U.S. patent application number 16/524104 was filed with the patent office on 2020-04-09 for directional neutron detector.
The applicant listed for this patent is David Edward Newman. Invention is credited to David Edward Newman.
Application Number | 20200110184 16/524104 |
Document ID | / |
Family ID | 62562399 |
Filed Date | 2020-04-09 |
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United States Patent
Application |
20200110184 |
Kind Code |
A1 |
Newman; David Edward |
April 9, 2020 |
Directional Neutron Detector
Abstract
A neutron detector that indicates the direction toward a neutron
source. The detector is a proton-recoil type of detector, in which
two different scintillators are positioned on both sides of a
hydrogenous target. Proton recoil signals from the two
scintillators indicate whether neutrons arrive from the left,
right, or center relative to the detector alignment. Surprisingly
high precision can be obtained by orienting the detector so that
the counting rates in the two scintillators are equal, at which
point the target layer is directly aligned with the source.
Disclosed are thick and thin target configurations, versions for
discriminating pulses from the two scintillators, options for
assembling a multi-detector stack and array, and multiple analysis
procedures for optimally locating the neutron source.
Inventors: |
Newman; David Edward;
(Poway, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Newman; David Edward |
Poway |
CA |
US |
|
|
Family ID: |
62562399 |
Appl. No.: |
16/524104 |
Filed: |
July 28, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15477083 |
Apr 1, 2017 |
10371837 |
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16524104 |
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62436013 |
Dec 19, 2016 |
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62443700 |
Jan 7, 2017 |
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62464778 |
Feb 28, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01T 3/06 20130101 |
International
Class: |
G01T 3/06 20060101
G01T003/06 |
Claims
1. A device for locating a radioactive source, comprising: a target
comprising a substantially planar layer comprising hydrogenous
material configured to emit a recoil proton responsive to a neutron
scattering event therein, the target being oriented parallel to a
centrally positioned target plane; a first scintillator comprising
a substantially planar layer of material configured to emit a light
pulse of a first type responsive to traversal by a charged
particle, the first scintillator being parallel to the target plane
and proximate to a first side of the target layer; a second
scintillator comprising a substantially planar layer of material
configured to emit a light pulse of a second type responsive to
traversal by a charged particle, the second scintillator being
parallel to the target plane and proximate to a second side of the
target layer, the second side being opposite to the first side of
the target layer; and a processor configured to count the first
type pulses and the second type pulses, and thereby determine
whether the radioactive source is located closer to the first
scintillator or the second scintillator.
2. The device of claim 1, wherein the processor is configured to
determine that the radioactive source is on the same side of the
target plane as the first scintillator when the count of second
type pulses is greater than the count of first type pulses, and to
determine that the radioactive source is on the same side of the
target plane as the second scintillator when the count of first
type pulses is greater than the count of second type pulses.
3. The device of claim 1, wherein the processor is configured to
determine that the radioactive source is aligned with the target
plane when the count of first type pulses is substantially equal to
the count of second type pulses.
4. The device of claim 1, wherein the processor is configured to
determine an angle between the target plane and a vector toward the
radioactive source by calculating a difference between the counts
of the first type and second type pulses or a ratio of the counts
of first type and second type pulses, and by comparing that
difference or ratio to a predetermined angular distribution that
relates the angle to the calculated difference or ratio.
5. The device of claim 4, wherein the processor is configured to
determine whether the radioactive source is in front or behind the
device by comparing the calculated difference or ratio at multiple
orientations of the device respectively.
6. The device of claim 1, wherein the processor is further
configured to determine an angular size of the radioactive source
by comparing an angular distribution of the counts of first type or
second type pulses to a predetermined correlation that relates the
angular size of the radioactive source to the distribution of
counts.
7. The device of claim 1, wherein the processor is configured to
determine a direction toward the radioactive source by calculating
a difference between the counts of first type and second type
pulses at a plurality of orientations respectively and then
determining an angle at which that difference approaches zero, or
by calculating a ratio of first type and second type pulses at a
plurality of orientations respectively and then determining an
angle at which that ratio approaches unity.
8. The device of claim 1, wherein the processor is configured to
determine a direction toward the radioactive source by calculating
a product comprising the count of first type pulses times the count
of second type pulses, and then determining an angle at which that
product is maximal.
9. The device of claim 1, wherein the processor is configured to
determine a direction toward the radioactive source by calculating
to a function based at least in part on the first type pulses and
the second type pulses.
10. The device of claim 1, wherein: the target comprises a material
that emits a light pulse of a third type, different from the first
and second type light pulses, when traversed by a charged particle;
the processor is configured to count first-type events comprising
simultaneous pulses from the target and the first scintillator, and
second-type events comprising simultaneous pulses from the target
and the second scintillator; and the processor is configured to
determine a direction toward the radioactive source according to a
difference between the count of first-type events and the count of
second-type events, or a ratio of the count of first-type events
divided by the count of second-type events.
11. The device of claim 1, wherein the first scintillator layer has
a thickness related to the stopping range of the recoil proton
therein, the second scintillator layer has a thickness related to
the stopping range of the recoil proton therein, and the target
layer has a thickness related to the stopping range of the recoil
proton therein.
12. The device of claim 1, wherein the target layer and the first
scintillator are substantially transparent to the second type light
pulses, and the target layer and the second scintillator are
substantially transparent to the first type light pulses.
13. The device of claim 1, further comprising a first light guide
optically coupled to the first scintillator, a second light guide
optically coupled to the second scintillator, and a barrier of
opaque material configured to optically isolate the first and
second light guides from each other.
14. A system comprising: a plurality of targets, each target
comprising a substantially planar layer of hydrogenous material
configured to emit a recoil proton responsive to a neutron
scattering therein, each target having a first side and a second
side which is opposite the first side; a plurality of first
scintillators, each first scintillator comprising a substantially
planar layer of non-hydrogenous material that emits a first light
pulse responsive to traversal by a charged particle, each first
scintillator being proximate to the first side of one target of the
plurality of targets respectively; a plurality of second
scintillators, each second scintillator comprising a substantially
planar layer of non-hydrogenous material that emits a second light
pulse responsive to traversal by a charged particle, each second
scintillator being proximate to the second side of one target of
the plurality of targets respectively; a centrally positioned
target plane oriented parallel to the targets, wherein the target
plane is a boundary between a first region and an opposite second
region; and a processor configured to calculate a first detection
rate according to the first light pulses and a second detection
rate according to the second light pulses, and to determine that a
neutron source is in the first region when the second detection
rate is greater than the first detection rate, and that the neutron
source is in the second region when the first detection rate is
greater than the second detection rate.
15. The system of claim 14, wherein the processor is further
configured to indicate that the neutron source is aligned with the
target plane when the first detection rate is substantially equal
to the second detection rate.
16. The system of claim 14, wherein each target of the plurality of
targets is configured to emit a third light pulse, different from
the first and second light pulses, when traversed by a charged
particle.
17. The system of claim 14, further including a light sensor
configured to detect the first light pulses and the second light
pulses, and wherein the processor is configured to distinguish the
first light pulses from the second light pulses by pulse shape
analysis.
18. The system of claim 14, further including a first light sensor,
a second light sensor, and a plurality of opaque layers wherein:
each opaque layer of the plurality of opaque layers is configured
to optically isolate the first scintillators from the second
scintillators; the first light sensor is configured to detect the
first light pulses; and the second light sensor is configured to
detect the second light pulses.
19. A device for locating a neutron source, comprising: a planar
hydrogenous target layer configured to emit a recoil proton when
struck by a neutron from the neutron source; a left scintillator
positioned on a left side of the target layer and configured to
detect recoil protons therein; a right scintillator positioned on a
right side of the target layer, the right side being opposite to
the left side, and configured to detect recoil protons therein; and
a processor configured to determine that the neutron source is on
the left side of the target layer when the right scintillator
detects more recoil protons than the left scintillator, and that
the neutron source is on the right side of the target layer when
the left scintillator detects more recoil protons than the right
scintillator.
20. The device of claim 19, wherein the processor is further
configured to determine that the neutron source is aligned with the
target layer when the left scintillator and the right scintillator
detect substantially equal numbers of recoil protons.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/477,083 entitled "Directional Neutron
Detector" and filed on Apr. 1, 2017, which claims the benefit of
U.S. Provisional Patent Application No. 62/436,013 entitled
"Gamma-Blind Neutron Detector" and filed on Dec. 19, 2016, and U.S.
Provisional Patent Application No. 62/443,700 entitled
"Dual-Scintillator Radiation Detector" and filed on Jan. 7, 2017,
and U.S. Provisional Patent Application No. 62/464,778 entitled
"Directional Neutron Detector" and filed on Feb. 28, 2017; the
entire disclosures of which are incorporated by reference as part
of the specification of this application.
FIELD OF INVENTION
[0002] The invention relates to neutron detectors, and particularly
to neutron detectors that indicate the direction of the
neutron.
BACKGROUND OF THE INVENTION
[0003] Detection of nuclear bomb material is an urgent national
priority. Nuclear weapons and their components can be transported
easily in shipping containers, trucks, and rail cars. Rogue states
and non-state adversaries could use clandestine delivery for
terrorism or extortion, with little risk of detection. The US
government has ordered that all cargo be scanned for nuclear
materials at border crossings and shipping ports, but there is yet
no suitable means for doing so.
[0004] Neutron radiation is a signature of plutonium, the key
component of most nuclear weapons. However, neutron radiation can
be shielded, greatly reducing the number of detectable particles.
To detect a shielded weapon or a small portion of smuggled
plutonium, the maximum information must be obtained from any
detectable neutrons. In addition, the neutron detector must reject
background radiation such as cosmic rays and gamma rays from
various radioactive materials in the environment. Roughly 1% of the
maritime containers entering US ports have detectable gamma
radiation, primarily due to items containing bentonite clay,
potassium, granite, and some lighting and electronic devices.
Neutron emitters are far less common than gamma emitters in cargo.
Only about 0.01% of the border shipments produce a detectable
neutron emission, due primarily to radioactive sources for
industrial inspections, well logging, and research.
[0005] Neutrons from plutonium typically have an energy of about 1
MeV, with a spread in energies from about 0.5 to about 5 MeV
generally. Neutrons in that energy range interact with matter
primarily by scattering from an atomic nucleus. For most nuclei,
the scattering can be either elastic or inelastic depending on the
nucleus and other factors. For hydrogen, however, only elastic
scattering is possible since .sup.1H has no excited nuclear states.
In n-p scattering, a variable amount of energy, about half of the
neutron energy on average, is transferred to the recoil proton. The
proton emerges with an energy and direction that depend on the
scattering angle. The recoil protons with the highest energy emerge
in a direction closest to the initial neutron direction, as
required by momentum conservation.
[0006] Gamma rays typically interact with matter by photoelectric
absorption, Compton scattering, or pair production, each of which
generates one or more energetic electrons (positrons being treated
as electrons herein). Electrons with 1-2 MeV typically have a
relatively low rate of energy deposition in matter, in contrast to
the recoil protons which have a very high energy deposition rate.
Accordingly, gamma-generated electrons have a much longer stopping
range (stopping distance) than the neutron-recoil protons.
Depending on the energy and the material, gamma-generated electrons
typically travel many millimeters or even centimeters before
stopping, whereas recoil protons typically stop in a few microns to
a few tens of microns.
[0007] A directional neutron detector would be a valuable
inspection tool by helping inspectors to localize a source of
neutron radiation. Determining the neutron direction would greatly
amplify the statistical power of each detection. For example,
during a 60-second vehicle scan, two or three detected neutrons
would probably not be sufficient to trigger an alarm, since
background neutrons are always present from cosmic rays and
environmental sources. But detecting two or three neutrons coming
from the same place in the vehicle would certainly be suspicious,
thereby prompting a secondary examination. For revealing neutron
threats, the overall effectiveness of a directional neutron
detector is about two orders of magnitude greater than a simple
non-directional detector due to the localization of the source.
[0008] What is needed, then, is a neutron detector that indicates
the neutron direction, focusing on the few-MeV energy range,
suitable for scanning whole containers and vehicles at shipping
ports and border crossings. Preferably such a detector would also
enable improved scanning of personnel in a walk-through portal
application, and would also lead to an improved direction-dependent
neutron survey meter. The detector should have high detection
efficiency for neutrons, yet have excellent rejection of gamma rays
and other non-neutron backgrounds. Preferably the detector uses no
scarce materials, and has low cost.
SUMMARY OF THE INVENTION
[0009] The invention is a proton-recoil neutron detector that
indicates the direction of the neutron. The detector comprises a
hydrogenous target material configured as a substantially planar
layer, with two substantially planar scintillator layers positioned
proximate to opposite sides of the target. An incident neutron
scatters in the target by n-p elastic scattering, thereby expelling
a recoil proton which passes into one of the scintillators, The two
scintillator layers are substantially parallel to each other, and
are configured to produce light pulses when traversed by a charged
particle such as the recoil proton. The scintillator struck by the
recoil proton then responsively emits a light pulse, which is
detected by a light sensor. The two scintillators thereby indicate
the direction from which the neutron arrives (or equivalently, the
direction to the neutron source) since they reveal the direction of
the recoil proton, which is related to the neutron direction by
momentum conservation.
[0010] For example, if the neutron arrives from the right side of
the detector and scatters in the target, the recoil proton is most
likely to emerge toward the left since it acquires a portion of the
neutron momentum. Hence the scintillator proximate to the left side
of the target would register the hit. If the neutron arrives from
the left, the recoil proton is highly likely to hit the
scintillator on the right. Furthermore, the detector can precisely
indicate the source direction, by being rotated until both
scintillators exhibit about the same counting rate. The two
scintillator signals indicate when the target layer is directly
aligned with the neutron source, because the recoil proton is then
equally likely to scatter into the first and second scintillators.
Thus the inventive detector indicates the left-right direction
toward the neutron source (relative to the detector orientation),
and also indicates when the detector is pointing directly toward
the source, according to the two scintillator detection rates. Such
a detector would be extremely useful for localizing a neutron
source such as a clandestine nuclear weapon, smuggled nuclear
components, or inadvertent contamination.
[0011] The first and second scintillators are proximate to opposite
sides of the target layer. Herein, a "planar" layer is a flat layer
in which the thickness is much smaller than the other two
dimensions. The term "opposite" has its geometrical meaning,
wherein the two sides or faces of a planar layer are substantially
orthogonal to the thickness direction and are on opposite sides of
the layer.
[0012] Neutron scattering involves a random scattering angle that
determines how much energy the recoil proton receives, and also
affects the recoil proton direction. The directional correlation
between the neutron and the recoil proton is strongest for the
highest energy protons since they receive most of the initial
neutron momentum, whereas the lowest energy protons have the
largest angular uncertainty. Beneficially, the highest energy
protons are also the ones most likely to escape from the target and
be counted, while the lowest energy protons are preferentially
absorbed in the target. Simulations show that, as a result of this
angular correlation effect, a small number of neutron detections
are sufficient to localize a neutron source to within a few
degrees.
[0013] Numerous configurations of the inventive detector are
disclosed herein. Versions are disclosed with thick or thin
targets, single or double targets, optional light guides and
reflectors and barriers in various relationships. Further versions
employ different methods to determine which scintillator is active,
involving different scintillator properties and multiple sensors.
Configurations of stacked detectors are disclosed comprising
multiple targets and multiple scintillators in stacked layers, to
obtain increased detection speed and efficiency. The invention
includes various methods for analyzing the scintillator data versus
angle to obtain high precision, and various fabrication methods as
detailed below. The common and essential feature among all the
versions of the invention is that there are two scintillators on
opposite sides of the hydrogenous target, so that the recoil proton
indicates the neutron direction according to which scintillator is
activated.
[0014] The hydrogenous target comprises any solid material that
includes hydrogen, preferably with a high density of hydrogen.
Example target materials include polyethylene or other polymer, a
hydrated crystal or mineral, or other hydrogen-bearing compound.
The target may comprise a precast sheet or may be deposited as a
vapor or may be applied as a liquid film which is then polymerized
or dried for example. Preferably the target is substantially
planar, since a flat geometry provides the best angular resolution.
The target may have any thickness. For example the target may be
very thin and have a thickness related to the recoil proton
stopping range, which is typically a few microns to a few tens of
microns. Or, the target may be thick enough to transport
scintillation light, which usually is in the range of 0.1 to 10 mm.
Usually the target is non-scintillating, to avoid producing
background light. But in some embodiments the target is itself a
scintillator which produces a light pulse different from that of
the other scintillators.
[0015] Each scintillator produces a light pulse when traversed by
the recoil proton. The scintillators are preferably parallel to
each other and to the target so that the finest angular resolution
is obtained. Any deviation from parallelism or planarity of the
scintillator layers would tend to broaden the angular resolution of
the detector accordingly. The scintillators are preferably very
thin, with a thickness related to the recoil proton stopping range.
The thinness enables rejection of gamma rays and other
minimum-ionizing backgrounds that produce very little light in
passing through the thin scintillators, whereas recoil protons have
a very high rate of ionization and thus generate a large light
signal. On the other hand, the scintillator should not be so thin
that the recoil proton produces insufficient light for reliable
detection. Often the scintillator thickness is equal to the maximum
expected proton stopping range, thereby obtaining the maximum light
from the proton track.
[0016] To indicate the neutron direction, the inventive detector
must determine which scintillator was struck. In a first version,
termed the "pulse-shape-discrimination" version, the two
scintillators comprise different materials that produce detectably
different light pulses with different duration or shape. In a
second "wavelength-discrimination" version, the two scintillators
emit light of detectably different wavelengths, which are then
separated by two optical filters and detected by two light sensors.
In a third "light-path-discrimination" version, the two
scintillators have the same composition and identical light
properties, but the light pulses from the two scintillators are
kept separate using reflectors and opaque barriers. In each case,
the output signals indicate which scintillator was hit, and thereby
indicate the direction of the incident neutron.
[0017] The scintillators are preferably hydrogen-free to avoid
producing recoil protons that would interfere with the directional
measurement. The scintillators are preferably planar layers so as
to obtain precise angular measurements. Suitable materials are any
inorganic scintillator such as CaF.sub.2 or BGO or cerium-activated
glass. Preferably the glass is free of lithium and boron, to avoid
neutron interactions that would interfere with the proton recoil
measurement. Hygroscopic scintillators such as CsI or NaI are
possible if well-sealed from environmental moisture. ZnS is
possible although it is not transparent to its own light, causing
problems with light transmission.
[0018] Although not preferred, it is possible that the
scintillators could contain hydrogen, such as plastic scintillator
sheets which have the advantage of low cost and easy assembly. The
disadvantage is that n-p scattering events in the scintillators
would partially, but not entirely, obscure the directional
measurement. The interference could be minimized by making the
scintillators much thinner than the proton stopping range, or
fabricating them with deuterium substituting for hydrogen, or
selecting a polymer with minimal hydrogen content.
[0019] The scintillator light is conveyed to the light sensor. In
the thick-target version, the target itself serves as the light
guide, and is optically coupled to one or both of the
scintillators, and is optically coupled to at least one sensor. In
the thin-target version, the light is carried by one or two
additional light guides, each comprising a non-hydrogenous
transparent body that is optically coupled to one or two
scintillators and to at least one sensor.
[0020] The light sensor or sensors are any transducers that produce
an electronic output signal related to the light received by the
sensor, such as photomultiplier tubes, silicon avalanche
photodiodes, and the like.
[0021] The detector can include a thin reflective layer between
each scintillator and its proximate target. The reflective layer
may assist in light transmission, or isolate the two scintillators
optically, or may protect the material surfaces for example.
However, such a reflective layer must be extremely thin to avoid
blocking the recoil protons. Preferably the reflective layer is
just thick enough to provide the reflectivity or opacity needed,
and no thicker. For example an aluminum deposit with a thickness of
20 to 200 nm may be suitable.
[0022] The detector can include an opaque barrier between two light
guides to isolate them optically. This is important for
light-path-discrimination wherein the two scintillators comprise
the same material with the same light pulse properties. The opaque
barrier ensures that the light from the first scintillator reaches
a first sensor, and the light from the second scintillator reaches
a second sensor with no cross-talk, that is, the two light paths
are isolated from each other. The barrier may also be reflective to
enhance light transmission. Aluminum foil or black paper are
possible choices.
[0023] Often the invention comprises a stack of detectors
comprising multiple targets and multiple scintillators of the first
and second types, all arranged in a particular sequence or order,
so as to indicate the direction from which a neutron has arrived,
according to whether a first or second type scintillator is active.
Such a stack provides increased detection efficiency due to the
large number of targets. Preferably the two scintillator types
alternate in position throughout the stack, with a first
scintillator to the left of a target and then a second scintillator
to the right of the target, and so on. Or the two scintillators
could be on alternate sides of successive targets, with the same
effect. Such alternation enables rejection of many penetrating
backgrounds, for example penetrating electrons or muons that
trigger both the first and second scintillator types, and thereby
can be rejected.
[0024] The invention includes an array of detectors, each detector
being oriented at a different angle. Each such detector can
comprise a single-target device or a double-target configuration or
a detector stack having many parallel targets and scintillators.
The detectors (or detector stacks) are oriented at different
angles, such as an angularly spaced-apart array of detector
orientations. The signals from the first and second scintillators
in each of the detectors can then be analyzed versus the detector
angle, to determine where the neutrons are coming from. If one of
the detectors in the array happens to be aimed directly or almost
directly at the source, then the two types of scintillators in that
detector will register nearly equal counting rates, thereby
providing enhanced angular precision in the source location. An
advantage of such an array is that the neutron source can be
localized without rotating the detector.
[0025] The invention includes methods to process the scintillator
signals to determine the neutron angle, and particularly to
localize the source precisely. The detector can be rotated until
the two scintillators exhibit the same counting rate, at which
point the source is then directly aligned with the target plane. Or
the detector can be rotated to find the two angles at which the two
scintillators produce about half their maximum counting rates, in
which case the source angle is the average between those two
half-maximum-rate angles. Or, a function such as the product of the
two counting rates can be formed, in which a clear localized peak
indicates the source direction. Or, the difference between the two
counting rates can be checked, in which a zero-cross condition
indicates the source azimuth.
[0026] Further inventive methods are disclosed below for assembling
the detector in both thin-target and thick-target configurations.
Methods are also provided for construction of the detector stack
configuration in various versions, including low-cost mass-produced
subassemblies that are easily put together in detector arrays of
unlimited size.
[0027] The inventive detector offers many advantages. First, it
provides a reliable left-right indication of the source direction
quickly, as soon as two or three neutron scattering events have
been detected. Second, with further data as a function of the
detector angle, the direction of the neutron source is determined
with surprising precision. Third, the detector is highly
insensitive to background gamma rays since the scintillators, being
only microns-thick, produce almost no light when traversed by a
Compton electron or other gamma-generated electron. Other
backgrounds can be rejected since they most probably will trigger
both scintillators.
[0028] Further advantages pertain to manufacturability and
economics. The inventive detector is simple to manufacture,
requiring only layered depositions onto a light guide substrate,
followed by attachment of the light sensor. The inventive detector
is readily expanded to arrays, including very large arrays,
suitable for rapid vehicle inspections and cargo scanning. The high
cost of prior-art detectors is due to their reliance on costly
scintillators, expensive light sensors, sealed tubes, fine wires,
gas-treatment facilities, and vanishing materials such as .sup.3He.
The inventive detector has none of these defects. The inventive
detector uses scintillator materials very sparingly and only in
micron-thin layers, it is compatible with a very wide variety of
light sensors including the slower and lower-cost solid state
sensors, and it has absolutely nothing to do with .sup.3He.
[0029] Many critical applications, previously deemed economically
infeasible, may now be addressed with the new low-cost directional
neutron detection system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a cross-section sketch of a neutron detector
according to the invention, in the thick-target configuration, with
the pulse-shape-discrimination version. A typical neutron
scattering event is shown. The various layers are shown
schematically. (Thin layers are not to scale. The thin layers are
shown greatly expanded.)
[0031] FIG. 2 is a cross-section sketch of a neutron detector
according to the invention, in the thin-target configuration, with
the wavelength-discrimination option. (Thin layers are not to
scale.)
[0032] FIG. 3 is a cross-section sketch of a neutron detector
according to the invention, in the thin-target configuration, with
the light-path-discrimination option. Reflectors and barriers
isolate light from the two scintillators. (Thin layers are not to
scale.)
[0033] FIG. 4 is a cross-section sketch of a neutron detector
according to the invention, in the double-target configuration,
with pulse-shape-discrimination. (Thin layers are not to
scale.)
[0034] FIG. 5 is a cross-section sketch of the inventive detector
stack in the thick-target configuration, with
wavelength-discrimination based on two optical filters. Any number
of layered detectors may be assembled by continuing in this
fashion. (Thin layers are not to scale.)
[0035] FIG. 6 is a cross-section sketch of the inventive detector
stack in the thin-target configuration, pulse-shape-discrimination,
with all of the light guides viewed by a single light sensor. (Thin
layers are not to scale.)
[0036] FIG. 7 is a cross-section sketch of the inventive detector
stack in the thin-target configuration with
light-path-discrimination in which reflectors and barriers keep the
light of each scintillator separate. (Thin layers are not to
scale.)
[0037] FIG. 8 is a perspective sketch of the inventive detector
stack, demonstrating how the stack can be rotated to determine the
neutron direction. (Thin layers are not to scale.)
[0038] FIG. 9 is a sketch of five inventive detector stacks
arranged in an array, with each detector oriented in a different
direction. The array determines the neutron direction in real time
without having to rotate the detector.
[0039] FIG. 10 is a sketch of the inventive detector stack with
trapezoidal light guides and a curved overall shape. The various
hydrogenous targets in the stack sample all different angles,
thereby obtaining data on neutron directions without having to
rotate the detector.
[0040] FIG. 11A is a graph showing how the simulated neutron
detection rate of the two scintillators varies with angle relative
to the neutron source. The source lies in the direction where the
two rates are about equal.
[0041] FIG. 11B is a graph showing how the product of the simulated
counting rates of the two scintillators exhibits a peak exactly in
the direction of the source.
[0042] FIG. 11C is a graph showing how the difference between the
simulated counting rates of the two scintillators exhibits a
zero-crossing at the angle of the neutron source.
[0043] FIG. 12 is a sketch in perspective and partially exploded
showing how various subassemblies of the stack configuration of
FIG. 7 can be prepared. (Thin layers are not to scale.)
[0044] FIG. 13 is a cross-section assembly sketch showing how the
subassemblies of FIG. 12 can be assembled into a stack according to
FIG. 7. (Thin layers are not to scale.)
[0045] FIG. 14 is a sketch in exploded perspective of the inventive
detector stack, with optional secondary light guides and a light
funnel.
[0046] FIG. 15 is a flowchart showing steps of an inventive method
for assembling the thick-target detector of FIG. 1.
[0047] FIG. 16 is a flowchart showing steps of an inventive method
for assembling the thin-target detector of FIG. 2.
[0048] FIG. 17 is a flowchart showing steps of an inventive method
for deriving the neutron direction from the A and B scintillator
counting rates.
DETAILED DESCRIPTION
[0049] The inventive target is usually a polymer such as CH.sub.2,
or it could alternatively be a hydrated mineral or ceramic layer
(for high temperature applications), or other compounds that
include hydrogen. The target could be any thickness, with different
advantages for each. A thin target, with a thickness related to the
recoil proton stopping range, is efficient in that many of the
recoil protons are able to escape from the thin target layer and be
detected. Such a thin target is easily applied in sheet or film
form, or deposited as a liquid layer that is then polymerized or
dried to achieve the desired thickness. Usually in the thin-target
configuration a pair of transparent non-scintillating
non-hydrogenous light guides are optically coupled to the two
scintillators respectively, to enable efficient transport of the
light to the sensor.
[0050] In the thick-target version, the target is a transparent
hydrogenous plate, usually a polymer, which is thick enough to
serve as a light guide, and may also provide structural strength to
the assembled system. Usually there is no need for additional light
guides since the thick target light guide carries the scintillator
light to the sensor. The thick target is less efficient than the
thin target, because the stopping range of a recoil proton is only
a small fraction of the total target thickness, so only those
neutron scattering events that occur very close to the surface of
the target are detectable. However, the thick-target configuration
has the advantages of simplicity and economy and ruggedness.
[0051] As a further alternative, a thin target could be split into
two separate layers. In this case the detector is centered on a
single transparent non-hydrogenous non-scintillating light guide.
The two scintillators are applied to the two sides of the light
guide, and two hydrogenous targets are affixed on the outside
surfaces of the two scintillators. Although the two scintillators
are proximate to two different targets in this case, it remains
true that one scintillator is to the left of its target while the
other scintillator is to the right of its target, so that the
relative counting rates of the two scintillators again indicate the
direction of the neutron. The advantage of the double-target
configuration is its extreme structural simplicity and ruggedness,
since the device consists typically of a simple glass plate with
scintillator material deposited on both sides, then coated in a
thin plastic target layer.
[0052] The inventive scintillators are preferably non-hydrogenous,
which largely rules out organic compounds. Organic scintillators
contain hydrogen which would lead to interference from proton
recoil events in the scintillators. Many suitable inorganic,
non-hydrogenous scintillators are available, depending on the type
of discrimination for determining which of the scintillators has
detected the recoil proton. For the pulse-shape-discrimination
version of the invention, the scintillators must produce light
pulses with different pulse durations. Suitable scintillators could
be CaF.sub.2(Eu) or BGO which have pulse durations of 900 and 300
ns respectively, and which are sufficiently distinct that standard
electronics can easily separate the two pulse types. Alternatively,
the two scintillators could be NaI(Tl) and CsI(Na) with pulse
durations of 230 and 460 ns respectively, although these two are
not as far apart. In addition the latter pair is hygroscopic, and
thus would require a hermetic enclosure. Non-activated CsI is an
option. Various glass formulations may be considered. Although
glass tends to be less bright than the other inorganics, thin
layers of glass scintillator are relatively cheap and can be
assembled in layered stacks easily. Also, the light pulse
properties are somewhat adjustable by varying the composition of
the glass. The glass may be activated with Ce or Tb, but preferably
not with B or Li, to avoid interference from neutron capture
reactions.
[0053] For the wavelength-discrimination version of the invention,
the two scintillators must produce light pulses with different
wavelength ranges. Suitable scintillators could be CaF.sub.2(Eu)
and unactivated BaF.sub.2 which have a primary wavelengths output
of 435 and 220-310 nm respectively. These are well separated in
wavelength, although the latter short wavelength may require use of
UVT materials for the waveguide and sensor, or a wavelength
shifter. To separately detect the light from the two scintillators,
a pair of light sensors with optical filters is used. Typically the
two filters would be a high-pass and a low-pass filter without
significant overlap. In this case, the two filters would have a 50%
cutoff at about 370-380 nm. Alternatively, if hygroscopic materials
can be accommodated, NaI(Tl) and CsI(Tl) emit at 415 and 540 nm
which could be easily separated with dichroic filters. An advantage
of dichroic filters is that they can be made to reflect, rather
than absorb, the out-of-band photons, thereby allowing the other
detector to receive them. There are many other scintillator
possibilities for both pulse shape and wavelength discrimination
besides these examples.
[0054] For the light-path-discrimination option, any
non-hydrogenous scintillators would do, so long as the two
scintillators are coupled to two separate light guides and viewed
by two separate light sensors. Typically, the detector includes a
number of reflective layers and opaque barriers to define each
light path separately, and to block light from the other path. An
advantage of the light-path-discrimination version is that the two
scintillators can use the same type of scintillation material,
thereby simplifying the fabrication process and also ensuring
similar performance for the two sides. The
light-path-discrimination version requires that several extra
reflective layers be applied to the various materials, in order to
keep the light paths separate, but this may not be a problem since
it is relatively easy to deposit a thin layer of aluminum or gold
to a surface.
[0055] As a further option, plastic scintillators may be considered
even though they contain hydrogen. Neutron scattering events in the
scintillators would simply add background counts to the target
recoil events, thereby diluting the measurement. But this may be
tolerable if the scintillators are particularly thin, or have a
relatively low hydrogen content compared to the target material.
The advantage is that plastic scintillators are economical and easy
to assemble in large arrays.
[0056] As a further alternative, the two scintillators could be
inorganic scintillators, while the target could comprise a plastic
scintillator. Since plastic scintillators are hydrogenous, such a
target would emit a characteristic light pulse upon each n-p
scattering as the recoil proton travels through the
target-scintillator. The light properties of the plastic
scintillator must be detectably different from the other two
scintillators, so that the target light pulses can be separated
from the other two scintillator signals. An acceptable event would
then include a target light pulse, plus one of the other
scintillator light pulses at the same time. Events with all three
scintillator pulses would be rejected as backgrounds. Additionally,
events with only the target scintillator pulse alone would be
tallied separately as a measure of the overall neutron flux.
[0057] The thin-target configuration of the detector includes two
non-hydrogenous light guides, one coupled to each of the
scintillators, to convey the light pulses to the light sensor. The
advantage of making the light guides hydrogen-free is that such a
material does not generate recoil protons which would interfere
with those from the target. Typically the light guides are made of
glass, with a thickness of about 0.1 mm to 10 mm depending on how
far the light is to be conveyed. The light guide may include a
wavelength shifter to reduce absorption in the light guide, or for
better matching to the acceptance bandwidth of the light sensor, or
to randomize the photon directions in the light guide, or for other
reason.
[0058] Light sensors are optically coupled to the light guides in
the thin-target configuration, or to the target in the thick-target
configuration. The light sensor is any device that produces
electrical signals responsive to scintillation light, thereby
indicating which of the scintillators was hit. The light sensors
may be photomultiplier tubes, solid state sensors such as SiPM
avalanche photodiodes, or other light transducers. In the
pulse-shape-discrimination version, the sensor must be fast enough
to resolve the pulse shape or duration. In the
wavelength-discrimination and the light-path-discrimination
versions, on the other hand, the light sensor can be a slow or
charge-integrating detector such as a CCD, provided that the noise
and backgrounds are low enough that the neutron events dominate.
One or more light sensors can view a single light guide or multiple
light guides together.
[0059] The invention includes a method for determining the neutron
direction, based on the signals in the two scintillators (which may
be termed scintillator-A and scintillator-B). In a first method,
the "rotated-detector" method, the directional detector is rotated,
usually about a vertical axis, while the scintillator-A and -B
detection rates are recorded or otherwise monitored. When the A and
B counting rates are equal, the detector is oriented directly
toward the source.
[0060] A simpler version of this method is to subtract one counting
rate from the other. This differential function has a clear
zero-cross at an angle corresponding directly to the source. An
advantage of the differential method is that it is not necessary to
find the exact orientation with equal counting rates, since a curve
fit to the other angle measurements can define the zero-cross point
precisely.
[0061] When the target is exactly aligned with the source, it is
impossible to tell if the source is in front or behind the
detector, since both scintillators are symmetrically positioned
relative to the target. However this is easily resolved by turning
the detector a few degrees and noting which scintillator then has a
higher counting rate. For example, a detector may be constructed
with scintillator-A to the left of the target and scintillator-B to
the right side of the target, as viewed by the operator from behind
the detector. First the detector is rotated until the two counting
rates are equal, and then is rotates 45 degrees to the left
(counter-clockwise as viewed from above). If scintillator-A has the
higher counting rate, the source must be in front of the detector,
and if scintillator-B is higher the source must be behind the
detector. Thus the front-back ambiguity is resolved.
[0062] The equal-counting-rates method and the zero-cross method
depend on the overall detection efficiency of the two scintillators
being about equal. Usually the detection rates can be equalized by
adjusting an electronic setting such as a threshold. But if that is
not possible, the two scintillators can be calibrated, or
"normalized", by dividing the counting rate of each scintillator by
the maximum counting rate seen at any angle. Normalizing in this
way eliminates any effects of differential efficiency between the
two scintillators.
[0063] As an alternative analysis method, the detector could be
rotated until the maximum scintillator-A counting rate is
determined, and then rotated back until the counting rate is
one-half the maximum value, and a first angle noted. Then the
operation could be repeated for scintillator-B, and a second angle
noted where the B counting rate is half its maximum value. The
neutron source direction is then the average of the two
"half-maximum" angles so determined. This method does not require
that the two scintillators be matched or normalized.
[0064] As a further method, a function of the two scintillator
rates can be formed that exhibits a peak or other distinct feature
at the source angle. For example, the product of the two counting
rates usually shows an obvious peak when centered on the source.
The peak is due to the neutron and proton having nearly equal
masses; it is nearly impossible for a neutron to scatter a proton
backwards. Therefore the detector will register counts in both
scintillators only when the target is nearly aligned with the
source.
[0065] All of the analysis methods should closely agree, if the
source subtends a small angle relative to the detector angular
resolution, which is normally a few degrees. If the various methods
give different answers, that means there are probably multiple
neutron sources present. Investigators might appreciate knowing
that, before approaching the inspection item any closer.
[0066] In some cases it may be inconvenient to rotate a detector to
determine the scintillator-A and -B counting rates versus angle.
Therefore the invention includes a "multiple-angled-detectors"
method, in which a plurality of directional detectors is arranged
in an array with each detector oriented at a different angle, and
the A and B counting rates are monitored for each of the detectors.
The various detectors exhibit a higher counting rate in the A or B
scintillator depending on whether each detector is pointing to the
left or right of the source, and the particular detector that shows
about equal counting rates in the A and B scintillators is pointing
directly toward the source. For example, successive detectors could
be oriented at 10 degrees, 20 degrees, 30 degrees, and so forth,
relative to some direction. With such an array there is no need to
rotate the detectors; one simply compares the A and B counting
rates for the different detectors to determine the neutron
direction. The various detectors may be mounted in a vertical
array, each pointing at a different angle. Or the detectors could
be arranged in a horizontal array, or in a circular mounting, or a
wall of detectors all pointing in different directions, or any
other spatial distribution so long as they point in different
directions and preferably do not obscure each other.
[0067] Often the detector is assembled in a multilayer stack with a
large number--typically tens to hundreds, possibly thousands--of
scintillator layers and target layers. Each scintillator is either
of a first type, positioned to a first side of a proximate target,
or of a second type, positioned to a second side of a proximate
target, the second side being opposite to the first side. The
entire stack may be viewed by one light sensor or a plurality of
light sensors, all viewing the entire stack at once or viewing
sections separately. Further light collectors may be arranged
around the stack to convey light from each of the light guide
layers to the light sensor. Light funnels can further collect the
light. The various light sensors can be arranged all around the
detector to view all the layered light guides.
[0068] As a further option, the inventive detector may comprise a
"curved stack" in which the various hydrogenous targets are all at
slightly different angles. To accomplish this, the light guides are
shaped like narrow truncated triangles or trapezoids, with one end
slightly wider than the other end. The two faces of the light guide
are not parallel to each other, but are at slightly different
angles. With such a trapezoidal-shaped light guide, the angular
orientation of each target in the detector points in a slightly
different direction, each target orientation being determined by
the accumulated rotation effect from all the trapezoidal light
guides. Each light guide then couples to a separate light sensor.
Solid-state sensors are good for this since they can be made long
and thin to fit the light guide. It is likely that one of the
hydrogenous targets in the stack will be oriented along the neutron
direction, in which case the A and B scintillators associated with
that particular target would register equal detection rates. All
the other targets would have more counts scattering to the left or
right. The target with equal left and right scattering thus points
directly toward the source.
[0069] The curved stack with trapezoidal light guides has another
advantage. Light in each trapezoidal light guide bounces off the
non-parallel surfaces of the light guide, and is successively
reflected toward the larger end of the trapezoid. With each
successive reflection off the light guide surfaces, the light will
be increasingly redirected outward to the larger end. By placing
the light sensor on the large end of the light guide, the light
collection is thus improved.
[0070] Since the invention is a directional detector, and since the
neutron changes direction when it scatters, only the first
scattering event is useful. Any neutron that scatters in material
around the detector and then is detected in the detector, would not
useful for localizing the source. Each scattering event introduces
a random scattering angle to the neutron direction. Scattering in
the vicinity of the source is acceptable, so long as the neutron
still arrives at the detector from about the source location. For
example, shield material around a source would cause local
scattering that effectively spreads out the source region, but the
inventive detector would still find the centroid of that larger
effective source region.
[0071] To minimize scattering ahead of the detector, any
unnecessary material should be removed from the region of the
detector facing the item to be inspected. This includes both
hydrogenous and non-hydrogenous material, since all nuclei can
scatter neutrons and ruin the directional correlation. For the same
reason, the inventive detector is preferably not deep enough to
promote double scattering in the detector. Here the detector depth
is the length of the target layer, as seen by the incoming neutron
when the target layer is aimed at the neutron source. Neutrons with
1-2 MeV have a scattering length in polyethylene of about 2 cm, so
a detector depth of 2-4 cm should be sufficient to scatter most of
the neutrons once, without generating many double-scattering
events. In some embodiments, the invention may comprise a large
array, for example forming a wall adjacent to an inspection item,
the wall having a thickness of 2-4 cm to limit double scattering,
and an overall length and height corresponding to the size of the
inspection item. Even more preferably, the detector array could
form a tunnel that nearly surrounds the inspection item, such as a
vehicle for example.
[0072] The directional neutron detector may be followed or backed
up by a second neutron detector which is not directional. The
secondary detector would catch any neutrons missed by the
directional detector. The secondary detector would also detect the
once-scattered neutrons that the directional detector has detected.
A wall of directional neutron detectors followed by a wall of
non-directional neutron detectors would provide a sensitive tool
for detecting nuclear materials. However, some neutrons may be
back-scattered from the secondary detector and could re-enter the
directional detector from the rear. Such wrong-way neutrons could
then produce random signals in the directional detector. The
backscattering interference may be mitigated by vetoing any event
that has counts in both the directional and secondary detectors at
the same time.
[0073] Turning now to the figures, FIG. 1 is a cross-section sketch
of the inventive detector in the thick-target configuration, with
the pulse-shape-discrimination option. The sketch is not to scale.
The thin layers (scintillators) are shown greatly expanded for
visibility. In an actual embodiment, the thin layers would be so
thin that they would not even show up in a sketch of this type.
[0074] The detector comprises a thick transparent hydrogenous
target layer 103, flanked by a thin first scintillator 101 and a
thin second scintillator 102. The scintillators 101 and 102 are
both optically coupled to the thick target 103, which is optically
coupled to the light sensor 105. The scintillators 101 and 102 are
substantially parallel to each other, and are positioned on
opposite sides of the target 103. The thick target 103 comprises a
hydrogen-rich material such as polyethylene, in a transparent form
such as cast polyethylene. Other hydrogenous materials would also
work including polystyrene, polycarbonate, and many other
transparent polymers. The thick target 103 has a thickness
determined by the need to transport light from the scintillators
101 and 102 to the sensor 105. In most cases, such a thickness is
far greater than the proton stopping distance, hence it is "thick".
Due to the short stopping range of recoil protons, only the
outermost thin regions of the target 103 are effective for
producing recoil protons that can be detected in the proximate
scintillator 101 or 102.
[0075] The sketch shows a neutron 120, shown as a solid arrow,
arriving from the left and scattering in the target 103, ejecting a
recoil proton 121 (hollow arrow) which passes into the second
scintillator 102. Responsively, the second scintillator 102 emits a
light pulse 122 which is characteristic of the second scintillator
102, and which propagates to the sensor 105, and is detected there.
In propagation, the light 122 usually reflects off the outer
surfaces of the two scintillators 101 and 102 as shown by a dashed
arrow. The light 122 spends most of the time in the thick target
103 because the scintillator layers 101 and 102 are extremely thin,
much thinner than they appear in the sketch.
[0076] The first scintillator 101 and second scintillator 102
comprise any two different non-hydrogenous scintillators that
differ in their light pulse duration. Preferably the scintillators
101 and 102 are not plastic or any hydrogenous scintillator
material, since recoil protons from the scintillators would
interfere with the directional measurement. A single light sensor
105 detects the light pulses from both scintillators 101 and 102,
responsively generating an electronic signal, and then electronics
(not shown) can separate the two scintillator signals according to
the electronic signal properties. Preferably the difference in
pulse duration is sufficient to unambiguously identify each proton
recoil event as coming from the first scintillator 101 or the
second scintillator 102. The scintillators 101 and 102 preferably
have a thickness related to the stopping range of the highest
energy recoil proton expected in the measurement. In this example
the scintillators 101 and 102 are 40 microns thick, which is
sufficient to stop a recoil proton with 2 MeV energy, yet is thin
enough that lightly-ionizing backgrounds (from gamma ray
interactions, cosmic ray muons, etc.) would generate very little
light in passing through the scintillator and thus are rejected
reliably on pulse height alone.
[0077] The light sensor 105 is any device that detects the
scintillator light pulse and responsively generates an electrical
signal. The electrical signal must indicate which scintillator, 101
or 102, generated the light, and therefore which direction the
neutron 120 arrived from. In the example of FIG. 1, the two
scintillators are discriminated according to the shape of the
pulse. Multiple sensors 105 may be mounted on all sides of the
layered assembly. However, if the sensor 105 includes a substantial
amount of neutron scattering material, then the sensor 105 is
preferably not positioned between the detector and the incoming
neutrons, to avoid blocking the neutrons. An optional light funnel
(not shown) may be added between the light guides 104 and the light
sensor 105. Examples of suitable sensors 105 include
photomultiplier tubes, semiconductor devices such as photodiodes
and phototransistors, and especially avalanche diodes such as SiPM
devices and the like. Preferably the sensor 105 provides high
photon detection efficiency, high gain, and very low noise so that
the light pulses from the two scintillators 101 and 102 can be
detected and identified as to which scintillator was hit.
Array-type sensors such as CCD and CMOS arrays are usually
time-integrating-type devices that have a readout interval far
longer than the pulse duration, and therefore would not be suitable
for the pulse-shape-discrimination version. Such slower sensor
types may be satisfactory for the wavelength-discrimination and
light-path-discrimination versions, if the noise and background
counting rate are sufficiently low. The sensor 105 may further
include an image intensifier (not shown) or other light amplifier
or electron amplifier to further enhance the signal.
[0078] FIG. 2 is a cross-section sketch of the inventive detector
in the thin-target configuration, wavelength-discrimination
version. Again the thin layers are shown greatly expanded for
visibility. Here the hydrogenous target 201 is a thin layer, as are
the first scintillator 201 and the second scintillator 202. The
scintillators 201 and 202 are inorganic scintillating materials
with different wavelength emission bands. The scintillators 201 and
202 are proximate to opposite faces of the thin target 201, and are
optically coupled to the two light guides 204, which are optically
coupled to two filters 2081 and 2082, which are coupled to two
sensors 2051 and 2052. The thin target 203 and the scintillators
201 and 202 are thin in that their thicknesses are related to the
stopping range of recoil protons in the respective materials.
Specifically, they are all 10 microns thick here. The 10 micron
target 203 is thin enough to let many or most of the recoil protons
escape, while the 10 micron scintillators 201 and 202 are
sufficient to encompass much of the recoil proton tracks. The
highest energy recoil protons may pass all the way through a 10
micron scintillator 201 or 202, but in this case that does not
matter since the light guides 204 are non-scintillating and thus
have no response. For example, the light guides 204 are
non-scintillating glass, 10 mm thick. The filters 2081 and 2082 are
highpass and lowpass filters respectively, configured to pass the
wavebands of scintillators 201 and 202 respectively and block any
light outside those bands.
[0079] The detector of FIG. 2 indicates the direction of an
incoming neutron that scatters in the target 203 according to which
scintillator 201 or 202 is hit by the recoil proton. When the
detector is rotated so that the neutron enters from the side of the
first scintillator 201, the recoil proton usually strikes the
second scintillator 202, and vice versa when the detector is
rotated so that the neutron enters from the side of the second
scintillator 202. When the detector is rotated so that the target
203 is aligned with the neutron source, an equal number of recoil
protons trigger the two scintillators 201 and 202. Thus the
preponderance of counts from one of the scintillators 201 or 202
indicates that the source is on the other side of the detector,
while equality of the two signal rates indicates that the source is
directly in the plane of the target 203.
[0080] FIG. 3 is a cross-section sketch of the inventive detector,
thin target configuration, light-path-discrimination version. Again
the thin layers are greatly expanded. The central thin target 303
is a polyethylene film coated on both sides with thin aluminum
reflector layers 306. The non-hydrogenous light guides 3041 and
3042 are each coated on one side with the first and second
scintillators 301 and 302 respectively. In this case the
scintillators 301 and 302 are made from the same type of material,
with the same light pulse properties. In the
light-path-discrimination version of the invention, the active
scintillator is identified according to which sensor, 3051 or 3052,
registers the event. Since the light paths from the two
scintillators are mutually isolated, there is no need to make the
two scintillators different. The purpose of the reflectors 306 is
to prevent light of each scintillator 301 and 302 from reaching the
opposite light guide 3042 and 3041 respectively. The sketch also
shows an opaque barrier 309 comprising black paper. The barrier 309
has no function if the detector is simply a single detector module
as shown. But if it is to be mounted in a detector stack
configuration, with numerous detector modules directly adjacent to
each other, then the barrier 309 is necessary to prevent light of
each detector module from crossing over into the adjacent module
and triggering the wrong sensor.
[0081] FIG. 4 is a sketch in cross-section of the inventive
detector, double-target configuration, with
pulse-shape-discrimination. A central non-hydrogenous transparent
light guide 404, such as glass, is flanked by a first scintillator
401 and, on the opposite side of the light guide 404, by a second
scintillator 402. Mounted on the first scintillator 401 is a thin
target 4021, and mounted on the second scintillator 402 is a second
thin target 4022. The positional relationship of the first
scintillator 401 relative to target 4031 is opposite to the
positional relationship of the second scintillator 402 relative to
target 4032; specifically, the first scintillator 401 is to the
right side of the target to which it is proximate (target 4031)
whereas the second scintillator 402 is to the left of the target to
which it is proximate (target 4032). Therefore, the two
scintillators 401 and 402 register recoil protons coming from the
right or left directions respectively, thereby indicating via
sensor 405 from which direction a neutron has arrived.
[0082] FIG. 5 is a sketch in cross-section of the inventive
detector stack configuration, thick-target configuration,
wavelength-discrimination version, with five modules showing
although the stack could be extended to as many modules as desired.
Thin layers are exaggerated. Only one module is labeled; the others
are identical. Thick target 503 is a transparent hydrogenous
polymer which has a first scintillator 501 coated on one side of
the target 503, and a different second scintillator 502 coated on
the other side of the target 503, so that light from both
scintillators 501 and 502 can propagate through the transparent
target 503 which serves as a light guide. The light may also
propagate through the scintillators 501 and 502. The light may pass
through adjacent modules since all the layers are transparent as
shown.
[0083] Two optical filters 5081 and 5082 are optically coupled to
all the light guides 503, and optionally to the scintillators 501
and 502 as well. The filters 5081 and 5082 are dichroic filters
configured to pass the light of the first scintillator 501 and the
second scintillator 502 respectively into sensor 5051 and 5052
respectively. Also the filters 5081 and 5082 are configured to
reflect the out-of-band light, so that it may propagate to the
other filter and be detected. The sensors 5051 and 5052 view the
entire stack at once, and therefore detect a neutron scattering
event anywhere in the stack.
[0084] Preferably the scintillators 501 and 502 are thick enough to
fully stop all recoil protons; otherwise a recoil proton might
enter the adjacent scintillator. Any event that triggers both types
of scintillators 501 and 502 must be rejected as directionally
ambiguous. The targets 503 preferably have a thickness sufficient
to propagate the light in both directions. The optimal thickness
depends on the material, the surface properties, the overall
detector size, and the sensitivity of the sensors 5051 and
5052.
[0085] The detector stack of FIG. 5 has higher neutron detection
efficiency than the single module of FIG. 1. For neutrons arriving
from the side (perpendicular to the target planes), the detection
efficiency is higher than that of FIG. 1 due to the extra targets,
but is less than five times that of FIG. 1 due to "shadowing", that
is, scattering of neutrons in one module before they can be
detected in another module. When the detector is aimed directly at
the source, on the other hand, the detection efficiency is fully
five times that of FIG. 1, since all five targets would then have
an unobstructed view of the source. Thus, beneficially, the
detection efficiency of the stack is highest for the critical
measurement with the detector aimed directly at the neutron source.
For this reason and others, the stack configuration provides a
statistical advantage that helps to rapidly localize the neutron
source.
[0086] FIG. 6 is a cross-section sketch of the inventive detector,
thin-target configuration, pulse-shape-discrimination version, with
five modules showing. Only one module is labeled; the others are
identical. Thin layers are exaggerated. Here each thin target 603
is flanked by a first scintillator 601 and a second scintillator
602 on opposite sides of the target 603. Light guides 604 are
optically coupled to each scintillator 601 and 602 to transport
light to the sensor 605 which views all of the light guides 604
together. In this configuration there is no requirement that the
thin scintillators 601 and 602 be sufficiently thick to fully stop
all the recoil protons, because any protons that pass all the way
through a scintillator layer 601 or 602 will harmlessly stop in the
adjacent light guide 604.
[0087] FIG. 7 is a cross-section sketch of the inventive detector
stack, thin-target configuration, light-path-discrimination
version, with three modules showing. Only one module is labeled;
the others are identical. Thin layers are shown exaggerated.
Reflectors and barriers are arranged to guide the light from each
scintillator to its respective sensor, and to prevent light from
reaching the adjacent sensors.
[0088] Three thin hydrogenous targets 703 have reflector layers 706
on both sides. A first scintillator 701 and a second scintillator
702 are proximate to the target 703 and are positioned on opposite
sides of each target 703. A light guide 704 is optically coupled to
each scintillator 701 and 702. Two sensors 7051 and 7052 are
coupled to the light guides 704 in a "ganged" manner such that
light from all of the first scintillators 701 goes to sensor 7051
and the light from all of the second scintillators 702 goes to
sensor 7052. Each light guide 704 has an additional end reflector
707 to prevent light from passing into the wrong sensor. Finally an
opaque barrier 709 separates adjacent light guides 704 from each
other, thereby keeping light from each scintillator 701 and 702
separated. In this way, the reflectors 706 and barriers 709 isolate
the light from each scintillator 701 and 702. The two sensors 7051
and 7052 register each recoil proton occurring anywhere in the
stack, and ascribe each event to a neutron arriving from the left
or from the right according to which scintillator 701 or 702 is
active, regardless of where in the stack the event occurred.
[0089] FIG. 8 is a sketch in perspective of the inventive detector
stack, comprising a series of layers in a particular order,
including a first scintillator 801, a hydrogenous target 803, and a
second scintillator 802 in each module, configured to indicate a
direction of a neutron 820, which arrives in this case from the
left. The detector is then rotated about a vertical axis as
indicated by a dashed arrow, while signals from the two
scintillators 801 and 802 are monitored. In the orientation shown
in the figure, the neutron 820 is likely to scatter in one of the
targets 803 and expel a recoil proton into the second scintillator
802. After a rotation of 180 degrees, however, the neutron 820
would scatter a proton into the first scintillator 801 instead, due
to the reversed positions of the scintillators 801 and 802 after
the rotation. And after a rotation of about 90 degrees, the plane
of the targets 803 would be aligned with the direction of the
neutron 820, at which point there would be equal probability for a
recoil proton to hit each of the first and second scintillators 801
and 802.
[0090] In most inspection applications, the azimuthal (horizontal)
angle of a neutron source is primarily needed, and therefore the
detector is rotated about a vertical axis (or yawed) as shown. In
an application where the elevation angle of the source is needed,
the detector could first be rotated 90 degrees about a longitudinal
axis (rolled 90 degrees) so that the target planes are horizontal,
and then the detector could be rotated about a lateral axis
(pitched) to acquire elevation data. To determine the full spatial
location of the source, the detector could be sequentially rotated
in horizontal and vertical directions, or a pair of detectors could
be rotated independently in the two directions, and the resulting
data subsequently merged.
[0091] FIG. 9 is a sketch of an array of five of the inventive
detectors 900, each with a sensor 905 included. The detectors 900
are shown spread out horizontally, as viewed from the top. Each
detector 900 may comprise a detector stack assembly, as suggested
by stripes. Each detector 900 is oriented at a different angle, for
example being oriented at spaced-apart angles separated by 10 or 20
degrees. Incoming neutrons (not shown) can scatter in any of the
detectors 900, generating signals that indicate the direction of
the neutron. By analyzing data from all the detectors 900 in the
array, the neutron direction can be discerned. If one particular
detector is aimed nearly at the neutron source, then that
particular detector will have the nearly same counting rate in its
two scintillators, thereby indicating that the neutron source is
nearly aligned with that particular detector.
[0092] Alternatively, the detectors 900 could be arranged in a
vertical array with detectors rotated successively like a barber
pole, so that each detector 900 points at a different azimuth. The
vertical array may be a desirable compact arrangement if space is
tight. Or, to scan a large item, a large number of detectors could
be arranged in a two-dimensional wall-like array, with each
detector having a unique position and angle, so that a large object
such as an entire vehicle could be scanned simultaneously.
[0093] FIG. 10 shows a version of the inventive curved detector
stack 1000. Here the light guides 1004 are shaped as truncated
triangles or trapezoids. Each light guide 1004 is coupled to a
light sensor 1005. The thin targets and scintillators, depicted as
stripes 1001, are all oriented at different angles due to the
non-parallel sides of the light guides 1004. The location of a
neutron source (not shown) can be found by analyzing the signals of
the various sensors 1005, without having to rotate the
detector.
[0094] FIGS. 11A, 11B, and 11C are graphs showing schematically how
the neutron direction may be obtained from the scintillator data. A
neutron source is positioned at a direction of zero degrees in the
simulation, and the inventive detector is rotated to determine
where the source is located. The horizontal axis is the rotation
angle of the detector relative to the actual neutron source
direction, and the vertical axis gives the simulated data and
analysis results. The simulation was carried out using the MCNP6
code with a 10 micron CH.sub.2 target between two CaF.sub.2
scintillators, and a 2 MeV neutron beam.
[0095] FIG. 11A shows the counting rate for the first scintillator
(S.sub.A) as a solid line, and for the second scintillator
(S.sub.B) as a dashed line, versus the detector angle in the
simulation. The two curves cross at about zero degrees, thereby
indicating the neutron source direction where the two scintillators
have about the same counting rate. This analysis assumes that the
two scintillators have about the same maximum counting rates at
large angles such as +-90 degrees, which is evidently true for this
case as can be seen in the graph. If, however, the maximum counting
rates of the two scintillators are different (due to different
scintillator thicknesses or photon efficiencies or transparencies
for example), then the data can be normalized by dividing each
value by the maximum rate seen at any angle. Usually it is not
necessary to normalize the data because usually the two
scintillators exhibit closely the same counting rates even if the
two scintillators have different properties, and any remaining
disparity can be canceled by adjusting electronic parameters such
as gains and thresholds and the like.
[0096] A second method for determining the neutron direction is to
determine the angle at which the first scintillator counts at half
its maximum rage, and then determining a second angle at which the
second scintillator counts at half its maximum rate, and the
average of these two angles is the source azimuth. Or, two standard
curves can be fit, one each for the angular data of each
scintillator's counts, and the angles where the two curves pass
through their half-maximum values could determine the first and
second angles. In either case, the two angles are then averaged,
and that average angle closely points toward the neutron source.
This "average-half-maximum" analysis method does not depend on
equal detection efficiency or normalization. Also, background can
be easily corrected for by subtracting the minimum count rate from
all the data.
[0097] FIG. 11B shows a third analysis method in which a peak is
derived by multiplying the two scintillator rates versus angle. The
product curve exhibits a peak at the source direction. This method
works best when the background rates are very low.
[0098] FIG. 11C shows another analysis method in which the counting
rate for the first scintillator is subtracted from the second
scintillator. The source is located at the angle where the
difference curve crosses zero. An advantage of this method is that
it is not necessary to find the angle where the two counting rates
are equal, since a standard curve shape can be fit to several angle
data points, and the fit curve would suffice to indicate the
zero-cross angle precisely. This method does not require that the
background rates are low, so long as both scintillators have about
the same background rates.
[0099] The methods of FIGS. 11A and 11C also indicate whether the
source is in front of the detector or behind it. The front-back
position of the source is easily determined according to which of
the two counting rates is larger. At an angle of -45 degrees, for
example, the first scintillator has a higher counting rate than the
second scintillator (S.sub.A>S.sub.B) thereby indicating at that
the source is in front of the detector. If the source were located
behind the detector, the curves would be inverted. For similar
reasons, the difference curve of FIG. 11C indicates whether the
source is in front or behind the detector. If the curve is rising
as the detector angle increases, the source is in front, and if the
curve is falling with angle, the source must be behind the
detector.
[0100] All of the analysis methods disclosed should give
substantially the same neutron direction, aside from statistical
fluctuations. If they do not agree, then it is likely that multiple
neutron sources are present, or that the neutron source is spread
out across a large angular range.
[0101] FIG. 12 is a partially exploded perspective sketch of
subassemblies of the inventive detector which may be useful for
constructing the detector stack configuration such as that of FIG.
7. Here a scintillator assembly 1241 comprises a transparent
non-hydrogenous non-scintillating light guide 1204 onto which a
first scintillator 1201 is applied. Also an end reflector 1207 is
deposited on one end of the light guide 1204. Separately, a target
assembly 1243 is prepared by depositing two thin reflector layers
1206 onto both sides of a thin film hydrogenous target 1203. Also a
barrier comprising aluminum foil 1249 is prepared. Multiple copies
of these subassemblies are then used to build the detector stack as
shown in the next figure.
[0102] FIG. 13 is an exploded assembly view of the inventive
detector stack of FIG. 7, thin-target configuration,
light-path-discrimination version, with two modules showing. Thin
layers are greatly expanded. The arrangement includes four copies
of the scintillator assembly 1241 of FIG. 12, but half of them are
turned upside-down so that they can detect recoil protons going in
the opposite direction. Hence the arrangement includes two
scintillator-A assemblies 1301 and two scintillator-B assemblies
1302, both of which use the same scintillator material and emit the
same type of light pulse, notwithstanding that they have different
labels and patterning. It is easier and cheaper to make the
scintillator assemblies all the same, and with
light-path-discrimination, there is no need to make them
different.
[0103] The stack detector is then constructed quite simply, by
arranging in sequence a scintillator-A assembly 1301, then a target
assembly 1303, then a scintillator-B assembly 1002, so that the two
scintillator layers are on opposite sides of the target. Then a
foil barrier 1309 is laid down, and the process is repeated for the
next module. This can be repeated for as many modules as desired.
Then the sensor 1305 is attached, preferably by clear epoxy, to the
scintillator-A assemblies so as to collect light from all the
scintillator-A layers. A second sensor (not shown) is then attached
to the scintillator-B assemblies to collect the light from all the
scintillator-B layers.
[0104] The sketch shows the scintillator-B assemblies 1302 slightly
elevated relative to the scintillator-A assemblies 1301 so that the
sensor 1305 contacts only the scintillator-A assemblies 1301, and
likewise the second sensor would contact only the scintillator-B
assemblies 1302. However this feature is not necessary, since each
scintillator-B assembly 1302 is shielded by an end reflector 1307,
and likewise each scintillator-A assembly has an end reflector.
Even if the sensor 1305 contacts both types of light guides, the
end reflectors will still guarantee that only one type of
scintillator will be viewed by each sensor. So the sensor 1305
could just as well contact all the light guides if that would
simplify construction. The stack so assembled can be tested for
channel cross-talk by placing a neutron source at 90 degrees to the
target planes, so that one type of scintillator should register
hits and the other should count zero.
[0105] FIG. 14 is an exploded sketch in perspective, of the
inventive stack 1421 with two secondary light guides 1424 coupled
to the exterior of the stack 1421 to collect light from each layer.
Also a light funnel 1428 collects light from the stack 1421 as well
as the secondary light guides 1424, and couples all the light
pulses into a sensor 1425. A neutron 1420 enters the stack 1421
from the top. The arrangement preferably has little or no material
in the way of the neutron 1420, thereby avoiding scattering or
absorption of the neutrons 1420 before they reach the stack
1421.
[0106] FIG. 15 is a flowchart showing how the inventive detector
can be built in the thick-target configuration,
pulse-shape-discrimination version, such as that of FIG. 1. First a
transparent hydrogenous thick-target such as a plate of
polyethylene or styrene or carbonate or acrylic or other polymer is
provided at step 1501. Then a first scintillator is deposited or
applied to one side of the target at 1502. The first scintillator
may be deposited by evaporation or sputtering or CVD or solvent
evaporation, or other deposition process suitable for the first
scintillator material.
[0107] If the deposition process is potentially damaging to the
target, the first scintillator may be applied instead to a backing
plate such as aluminum foil, or other robust material, which is
preferably highly reflective so that it will assist in light
propagation when the detector is finally assembled. The first
scintillator with backing plate may then be pressed to the target,
closing the gap so that the recoil protons can get into the first
scintillator from the target, and so the light pulse can get into
the target from the scintillator. If pressing the first
scintillator against the target does not provide sufficient optical
coupling for the scintillator light to pass efficiently between the
first scintillator and the target, then a film of an optical
coupling material such as a gel or clear epoxy may be applied to
close the joint. Preferably such a material contains abundant
hydrogen, in which case the hydrogenous coupling material would not
comprise an obstruction to the recoil protons, but would serve as
an extension of the hydrogenous target itself. In other words, if
the coupling material is hydrogenous, there is no need to keep the
coupling layer thin.
[0108] Then at 1503 the other scintillator is deposited or applied
to the opposite side of the target, and a light sensor is attached
to the target at 1504 to receive the light from both scintillators.
The entire detector is then wrapped in a light-tight cover such as
foil and tape at 1505. In use, the sensor generates signals related
to the light pulses, which can be discriminated electronically at
1506, thereby indicating which scintillator was hit by the recoil
proton, and thus the general direction toward the neutron
source.
[0109] FIG. 16 is a flowchart showing how the inventive detector
can be built in the thin-target, wavelength-discrimination
configuration such as FIG. 2. First a non-scintillating,
non-hydrogenous, transparent light guide is prepared 1601, and a
thin layer of the first scintillator material is deposited on it at
1602. Optionally, a thin reflective layer may then be deposited on
the first scintillator layer. A second light guide is prepared at
1603, and the second scintillator layer is deposited or otherwise
affixed to it at 1604. The second scintillator produces light with
a different wavelength band than the first scintillator. Then a
thin hydrogenous target is deposited or affixed to one of the
scintillators at 1605, or alternatively a half-thickness of the
target material can be deposited on both of the scintillators. In
any case, the two subassemblies are then pressed together so that
both of the scintillators are proximate to the target but on
opposite sides of the target at 1606. Two optical filters are then
provided, one a lowpass filter and the other a highpass, each
configured to pass only the light from one of the scintillators.
The filters are then coupled to both of the light guides at 1607,
and two sensors are attached to the filters at 1608 if not
previously attached to them. Alternatively, each filter could be
attached to its respective light guide alone, rather than to both
of the light guides, thereby further separating the two light
pulses.
[0110] FIG. 17 is a flowchart showing how to use the inventive
detector to determine the direction of neutrons. First at 1701 a
directional detector is provided, such as any of the detector
configurations described herein. Then at 1702 the detector is
oriented at a particular starting angle, and at 1703 the number of
light pulses from first-scintillator events and second-scintillator
events (here termed A-pulses and B-pulses respectively) are counted
or tallied. Then at 1704 the detector is rotated to other angles,
which may be equally-spaced stepped-apart angles, or other sequence
of orientations, and the A-pulses and B-pulses are tallied at each
orientation. (Alternatively, a plurality of similar detectors could
be all oriented at different angles, and their tallies analyzed in
the same way, without the need for rotating.) The front-to-back
ambiguity is resolved at 1705 by noting the sign of the difference
A-B versus angle, which depends on whether the neutron source is in
front of or behind the detector. A particular angle is then found
at 1706 by rotating the detector until the counting rates for the
A-pulses and B-pulses are substantially equal. The detector then
points directly at the neutron source. As an optional confirmation
analysis, the detector could be rotated until the A-pulse rate is
about half of the maximum A-pulse rate seen at any orientation, and
the angle noted at 1707. Then the same determination is repeated
for the B-pulses, and the two angles (corresponding to the
half-maximum rates) are averaged to obtain a second determination
of the source direction. A third determination can be made at 1708
by finding a peak in a particular function, such as the product of
the A and B counting rates versus angle. A fourth determination can
be made at 1709 by finding the zero-crossing of a function such as
the difference between the A and B counting rates versus angle,
which passes through zero when the detector is aligned with the
source.
[0111] The inventive directional neutron detector will enable
inspectors to detect and localize neutron sources at critical
national inspection sites such as border crossings and shipping
ports, as well as places where neutron sources may be present such
as nuclear reactors and research facilities. By indicating the
direction of the neutron source, the detector amplifies the
statistical power of each detection, rapidly distinguishing a point
source from a uniform background. As a multi-detector array, the
invention is suitable for large-item inspections such as
whole-vehicle scanners. In smaller configurations, the invention
enables efficient localization of contaminants or smuggled material
on workers in a walk-through portal. As a hand-held survey monitor,
the invention indicates to the operator whether a neutron source is
to the left or right of the detector, and also indicates when the
detector is directly pointing toward the source. The invention
enables rapid, efficient, and precise neutron source localization
that is not possible with prior art.
[0112] The embodiments and examples provided herein illustrate the
principles of the invention and its practical application, thereby
enabling one of ordinary skill in the art to best utilize the
invention. Many other variations and modifications and other uses
will become apparent to those skilled in the art, without departing
from the scope of the invention, which is to be defined by the
appended claims.
* * * * *