U.S. patent application number 10/861926 was filed with the patent office on 2005-06-23 for neutron detector using neutron absorbing scintillating particulates in plastic.
This patent application is currently assigned to Neutron Sciences, Inc.. Invention is credited to Wallace, Steven.
Application Number | 20050135535 10/861926 |
Document ID | / |
Family ID | 34681270 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050135535 |
Kind Code |
A1 |
Wallace, Steven |
June 23, 2005 |
Neutron detector using neutron absorbing scintillating particulates
in plastic
Abstract
A neutron detector composed of a matrix of scintillating
particles imbedded in a lithiated glass is disclosed. The neutron
detector detects the neutrons by absorbing the neutron in the
.sup.6Li isotope which has been enriched from the natural isotopic
ratio to a commercial ninety five percent. The utility of the
detector is optimized by suitably selecting scintillating particle
sizes in the range of the alpha and the triton. Nominal particle
sizes are in the range of five to twenty five microns depending
upon the specific scintillating particle selected.
Inventors: |
Wallace, Steven; (Knoxville,
TN) |
Correspondence
Address: |
PITTS AND BRITTIAN P C
P O BOX 51295
KNOXVILLE
TN
37950-1295
US
|
Assignee: |
Neutron Sciences, Inc.
1256 Lovell View Drive
Knoxville
TN
37932
|
Family ID: |
34681270 |
Appl. No.: |
10/861926 |
Filed: |
June 4, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60476001 |
Jun 5, 2003 |
|
|
|
Current U.S.
Class: |
376/153 |
Current CPC
Class: |
G01T 3/06 20130101 |
Class at
Publication: |
376/153 |
International
Class: |
G01T 001/00 |
Claims
Having thus described the aforementioned invention, we claim:
1. A neutron detector comprising: a glass medium; a first material
which yields at least one of a triton, an alpha particle and a
fission fragment when said first material absorbs a neutron, said
first material being incorporated into said glass medium; a second
material embedded within said glass medium, said second material
consisting of scintillating particles which scintillate when
traversed by said at least one of a triton, an alpha particle and a
fission fragment, said second material being defined by small
particulates such that said glass medium surrounding said small
particulates when emitting charged particles from the constituent
within said glass medium is thin relative to a range of said
charged particles; a transparent plastic into which said glass
medium, said first material and said second material are dispersed;
and a surface coating disposed on said transparent plastic for
reflecting scintillation light pulses.
2. The neutron detector of claim 1 wherein said glass medium
contains a constituent that absorbs a neutron and subsequently and
promptly emits a charged particle from the group consisting of Li-6
and B-10.
3. The neutron detector of claim 2 wherein said glass medium is a
lithiated sol-gel glass, and wherein a composite mass of said
lithiated sol-gel glass and said scintillating particles is
polymerized to a mass and heat-treated to form a rigid
structure.
4. The neutron detector of claim 3 wherein said scintillating
particulates are uniformly mixed within said glass medium.
5. The neutron detector of claim 2 wherein said scintillating
particulates are finely powdered and mixed into said glass medium
to form a glass medium/scintillation particulate mixture, said
glass medium being in a melted state, said glass
medium/scintillation particulate mixture then being solidified.
6. The neutron detector of claim 2 wherein said scintillating
particulates are finely powdered, and wherein said glass medium is
powdered, said scintillating particulates and said glass medium
being mixed to form an aggregate mixture, said aggregate mixture
being melted and then solidified.
7. The neutron detector of claim 2 wherein said scintillating
particulates are finely powdered, said scintillating particulates
being mixed into a sol-gel precursor to glass, said sol-gel
precursor containing a constituent of said glass medium, said
scintillating particulates being locked into said glass medium as
polymerization occurs.
8. The neutron detector of claim 2 wherein said scintillating
particulates are overcoated by said glass medium, said
scintillating particles being processed to a size in the range of
from nominally five to twenty five microns in diameter referenced
to spherical particulates, said range being selected to optimally
absorb charged particles emitted within said glass medium.
9. The neutron detector of claim 2 wherein said scintillating
particles are selecting from the group consisting of: cerium doped
strontium sulfide, bismuth doped strontium sulfide, cerium
activated calcium sulfide, europium activated calcium sulfide,
bismuth germanate, cerium activated yttrium silicate, aluminum
perovskite, cerium activated yttrium aluminum garnet, terbium
activated yttrium aluminum garnet, cerium activated lutetium
oxyorthosilicate, europium activated yttrium oxide, europium
activated calcium fluoride, gallium activated zinc oxide,
thallium-activated cesium iodide, europium activated lanthanum
oxsulfide, manganese-lead activated calcium silicate,
europium-activated gadolinium oxysulfide, europium activated indium
borate, cerium-activated calcium sulfide, and zinc sulfide activate
with silver.
10. The neutron detector of claim 9 wherein said scintillating
particles are mechanically sized in the range of from five to
twenty five microns, said range being selected to optimally absorb
charged particles emitted within said glass medium and such that
charged particles are subjected to ionization from reaction
products of absorption in said glass medium.
11. The neutron detector of claim 1 wherein said scintillating
particulates are a scintillating molecular compound including at
least one of PPO and POPOP, said scintillating molecular compound
having attached a molecular entity containing lithium such that
each of said scintillating particles is a mass composed of
fluor/lithium molecules distributed within said transparent
plastic.
12. The neutron detector of claim 1 wherein said transparent
plastic includes a plurality of transparent plastic elements, said
glass medium, said first material and said second scintillator
material being dispersed in said plurality of transparent plastic
elements, whereby detection of neutrons in each of said plurality
of transparent plastic elements is identified individually and the
occurrence of the absorption of a neutron is temporally identified
with respect to a repeating fiducial timing signal thus allowing
each neutron detection event in each of said plurality of
transparent plastic elements to be analyzed with respect to each
other.
13. The neutron detector of claim 2 wherein said glass medium
defines a rigid structure and is mechanically pulverized into glass
particulates in the range five to twenty five microns, such as
particulates being a composite of the lithiated glass and said
scintillating particles.
14. The neutron detector of claim 13 wherein said glass medium
comprises composite particles such that absorption of a neutron
results in an ionization path yielding a light pulse characteristic
of said scintillating particles.
15. The neutron detector of claim 1 wherein said transparent
plastic is selected from the group consisting of at least
polystyrene, polyvinyl toluene and polymethylmethacrylate.
16. The neutron detector of claim 1 wherein said surface coating
contains titanium dioxide.
17. The neutron detector of claim 1 further comprising at least one
wavelength shifting fiber and at least one light detecting element,
said wavelength shifting fiber being secured in optical
communication between said transparent plastic and said light
detecting element, whereby scintillation light generated within
said transparent plastic travels through said wavelength shifting
fiber toward said light detecting element.
18. The neutron detector of claim 17 wherein said light detecting
element is selected from the group consisting of a photomultiplier
tube, a multi-anode photomultiplier, a silicon photodiode, a
microchannel plate, and an avalanche photodiode.
19. The neutron detector of claim 17 wherein said transparent
plastic includes an array of transparent plastic slabs, each of
said array of transparent plastic slabs for detecting neutrons,
each of said array of transparent plastic slabs having one said
wavelength shifting optical fiber.
20. The neutron detector of claim 19 wherein said array of
transparent plastic slabs is an 8.times.8 array of said transparent
plastic slabs, each of said 8.times.8 array of transparent plastic
slabs being optically connected to a single multi-anode
photomultiplier tube, said multi-anode photomultiplier tube
defining 64 individual pixels of detection, a direction of a
neutron source being ascertainable by determining a relative
intensity of detected neutrons in each of said 8.times.8 array of
transparent plastic slabs, a portion of said 8.times.8 array of
transparent plastic slabs facing the neutron source yielding the
highest intensity of counted signals.
21. The neutron detector of claim 1 wherein said transparent
plastic includes a plurality of transparent plastic elements, said
glass medium, said first material and said second scintillator
material being dispersed in said plurality of transparent plastic
elements, each of said plurality of transparent plastic elements
being optically coupled to a multi-anode photomultiplier, whereby
detection of neutrons in each of said plurality of transparent
plastic elements is identified individually and the occurrence of
the absorption of a neutron is temporally identified with respect
to a repeating fiducial timing signal synchronized with a pulsing
of an external neutron source thus allowing each neutron detection
event in each of said plurality of transparent plastic elements to
be analyzed temporally with respect to each other.
22. The neutron detector of claim 21 wherein each of said plurality
of transparent plastic elements is configured to enclose a volume
allowing a high probability of capture of all neutrons borne from
spontaneous fission events and induced fissions events from active
external injection of neutrons from a pulse neutron tube.
23. The neutron detector of claim 22 wherein neutron event data is
collected and stored in a record such that correlated neutron
events may be calculated to acquire a quantitative measurement of a
quantity of material spontaneously emitting neutrons and a
quantitative measurement of fissile content within said enclosed
volume having been stimulated to emit neutrons.
24. The neutron detector of claim 23 wherein individual neutron
detection events are correlated in time with neutron pulses from
either of a pulsed deuterium-deuterium and a deuterium-tritium
neutron tube.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/476,001, filed Jun. 5, 2003.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] 1. Field of Invention
[0004] The field of invention is neutron detectors. More
particularly, the present invention relates to a neutron detector
having solid absorbers.
[0005] 2. Description of the Related Art
[0006] Neutron detectors are useful in many applications. One such
application is the monitoring of fissile material in storage
containers, in spent nuclear fuel and in waste. Limiting the
unauthorized transfer of fissile material is endorsed by all
countries participating in the International Atomic Energy Agency
(IAEA) program. The purpose of the IAEA program is to limit access
to those materials needed for constructing a nuclear weapon of mass
destruction.
[0007] A major effort is being made to provide robust neutron
detection instrumentation systems at all locations where fissile
material from spent nuclear fuel and excess plutonium from obsolete
nuclear weapons are stored. Quantitative measurements are required
for nuclear accountability of the fissile mass placed in
criticality-safe storage containers. The .sup.240PU isotope is
present in these materials and is a spontaneous emitter of about
1000 neutrons/second/gram of the isotope. Fissile material emits
high energy gamma radiation and this, combined with the measurement
of neutrons, allows real-time monitoring of the stored
material.
[0008] Fissile material containing .sup.235U and .sup.239PU can be
detected by the measurement of excess neutrons when materials
containing these isotopes are subjected to an external source of
neutrons. The measurement techniques include the differential
die-away technique, the Californium shuffler and the AmLi
add-a-source. Typical of the art are those techniques described by
Phillip Rinard, Calculating Shuffler Count Rates, LA-13815-MS,
August 2001 and Greg Becker et al., Transuranic and Low-Level Boxed
Waste Form Nondestructive Assay Technology Overview and Assessment,
INEEL/EXT-99-00121, February 1999. Each technique is principally
developed by the Los Alamos National Laboratory.
[0009] Another industry in which neutron detectors are widely used
is the oil industry, in which neutron detectors are used to detect
potential oil yielding sites. Oil producing formations deep in the
earth emit neutrons at a different rate than water bearing
formations or non-fluid bearing rock. A device using neutron
detection for logging oil wells is disclosed in U.S. Pat. No.
4,641,028 issued to Taylor et al., on Feb. 3, 1987.
[0010] The '028 patent teaches a well logging instrument for use in
a cased well bore. The '028 device contains a sealed source of fast
neutrons and two identical thermal neutron detectors with a volume
of four atmospheres of .sup.3He gas. The formation surrounding the
cased well bore is bombarded with high energy neutrons and the two
thermal neutron detectors are spaced apart from one another and
from the source to receive slowed down or thermal neutrons from the
surrounding formations. The epithermal or fast neutrons striking
the formation are slowed down by fluids containing great quantities
of hydrogen or chlorine atoms, creating thermal or slow neutrons,
which the detectors respond to logarithmically and independently.
The counting rate of each detector is processed independently to
count rate meters and to a recorder to present two outputs of
information. The presence or absence of hydrogen and chlorine atoms
in the formations is detected by each detector.
[0011] U.S. Pat. No. 5,532,482 issued to Stephenson on Jul. 2,
1996, teaches a method for determining a characteristic of an
underground formation. The '482 method includes the steps of
irradiating the formation with high-energy neutrons and detecting
neutrons scattered by the formation. The detected neutrons have
energies above epithermal to determine the nature of the formation
matrix. Stephenson teaches that epithermal neutrons can also be
detected to determine formation porosity. Stephenson utilizes an
apparatus having a high-energy neutron source, typically a D-T
accelerator producing 14 MeV neutrons, and detectors such as
.sup.4He-filled proportional counters for detecting neutrons having
energies above epithermal and .sup.3He-filled proportional counters
for detecting epithermal neutrons.
[0012] In addition to the oil industry, neutron detectors are also
commonly used in the medical industry. Neutron detectors are also
useful for surveillance in nuclear facilities and weapons storage.
While several specific utilities of neutron detectors are
mentioned, it is well known to those skilled in the neutron
detection art that neutron detectors are useful in many
applications.
[0013] Neutrons are uncharged particles that can travel through
matter without ionizing the matter. Because neutrons travel through
matter in such a manner, they are difficult to detect directly.
Some other evidence of a neutron event must be detected in order to
determine its existence. An indirect method detects the result of a
neutron event and not the neutron event itself.
[0014] The use of indirect detection of neutrons is known in the
art. For example, a neutron detector as disclosed in U.S. Pat. No.
5,334,840 issued to Newacheck et al., on Aug. 2, 1994. The '840
neutron detector detects photons of light emitted by carbon
infiltrated boron nitride in its hexagonal form when the compound
is bombarded by neutrons. The amount of light detected correlates
to the number of neutrons bombarding the boron nitride.
[0015] Another neutron detector commercially available utilizes
.sup.3He as the neutron absorber, such as in the '028 device
described above. When bombarded by neutrons, .sup.3He decomposes
into H and H.sub.3 having combined kinetic energies of 764 keV. The
ionization of the gas electrons can be detected using conventional
methods well known in the art and further described below. This
type of neutron detector requires a long collection time for the
resulting ionization, requiring integrating and differentiating
time constants of between 1 and 5 microseconds for the best
results.
[0016] Other gas mixtures are commercially available that have
varying resolution or charge per pulse yields depending on the
gases used.
[0017] Neutron detection for monitoring the dose of thermal
neutrons given to patients receiving boron neutron-capture therapy
has used .sup.6Li and a cerium activator in a glass fiber. See M.
Bliss et al., "Real-Time Dosimetry for Boron Neutron-Capture
Therapy," IEEE Trans. Nucl. Sci., Vol. 42, No. 4, 639-43 (1995).
Hiller et al., in U.S. Pat. No. 5,973,328, issued on Oct. 26, 1999,
improve this technique by allowing a cerium-activated glass fiber
to be coated with fissionable elements. A wet chemistry method of
placing radioactive fissile elements into glass--which in the
vitrified state does not pose a hazard--as described in the '328
patent using sol-gel based technology, is a significant benefit. M.
Ghioni et al., "Compact Active Quenching Circuit for Fast Photon
Counting with Avalanche Photodiodes," Rev. Sci. Instr., 67, 3440-48
(1996), describe an avalanche photodiode implementation for
detecting neutron induced ionization and optical pulse
detection.
[0018] The '328 device introduced sol-gel techniques unique in the
art of neutron detection. Sol-gel chemistry was first discovered in
the late 1800s. This area of chemistry received renewed interest
when the process was found useful in producing monolithic inorganic
gels at low temperatures that could be converted to glasses without
a high temperature melting process. C. J. Brinker et al., "Sol-Gel
Science: The Physics and Chemistry of Sol-Gel Processing" (Academic
Press, Inc., New York 1990) provide a comprehensive explanation of
sol-gel chemistry. Sheng Dai et al., "Spectroscopic Investigation
of the Photochemistry of Uranyl-Doped Sol-Gel Glasses Immersed in
Ethanol," Inorg. Chem., 35, 7786-90 (1996), provide further detail
disclosing uranyl-doped sol-gel glasses.
[0019] Emissions detectors such as microchannel plates,
channeltrons, and avalanche photodiodes (APDS) are in common use
for detecting ultraviolet (UV) light and fissioned charged
particles such as electrons or protons. Microchannel plates are
commercially available and well known in the art. Typically a
microchannel plate is formed from lead glass having a uniform
porous structure of millions of tiny holes or microchannels. Each
microchannel functions as a channel electron multiplier, relatively
independent of adjacent channels. A thin metal electrode is
vacuum-deposited on both the input and output surfaces to
electrically connect channels in parallel. Microchannel plates can
be assembled in stacked series to enhance gain and performance.
[0020] The microchannel plates serve to amplify emissions from
fissionable material resulting from the bombardment of neutrons.
The amplified signal is then detected and recorded. The signal
frequency is proportional to the charged particle emissions, which
are proportional to the amount of neutrons bombarding the
fissionable material.
[0021] Typically due to the exotic materials and sensitivity of the
equipment, the neutron detectors currently available are expensive
and difficult to maintain. For example .sup.3He is an extremely
rare stable isotope and must be separated at considerable expense
from the radioactive gas tritium. Furthermore, the use of a gas
absorber results in a slower response time than a solid absorber as
disclosed herein. The '328 device thus incorporates fissionable
material into a sol-gel composition in combination with an emission
detector.
[0022] Neutron scattering is a powerful tool for conducting
scientific studies of the physical geometry of molecules important
in biology and material science. Protein structure and the
structure of superconductors are of immediate practical importance.
The detection of the scattered neutrons is an area where
advancements are necessary. Specifically of concern are the ability
to locate the scattered neutrons and the ability to rapidly process
the neutron signals as the detection rate becomes very rapid.
[0023] A major research facility using spallation constructed by
the Department of Energy is the Spallation Neutron Source (SNS)
facility in Oak Ridge, Tenn. Spallation is a nuclear reaction in
which incident particles bombard an atomic nucleus to eject
particles from the nucleus. The SNS is designed to have an output
of pulsed neutrons that is the most intense in the world of its
type. The SNS is provided with multiple experimental stations using
pulsed neutrons. One such experimental station locates the neutrons
diffracted from a target on an x-y plane. The time of the neutron
absorption on the x-y plane surface is then referenced to a
fiducial timing signal to an accuracy of within 100 nanoseconds of
the absorption event.
[0024] Mori et al., "Measurement of Neutron and .gamma.-ray
Intensity Distributions with an Optical Fiber-Scintillator
Detector," Nuclear Instruments and Methods in Physics Research, A
422, 129-132 (1999), describe a ZnS(Ag) scintillator with .sup.6Li
on the tip of an optical fiber for locating neutrons with a
position resolution of 1 mm in a 10 minute interrogation and within
a volume wherein the tip is extendable one meter. Gorin et al.,
"Development of Scintillation Imaging Device for Cold Neutrons,"
Nuclear Instruments and Methods in Physics Research, A 479, 456-460
(2002), have described using ZnS(Ag) and .sup.6LiF coupled to
wavelength-shifting fibers. Gorin et al., employ arrays of fibers
in two planes rotated ninety degrees relative to each other such
that an absorbed neutron is located in the plane to a resolution of
1 mm.
[0025] Wallace et al., Nuclear Instruments and Methods A 483 (2002)
764-773 report the gamma insensitivity of the thin film lithiated
glass. This specificity of the lithiated glass for generating a
signal in the presence of gamma radiation has application to the
monitoring of spent nuclear fuel rods and for the determination of
the fissile mass within remotely handled transuranic waste.
[0026] Other methods and devices have been developed for neutron
detection. Typical of the art are those methods and devices
disclosed in the following U.S. patents and patent
applications:
1 Patent/Appl No. Inventor(s) Issue/Filing Date 3,222,521 K.
Einfeld Dec. 7, 1965 4,365,159 C. A. Young Dec. 21, 1982 4,481,421
C. A. Young et al. Nov. 6, 1984 5,289,510 J. T. Mihalczo Feb. 22,
1994 5,336,889 K. J. Hofstetter Aug. 9, 1994 5,345,084 R. C. Byrd
Sep. 6, 1994 5,659,177 R. L. Schulte et al. Aug. 19, 1997 5,726,453
R. G. Lott et al. Mar. 10, 1998 5,880,471 J. Schelton et al. Mar.
9, 1999 5,968,425 A. Bross et al. Oct. 19, 1999 5,973,328 J. M.
Hiller et al. Oct. 26, 1999 6,134,289 A. J. Peurrung et al. Oct.
17, 2000 6,218,670 J. C. Yun et al. Apr. 17, 2001 2003/0178574 S.
Wallace et al. Sep. 25, 2003
[0027] Of these patents, Einfeld ('521) teaches a method and
apparatus for the non-destructive testing of a substance to
determine the concentration of two or more fissionable isotopes in
the substance. Einfeld teaches generation of first and second
neutron spectrums, each having a unique mean energy. Determination
of the number of fissions as a function of the neutron spectrum
applied follows from the counting of the prompt and/or delayed
neutrons produced by the fissions.
[0028] Young ('159) teaches a neutron detection apparatus including
a selected number of flat surfaces of .sup.6Li foil. A gas mixture
is in contact with each of the flat surfaces for selectively
reacting to charged particles emitted by or radiated from the
lithium foil. A container is provided to seal the lithium foil and
the gas mixture in a volume from which water vapor and atmospheric
gases are excluded, the container having one or more walls capable
of transmitting neutrons. Monitoring equipment in contact with the
gas mixture detects reactions taking place in the gas mixture and,
in response to such reactions, provides notice of the flux of
neutrons passing through the volume of the detector.
[0029] Similarly, Young et al., ('421) teach a neutron detection
apparatus is provided including a selected number of surfaces of
.sup.6Li coated wire mesh in contact with a gas mixture for
selectively reacting to charged particles emitted or radiated by
the .sup.6Li coated mesh. As in the '159 device, a container is
provided to seal the .sup.6Li coated mesh and the gas mixture in a
volume from which water vapor and atmospheric gases are excluded,
the container having one or more walls capable of transmitting to
neutrons. Monitoring equipment in contact with the gas mixture
detects the generation of charged particles in the gas mixture and,
in response to such charged particles, provides an indication of
the flux of neutrons passing through the volume of the
detector.
[0030] In the '510 patent, Mihalzco teaches nuclear reaction
detectors capable of position sensitivity with sub-millimeter
resolution in two dimensions. The nuclear reaction detectors
include two arrays of scintillation or wavelength shifting optical
fibers. Each array is formed of a plurality of optical fibers
disposed in a side-by-side relationship. The two arrays are
disposed in X-- and Y-directions with respect to each other, with a
layer of nuclear reactive material disposed between and operatively
associated with surface regions of the optical fiber arrays. Each
nuclear reaction occurring in the layer of nuclear reactive
material produces energetic particles for simultaneously providing
a light pulse in a single optical fiber in the X-oriented array and
in a single optical fiber in the Y-oriented array. These pulses of
light are transmitted to a signal producing circuit for providing
signals indicative of the X-Y coordinates of each nuclear
event.
[0031] The nuclear reactive material of the '510 patent is doped
with a phosphor such as calcium tungstate, magnesium tungstate,
zinc silicate, zinc sulfide, cadmium tungstate, and cadmium borate.
Mihalzco further teaches that a compound such as .sup.6LiF or glass
or plastic scintillators containing .sup.235U, .sup.10B, or
.sup.238U provides a concentration of the phosphor dopant in the
layer of nuclear reactive material sufficient to assure that an
adequate distribution of phosphor to be contacted by and react with
the energetic particle is produced from each nuclear reaction.
Normally, a concentration of the phosphor dopant in the range of
about 100 ppm to about 2 percent by volume is adequate for the
purposes of the Mihalzco detectors.
[0032] Hofstetter ('889) discloses a gamma radiation detector using
a radioluminescent composition. The detector includes a
radioluminescent composition that emits light in a characteristic
wavelength region when exposed to .gamma. radiation. The
composition contains a scintillant such as anglesite (PbSO.sub.4)
or cerussite (PbCO.sub.3) incorporated into an inert, porous glass
matrix via a sol-gel process. Particles of radiation-sensitive
scintillant are added to a sol solution. The mixture is polymerized
to form a gel, and then dried under conditions that preserve the
structural integrity and radiation sensitivity of the scintillant.
The final product is a composition containing the
uniformly-dispersed scintillant in an inert, optically transparent
and highly porous matrix. Hofstetter describes the resulting
composition as chemically inert and substantially impervious to
environmental conditions.
[0033] In the Byrd ('084) device, a plurality of omnidirectional
radiation detectors is arranged in a closely packed symmetrical
pattern to form a segmented detector. The output radiation counts
from these detectors are arithmetically combined to provide the
direction of a source of incident radiation. Output counts from
paired detectors are subtracted to yield a vector direction toward
the radiation source. The counts from all of the detectors are
combined to yield an output signal functionally related to the
radiation source strength.
[0034] R. L. Schulte et al., ('177) teach a directional thermal
neutron detector for detecting thermal neutrons and determining the
direction of the thermal neutron source. The directional detector
includes an array of individual thermal neutron detector modules,
each of which comprises front and back planar silicon detectors
between which is disposed a gadolinium foil. The array comprises a
plurality of individual detector modules angularly displaced with
respect to each other. The direction of the thermal neutron source
is determined by comparing the magnitudes of the output signals
from the plurality of angularly displaced detector modules. Each
thermal neutron detector module is segmented into four quadrants to
reduce its capacitance and resultant noise. The thickness of the
gadolinium foil in each thermal neutron detector module is at least
15 microns thick, to improve the front-to-back silicon detector
counting ratio to ascertain the side (front or back) from which
thermal neutrons are arriving at the detector. The thick gadolinium
foil makes each detector module substantially opaque to thermal
neutrons, and the detector modules are positioned relative to each
other in the array to shield one another from thermal neutrons,
thereby enhancing the angular resolution of the directional thermal
neutron detector. Gamma rays are discriminated against by using
coincidence signal processing within the elements of the detector
sandwich to reduce the gamma ray contribution to the total
signal.
[0035] Lott et al., ('453) disclose a radiation resistant solid
state neutron detector. The '453 detector uses a neutron converter
material such as boron or lithium to react with neutrons to create
charged particles that are received in a semiconductor active
region of the detector. The active thickness of the detector is
smaller than the range of the charged particles. Since most of the
radiation damage produced by impinging charged particles occurs
near the end of the range of the particles, displacement damage
predominantly occurs outside of the active region. Although the
charged particles pass through the semiconductor material, the
particles cause electron excitation within the semiconductor
material, the electron excitation being detected in the form of an
electronic pulse. The '453 detector is provided to increase
resistance to radiation damage, improve high temperature operation,
and to obtain real time measurements of neutron flux in reactor
cavities and other previously inaccessible locations.
[0036] Schelton et al., ('471) disclose a neutron detector for the
detection of thermal neutrons. The '471 neutron detector includes
.sup.6LiF layers for the conversion of the neutrons to ionizing
radiation. The .sup.6LiF layers are surrounded by layers for
detecting the ionizing radiation generated by the neutrons in the
.sup.6LiF layers.
[0037] The '425 patent issued to Bross et al., discloses methods
for the continuous production of the plastic scintillator material.
The methods employ either two major steps (tumble-mix) or a single
major step (inline-coloring or inline-doping). Using the two step
method, the polymer pellets are mixed with silicone oil, and the
mixture is then tumble mixed with the dopants necessary to yield
the proper response from the scintillator material. The mixture is
then placed in a compounder and compounded in an inert gas
atmosphere. The resultant scintillator material is then extruded
and pelletized or formed. When only a single step is employed, the
polymer pellets and dopants are metered into an inline-coloring
extruding system. The mixture is then processed under a inert gas
atmosphere, usually argon or nitrogen, to form plastic scintillator
material in the form of either scintillator pellets, for subsequent
processing, or as material in the direct formation of the final
scintillator shape or form
[0038] Peurrung et al., ('289) teach a system for measuring a
thermal neutron emission from a neutron source. The '289 device
includes a reflector/moderator proximate the neutron source that
reflects and moderates neutrons from the neutron source. The
reflector/moderator further directs thermal neutrons toward an
unmoderated thermal neutron detector.
[0039] U.S. Pat. No. 5,973,328 issued to J. M. Hiller et al.,
discloses a neutron detector composed of fissionable material
having ions of lithium, uranium, thorium, plutonium, or neptunium,
contained within a glass film fabricated using a sol-gel method
combined with a particle detector. When the glass film is bombarded
with neutrons, the fissionable material emits fission particles and
electrons. The '328 patent further discloses prompt emitting
activated elements yielding a high energy electron contained within
a sol-gel glass film in combination with a particle detector. The
emissions resulting from neutron bombardment can then be detected
using standard UV and particle detection methods well known in the
art, such as microchannel plates, channeltrons, and silicon
avalanche photodiodes.
[0040] Currently pending U.S. patent application 2003/0178574 filed
by the inventor of the present application, along with A. Stephan,
S. Dai and H. J. Im, discloses a neutron detector composed of a
matrix of scintillating particles imbedded in a lithiated glass.
The neutron detector detects the neutrons by absorbing the neutron
in the .sup.6Li isotope which has been enriched from the natural
isotopic ratio to a commercial ninety five percent. The utility of
the '574 detector is optimized by suitably selecting scintillating
particle sizes in the range of the alpha and the triton. Nominal
particle sizes are in the range of five to twenty five microns
depending upon the specific scintillating particle selected.
[0041] Other references of interest in the art of neutron detection
include:
[0042] H. Krinninger et al., "Pulsed Neutron Method for
Non-Destructive and Simultaneous Determination of the .sup.235U and
.sup.239Pu Contents of Irradiated and Non-Irradiated Reactor Fuel
Elements," Nucl. Instr. Meth. 73, 13-33 (1969);
[0043] M. Zanarini et al., "Evaluation of Hydrogen Content in
Metallic Samples by Neutron Computed Tomography," IEEE Trans. Nucl.
Sci., 42, 580-84 (1995);
[0044] C. M. Logan et al., "Observed Penetration of 14-MeV Neutrons
in Various Materials," Nucl. Sci. Eng. 115, 38-42 (1993);
[0045] H. Jaeger et al., "Two-Detector Coincidence Routing Circuit
for Personal Computer-Based Multichannel Analyzer," Rev. Sci.
Instrum. 66, 3069-70 (1995);
[0046] E. J. T. Burns et al., "A Solenoidal and Monocusp Ion Source
(SAMIS)," Rev. Sci. Instr., 67, 1657-60 (1996);
[0047] S. T. Coyle et al., "A Low Cost Preamplifier for Fast Pulses
From Microchannel Plates," Rev. Sci. Instr., 66 4000-01 (1995);
[0048] Y. G. Kudenko et al., "Extruded Plastic Counters with WLS
Fiber Readout," Nucl. Inst. And Meth. A 469, 340-346 (2001);
[0049] C1207-97 Standard Test Method for Nondestructive Assay of
Plutonium in Scrap and Waste by Passive Neutron Coincidence
Counting, ASTM International;
[0050] W. Harker et al., "Demonstration Neutron Multiplicity
Counter Coincidence Counting Software for Authentication," Los
Alamos National Laboratory Report LA-UR-01-4186, July 2001;
[0051] R. Hogle et al., "APNEA list mode data acquisition and
real-time event processing," 5th Nondestructive Assay and
Nondestructive Examination Waste Characterization Conference,
January 1-16, Salt Lake City, Utah., 1997;
[0052] B. D. Lebedev et al., "Monte Carlo Calculation to Optimize
the Neutron Multiplicity Counter for Measurement of Representative
Plutonium Items in AT 400 Container," Proceedings of the INMM
43.sup.rd Annual Meeting, 2002;
[0053] S. Croft et al., "Principles of Fast Neutron Detector
Package Design for Differential Dieaway Technique Assay,"
Proceeding of the INMM 43.sup.rd Annual Meeting, 2002;
[0054] Yun Chan Kang et al., "Y.sub.2SiO.sub.5:Ce Phosphor
Particles 0.5-1.4 micrometer in Size with Spherical Morphology," J.
Solid State Chem., 146 (1999) 168-175;
[0055] A. P. Bartkoetal et al., "Observation of dipolar emission
patterns from isolated Eu.sup.3+:Y.sub.2O.sub.3 doped nanocrystals:
new evidence for single ion luminescence," Chemical Physics Letters
358 (2000) 459-465; and
[0056] J. Y. Choe et al., "Cathodluminescence study of novel
sol-gel derived Y.sub.3-xAl.sub.5O.sub.12:Tb.sub.x phosphors,"
Journal of Luminescence 93 (2001) 119-128.
[0057] The trafficking in fissile material capable of being
fabricated into a nuclear weapon of mass destruction is publicized
to be so profitable that significant resources are being expended
to prevent the diversion of material for clandestine sales. Iraq
has been the focus of international attention because that
government has actively sought to acquire fissile material. Other
nations in the Mid East may want to clandestinely acquire fissile
material so as to have the option of developing a nuclear weapon in
the future. Such a clandestine hording of material would be
virtually impossible to detect if placed in cold storage without
the generation of new structures that would be detected by
satellite surveillance. Neutron detectors offer the means to
observe fissile material using active and passive measurement
techniques.
[0058] Two attributes are necessarily measured in all systems used
for the unambiguous identification of fissile material. These
include gamma radiation and the active and passive techniques of
measuring for the presence of neutrons.
[0059] The development of systems for measuring gamma rays starting
at 59.5 keV associated with Americium and extending up to 414 keV
for Plutonium is mature. The Los Alamos National Laboratory and the
Lawrence Livermore National Laboratory have developed sodium iodide
(NaI) and high purity germanium (HPGE) gamma ray detection systems
that are field portable and operator friendly for measuring
radiation in the desert. Large neutron detectors have not had as
much effort expended in the development of field detectors.
Improved technology exists to provide a solid-state neutron
detector for field measurement of neutrons using aerial surveys.
The same detector can be truck mounted allowing surveys having a
standoff from the buildings being observed.
[0060] The most useful rapid means of surveying a large area for
radiation is an aerial survey using a helicopter. A large volume
neutron detector that can be towed below a helicopter allows a
rapid survey to be conducted of any facility and the surrounding
buildings without a need to place personnel with instruments at
risk. However, large neutron detectors have not been developed for
this application. The same detectors could be placed into trucks
and any activity seen in the air can be confirmed by having surveys
made around the perimeter of the hot buildings.
[0061] With reference again to the '328 patent, which discloses the
means for producing a glass containing a high loading of .sup.6Li,
the glass is pipetted upon the surface of a silicon charged
particle detector and the triton and alpha particles from neutron
absorption enter the detector signaling the presence of neutrons.
The enhancement on the technology takes the original technique of
generating a lithiated glass using a sol-gel process and couples it
with the manufacture of a composite material where micron size
organic and inorganic scintillating particles are embedded within
the glass. This method has been demonstrated. A further development
of the technology is the placement of a thin 1 mm layer of this
scintillating composite between two arrays of orthogonal wavelength
shifting fibers. The basic 2-D imaging technique using a
scintillation layer between wavelength shifting fibers is reported
by Gorin, A., et al., Nuclear Instruments and Methods A 479, 456
(2002).
[0062] Using high speed coincidence circuitry based upon positron
emission tomography the location of neutron absorption can be
located on two of the fibers allowing a 1 mm resolution on a tile
of one square meter. The scintillation output in the inorganic
particles is roughly 100,000 photons. The spherical wave from the
ionization trail in the particle couples to a fiber above and a
fiber below the glass film and the x-y coordinate is recorded in a
high speed data acquisition system. Additionally the time of the
detection of a neutron is to be made to within 100 nanoseconds of a
fiducial timing signal. Multiple detectors are to be tiles so that
at about thirty square meters of detector area are monitored. The
final product is a system to be used at the Spallation Neutron
Source for making neutron diffraction measurements of cold
neutrons.
[0063] Specific reference is made to the MINOS neutrino oscillation
detector under construction at Fermi National Accelerator
Laboratory. In particular, reference is made to
http://www-numi.fnal.gov/minwork/info/mi- nos.sub.13 tdr.html,
Chapter 5, Scintillator detector fabrication detailing the MINOS
scintillator system. Sections 5.1 through 5.5 which is a detector
which uses an extruded plastic scintillator read out by wavelength
shifting (WLS) fibers coupled to multi-pixel photodetectors. The
polystyrene used for the MINOS detector contains a fluor at a 175
ppm doping concentration.
[0064] Coincidence detection of neutrons can be automated into
hardware as can be licensed from Los Alamos National Laboratory. In
an alternate method, every neutron that is in every neutron
detector in a plurality of detectors with respect to fiducial
timing markers is recorded. Post-processing the millions of
detected neutrons that are taken in making a fissile material mass
measurement has been developed for field measurements of
transuranic waste destined for the Waste Isolation Pilot Plant in
Carlsbad, N. Mex. The entire list of events is sorted in a few
minutes for tagging the spontaneous neutron events and by use of
calibration standards giving the fissile waste mass in a fifty-five
gallon waste drum. The same coincidence method is used for small
one and five gallon containers used for storing enriched uranium
oxide and plutonium oxide. These systems are based upon using
.sup.3He tubes containing the gas at a pressure of three or four
atmospheres embedded in polyethylene walls surrounding the material
being measured for fissile mass. The polyethylene is thermalizing
the neutrons which then enter the .sup.3He tubes. The tubes may be
concentric layers up to three deep to achieve high capture
efficiency. The present invention proposes to replace the
polyethylene with polystyrene and the .sup.6Li glass/scintillation
particulates for the .sup.3He tubes. A key figure that is a measure
of the power of the system to design the dimensions of walls and
the placement of the .sup.3He tubes in such a way as to thermalize
the neutrons and capture them in the shortest interval after their
creation. The use of polystyrene as the matrix for placement of the
.sup.6Li glass/scintillating particulates allows great design
flexibility in the selection of the density per cubic centimeter to
minimize the time for thermalization and neutron capture for
differing enclosed volumes.
BRIEF SUMMARY OF THE INVENTION
[0065] A method for manufacturing a neutron absorber/scintillating
particle matrix is disclosed. The method of the present invention
utilizes scintillating/lithiated glass composite particles having a
light emission pulse width output that is nominally less that 100
nanoseconds full-width-half-maximum (FWHM). As a result, monitoring
high count rates using a large volume efficient detector is
accomplished. In the present method, .sup.6Li glass/scintillating
particulates are coupled as a dilute homogeneous distribution
within an optically clear plastic matrix.
[0066] The present invention describes the means for manufacturing
a neutron detector so that a large volume can be observed for the
detection of absorbed neutrons. Neutron absorbing
lithiated/scintillating particulates are uniformly distributed
within a clear, optically-transparent plastic matrix. Scintillation
emission is detected by either direct coupling of the
optically-transparent plastic to a photomultiplier or via a
wavelength shifting fiber coupled between the optically-transparent
plastic block and a photomultiplier. Multiple blocks may each have
a dedicated wavelength shifting fiber. In one embodiment, a
plurality of fibers is coupled to the face of one multi-anode
photomultiplier allowing for a lower cost monitoring system. The
volume that can be made sensitive to this neutron detector design
is very large. The present invention provides a means for
interrogating large volumes of scintillation material for neutron
absorption using a basic design as a building block that is
replicated as required.
[0067] The invention has application to the quantitation of the
fissile mass within a container using the neutron absorbing plastic
in place of .sup.3He tubes. A technology is commercialized that was
developed over several years at the Los Alamos National Laboratory.
The present invention offers an alternate means of detecting
neutrons having the advantage that the cost of the neutron
absorbing element is lower cost than the gas tubes and can be
produced in large quantity by an extrusion process.
[0068] The building block element of the present invention is a
rectangular block of polystyrene doped with a dilute density of
lithiated/scintillator particulates. Polystyrene is a cost
effective plastic, but other clear plastics such as
polymethylmethacrylate are likewise useful. In particular, the
present invention is based upon the hardware being fielded for the
detection of neutrino oscillations and is further based upon the
use of a pressure extruded polystyrene block with a thin
over-coated layer of polystyrene containing titanium dioxide. The
cross section of the extrusion is rectangular and has a groove
running the length of the extruded bar. The extruded bar is cut to
a five meters length and a wavelength shifting fiber is cemented in
the groove and the fiber is extended beyond the groove to allow
placement against the face of a multi-anode photomultiplier. A
wavelength shifting fiber is placed in the groove and secured with
an optically transparent epoxy. Whereas the MINOS polystyrene
blocks contain a fluor in a concentration of 175 ppm, the present
invention uses a fine dilute dispersion of neutron absorbing
composite particles composed of lithiated glass/scintillating
particles. The volumetric density of the particulates is selected
so as to be designed to absorb a fraction of the neutrons entering
the block. Such a designed density is engineered as to the percent
absorption by the .sup.6Li using the neutronic code MCNP5 or MCNPX,
programs written at the Los Alamos National Laboratory.
[0069] A scintillation pulse occurs when an ionization path is
created in the scintillating particle. This occurs when a neutron
is absorbed in the proximate lithiated glass within the volume of
the clear polystyrene. The scintillation pulse reflects internally
from the titanium dioxide walls and impinges upon the wavelength
shifting optical fiber located in the extruded groove. The optical
fiber in turn guides a fraction of the scintillation light emission
to a location on the face of the multi-anode photomultiplier.
[0070] The present invention advances the technology of detecting
fissile material by utilizing the processing of the temporal
detection of neutrons using timing circuitry used in Positron
Emission Tomography (PET) scanners manufactured by CTI Molecular
Imaging, Inc., Knoxville, Tenn. The use of analyzing the temporal
distribution of the neutron detection moments is well known. The
relative distribution of singles, doubles and triples within a
window of time with respect to the occurrence of each neutron is
used for the measurement of the mass of fissile plutonium. In
particular, the .sup.240PU isotope spontaneously fissions, yielding
about 1040 neutrons per second per gram of .sup.240Pu. The fission
process yields a mean number of neutrons greater than two, and the
tabulated value is designated eta. In measuring the fissile content
of a container of plutonium, waste neutrons are also present from
alpha particles interacting with light nuclei with an emission of
one neutron. Since spontaneous emission has an average of a nominal
2.5 to 2.8 neutrons created and since these neutrons are born
simultaneously, the neutrons are highly correlated with respect to
their temporal detection. The process that results in the
absorption in the .sup.6Li atom is one of a random walk. For
example, if the spontaneous fission results in three neutrons being
emitted and they possess kinetic energy of 900 keV, then the
neutrons split off in different directions and are absorbed into
the wall of an enclosure composed of blocks of polystyrene
containing the .sup.6Li. Polystyrene is a compound having a formula
showing that the material is one half hydrogen and one half carbon.
The neutrons bounce off the hydrogen and the carbon and momentum is
exchanged so that with a few tens of energy exchanges, the neutrons
move through the plastic as if a gas at the temperature of the
plastic. A property of the .sup.6Li is that as these neutrons lose
energy so as to come into thermal equilibrium with the polystyrene,
the cross-section for the capture by the .sup.6Li atom becomes very
high relative to the range of the neutrons. That is to say, thermal
neutrons cannot penetrate through a .sup.6Li wall thicker than a
fraction of a millimeter without being captured. The dispersion of
the .sup.6Li in the polystyrene is quantified in milligrams per
cubic centimeter and it is this density that is determined using
the Monte Carlo codes to design a detector of dimensions so that
thermalization and capture occur with a probability near one for
the spontaneous neutrons. In this manner neutrons detected closely
in time can be attributed as being of a common origin, i.e. from a
spontaneous fission.
[0071] The present invention allows the direct measurement of the
detection of neutrons that are so proximate in time as to be
attributed with a high probability to a fission event. The present
state-of-the-art for the measurement neutrons correlated in time
uses .sup.3He tubes to capture the neutrons arranged in concentric
rings about interrogation volume. Extensive modeling is made for
these systems as can be seen in the Proceedings of the INMM
43.sup.rd Annual Meeting, 2002, paper Monte Carlo Calculation to
Optimize the Neutron Multiplicity Counter for Measurement of
Representative Items in AT400 Container and paper Principles of
Fast Neutron Detector Package Design for Differential Dieaway
Technique Assay, see www.inmm.org. The present invention replaces
the .sup.3He tubes with the slabs of polyethylene containing the
dilute uniform dispersion of lithiated glass/scintillation
particles. Neutrons are also present in fissile material from an
alpha particle impacting a light atomic number atom. A single
neutron is emitted. An ASTM Standard Test Method describes the
nondestructive assay of scrap or waste for plutonium content using
passive thermal-neutron coincidence counting, C1207-97.
[0072] A neutron detector composed of a matrix of scintillating
particles imbedded in a lithiated glass is disclosed. The lithiated
glass is formed through one of several methods including, but not
limited to: mixing scintillating particles into a high temperature
liquid; mixing powdered lithium glass and scintillating
particulates and melting the mixture to fuse the particulates in
the glass; and polymerizing a mixture of scintillating particulates
in a sol-gel lithiated glass precursor. The neutron detector is
provided for detecting neutrons by absorbing the neutrons in a
.sup.6Li isotope enriched from its natural isotopic ratio to
approximately ninety-five percent (95%). The utility of the
detector is optimized by suitably selecting scintillating particle
sizes in the range of the alpha and the triton. Nominal particle
sizes are in the range of five to twenty-five (5-25) microns,
depending upon the specific scintillating particle selected.
[0073] The neutron absorber/ scintillating particle matrix utilizes
scintillating particles having a scintillating pulse width output
less than 100 nanoseconds. Coupling of the .sup.6Li to the
scintillating particulates is accomplished as a homogeneous
distribution of the .sup.6Li within a glass produced using sol-gel
chemistry and into which scintillating particulates are embedded.
The lithiated glass is useful for generating a signal in the
presence of gamma radiation. The material is useful in
manufacturing a neutron detector so that a large area is observable
for detecting neutrons at a relatively high resolution.
[0074] The neutron detecting material is fabricated from a
scintillating material and a matrix material. The matrix material
is provided to fill spaces between particles of the scintillating
material. The matrix material is a glass having a volumetrically
high loading of a neutron absorbing material.
[0075] As a neutron is absorbed in the matrix material, ions having
a high kinetic energy are created. The ions then transverse from
the matrix material into the scintillating material, creating an
ionized path. As a result of the ionization path within the
scintillating material, a scintillation output is generated. Upon
relaxation of the ions into a non-ionized state, photons are
yielded. The photons are of a wavelength and duration
characteristic of the scintillator material.
[0076] A layer of material defines first and second opposing
surfaces. A light reflecting layer is coated on the first surface
of the material. The light reflecting layer is provided for
reflecting photons back into the material to be transmitted through
the second surface to then be detected. A detector is provided for
detecting the photons. The detector is disposed proximate the
material layer second surface. A scintillating layer is selectively
positioned between the material layer and the detector.
[0077] In an alternate employment of the material of the present
invention, the second surface of a layer of the neutron detecting
material is disposed on a quartz disk to form a rigid surface.
Optics composed of at least one lens are placed between the
material and the detector. The optics also include at least one
turning mirror as required. The optics are optimized to focus an
image on the receptor surface of the detector. In this
configuration, an incident beam of neutrons is passed through a
sample and toward the material. Scintillation pulses from an
incident beam of neutrons passing through a sample are directed
through the optics toward the detector.
[0078] In a further embodiment utilizing a layer of the neutron
detecting material, a reflective layer such as chrome is disposed
on the first surface of the material layer. A scintillating layer
such as yttrium aluminum garnet is disposed on the second surface
of the material layer. A detector is disposed proximate the
scintillating layer as described in either of the prior
embodiments, or other conventional detector arrangements.
[0079] In one such conventional detector arrangement, two planar
arrays of fibers are provided. The two fiber arrays are disposed
orthogonally with respect to each other, with a first array
disposed in a local x-direction, and a second array disposed in a
local y-direction. A plurality of PMTs is associated with each
array of fibers for receiving optical pulses from the individual
fibers. A neutron detecting material layer is disposed between the
proximal ends of each layer of fibers. Scintillation activity
generated by the detection of neutrons in the material is detected
by a fiber in each array. The location of the scintillation
activity is determined to be the coordinates of the fiber in the
first array and the fiber of the second array.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0080] The above-mentioned features of the invention will become
more clearly understood from the following detailed description of
the invention read together with the drawings in which:
[0081] FIG. 1 is a perspective illustration of a plastic neutron
absorbing element of the present invention. Preferably polystyrene
containing the lithiated glass/scintillating particles manufactured
by an extrusion process;
[0082] FIG. 2 is a perspective illustration of an enlarged portion
of the transparent plastic with the lithiated glass/scintillating
particulates dispersed therein;
[0083] FIG. 3 illustrates a particulate of a nominal twenty micron
diameter made up of an aggregate of smaller scintillating particles
within a lithiated glass matrix. is an elevation view of the
neutron detecting material of the present invention showing the
neutron detecting composite structure and one preferred means of
coupling to light detecting and processing elements;
[0084] FIG. 4 illustrates the pulse shape for BC-400 sol-gel
glass;
[0085] FIG. 5 illustrates the pulse shape for CaS:Ce sol-gel
glass;
[0086] FIG. 6 illustrates the pulse shape for CaSiO3:Mn,Pb sol-gel
glass;
[0087] FIG. 7 illustrates the pulse shape for La.sub.2O.sub.2S:Eu
sol-gel glass;
[0088] FIG. 8 illustrates the pulse shape for Lu.sub.2SiO.sub.5:Ce
(furnished by CTMI, Knoxville, Tenn.) sol-gel glass;
[0089] FIG. 9 illustrates the pulse shape for polystyrene-POPOP-PPO
(furnished by Fermi National Laboratory) sol-gel glass;
[0090] FIG. 10 illustrates the pulse shape for Y.sub.2SiO.sub.5:Ce
(P-47) sol-gel glass;
[0091] FIG. 11 illustrates the pulse shape for
Y.sub.3Al.sub.5O.sub.12:Ce (YAG) sol-gel glass; and
[0092] FIG. 12 illustrates the pulse shape for ZnO:Ga sol-gel
glass.
DETAILED DESCRIPTION OF THE INVENTION
[0093] A neutron detector composed of a matrix of scintillating
particles imbedded in plastic is disclosed. The present invention
provides a means for manufacturing a neutron absorber/scintillating
particle matrix utilizing scintillating particles having emission
properties superior to ZnS(Ag). Specific applications of the
material of the present invention include, but are not limited to,
monitoring spent nuclear fuel rods and determining fissile mass
within remote handle transuranic waste. The material of the present
invention is also useful in manufacturing a neutron detector so
that a large area is observable for detecting neutrons at a
relatively high resolution. For instance, in one application, a
resolution of approximately 1 mm is accomplished. The material of
the present invention is capable of being replicated in order to
provide a relatively large area (square meters) sensitive to
neutrons.
[0094] The present invention is an extension of the technology
disclosed in the above-referenced '328 patent issued to Hiller et
al., and incorporated herein by reference, with which there is at
least one common inventor to the present invention. As a point of
reference for description of the present invention, the '328 patent
teaches a neutron detector composed of fissionable material
contained within a glass film and combined with a particle
detector. The fissionable material incorporates ions of an element
selected from the group consisting of lithium, uranium, thorium,
plutonium, and neptunium. The neutron detector of the '328 patent
is fabricated using a sol-gel method. When the glass film is
bombarded with neutrons, the fissionable material emits fission
particles and electrons. Hiller et al., further disclose the prompt
emission of activated elements yielding a high-energy electron
contained within a sol-gel glass film in combination with a
particle detector. The emissions resulting from neutron bombardment
are then detected using standard optical and particle detection
methods well known in the art, such as with microchannel plates,
photomultiplier tubes, and silicon avalanche photodiodes.
[0095] The neutron detecting material of the present invention is
illustrated schematically at 10 in the Figures. The neutron
detecting material, or material 10, is fabricated from a
scintillating material 12 and a matrix material 14. The matrix
material 14 is provided to fill spaces between particles of the
scintillating material 12. In the present practical demonstration
of the invention the preferred neutron absorbing element is
.sup.6Li. The scintillating material 12 is in the present case
demonstrated using yttrium silicate and yttrium aluminum
garnet.
[0096] In the illustrated embodiment in FIG. 1, the neutron
detecting material 10 is formed into slabs such as through an
extrusion process. Each slab of material 10 is optically connected
to a detecting element 16 such as via the illustrated wavelength
shifting fibers 18. Data from the detecting element 16 is delivered
to a processor 20 for storage, processing and output. In the
illustrated embodiment, two slabs of material 10 are shown in
optical communication with a single detecting element 16. However,
it will be understood that more or fewer than two may be optically
connected, with limits only based on the characteristics of the
selected detecting element 16. Typical light detecting elements 16
include photomultiplier tubes, multi-anode photomultipliers,
silicon photodiodes, microchannel plates, and avalanche
photodiodes.
[0097] As a neutron is absorbed in the matrix material 14, ions
having a high kinetic energy are created. The ions then transverse
from the matrix material 14 into the scintillating material 12,
creating an ionized path. As a result of the ionization path within
the scintillating material 12, a scintillation output is generated.
Upon relaxation of the ions into a non-ionized state, photons are
yielded. The photons are of a wavelength and duration
characteristic of the scintillator material 12.
[0098] Although not illustrated, the neutron detecting material 10
of the present invention is also useful in a detector arrangement
similar to that disclosed by Byrd in the aforementioned U.S. Pat.
No. 5,345,084. In such an arrangement, an array of neutron
detectors of the present invention is used for providing the
direction of a neutron source. The array is augmented with boron
loaded polyethylene collimation to enhance discrimination with
respect to determining the direction of the source of neutrons.
Such arrangement is useful, for example, for locating a mass of
plutonium within a drum, and for measuring the curium concentration
along the length of a spent nuclear fuel rod.
[0099] Various matrix materials 14 have been used to fabricate the
neutron detecting material 10 of the present invention. Several
scintillation materials have been tested in lithiated sol-gel for
light pulse output in a gamma free port at the Intense Pulsed
Neutron Source located at the Argonne National Laboratory.
Scintillation materials used for testing include: polystyrene doped
with PPO (2,5-diphenyloxazole) as a primary flour and POPOP
(1,4-bis(5-phenyloxazol-2-yl)benzene) as a secondary flour;
Lu.sub.2SiO.sub.5:Ce; Y.sub.2SiO.sub.5:Ce; ZnO:Ga;
Y.sub.3Al.sub.5O.sub.12:Ce; CaSiO.sub.3:Mn,Pb; La.sub.2O.sub.2S:Eu;
BC-400 and CaS:Ce. Each scintillation material was individually
mixed with a lithiated sol-gel, the sol-gel then being placed upon
a 3.8 cm quartz disk of a nominal one (1) mm thickness. The disk
was coupled to a PMT using a commercial optical couplant available
from St. Gorbain. Y.sub.2O.sub.3:Eu was evaluated using a Cf-252
source. The neutron detecting material 10 of the present invention
was disposed on a quartz disk which was coupled to the face of a
Hamamatsu R580 PMT. Neutrons scattered out of the beam line into
the neutron detecting material 10 generated scintillation pulses
upon absorption by the .sup.6Li and the ionization in the
scintillation particulates from the stopping of the triton and
alpha particle. A LeCroy LT344 500 MHz digitizing oscilloscope
recorded the scintillation pulse from the photomultiplier. Each
scintillation material had a ten percent to ninety percent
(10%-90%) of peak rise time of less than sixteen nanoseconds and a
full-width-half-maximum duration of less than seventy nanoseconds.
The scintillation pulse width of the selected materials was based
upon literature and vendor reported measurements. Other materials
are commercially available having similar fluorescent output pulse
widths.
[0100] FIGS. 4-12 illustrate a further reduction to practice of the
present invention. Each figure illustrates the response from
lithiated glass absorbing a neutron containing a selected
scintillating material. Specifically, FIG. 4 illustrates the pulse
shape for BC-400 sol-gel glass; FIG. 5 illustrates the pulse shape
for CaS:Ce sol-gel glass; FIG. 6 illustrates the pulse shape for
CaSiO3:Mn,Pb sol-gel glass; FIG. 7 illustrates the pulse shape for
La.sub.2O.sub.2S:Eu sol-gel glass; FIG. 8 illustrates the pulse
shape for Lu.sub.2SiO.sub.5:Ce (furnished by CTMI, Knoxville,
Tenn.) sol-gel glass; FIG. 9 illustrates the pulse shape for
polystyrene-POPOP-PPO (furnished by Fermi National Laboratory)
sol-gel glass; FIG. 10 illustrates the pulse shape for
Y.sub.2SiO.sub.5:Ce (P-47) sol-gel glass; FIG. 11 illustrates the
pulse shape for Y.sub.3Al.sub.5O.sub.12:Ce (YAG) sol-gel glass; and
FIG. 12 illustrates the pulse shape for ZnO:Ga sol-gel glass.
Except as noted above, the scintillating powders are commercially
available. Other inorganic compounds from the same commercial
sources are available having fast--less than 100 nanosecond
fwhm--scintillation pulse output and differing wavelength emission
allowing selection to be made to the sensitivity of the selected
detector.
[0101] The lithiated glass/scintillating particulate material is
manufactured by mixing polystyrene beads with an active neutron
powder. The furnace is for the high temperature fusing of the glass
to the scintillating particles. After a lithiated
glass/scintillating particles boule is complete, the boule is
ground for the blending and fusing into the polystyrene into an
extruded shape as is used in the MINOS detector. Optical wavelength
shifting fiber and at least one Hamamatsu multi-anode
photomultiplier tube are used. A data recorder is used for
presenting the detection events.
[0102] An alternate clear plastic is polymethylmethacrylate (PMMA),
which is available in solutions that can be mixed with the
particulates and then allowed to evaporate. This approach can give
a cost effective means of loading the neutron absorbing
glass/scintillating particulates in the plastic without the expense
of the higher temperature fusion of a resin.
[0103] Powdered lithiated glass/organic scintillator are mixed into
polyethylene resin and is subsequently extruded into a bar stock
that uses the dies, such as those developed for the MINOS neutrino
oscillator detector being installed at the Fermi National
Accelerator Laboratory. An in-line production process is used,
where fluors are first tumbled with dry polystyrene pellets which
have been held under an argon atmosphere for several days prior to
extrusion. The PPO and POPOP are pre-measured into packages for
addition to a set amount of polystyrene pellets during the mixing
process. It is envisioned that this process is useful for the
replacement of the PPO and POPOP by a finely ground powder of the
lithiated glass/scintillating particles manufactured using the
patented sol-gel process.
[0104] The extrusion uses the process that coats the polystyrene
bar with titanium diode to maximize the coupling of the
scintillation pulses to the wavelength shifting fiber. Optical
fiber is coupled into the extruded groove in the block and the use
of a multianode photomultiplier is used to operate two or more
blocks of detector simultaneously.
[0105] A Cf-252 source is provided for use in testing developmental
neutron detectors. A dual channel 2 Gs/sec LeCroy digital storage
oscilloscope is used to record simultaneous scintillation from two
neutron detecting fibers. Measurements are made with one, then the
other and finally with two of the neutron detecting polystyrene
slabs facing the Cf-252 source. The measurements establish the
range of the neutrons by noting the passage of fast neutrons
through the first polyethylene slab into the second.
[0106] Electronic processing is used to show that data can be
recorded in both channels for post-processing. High speed storage
of the neutron events from more than one channel is needed for
coupling with data recorded concurrently from a global positioning
satellite (GPS) signal. Multiple channel recording from a
multi-anode photomultiplier is recordable.
[0107] After testing with ZnS:Ag and other inorganic scintillators,
the key measure is the selection of the inorganic which has the
best efficiency for coupling to the wavelength shifting fiber. This
evaluation process is used for making samples where the one mm
square fiber is coupled to the glass suspended in plastic in small
bore NMR tubes.
[0108] MCNP calculations have been used for calculating the range
of cold neutrons in the lithiated glass containing inorganic
particles. Calculations are made to select the loading of .sup.6Li
per cubic centimeter of polystyrene. Initially a loading of 2.5
milligrams of .sup.6Li per cc is used and the efficiency with
energy and the absorption per one centimeter depth steps is
calculated. The aerial detector models a detector of twenty five
blocks of four centimeter width by one meter long. Thirty layers of
one centimeter thickness is used. MCNP calculations are then
compared with measurements using the Cf-252 source. The Cf-252
spontaneous fission neutrons are similar in spectral output to that
from Pu-240 and the active fissioning of U-235.
[0109] Neutron detection on the boundary perimeters of nuclear
reactors is the most useful non-intelligence application of the
solid-state large volume neutron detector. All 103 commercial
nuclear power plants in the United States have active monitoring of
the radiation exposure sustained by the staff. The large neutron
detectors can be incorporated into the infrastructure so that any
movement of the spent nuclear fuel during the fuel reloading
process can be observed in real-time. From a dispersion perspective
the most vulnerable period for creating a radiological is during
the period when the spent fuel in being transported. Adding the
large neutron detectors would give a real-time readout of the
extent of any dispersion should the spent fuel be breached. The
detectors have the additional use of allowing security forces at
the facility to actively and remotely verify that the operations
are not deviating from an approved procedure. By observing the
neutron activity using the monitors, the security forces can follow
each step in the transfer of the spent fuel from removal to
relocation in storage.
[0110] The security enhancement that applies to commercial reactors
uses the detectors in a strictly passive role. Neutrons that are
spontaneously emitted from the spent fuel are detected by the
plastic detectors. Another very useful method for detecting hidden
fissile material is the active interrogation method. This technique
has application to the ports of entry using the large cargo
containers. Several neutron detectors are placed above and along
the sides of the large shipping containers. A deuterium-tritium
(D-T) pulsed 14 MeV neutron source as can be purchased by M F
Physics in activated while the source is moved along a rail above
the container. The between pulse gaps are used to log data from all
the neutron detectors. As soon as the D-T gun has made its run to
the end of the trailer and returned to the home position, all the
data from a nominal nine detectors are read and processed within
the time it takes for the trailer to be moved out and a second
trailer entered into the inspection booth, the decision is made as
to whether to impound the container. All the steps described have
already been demonstrated as neutron scattering from several of the
elements found present in explosives and drugs have characteristic
emission of MeV energy gamma rays. Large sodium iodide detectors
have been used for observing the presence of the lines. Just as
gamma aerial surveys can be transitioned to neutron surveys, the
neutron detectors proposed for development in the submission can be
transitioned easily to the interrogation of cargo shipping
containers now having inspections being made for drugs.
[0111] Advanced neutron detectors are ideal for the integration as
solid-state detectors in SNS [Spallation Neutron Source], for
improving personnel dosimetry, and for providing non-destructive
assay of equipment and nuclear fuel packages that contain fissile
materials. The present invention is directed to a novel technology
to synthesize highly efficient and less expensive neutron detectors
via sol-gel processing, that enables the evaluation of neutron
radiation fields in measurement regimes that were likely impossible
with previous technology.
[0112] From the foregoing description, it will be recognized by
those skilled in the art that a neutron detector composed of a
matrix of scintillating particles imbedded in a lithiated glass has
been disclosed. The neutron detector is provided for detecting
neutrons by absorbing the neutrons in a .sup.6Li isotope enriched
from its natural isotopic ratio to approximately ninety-five
percent (95%). The range of scintillating particles amenable to
detection using the present invention is limited only by
compatibility with current sol-gel methodology. Compatibility with
high temperature fusion into the lithiated glass is needed for some
scintillants. The present invention provides a means for
manufacturing a neutron absorber/scintillating particle matrix
utilizing scintillating particles having emission properties
superior to ZnS(Ag). The scintillating pulse width output is less
than 100 nanoseconds. Coupling of the .sup.6Li to the scintillating
particulates is accomplished as a homogeneous distribution of the
.sup.6Li within glass produced using sol-gel chemistry and into
which scintillating particulates are embedded. The lithiated glass
of the present invention is useful for generating a signal in the
presence of gamma radiation. The material of the present invention
is also useful in manufacturing a neutron detector so that a large
area is observable for detecting neutrons at a relatively high
resolution.
[0113] While the present invention has been illustrated by
description of several embodiments and while the illustrative
embodiments have been described in considerable detail, it is not
the intention of the applicant to restrict or in any way limit the
scope of the appended claims to such detail. Additional advantages
and modifications will readily appear to those skilled in the art.
The invention in its broader aspects is therefore not limited to
the specific details, representative apparatus and methods, and
illustrative examples shown and described. Accordingly, departures
may be made from such details without departing from the spirit or
scope of applicant's general inventive concept.
* * * * *
References