% of the hydrogen
nuclei in the neutron moderator are the .sup.2H isotope.
[0022] Light guide 22 can be coupled to neutron scintillator 14 to
direct the scintillation to optical detector 26. Light guide 22 can
collect scintillation photons from a relatively large scintillation
surface area and direct them to the smaller area of the detector
26. This can result in a higher scintillation collection efficiency
for a given detector surface area. Although other configurations
are possible, the depicted configuration where light guide 22 can
be parallel to the surface of scintillator 14 (which can be
perpendicular to the detection surface of detector 26) provides a
compact structure suitable for a handheld unit.
[0023] In addition to guiding scintillation photons to optical
detector 26, light guide 22 can optionally serve one or both of the
following additional functions.
[0024] First the light guide material can act as a moderator or
thermalizer of the fast neutrons, thus slowing them to thermal
energies so that they can be efficiently captured by neutron
scintillator 14. Thus, light guide 22 can include any neutron
moderator described above that can meet the transparency criterion,
e.g., typically hydrogenous materials such as water, organic
solvents, transparent organic polymers (e.g., polyacrylics,
polystyrenes, polycarbonates, polyalkylsiloxanes) composites or
gels of water or organic solvents with polymers, mineral oil, and
the like. Typically, the material of light guide 22 can be a solid,
e.g., an organic polymer, generally a polyacrylate, e.g. in some
embodiments, polymethyl methacrylate. In various embodiments, the
hydrogen nuclei in the material of light guide 22 can be enriched
in the .sup.2H isotope, i.e., the fraction of .sup.2H in the
moderator is above natural abundance level. In some embodiments, at
least about 50%, more typically at least about 90%, or preferably
at least about 95% of the hydrogen nuclei in the neutron moderator
are the .sup.2H isotope.
[0025] Second, the material of the light guide, described in the
preceding paragraph, can have a finite efficiency for scintillating
in response to fast neutrons, for example, when fast neutrons
strike a hydrogen nuclei, the hydrogen nuclei can be scattered with
sufficient energy to give an ionizing signal, which can be detected
by optical detector 26. In some embodiments, light guide 22
functions as a fast neutron scintillator and thus encompasses
neutron scintillator 14. Thus, in various embodiments, apparatus 10
can detect fast neutrons, thermal neutrons, or fast and thermal
neutrons depending on the materials and selection of light guide 22
and neutron scintillator 14.
[0026] The gamma ray detector 18 can be any of a variety of gamma
ray scintillators known to the art, e.g., sodium iodide doped with
thallium (Na(Tl), cesium iodide doped with thallium (CsI(Tl)),
bismuth germanate (BGO), barium fluoride (BaF.sub.2), lutetium
oxyorthosilicate doped with cesium (LSO(Ce)), cadmium tungstate
(CWO), yttrium aluminum perovskite doped with cerium (YAP(Ce)),
gadolinium silicate doped with cerium (GSO), and the like. For
example, NaI(Tl) can be fast, efficient and inexpensive, but can be
hygroscopic and is typically sealed against moisture.
Non-hygroscopic crystals such as BaF.sub.2, BGO or LSO, and the
like, can also be employed. Such materials are typically selected
to have good efficiency for detecting gamma rays from dirty bombs;
for example, a 662 keV gamma ray from .sup.137Cs (often cited as a
radiological threat in a dirty bomb) can have more than an 80%
absorption efficiency in a 2.5 cm (1 inch) thick crystal of LSO,
which can produce about 10,000 detectable optical photons.
Generally, the gamma ray scintillator includes one of NaI(Tl),
CsI(Tl), BGO, BaF.sub.2, LSO, or CdWO.sub.4, or more typically,
BGO, BaF.sub.2, or LSO. In some embodiments, the gamma ray
scintillator is BaF.sub.2, and in other embodiments, the gamma ray
scintillator is LSO.
[0027] In various embodiments, gamma-ray scintillator 18 and the
light guide 22 are transparent to the optical wavelengths generated
by any of the scintillation events. As used herein, the terms
"transparent" and "transparency" refer to the transmittance per
unit path length in a material of light, e.g., scintillation light.
Typically, a material transparent to scintillation light transmits,
per meter of material, at least about 90%, generally about 95%, and
more typically about 98% of scintillation. Typically, the
scintillation transmitted is in a range from about 400 nanometers
(nm) to about 600 nm, generally from about 350 to about 600 nm, or
more typically from about 300 to about 600 nm. Thus, in some
embodiments, transparent materials (e.g., the light guides, the
gamma ray scintillator, and the like) transmit about 95%/meter of
scintillation between about 350 nm and about 600 nm, or more
typically, transmit about 98% of scintillation between about 300 nm
and about 600 nm.
[0028] In various embodiments, the respective refractive indices of
the scintillator 18 and the light guide 22 can be in the same
range, e.g., between about 1.4 to about 2.4, or more typically,
between about 1.5 to about 1.8, and can generally be selected to be
similar to minimize reflections at the interface between
scintillator 18 and light guide 22.
[0029] Thus, in various embodiments, light guide 22 and/or gamma
ray scintillator 18 are transparent to scintillation, which can
benefit the efficiency of detection at optical detector 26.
Further, it can allow the use of a single optical detector 26
because the light from multiple scintillation sources can be
collected and delivered on the optical face of detector 26. For
example, as depicted in FIG. 1, scintillation from thermal neutrons
interacting with neutron scintillator 14 can travel through light
guide 22 and gamma ray scintillator 18 to detector 26. In
embodiments where light guide 22 can also function as a fast
neutron scintillator, its scintillation can also travel through
gamma ray scintillator 18 to detector 26, and thus scintillation
from three sources (fast neutrons in light guide 22, slow neutrons
in scintillator 14, and gamma rays in scintillator 18) can be
detected by a single optical detector 26. Further, in some
embodiments, alternate arrangements of these components can be
possible, for example, the order of light guide 22 and gamma ray
scintillator 18 can be reversed and gamma ray scintillation can
travel from scintillator 18 through light guide 22 to detector
26.
[0030] In various embodiments, where two or more types of
scintillation are detected at detector 26, they can be
distinguished according to their temporal characteristics, i.e., as
a function of time. For example, in embodiments of apparatus 10
equipped to detect fast neutrons, thermal neutrons, and gamma rays,
controller 70 can be programmed to sort detected signals according
to features of their temporal characteristics, e.g., rise times,
decay times, and the like. For example, in some embodiments,
employing polymethyl methacrylate for light guide 22 gives a fast
neutron scintillation decay time of about 2 nanoseconds; employing
LSO for scintillator 18 gives a gamma ray scintillation decay time
of about 40 nanoseconds (20 times slower); and employing the
.sup.6LiF/ZnS in scintillator 14 gives a thermal neutron
scintillation decay time of about 30 microseconds (about 15000
times slower than fast neutron scintillation decay and about 700
times slower than gamma ray scintillation decay). Standard
rise-time detection circuits known to the art can easily
distinguish such temporally separated signals, and thus multiple
scintillation types can be sorted, typically unambiguously, by
controller 70, to yield separate data, e.g., pulse height spectra
for each scintillation type. Standard circuits known to the art can
be employed by controller 70 which can be fast enough so that
substantially all signals from multiple scintillation sources can
be processed.
[0031] FIG. 2 depicts optional X-ray fluorescence (XRF) detector 40
coupled to controller 70 for detecting high atomic weight (high Z)
materials 54 that can shield radioactive materials, e.g. gamma ray
source 56.
[0032] The XRF analyzer 40 can be easily adapted from commercial
XRF detectors known to the art, for example, the Xli XRF analyzer,
Niton LLC, Billerica, Massachusetts. The XLi is a hand-held unit
weighing less than 1 kg (2 pounds) that contains radioactive
fluorescing sources, for example, it can contain a strong source of
.sup.57Co, which emits a 122 keV gamma ray that can excite the
characteristic x-ray of various high-Z, heavy elements, including
tungsten, lead, uranium, plutonium, and the like. Emitted X-ray
fluorescence radiation can be detected in a detector, e.g., a
cooled CdTe detector, which can have excellent efficiency and
resolution for detecting the characteristic X-rays of high-Z
materials. The processed information can be displayed, e.g., in a
liquid crystal display. The collected information, including the
pulse height spectra, can be stored in unit 70, can be telemetered
to a remote location, and can automatically alert the operator to a
potential hazard.
[0033] Thus, XRF analyzer 40 can optionally include a radioactive
source 48 (typically encased in shield 64) to stimulate X-ray
fluorescence in target materials, e.g., shield material 54
surrounding radioactive source 56 in bomb 52. For example, in one
embodiment radioactive source 48 (depicted in FIG. 2 as optional
dual sources) can be .sup.57Co, which can emit 122 keV gamma rays
in about 90% of its decays. The 122 keV gamma rays can be efficient
exciters of the K X-rays of high atomic weight/high Z material 54
that can be suitable as shielding for radioactive source 56, for
example, high Z materials such as tungsten, lead, uranium,
plutonium, and the like. XRF analyzer 40 includes a detector 60,
which can be any X-ray detector known to the art, for example in
various embodiments detector 60 can be a CdTe (cadmium telluride)
semiconductor detector, about 2 mm thick, coupled to a preamplifier
68. A 2 mm thick CdTe detector can have an intrinsic efficiency of
more than about 80% for detecting the K rays of high atomic
weight/high Z elements. The energy resolution of commercially
available CdTe detectors can be greater than about 2 keV for 100
keV gamma rays, which can be sufficient to separate the K X-rays of
various heavy elements and identify, at least in part, the
elemental composition of the shielding material 54. One skilled in
the art can determine that for some embodiments, a commercially
available 100 mCi ring source of .sup.57Co, together with a 1
cm.sup.2, CdTe detector 2 mm thick, can determine the presence of a
lead shield inside a container of steel up to 6.4 mm (1/4 inch)
thick, at a distance of one foot from the detector.
[0034] Each possible radiation detection combination is
contemplated in various embodiments of the method and apparatus.
For example, included in various embodiments are XRF and fast
neutron detection; XRF and thermal neutron detection; XRF and gamma
ray detection; XRF, fast neutron, and gamma ray detection; XRF,
thermal neutron, and gamma ray detection; XRF, fast neutron,
thermal neutron, and gamma ray detection; fast neutron and gamma
ray detection; thermal neutron and gamma ray detection; fast
neutron and thermal neutron detection; fast neutron, thermal
neutron, and gamma ray detection; and the like. Further, each of
these are contemplated in various embodiments as automatically
controlled, e.g., by a single controller 70, and adapted for
handheld operation, e.g., in a single handheld unit.
[0035] In other embodiments, one or more detectors can be coupled
with controller 70 by an umbilical cord or a wireless communication
link, and the like. For example, a single handheld apparatus can
include a controller and an XRF analyzer combined with a
gamma/neutron detector subunit; the subunit can be detached from
the main unit containing the controller and the XRF unit, and can
communicate with the controller via an umbilical cord or a wireless
communication link. This can allow for more flexible detection
usage, for example, a detachable gamma/neutron probe can be
employed to search difficult to reach areas in vehicles or confined
spaces.
[0036] FIG. 3 depicts an embodiment of a neutron detector apparatus
120 employing an alternating arrangement of light guides 82 and
thermal neutron scintillator sheets 80. Each thermal neutron
scintillator sheet 80 is sandwiched between a corresponding pair of
light guides 82, such that one major surface of the thermal neutron
scintillator sheet is positioned adjacent to a first light guide
and the opposite major surface of the thermal neutron scintillator
sheet is positioned adjacent to a second light guide. Light guides
120 are optically coupled to a common detector 26, which generates
a signal representative of the intensity of light received thereby.
Thermal neutron scintillator sheets 80 and light guides 82 may be
flat planes, as depicted in FIG. 3, or may alternatively be curved
along one or both major axes to assume (for example) a cylindrical
or hemispherical shape.
[0037] Light guides 82 may be formed from any suitable material (or
combination of materials) that is substantially transparent at the
wavelength of interest, i.e., the wavelength of the light emitted
by scintillator sheets 80 responsive to capture of a thermal
neutron. If detection of fast neutrons is also desired, light
guides 82 may be formed from or doped with a hydrogenous material
that effects thermalization of fast neutrons passing therethrough
so that the resultant thermalized neutrons can interact with
thermal neutron scintillator sheets 80. In one illustrative
embodiment, light guides 82 are fabricated from polymethyl
methacrylate (PMMA).
[0038] As known in the art, thermal neutron scintillator sheets 80
may be formed from thin (.about.500 .mu.m thick) sheets of
.sup.6LiF:ZnS, which are commercially available Applied
Scintillation Technologies (Harlow, United Kingdom). The
scintillator sheets 80 and adjacent light guides 82 may be
physically bonded with a optically transparent layer of epoxy or
other material that facilitates good optical coupling from the
scintillator sheets to the light guides.
[0039] In operation, a thermal neutron captured by one of the
thermal neutron scintillator sheets 80 will cause light to be
simultaneously emitted in opposite transverse directions (the
component of travel along an axis locally normal to scintillator
sheet 80), such that a first portion of the emitted light will
travel to and be coupled into the light guide 82 positioned
adjacent to one of the major surfaces of scintillator sheet and a
second portion of the emitted light will travel to and be optically
coupled into the light guide 82 positioned adjacent to the opposite
major surface. The sandwiched or layered geometry of neutron
detector apparatus 120 advantageously permits capture and
subsequent detection (by common detector 26) of substantially all
the light produced by a thermal neutron capture event, thereby
exhibiting improved signal-to-noise ratio (and lower limits of
detection/quantitation) relative to prior art designs that capture
and detect only a portion of the emitted light. Furthermore,
neutron detector apparatus 120 removes the need to provide multiple
detectors to detect scintillation light emitted in different
directions, thereby avoiding the associated cost and space
requirements and enabling the design of a relatively inexpensive
and compact device.
[0040] Discrimination against signals arising from other forms of
radiation, such as gamma rays, may be effected in neutron detector
apparatus 120 by selection of a scintillator material that is
substantially insensitive to radiation other than thermal neutrons
and/or by filtering of the signal based on temporal characteristics
to remove components of the signal attributable to non-neutron
radiation.
[0041] FIG. 4 depicts the components of an apparatus 130 for
selective detection of neutrons and gamma rays viewed by a single
optical detector 26. In applications in which gamma ray and
neutrons are desired to be detected separately, a gamma ray
scintillation detector 18 can be attached to one end of the light
guides/scintillators 80/82. The signals from the gamma ray detector
and neutron detector are separated by their different temporal
characteristics as described above. If the portion of apparatus 130
defined by light guides/scintillators 80/82 is long, for example,
more than about 30 cm in length, it may be advantageous to put an
optical detector on both ends of the combined gamma ray and neutron
detector. The signals from two optical detectors can be added and
the combined signal can be separately analyzed into neutron and
gamma ray signals according to temporal characteristics as
described above.
[0042] Further embodiments of the apparatus 130 can be useful for
applications in which it is desired to detect fast neutrons. In
some embodiments, the neutron scintillators 82 can be made out of a
material, e.g., organic polymer, that scintillates in response to
fast neutrons. In other embodiments, the .sup.6LiF:ZnS neutron
scintillator material can be suspended in a liquid scintillator,
e.g., water, organic solvents, mineral oil, and the like, wherein
the decay time of scintillation light emitted when a gamma ray or
electron is detected can be significantly different from the decay
time of scintillation light emitted when a fast proton (e.g., due
to fast neutron scintillation) is detected. Since the two decay
time constants of the liquid scintillator differ significantly from
the decay time constants of the gamma ray detector 18 or light
guides/scintillators 80/82, it can be possible to separate all four
signals and therefore completely discriminate fast neutrons,
thermal neutrons, and gamma rays using a single optical detector
(or one or more optical detectors, the outputs of which are added
together).
[0043] FIG. 5 depicts an isometric drawing of an embodiment of a
new neutron scintillator/light guide apparatus 150. Four sheets,
110, 112, 114 and 116 of optically transparent polymethyl
methacrylate, about 5.1 cm wide by about 30.5 cm long by about 1.25
cm thick, polished on all sides, have thermal neutron scintillator
material .sup.6LiF:ZnS 116 layered between each 5.1 cm.times.30.5
cm side and on the top and bottom. The four slabs with their Li6F:
ZnS screens make a multilayer sandwich, 150, 5.1 cm.times.30.5 cm
by about 5.6 cm high. Apparatus 150 can be coupled to an optical
detector, for example, apparatus 150 can replace neutron
scintillators/light guides 80/82 in FIG. 4. As above, for
applications that require very long detectors and/or detection of
faint signals, it can be useful to attach a second optical
detector, e.g., a photomultiplier tube, to each end of such a light
guide/scintillator apparatus so as to increase the amount of light
detected by employing two detectors.
[0044] Monte Carlo simulations, confirmed by experiment, show that
polymethyl methacrylate can be about 75% as effective as
high-density polyethylene for thermalizing neutrons. Thus, the
neutron scintillator/light guide 150 can be an efficient neutron
detector as shown. It can be made about 30% more effective by
covering the length of the detector with a layer of neutron
moderator 134, e.g., high density polyethylene, and still more
effective by placing a layer of neutron-scintillator material
between the light guide/scintillator 150 and the neutron moderator
134.
[0045] The neutron selectivity over gamma rays of light
guide/scintillator 150 was measured at of 5.times.10.sup.8:1.
Commercial .sup.3He gas proportional counters, the current "gold
standard" of neutron detectors, have rejection ratios ranging from
10.sup.3 to 10.sup.6. Thus, the detector can have a gamma ray
rejection ratio that is more than 1000 times greater than the best
current commercial .sup.3He detectors.
[0046] As noted above, selectivity for neutrons over gamma rays can
be essential for detecting neutron sources, e.g., plutonium, while
minimizing false alarms from gamma ray sources. For example, one
current security standard desires a neutron detector to detect the
presence of 0.455 kg (1 pound) of plutonium at a distance of 2
meters. 0.455 kg (1 pound) of plutonium emits approximately 20,000
fast neutrons per second. At 2 meters, there are at most 0.04
neutrons crossing per cm.sup.2 of the detector per second. If the
efficiency for detecting the neutron is 50%, which can be attained
for light guide/scintillator 150, then the count rate is only
0.02/sec/cm.sup.2. If the efficiency of the neutron detector for
detecting gamma rays is 10.sup.-3, then 20 gamma rays/sec/cm.sup.2,
from a modest source, will give the same signal as the neutrons
from 0.455 kg (1 pound) of plutonium, and trigger an alert. Neutron
light guide/scintillator 150, with an efficiency for detecting
gamma rays of only 2.times.10.sup.-9, will typically not be alerted
by modest gamma ray sources compared to the preceding security
standard for neutron emission from plutonium. In fact, neutron
light guide/scintillator 150, will typically not detect a gamma ray
source as equivalent to the neutron/plutonium security standard
unless the gamma ray source is itself a serious health risk.
[0047] The light guide/scintillator 150 has other practical
advantages over conventional .sup.3He detectors. Commercial
.sup.3He detectors typically have only about 10% efficiency for
detecting neutrons unless surrounded by a thick neutron moderator
such as a 5.1 cm thick cover of high density polyethylene. The
disclosed neutron detectors, with intrinsic neutron moderation
provided by the light guide, e.g., the polymethyl methacrylate
light guides 110, 112, 114 and 116 in neutron light
guide/scintillator 150, can have an efficiency of almost 40%
without a high density polyethylene cover. Further, if necessary to
achieve the efficiency of a fully moderated .sup.3He detector, the
disclosed neutron detectors can employ a much thinner moderator
(e.g., polyethylene) to obtain full moderation. Thus, the detectors
disclosed herein can be significantly lighter than a commercial
.sup.3He detector of the same efficiency, which is of central
importance for adapting a device to handheld use.
[0048] Also, light guide/scintillator 150 can be very robust and
can be free of travel restrictions. A .sup.3He detector contains
the isotope .sup.3He at a pressure typically from about two to
about four atmospheres. In many situations, transportation
regulations require special procedures for transporting such
detectors.
[0049] Also, commercial .sup.3He detector are limited to an
operating temperature range from +10.degree. C. to +50.degree. C.,
where detection can still be affected by changes in temperature.
Light guide/scintillator 150 can be insensitive to temperature
change over a range of at least about -10.degree. C. to about
50.degree. C.
[0050] Still another advantage is that the disclosed detector, in
sizes large enough to meet Homeland Security requirements, can be
less costly than commercial .sup.3He detectors of comparable
efficiency because the cost of comparable materials, e.g., the
light guide material, are typically much less expensive compared to
the cost of .sup.3He in a conventional detector.
[0051] One skilled in the art will appreciated that many possible
arrangements of one or more light guides and one or more neutron
scintillation layers can be combined with an optical detector to
form a neutron detector, for example, a neutron scintillation layer
can be applied to the front of a block of light guide material, and
an optical detector can be coupled to the back of the block of
light guide material. However, arrangements of multiple layers of
light guides and neutron scintillators in combination with one or
more optical detectors as provided in FIGS. 3-5 are particularly
effective as described above.
[0052] FIG. 6 depicts another embodiment of neutron
scintillator/light guide apparatus 168 where multiple light guide
segments 160 are employed to provide the neutron detector with
directional capability. Light guide segments 160 are arranged in
the form of a hexagon 164 segmented into six pie-like sections. The
.sup.6LiF:ZnS thermal neutron scintillator material 166 can be
applied to surround each light guide segment 160. Scintillation
light collected from each segment, whether from fast neutron
scintillation in the light guide material, thermal neutron
scintillation in material 166, or both, can be detected separately,
for example by employing a segmented optical detector, which is
commercially available, or with separate optical detectors. The
light collected at the different segments can be correlated with
the direction of a neutron source, e.g., by appropriate modeling or
by conducting calibration experiments. One skilled in the art will
appreciate that the hexagonal segmentation shown in FIG. 6 is one
of many configurations that can allow differential detection of
scintillation based on the direction of the neutron source compared
to the detector; for example, the arrangement of neutron
scintillator material 80 and light guides 82 in FIG. 4 or 5 can
have the same function.
[0053] FIG. 7 depicts apparatus 700 in which neutron and gamma
detectors and an--ray fluorescent analyzer are integrated with a
controller into a single, compact unit adapted for handheld
Homeland Security bomb detection. Apparatus 700 which has been
designed through experimental test and Monte Carlo computer
simulation. Apparatus 700 includes a selective neutron detector
that is insensitive to gamma rays; a selective gamma ray detector
that is insensitive to neutrons; and an XRF detector capable of
finding shielding material at least about 30.5 cm (12 inches)
inside a box made of 3.1 mm (1/8 inch) steel.
[0054] The neutron detector, with overall dimensions of 5.1 cm by
5.1 cm by 25.4 cm, consists of 4 sheets of polished, transparent
polymethyl methacrylate light guides 710, each 1.25 cm by 5.1 cm by
25.4 cm, with 0.43 mm thick .sup.6LiF/ZnS neutron scintillators 712
covering all faces of guides 710 but the ends that are abutting the
face of a 5.1 cm optical detector 714, which is a photomultiplier.
The outside of the detector is covered by a neutron moderator 716
of 1.25 cm thick high-density polyethylene, which, together with
the polymethyl methacrylate light guides 710, moderate incoming
fast neutrons so that they are efficiently captured by
.sup.6LiF/ZnS neutron scintillators 712.
[0055] Gamma-ray scintillator 718 is a 5.1 cm diameter, 5.1 cm long
single crystal of BaF.sub.2, which can have a good efficiency for
detecting gamma rays and a good energy resolution for identifying
the emitting isotope. A thin window 720 of, for example, aluminum
or plastic about 0.8 mm thick, in front of gamma-ray scintillator
718 and parallel to optical detector 714 can adapt the gamma
detector to be sensitive to gamma radiation from 50 keV to several
MeV. One skilled in the art will know how to select windows of
other materials or thicknesses to adapt the gamma detector to other
radiation ranges. In the depicted embodiment, scintillator 718 is
located opposite detector 714 from guide/scintillators 710/712. (In
other embodiments, higher energy resolution of the BaF.sub.2 gamma
scintillator 718 can be obtained by placing scintillator 718
between detector 714 and guide/scintillators 710/712. A thin layer
of, of, for example, aluminum or plastic about 0.8 mm thick, can be
placed as a band around the BaF.sub.2 gamma scintillator 718,
perpendicular to the face of detector 714.) The scintillation light
from the BaF.sub.2 is transmitted through light guides 710 to
detector 714.
[0056] The signals from the BaF.sub.2 gamma scintillator 718 are
separated from those from the .sup.6LiF/ZnS neutron scintillators
712 by their different decay times of 0.63 microseconds and
.about.30 microseconds, respectively.
[0057] The neutron/gamma assembly 722 is fitted as the top of a
modified model XLp XRF analyzer 724 (Niton, ibid), which employs
digitized pulse processing to analyze two detector 714 and XRF
detector 726 simultaneously, storing the spectra and results of
4,096 channels data, all of which can be telemetered wirelessly to
central command points.
[0058] XRF analyzer 724 uses a 100 mCi, well-shielded, .sup.57Co
source 726 that emits, when shutter 728 is opened by trigger 730,
122 keV gamma rays for exciting the characteristic X-rays of
heavy-element shielding; the characteristic X-rays are detected in
large-area CdTe detectors 732. The size of apparatus 700 is similar
to that of a large cordless drill, with a weight of about 3 kg,
including a battery power supply. A full battery charge can give up
to 12 hours of continuous operation or more.
[0059] Controller 734 operates the detectors of apparatus 700 and
displays radiation detection results on display screen 736. A
portable power source 738, e.g., a battery or fuel cell, can be
included.
[0060] In various embodiments each detector/analyzer can operate
separately from each other or the controller via a modular design.
For example, the neutron/gamma-ray detectors can be a detachable
module from a base unit including the XRF analyzer and the
controller, and the/gamma-ray detectors can communicate with the
controller via an umbilical cord, wireless communication, and the
like. Thus, the/gamma-ray detectors can be an entirely independent
module or preferably can dock with the balance of apparatus 700.
One skilled in the art can provide for such remote operation, for
example, in the case of umbilical cord operation, employing
suitable preamplifier circuitry or in the case of wireless
operation, coupling off-the-shelf wireless communication modules
with the controller and the XRF detector.
[0061] Government agencies can establish desired detection
specifications, for example, for antiterrorism purposes,
environmental monitoring, and the like. Various embodiments can
meet one or more of the following specifications, including, for
example: [0062] 1. Detect in 10 seconds, at a distance of 2 meters,
an unshielded neutron source that emits 20,000 or more neutrons per
second; [0063] 2. Detect in 10 seconds, at a distance of 2 meters,
an unshielded, 10 .mu.Ci .sup.137Cs source;
[0064] 3. Identify a specific radioisotope based on emitted gamma
rays; and [0065] 4. Detect high Z shielding up to 1 foot (30.5 cm)
from the detector and behind as much as 1/4'' (6.4 mm) of steel or
material with equivalent absorption.
[0066] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *