U.S. patent application number 14/953803 was filed with the patent office on 2017-07-13 for compositions and methods for determining directionality of radiation.
The applicant listed for this patent is Sagamore/Adams Laboratories LLC. Invention is credited to Rusi P. Taleyarkhan.
Application Number | 20170199288 14/953803 |
Document ID | / |
Family ID | 43032777 |
Filed Date | 2017-07-13 |
United States Patent
Application |
20170199288 |
Kind Code |
A1 |
Taleyarkhan; Rusi P. |
July 13, 2017 |
Compositions and Methods for Determining Directionality of
Radiation
Abstract
A method of determining directionality of radiation is disclosed
which comprises dividing the tensioned metastable fluid liquid
volume adjacent to a radioactive source into a plurality of
sectors, determining the opposing sector ratio of the respective
sector and determining the direction of the radiation based on the
opposing sector ratios of the plurality of sectors. The method
further comprising determining directionality of incoming radiation
from the tension pressure assisted elongation of bubble shapes
pointing towards direction of radiation particles that interacted
with nuclei of tensioned metastable fluid detector system. A device
capable of carrying out these methods is also disclosed.
Inventors: |
Taleyarkhan; Rusi P.;
(Lafayette, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sagamore/Adams Laboratories LLC |
Chicago |
IL |
US |
|
|
Family ID: |
43032777 |
Appl. No.: |
14/953803 |
Filed: |
November 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13875974 |
May 2, 2013 |
9201151 |
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14953803 |
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12988949 |
Oct 21, 2010 |
8436316 |
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PCT/US10/32991 |
Apr 29, 2010 |
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13875974 |
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61174159 |
Apr 30, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01T 5/06 20130101; G01T
1/12 20130101 |
International
Class: |
G01T 5/06 20060101
G01T005/06 |
Goverment Interests
INVENTION MADE WITH U.S. GOVERNMENT SUPPORT
[0002] This invention was made with Government Support under
Contract No. HROO11-05-C-0141 awarded by the Defense Advanced
Projects Research Agency. The Government has certain rights in this
invention.
Claims
1-44. (canceled)
45. A method of determining directionality of radiation comprising,
creating a volume of a tensioned metastable fluid; placing the
tensioned metastable fluid volume in the proximity of a radiation
source; detecting the location of radiation induced cavitations
within the tensioned metastable fluid within at least two sections
of the metastable fluid; determining the number of cavitation
events in the at least two sections of the metastable fluid; and
determining the direction of the radiation source based on the
radiation induced cavitations within the tensioned metastable
fluid, wherein the direction is from the section with the least
number of cavitation events toward the section with the higher
number of cavitation events.
46. The method of determining directionality of radiation of claim
45, wherein the tensioned metastable fluid is an acoustically
tensioned metastable fluid.
47. The method of determining directionality of radiation of claim
45, wherein the step of detecting the location of radiation induced
cavitations comprises detecting the time delay of the arrival of
cavitation induced shock signals by processing signals obtained
from a plurality of signal detection transducers mounted on the
chamber.
48. The method of determining directionality of radiation of claim
45, wherein the step of detecting the location of radiation induced
cavitations comprises detecting cavitation induced shock signals by
processing signals obtained from a plurality of signal detection
transducers mounted on the chamber wherein the processing further
comprises a step to minimize bias.
49. The method of determining directionality of radiation of claim
45, wherein the step of detecting the location of radiation induced
cavitations comprises detecting cavitation induced shock signals by
processing signals obtained from a plurality of signal detection
transducers mounted on the chamber wherein the processing further
comprises a step to minimize bias that includes the step of
detecting signals from the signal detection transducers that are
above a threshold voltage level, wherein the threshold voltage
level can be determined from an asymptotic response comparison of
all transducers.
50. The method of determining directionality of radiation of claim
45, wherein the step of detecting the location of radiation induced
cavitations comprises the method of detecting the location of
cavitations by a hyperbolic positioning method.
51. The method of determining directionality of radiation of claim
45, wherein the step of detecting the location of radiation
involves determining a ratio of cavitations occurring in at least
two regions of the chamber.
52. The method of determining directionality of radiation of claim
45, wherein the method further comprises comparing cavitation
events in opposing sectors without including the event counts in a
volume of space that includes at least a portion of the centerline
vertical axis.
53. The method of determining directionality of radiation of claim
45, wherein the method further comprises using pressure differences
to amplify the elongation of cavitation bubbles to coincide with
direction of energy transfer to liquid molecules from the incoming
radiation.
54. A device for determining directionality of incident radiation
comprising: a sealed chamber holding a fluid, a control system in
communication with a mechanism for deforming the chamber that
includes at least one drive transducer and the resonance frequency
of the at least one drive transducer is substantially similar to
the resonance frequency of the chamber; wherein the control system
and the mechanism for deforming the chamber operate together to
induce and maintain a tension metastable state in the fluid that is
sufficient to allow the nucleation of bubbles when the fluid
molecules are struck by incident nuclear particles, and a plurality
of signal detection transducers spaced apart within the chamber in
electronic communication with a system for determining the location
of bubble cavitation events within the fluid volume.
55. The device for determining the directionality of incident
radiation of claim 54, wherein the fluid in the chamber is selected
from the group of fluids consisting of acetone, fluorocarbon,
chlorofluorocarbon, benzene, isopentane, trimethyl borate, water
and their mixtures.
56. The device for determining the directionality of incident
radiation of claim 54, wherein the mechanism for deforming the
chamber includes at least one transducer mounted to the chamber
such that it surrounds the circumference of the chamber around the
mid plane or in a plane corresponding to a desired oscillating
tension/compression pressure field.
57. The device for directionality of incident radiation of claim
54, wherein the mechanism for deforming the chamber includes
multiple transducers mounted to the chamber at discrete locations
in a plane corresponding to a desired oscillating
tension/compression pressure field.
58. The device for determining the directionality of incident
radiation of claim 54, wherein the plurality of signal detection
transducers spaced apart within the chamber in electronic
communication with a system for determining the location of
cavitation events within fluid volume include at least four signal
detection transducers.
59. The device for determining the directionality of incident
radiation of claim 54, wherein the plurality of signal detection
transducers spaced apart within the chamber in electronic
communication with a system for determining the location of
cavitation events within fluid volume further include at least
three signal detection transducers in the same plane and at least
one signal detection transducer that is outside the plane.
60. The device for determining the directionality of incident
radiation of claim 54, wherein the system for determining the
location of bubbles within the fluid volume include a signal
processing system comprising a high-pass filter circuit that
removes the baseline drive frequency signal.
61. The device for determining the directionality of incident
radiation of claim 54, wherein the system for determining the
location of bubbles within the fluid volume includes a signal
processing system that compares filtered signals from the signal
detection transducers to determine the arrival time delay of bubble
signals at the signal detection transducers that employs a
positioning algorithm to determine position of imploding bubbles
within the chamber.
62. The device for determining the directionality of incident
radiation of claim 54, wherein the system for determining the
location of bubbles within the fluid includes a signal processing
system that determines the number and location of bubble
cavitations in the chamber.
63. The device for determining the directionality of incident
radiation of claim 54, wherein the system for determining the
location of bubbles within the fluid includes a signal processing
system that includes a visual monitoring system that captures
real-time bubble formation within fluid volume and determines
directionality from major axis of elongated cavitation bubbles.
31. The device for determining the directionality of incident
radiation of claim 54, wherein the chamber has a size and shape
that allows for the directional detection of radiation that permits
down scattering assisted collection of cavitation events in various
regions of the chamber.
Description
PRIORITY
[0001] This application is a continuation of U.S. application Ser.
No. 13/875,974, filed on May 2, 2013, now U.S. Pat. No. 9,201,151
which is a continuation of U.S. application Ser. No. 12/988,949,
filed on Oct. 21, 2012, now U.S. Pat. No. 8,436,316 which is a
nation stage entry of PCT/US10/32991, filed on Apr. 29, 2010 which
claims the benefit of U.S. Provisional Application No. 61/174,159
filed Apr. 30, 2009, the entire contents of which are incorporated
herein by reference.
TECHNICAL FIELD
[0003] The present disclosure relates to a method of determining
directionality of radiation. More specifically, the present
disclosure relates to a method of determining directionality of
radiation using a tensioned metastable fluid detection system.
BACKGROUND
[0004] Radiation cannot be detected by human senses. A variety of
handheld and laboratory instruments is available for detecting and
measuring radiation, such as Geiger counters. However, these
devices do not provide information about the direction from which
the radiation emanates.
BRIEF SUMMARY
[0005] This disclosure provides compositions and methods for
determining the direction of incoming radiation.
[0006] One method of determining directionality of radiation
involves dividing a tensioned metastable volume of fluid in a
chamber into a plurality of sectors, placing the fluid in the
proximity of a radiation source and detecting radiation induced
cavitation nucleation events in various regions or sectors within
the chamber, determining the opposing sector ratio of the number of
cavitations in the respective sectors and determining the direction
of the radioactive source based on the opposing sector cavitation
ratios in the plurality of sectors.
[0007] A neutron detection system is disclosed that has the ability
to give directional information about the source. Rather than
relying on neutron or other radiation (e.g., photon) interactions
that give no directional information about the origin of the
radiation being detected, an acoustic tensioned metastable fluid
detection (ATMFD) system can be used to show which direction the
radiation is coming from.
[0008] While the ATMFD system is operating, the probability that a
neutron/radiation induced cavitation event will occur is a function
of the tensioned or negative pressure in the fluid and the
neutron/radiation flux. One embodiment relies upon an acoustic
tensioned metastable fluid in which the pressure profile is nearly
axially symmetric on a horizontal plane such that all points that
are equally distant from the center will have substantially the
same negative pressure. In such a system the cavitation probability
is a function of neutron/radiation flux. Since neutron/radiation
flux from a source decreases with distance and with the degree of
down scattering and absorption, the side sector of the detector
closest to the source has a higher probability of detection. By
detecting the location of a sample set of cavitation events,
directional information can be determined by observing an imbalance
in the locations of cavitation events.
[0009] The location of a neutron/radiation detection nucleation
site can be determined by recording the time at which the resulting
cavitation induced shockwave reaches various locations on the
detector wall. Any number of transducers can be incorporated into
the detector in order to determine the location of the source of
cavitation induced shock waves. Any suitable number of transducers
can be used so long as directional information can be obtained. For
example, four (approximately 7 mm OD) piezoelectric transducers can
be used to detect the arrival of the shockwave from the cavitation
event. At least two, preferably three or more of the signal
detection transducers can be in a plane and one or more signal
detection transducers can be outside that plane. The signals from
the four transducers can then be processed to derive desired
information on directionality. Additionally, directional
information may also be obtained from monitoring of the bubble
shapes at the time, and after cavitation events occur. Such
cavitation bubbles generated from neutron/radiation strike on to
nuclei of atoms in the acoustically tensioned pressure field of the
ATMFD liquid preferentially extend themselves in elliptical like
shapes pointing in the direction of incoming radiation.
[0010] The preferred ATMFD systems described herein have the
ability to:
[0011] Detect SNM neutrons over eight orders of magnitude,
[0012] Detect alpha particles,
[0013] Maintain virtually complete insensitivity to gamma
photons,
[0014] Operate with intrinsic efficiency of about 90%,
[0015] Provide real-time directional information of incoming
radiation.
[0016] Benchmarking and qualification studies have been conducted
with Pu-based neutron-gamma and photon light sources. This
disclosure provides the modeling cum-experimental framework along
with a demonstration of the operation of the ATMFD system.
BRIEF DESCRIPTION OF THE FIGURES
[0017] FIG. 1A provides schematic views of ATMFD resonant chambers
having multiple disk transducers positioned with the hollow
cylinder transducer.
[0018] FIG. 1B provides a schematic view of ATMFD resonant chamber
with multiple transducers positioned spaced apart.
[0019] FIG. 2 is a schematic view of pressure distributions in a
chamber at about 4.5 W and about 10 W.
[0020] FIG. 3 is a not-to-scale view of the geometry of an MCNP
input deck.
[0021] FIG. 4 is a schematic view of neutron flux ratio at large
standoff for acetone and Freon-113.
[0022] FIG. 5 is a schematic view of the screenshot of shock pulses
before signal processing. The vertical scale is about 500 mV/div.
The horizontal scale is about 100 ms/div.
[0023] FIG. 6 is a schematic view of a screenshot of shock pulses
analyzed for directionality information. The vertical scale is
about 500 mV/div. The horizontal scale is about 5 .mu.s/div.
[0024] FIG. 7 is a schematic view of the experimental setup with
directionality automation.
[0025] FIG. 8 is an axial cross-section view of positions of
detection events using a PuBe source about -20.3 cm away from the
center axis of chamber in line with Mic 1 and Mic 3.
[0026] FIG. 9 is a schematic view of the determination of source
direction within .+-.30.degree.. Comparison of experimental results
with MCNP simulation.
[0027] FIG. 10 is a schematic view of all neutron detection events
as recorded, seen in rz plane, overlaid with COMSOL.TM. simulations
at about 4.5 W.
[0028] FIG. 11A is a schematic view of pressure distributions in
chamber about 4.5 W.
[0029] FIG. 11B is a schematic view of pressure distributions in
chamber about 10 W.
[0030] FIG. 12 is a not-to-scale view of the geometry of MCNP
input.
[0031] FIG. 13 is a schematic view of the oscilloscope trace of
cavitation shock waves. The vertical scale of first signal is about
20 mV and the second signal is about 200 mV. The horizontal scale
is about 50 ms/div.
[0032] FIG. 14 is a schematic view of the screenshot of shock
pulses analyzed for directionality information.
[0033] FIG. 15 is a schematic view of the trigger rate recorded by
each transducer for various triggering levels.
[0034] FIG. 16 is a schematic view of the experimental setup with
directionality automation.
[0035] FIG. 17 is a schematic view of data taken manually with 4
channel 100 MHz 100 MSa/s oscilloscope.
[0036] FIG. 18 is a schematic view of positions of cavitation
events using PuBe source about -35.5 cm away from center axis of
chamber in line with Mic 1 and Mic 3.
[0037] FIG. 19 is a schematic view of positions of cavitation
events using PuBe source about +35.5 cm away from the center axis
of the chamber in line with Mic 1 and Mic 3.
[0038] FIG. 20 is a schematic view of all cavitation events as
recorded, seen in the xz plane. Data taken with source about -35.5
cm and 35.5 cm away from the center of chamber on x-axis with Mics
1 and 3.
[0039] FIG. 21A is a schematic view of radial distribution of
cavitation events, separated into two sections (closest to source
and furthest from source) data taken with source about 35.5 cm away
from the center of the chamber on the x-axis.
[0040] FIG. 21B is a schematic view of radial distribution of
cavitation events, separated into two sections (closest to source
and furthest from source) data taken with source about 35.5 cm away
from the center of the chamber on the x-axis.
[0041] FIG. 22 is a picture of an elongated bubble in an ATMFD
pointing to the source of incoming (in this case neutrons from a
Pu--Be isotopic source) radiation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] In one method the capacity to detect nuclear particles can
be provided by tensioned metastable fluid states which can be
attained via tailored resonant acoustic systems such as acoustic
tensioned metastable fluid detector (ATMFD). Radiation detection in
tensioned metastable fluids can be accomplished via
macro-mechanical manifestations derived from the femtoscale nuclear
interactions. Incident nuclear particles can interact with the
dynamically tensioned metastable fluid wherein the intermolecular
bonds are sufficiently weakened such that the recoil of ionized
nuclei generates nano-scale vapor cavities which grow to visible
scales. Ionized nuclei form preferentially closest to the incoming
radiation, thereby providing for the first time the capability to
ascertain information on directionality of incoming radiation.
[0043] The present disclosure provides advancements in the
detection of a broader range of nuclear particles, the detection of
neutrons over an energy range of eight orders of magnitude,
improved intrinsic detection efficiencies beyond 90%, and in
ascertaining directionality information of incoming radiation than
has previously been possible. In one example, the present
disclosure provides a composition and method which increases the
accuracy and precision of ascertaining directionality information
utilizing enhanced signal processing cum-signal analysis, refined
computational algorithms, and on demand enlargement of the detector
sensitive volume.
[0044] Advances in the development of ATMFD systems can be
accomplished through the use of a combination of experimental and
theoretical modeling. Modeling methodologies include Monte-Carlo
based nuclear particle transport using MCNP5 and complex
multi-physics based assessments accounting for acoustic,
structural, and electromagnetic coupling of the ATMFD system via
COMSOL.TM.'s Multi-physics simulation platform. Benchmarking and
qualification studies have been conducted with special nuclear
materials (SNMs), including Pu-based neutron-gamma sources. The
results show that the ATMFD system, in its current configuration,
is capable of locating the direction of a radioactive source at
least to within about 30.degree. with about 80% confidence or
more.
A First Embodiment
[0045] Radiation detection in tensioned metastable fluids is based,
in part, on the principle that incident nuclear particles interact
with a tensioned fluid wherein the intermolecular bonds are
sufficiently weakened such that nuclear particles are capable of
triggering a localized explosive phase change in the fluid. A
liquid in a tensile state is metastable below its thermal
equilibrium state, unlike a superheated liquid which is in a state
of thermal superheat which is above its normal boiling point.
Tension in fluids is analogous to the stretching of solid
structures. The energy required to tear apart the intermolecular
bonds of a solid decreases as the tension in the structure
increases. In an analogous manner, the excess trigger energy
required to break the intermolecular bonds between liquid molecules
decreases with increasing tension metastability; eventually
resulting in spontaneous triggering of explosive phase change at
the spinodal limit of tension. Below this stability limit, excess
energy is required to trigger phase change of the tensioned
metastable fluid. This excess energy can be provided via
interaction with nuclear particles (e.g., neutrons, alphas,
photons, betas, fission products, etc.) or even with visible light
photons. This property enables the amplification of femto-scale
nuclear scale particles to relatively large (.times.10.sup.13)
macroscopic scales therefore allowing for new low-cost,
ultra-sensitive detectors for nuclear engineering and scientific
applications such as the acoustically tensioned metastable fluid
based detection system (ATMFD) described herein.
[0046] This disclosure is directed to the directional detection of
neutrons in the MeV range (e.g., those neutrons emitted by special
nuclear materials such as U, Pu, Cf, Am, Cm, etc.) via direct
knock-on collisions with the detector fluid.
[0047] The ATMFD approach appears capable of not only detecting the
energy and intensity of incident radiation, but also ascertaining
information on the location of the radioactive source, a feature of
significant potential use in widespread fields, including
identifying the tell-tale neutron emission signatures from SNMs for
homeland security. Directional information is ascertainable in the
ATMFD system due to the increased probability that a neutron
induced detection event will occur in the regional sector of the
tensioned fluid volume nearest the source. The probability that a
neutron induced detection event will occur is a function of the
negative pressure in the detector fluid and the neutron flux and
energy. Since the pressure profile is nearly axi-symmetric, the
probability of detection events is a function of the neutron flux
and energy. Since neutron flux of a given energy from a source
decreases with distance and with the degree of down scattering and
absorption, the side of the sensitive, volume nearest the source
has the highest probability of detection. Detecting the location of
these detection events inside the detector allows the user to
ascertain information on the direction of the radioactive source.
The present disclosure provides an improved mechanistic treatment
of directionality determination.
NOMENCLATURE
[0048] ATMFD--Acoustic tension metastable fluid detector
[0049] COMSOL.TM.--COMSOL Multiphysics.TM.
[0050] GPIB--General purpose interface bus (IEEE 488)
[0051] GPS--Global positioning system
[0052] LabVIEW.TM.--Graphical programming language
[0053] LET--Linear energy transfer (also dE/dx)
[0054] PuBe--Plutonium beryllium neutron source
[0055] PZT--Lead zirconate titanate
[0056] Mic--Microphone
[0057] MCNP5--Monte Carlo n-Particle Version 5
[0058] OD--Outer diameter
[0059] SDD--Superheated droplet detector
[0060] SNM--Special nuclear material
[0061] TDOA--Time difference of arrival (also 't)
[0062] TMFD--Tension metastable fluid detector
[0063] V1--Sensitive volume (or sector) nearest the source
[0064] V2--Sensitive volume (or sector) farthest from source
[0065] XatMaxY--Time in of the highest peak relative to the trigger
point.
[0066] XatMinY--Time in of the lowest peak relative to the trigger
point
ATMFD Design
[0067] Any suitable fluid chamber can be used so long as the
chamber can be used to create a tensioned metastable fluid,
preferably an acoustic tensioned metastable fluid, in which a fluid
pressure profile can be created that is nearly axially symmetric
such that all points that are equally distant from the central axis
will have substantially the same negative pressure, a cylinder for
example. Due to manufacturing issues, cylinders of glass can
involve slight (.about.10-100 micron type) deviations in thickness
and diameter along the circumference and length. As a consequence,
the true central axis of a resonance chamber can shift from the
centerline. Such a shift can create asymmetry in the oscillating
pressure profiles in the radial and axial directions. Such
variations can be accounted for up-front by system
characterization. For example by transient oscillating pressure
mapping over a range of frequencies of interest to find the true
central axis. For practical systems, the fluctuating pressures from
the geometrical central axis will generally be substantially the
same, albeit, somewhat skewed but to a known level such that
adjustments can be made when deriving directionality related
information. In certain embodiments suitable chambers will have the
characteristic that they can be mechanically deformed in transient
fashion from pulses from an external transducer, in a manner that
generates a standing acoustic wave in the fluid housed by the
chamber. The chamber in certain embodiments has a size and shape
that allows for the directional detection of radiation that permits
down scattering assisted collection of bubble cavitation events in
various regions of the chamber. The pressure wave can consist of
oscillating positive and negative pressures, such that the negative
pressures are in a range above the spinodal limit of tension but
which allows the energy released by interaction of nuclear
particles with fluid molecules to trigger a phase change also known
as a bubble nucleation or cavitation event. Generally, when
freon-113 is the fluid, the negative pressure is thought be about
-2.5 bar or lower in the presence of 4 MeV neutrons from an SNM
such as from a Pu--Be source. Where acetone is the test liquid the
negative pressure is about -3.5 bar or lower. The required negative
pressure is variable with the external neutron energy and can be
calibrated a priori by comparing against neutron sources of known
energy (e.g., from accelerator systems or mixtures of alpha
emitting isotopic sources such as Am--Be, Am--Li, Am--B, Am--C,
Am-Fl and the like). Suitable chambers can be manufactured from
quartz, glass (preferably Pyrex glass), ceramics, polycarbonates,
and a number of metals, as is known in the art. In one embodiment,
a resonant acoustic chamber can have an outside diameter of
approximately 70 mm and 150 mm long cylindrical quartz tube having
a hemispherical top and bottom. A schematic of this ATMFD is shown
in FIG. 1A. Other dimensions can be chosen as well to fit needs
related to frequency of operation. The chamber can be filled with a
fluid and is generally sealed. The chamber can be adapted with a
mechanism for focusing acoustic energy within the fluid in the
chamber. Acoustic energy can be focused within the fluid inside the
chamber by any suitable means, for example a hollow glass or quartz
reflector placed at the top of the test fluid and a similar hollow
glass or quartz reflector placed at the bottom of the chamber can
be used. Plastics, polytetrafluoroethylene or polycarbonates may be
used if they are not attacked chemically by the working fluid. For
example, a concentrically ring shaped piezoelectric transducer made
of lead-zirconate titanate (PZT) can be affixed by standard methods
(mechanical or epoxy glue-based) to the outside of the chamber and
used to power the acoustic resonance chamber. Suitable transducers
can be made of any material that can induce acoustic resonance
within the fluid, suitable materials include ceramic materials such
as barium titanate, lead zirconate titanate (PZT), among other
materials, as are known. It is not necessary to use a concentric
ring shaped hollow cylinder. This is especially true for large
diameter ATMFDs where large circular concentric ring transducers
become increasingly more difficult to procure. As an alternate,
multiple disk such as circular, rectangular or other shaped
transducers may be positioned as shown schematically in FIG. 1B
either together with the hollow cylinder as in FIG. 1A, or by
themselves as shown in FIG. 1B. In such a situation, about 4 such
disks are positioned in a given plane and act as drive transducers.
The fifth is mounted at a higher elevation and may be of
significant smaller size--the purpose being to receive shock
signals. The four in the same plane act to not only provide drive
power but also to receive shock signals from imploding bubbles. In
both cases, the thickness and size for a given material control the
capacitance and resonance frequency of the transducers. For
example, for a hollow ring transducer, the capacitance is directly
proportional to the height of the ring and inversely proportional
to the natural logarithm of the ratio of outer to inner diameters
of the hollow cylinder, respectively. For a circular disk
transducer which is polarized in either planar or the thickness
direction, the capacitance is directly proportional to the square
of the diameter and inversely proportional to the thickness. These
transducers are best utilized in such manner that their resonance
matches the mechanical resonance of the test cell enclosure. For
the 70 mm OD and 150 mm long test cell shown in FIGS. 1A and 1B the
mechanical resonance frequency (when filled with acetone) amounts
to around 20 kHz and the capacitance of the ring transducer is
around 20 nF. For the disk transducers of FIG. 1B, the disk
transducers should be selected for the 70 mm OD test cell with a
capacitance around 20 nF as well but with dimensions selected to
provide a resonance frequency of around 20 kHz. For larger diameter
systems the mechanical resonance will roughly vary inversely with
the ratio of diameters of the systems involved to a good first
order approximation (e.g., for a system with a diameter of 140 mm
OD, the mechanical resonance thus may be expected to drop down to
approximately 10 kHz) and the capacitance of the transducers must
therefore, be adjusted accordingly to bring the resonance frequency
of the transducer to become close to 10 kHz as well, such that
maximum efficiency of drive power is attainable. A more refined
estimate of mechanical resonance of the system--one that includes
multi-dimensional 3-D effects, may be estimated via direct pressure
mapping of the test cell at various elevations in the test liquid
over a range of frequencies wherein, one would readily find the
frequency at which pressure oscillations reach their highest
levels. Alternately, a multi-physics modeling and simulation scheme
may also be employed, as shown later with use of the COMSOL.TM.
multiphysics simulation platform.
[0068] Transducers can be affixed to the chamber using a coupling
agent such as an adhesive. Suitable coupling agents will have a
suitable impedance which essentially matches the product of density
and sound velocity in the medium to the driver transducer and the
driven structure receiving the mechanical impulses from the high
frequency oscillating transducers. The coupling agent is chosen so
that it minimizes acoustic energy scattering and/or wasting, such
as by dissipation into heat. As an example, epoxy can be used to
affix the transducers to the chamber walls. Trapped bubbles which
adversely affect the coupling are to be avoided when using epoxy.
Glass frit can be mixed in with the epoxy to improve the coupling
impedance but the quantity of glass used must be limited so as not
to overly weaken the epoxy bonding properties. A product called
Stycast.TM. can also be used to affix transducers. Metals that are
liquids at room temperature, such as galinstan--a eutectic mixture
of Ga, In and Sn or other fluids with extremely low vapor pressure,
such as tetradecane or glycerine can also be used as coupling
agents. Acoustic energy is transmitted readily through such agents.
The edge of the cavity holding such fluids that transmit acoustic
energy from the transducer to the glass wall may be sealed around
the edge with epoxies or silicon rubber (RTV-like) cements.
[0069] Many fluids may find use in the ATMFD such fluids include
acetone, freon, benzene, isopentane, trimethyl borate, water, and
the like, are all contemplated for use in the present invention.
Detector fluids having higher hydrogen contents can be used to
increase the effect of down scattering on the neutron flux ratio
which in turn may enhance the resolution of directionality
determination at large standoff.
[0070] The ATMFD devices can be refurbished such as by replacing
transducers or refilling the fluid in the gap between the chamber
and the transducer. In the process care must be exercised during
removal of epoxy or during refilling so as not to destroy the PZT
transducers which can easily crack or malfunction.
[0071] To operate the device a sinusoidal signal amplified by a
linear amplifier can be used to drive the piezoelectric transducer
which can be polarized in the radial or axial direction. A
piezoelectric material when extended in a given direction will
shrink in the direction at right angles to that first direction.
The need for driving perturbations largely in the radial direction
was the reason for choosing hollow cylindrical PZT transducers
polarized in the radial direction; the electrode leads are on the
inner and outer surfaces of the rings. Vertically polarized hollow
cylindrical PZTs are also available where the electrode leads are
on the lips and may also be used. Alternatively, banks of flat disk
piezoelectric transducers may also be affixed (mechanically or via
glue/epoxy) to the glass surface and then driven individually or in
parallel. Such a bank of drive transducers serve a dual purpose;
first, to provide drive power to the ATMFD, and then, also to serve
as pickup devices for the shock signals arriving from collapsing
cavitation bubbles; the shock signals being superimposed on to the
main drive frequency. In such a case, the need for additional small
microphones is dispensed with, or alternately, may be used to
derive more acoustic information related to ATMFD performance and
directionality monitoring. When in resonance, the mechanical
deformation of the quartz/glass/ceramic/metal chamber of the
present dimensions can be used to generate a standing acoustic wave
consisting of oscillating positive and negative (i.e., sub-vacuum)
pressures in the 20 kHz range. During the time the fluid molecules
are under negative pressure, the state is metastable whereupon,
nuclear particle strikes from incident radiation can be
generated.
Modeling and Simulation
[0072] Two simulation tools can be used in the characterization of
the particle strikes in the ATMFD system: COMSOL Multphysics.TM.
(referred to from here on as COMSOL.TM.)--a finite element
multiphysics program and MCNP5--a nuclear particle transport code.
The COMSOL.TM. numerical model can be used to solve the complex
multiphysics problems of acoustic-structural interactions,
including highly-transient variations, structural dynamics, strong
multidimensional aspects, and electromagnetic coupling. MCNP5 can
be used to evaluate in 3D the spatial and energy dependent physical
aspects affecting the neutron/radiation transport and energy
spectrum over the sensitive volume of the ATMFD.
Finite Element Simulation
[0073] A model of the resonant acoustic chamber can be developed
utilizing COMSOL's.TM. structural mechanics module including
stress-strain and piezoelectric effects analysis and the acoustic
wave transport module together with electromagnetic coupling. Due
to the complexity of the problem, the COMSOL.TM. model utilizes
finite element methods to solve the problem in the frequency
domain.
[0074] Similar models using ethylene glycol and acetone as the host
liquid have been benchmarked against experimental data for the
pressure distribution and frequency spectra response. The ATMFD
system was modeled as axi-symmetric, symmetric about a central
axis. The detector fluid used in the model was pure acetone at
about 25.degree. C. FIG. 2 shows the relationship between
variations in the drive power applied to the PZT and the spatial
characteristics of the sensitive volume of the chamber at the
resonant frequency of about 18.78 kHz. In the current detector
configuration, the sensitive volume of the chamber can be defined
as the volume of the chamber in which the oscillating negative
pressure fluctuations are at or below -3.5 bar, which is the
threshold negative pressure for detection of fast (MeV) neutrons in
acetone.
[0075] As can be seen from FIG. 2, a modest doubling in drive power
from about 4.5 W to 10 W resulted in a linear increase of the
sensitive volume from about 50 cm.sup.3 to about 100 cm.sup.3.
Larger sensitive volumes in the detector not only increase the
effective detection efficiency, allowing for more neutrons to
interact in the sensitive volume of the chamber, but also increase
the radial dimension of the sensitive volume which can be used to
increase the resolution of directional information and increase
detection efficiency.
Monte Carlo Simulation
[0076] Nuclear particle transport assessments can be performed
using the MCNP5 code developed at Los Alamos National Laboratory,
New Mexico, USA. The model consists of the ATMFD's resonant chamber
and a PuBe neutron source (emitting about 2.times.10.sup.6 n/s) at
a distance of about 20.3 cm from the central axis of the chamber.
The chamber can be modeled as axi-symmetric. Suitable structural
materials including the reflectors can be quartz, the piezoelectric
transducers can be lead zirconate titanate (PZT), and the working
fluid can be acetone (C.sub.3H.sub.60). The portion above the top
reflector, inside the top and bottom reflectors and outside the
chamber can be modeled as air.
[0077] According to this method two regional sensitive volumes are
defined in the detector fluid as seen in FIG. 3. The cylinder
(r=about 1.25 cm and h=about 4 cm) defining the sensitive volume
can be divided into two halves; one half facing the source (V1) and
one half facing away from the source (V2). The two half cylinders
formed the neutron tally volumes. The neutron energy spectrum for a
bare PuBe source can be used and all cross sections are evaluated
at about 300.degree. K. All assessments can be calculated to within
about 1% relative error.
[0078] Results of MCNP5 simulations demonstrate about a 23% higher
neutron flux in V1 relative to V2. By comparing the solid angles
subtended by the two sensitive volumes to the PuBe source, the
spatial effects by themselves result in -13% higher flux in V1 than
V2. In comparison, down scattering accounts for about a 10% higher
fast neutron flux in V1. This shows that as the source to detector
distance increases, effectively reducing and eventually nullifying
any possible contributions from spatial effects, the detection in
the ATMFD will still be preferential to and discernable as favoring
the region nearest the source.
[0079] Calculations based on the exponential attenuation law allow
for a quantitative estimate of the relationship between the effect
of down scattering on the neutron flux ratio and the size of the
sensitive volume. A second fluid, Freon-113 (commonly used in
tension metastable fluid based detector systems), is included for
comparison. The PuBe source emits neutrons with an average energy
of about 4 MeV which have a mean free path, A, of about 5 cm in
acetone and about 10 cm in Freon-113. The results are shown in FIG.
4. The effects of down scattering on the neutron flux ratio
increases proportionally with the sensitive volume size. The
ability to increase the amount of directional information available
is observed when the source is far enough away such that solid
angle effects on the magnitude of the neutron flux are small. The
effects of down scattering are also dependent on the composition of
the detector fluid. As seen from FIG. 4, down scattering has a
larger effect in acetone compared to Freon-113. This is primarily
attributable to the higher hydrogen content (and therefore lower A)
of MeV neutrons in acetone. Different detector fluids having higher
hydrogen contents can be used to increase the effect of down
scattering on the neutron flux ratio which in turn may enhance the
resolution of directionality determination at large standoff. Thus,
fluids such as acetone, freon, benzene, isopentane, trimethyl
borate, water and the like are all contemplated for use in the
present invention.
Automation
[0080] The ability to decipher directionality requires acquisition
of hundreds to thousands of detection signals and then rapid
analysis to then result in an answer with high enough confidence
(e.g., >75%) within seconds to minutes. Automation is highly
desirable for practical systems. The violent collapse of the
imploding vapor cavity formed by a nuclear particle interaction
causes an audible click that can be heard several feet away from
the chamber. The audible clicks from the collapsing vapor cavities
can be recorded using four tiny MHz response piezoelectric
transducers which can be affixed to the outside of the resonant
chamber. Hardware and LabVIEW.TM. based virtual instrument software
based control systems are developed to record these detection
events and to extract information on the direction of the
radioactive source.
[0081] The electrical signals from the piezoelectric transducers
can be sent through a third order Butterworth high pass filter to
eliminate the dominate drive frequency therefore isolating the high
frequency components. The signals from the filter can then be sent
to an Agilent.TM. 100 MHz digital storage oscilloscope for display,
storage, and further signal processing. Screen shots of actual
signals used in the experiment are shown in FIG. 4 and FIG. 5. The
peaks in the signal are results of recorded neutron detection
events which resulted in imploding vapor cavities thereby radiating
shock signals that are detected by the PZT transducers.
[0082] A LabVIEW.TM. based virtual instrument (VI) can be designed
as a graphical user interface to not only control the operation of
the ATMFD system but also collect and analyze experimental data.
Experimental data can be acquired by the LabVIEW.TM. virtual
instrument via a GPIB interface with the oscilloscope. In one
method once the data are acquired from the oscilloscope, the
virtual instrument used two statistical techniques to validate the
acquired electrical signal as a neutron detection event. Validation
of the neutron detection events can take advantage of two
characteristics of the shock traces. As can be seen from FIG. 6,
the shock traces from neutron detection events have a high
frequency (about 250 kHz) sinusoidal pulse shape. A further
characteristic of the shock traces is that they are symmetrical. A
measure of the symmetry of the shock traces, known as the skewness,
can be calculated to determine if the electrical signal is that of
a neutron detection event. This technique takes advantage of the
random nature of noise (both electrical and mechanical) and helps
eliminate false positives. The second technique that can be used is
a measure of the similarity of two of the shock traces. In the case
of a true neutron detection event, the recorded shock trace as seen
by each transducer shock monitor should have substantially the same
shape. The cross-correlation, or sliding dot product, of two of the
shock traces can be calculated to ascertain how well the two
individual shock traces match. The cross-correlation method also
allows for the calculation of the time difference of arrival
between the two shock traces. The time difference of arrival (T) is
shown graphically in FIG. 6. The time difference of arrivals can
then be analyzed with a hyperbolic positioning algorithm to
calculate the location of the neutron detection event in the
resonant chamber. The LabVIEW.TM. virtual instrument then utilizes
the locations of the neutron detection events to ascertain the
direction of the neutron source and display it graphically to the
user. The integral detection system designed for the ATMFD, from
neutron detection event to ascertaining the direction of the
neutron source, can be performed in near real time on the
millisecond (ms) time scale.
Directionality Determination Experimentation and Results
[0083] The experimental setup utilizes preferably, a quartz ATMFD
chamber with OD of about 6.9 cm although, other shapes such as
spheres and conical types made of Pyrex glass have also been tested
successfully, as described in MCNP and COMSOL.TM. models. The
liquid used in the chamber was pure acetone at about 25.degree. C.
and under about 20 inches Hg of vacuum. The chamber was operated
with a wave-form generator (Agilent, model 33120A) and a linear
amplifier (Piezo Systems, Inc. model EPA-104). The resonant
frequency was found at about 18.3 kHz, and the drive voltage used
was about 96 V. Experimental data was taken utilizing an
oscilloscope which recorded the shock traces. The LabVIEW.TM.
program was used to control the operation of the oscilloscope,
collect data, and perform signal processing and analysis.
Communication with the oscilloscope was accomplished via a GPIB
interface. Four piezoelectric transducers were placed at right
angles to each other on the same XY plane, with the exception of
the fourth transducer which was placed with a positive Z component
to allow for 3-D positioning. The setup is shown in FIG. 7.
[0084] Experimentation was performed with about a 1 Ci PuBe
neutron-gamma source (emitting about 2.times.10.sup.6 n/s) located
about -20.3 cm and 20.3 cm from the center of the chamber on axis
with Mic 1 and Mic 3. The TDOAs recorded were used to calculate the
positions of the nucleation events and are shown in FIG. 8. Due to
the high Q-factor of the system, small variations in the
construction of the chamber result in a small variation between the
geometrical center and the center of the sensitive volume that must
be taken into account. The average position of the nucleation
events in the XY plane was used to ascertain the center of the
sensitive volume. The positions were adjusted accordingly. The
graphs were first divided into two substantially equally sized
semi-hemispherical volumes. The volume closest to the radioactive
source, V1, contained about 55.2% (.+-.2.5%) of the detection
events, and only about 44.8% (.+-.2.2%) of the detection events
occur in V2. The resulting ratio of neutron detection events is
given as about 1.23 (.+-.0.07). As mentioned previously, the
predicted ratio given by the MCNP model is about 1.23.
[0085] Further analysis of the positions of the neutron detection
events was performed to determine the ability of the detector to
better resolve the angular direction of the radioactive source. The
sensitive volume was divided into 6 separate about 600 angular
sectors. The total number of neutron detection events in each
sector was calculated and compared to the number of neutron
detection events occurring in the opposing sector. Similarly, a
cylindrical mesh tally of the neutron flux in the sensitive volume
was added to the MCNP simulation. When comparing opposing sectors,
the sector nearest the radioactive source was observed to contain
about 57.8% (.+-.4.5%) of the neutron detection events, and only
about 42.2% (.+-.3.7%) of the neutron detection events occur in the
sector furthest away from the radioactive source. The resulting
ratio of neutron detection events was given as about 1.37
(.+-.0.13) which again correlates with our prediction based on
simulations of about 1.38. Again it is noted that the experimental
results correlated to the theoretical model estimates within one
standard deviation.
[0086] A study was performed to investigate the ability of the
ATMFD system to detect the direction of a radioactive source
positioned in an unknown location. The opposing sector ratios for
all sectors were calculated and plotted in FIG. 9 in order to
correctly determine the correct radioactive source direction. It
was exceptionally clear that the sector pointing in the. direction
of the source had the highest opposing sector ratio. Logically, the
second highest opposing sector ratio occurs in the sector directly
adjoining the source direction sector. The sector ratio in the
adjoining sector was given as about 1.16 (.+-.0.11). The ratio
predicted by the MCNP simulation is about 1.18, within one standard
deviation of the experimental result. Analysis of the results shows
that the ATMFD system is capable of locating the direction of the
radioactive source to within 30.degree. with about 80%
confidence.
[0087] Based on the principle that a higher the pressure amplitude
in the liquid provides a higher the probability that a neutron
induced nucleation event will occur, the pressure field inside the
chamber was mapped by the distribution density and profile of
neutron-induced bubble nucleation sites. Experimentation was done
with the PuBe source about -20.3 cm and 20.3 cm from the center of
the chamber on axis with Mic 1 and Mic 3 to prevent the directional
nature of the detector from becoming a factor. The positions of the
detection event sites were plotted in the RZ plane and overlaid on
top of the sensitive volume pressure field predicted by the
COMSOL.TM. model. The results are shown in FIG. 10. Analysis of the
results showed that the neutron induced detection events primarily
occurred at pressures lower than about -4 bar, which correlates
with the previously measured threshold of about -3.5 bar. It was
also apparent that substantially all of the neutron detection
events occurred within a radius of about 1.25 cm from the
centerline of the ATMFD. Therefore the value of about 1.25 cm was
used for the MCNP assessments.
[0088] This work demonstrates directionality determinations and
also shows that the ATMFD system can be tailored to be insensitive
to gamma radiation and that by changing the detector liquid to
Freon-113 and trimethyl borate the ATMFD system can also be
simultaneously used to detect neutrons with energies spanning eight
orders of magnitude while operating with nearly 90% intrinsic
detection efficiency. This is enabled via (n,p) reactions with Cl
atoms of freon, and, (n,alpha) reactions with boron atoms in
trimethyl borate.
[0089] A method for determining the direction of incoming radiation
in near real time is described. The experimental evidence presented
herein has shown that the locations of the neutron detection events
occur preferentially on the side of the detector nearest the source
with a ratio of about 1.23 (.+-.0.07):1 which corresponds with our
Monte Carlo based simulations (about 1.23:1). Calculations have
been performed which show that the increase in solid angle from the
sensitive volume nearest the source to furthest from the source
accounts for about a 13% reduction in neutron flux. The down
scattering of the neutrons through the acetone accounts for about a
10% reduction. The directional information may be intrinsically
obtained with the ATMFD technology even when the source is far
enough away such that solid angle effects on the magnitude of the
neutron flux are negligible. These same calculations prove that the
reduction of the neutron flux due to down scattering increases as
the sensitive volume increases, therefore providing an avenue for
increasing the accuracy and precision of the determination of the
source direction at large standoffs. The COMSOL.TM. coupled physics
simulation is benchmarked with experimental neutron detection data
and can have the ability to scale the sensitive volume of the
detector by increasing the drive power therefore yielding increased
accuracy and precision of the determination of the source direction
and enhanced effective detection efficiency.
[0090] Further analysis of the locations of the neutron detection
events can yield improved methods of directionality determination
via opposing neutron flux sector ratios. The results show that the
ATMFD system, in its current configuration, is capable of locating
the direction of a radioactive source to within about 30.degree.
with about 80% confidence.
A Second Embodiment
[0091] In another embodiment, tension metastable fluid states offer
a potential for advancements in radiation detection. Such
metastable fluid states can be attained using tailored resonant
acoustics to result in acoustic tension metastable fluid detection
(ATMFD) systems. Present-day neutron detectors sometimes may be
bulky, expensive, require different detector systems for various
neutron energy groups and are not suited for providing information
on which direction neutron radiation arrived. Radiation detection
in ATMFD systems is based on the principle that incident nuclear
particles interact with the dynamically tensioned fluid wherein the
intermolecular bonds are sufficiently weakened such that even
fundamental particles can be detected over eight orders of
magnitude or more in energy with intrinsic efficiencies far above
conventional detection systems. In the case of neutron-nuclei
interactions the ionized recoil nucleus ejected from the target
atom locally deposits its energy, effectively seeding the formation
of vapor nuclei that grow from the sub-nano scale to visible scales
such that it becomes possible to record the rate and timing of
incoming radiation (neutrons, alphas, and photons). Nuclei form
preferentially in the direction of incoming radiation. Imploding
nuclei then result in shock waves that are readily possible to not
only directly hear but also to monitor electronically at various
points of the detector using time difference of arrival (TDOA)
methods. In conjunction with hyperbolic positioning, the
convolution of the resulting spatio-temporal information provides
for the first time not just the rate of incident neutron radiation
but also its directionality.
[0092] The development of intrinsic-efficiency, low-cost, and
rugged, ATMFD systems can be accomplished using a combination of
experimentation-cum-theoretical modeling. Modeling methodologies
include Monte-Carlo based nuclear particle transport using MCNP5,
and also complex multi-dimensional electromagnetic-cum-fluid
structural assessments with COMSOL.TM.'s Multi-physics simulation
platform. ATMFD system automation was accomplished with the
programming of virtual instrument (VI) control algorithms using
LabVIEW.TM. software.
[0093] Liquids like solids can withstand tension (i.e., they can
sustain sub-vacuum pressures before tearing apart). A liquid in a
tensile state is metastable below its thermal equilibrium state,
unlike a metastable liquid in a state of thermal superheat which is
above its normal boiling point. Tension in fluids is analogous to
the stretching (versus compression) of solid structures. The energy
required to tear apart the intermolecular bonds of a solid
decreases as the structure is stretched. In an analogous manner,
the energy required to break the bonds between liquid molecules
decreases with increasing tension metastability; eventually
resulting in spontaneous triggering of explosive phase change at
the spinodal (thermodynamic stability) limit of tension.
[0094] Explosive phase change can be triggered in metastable
liquids below the stability limit. This triggering causes explosive
vaporization of fast nucleating and expanding vapor pockets. The
three possible methods of triggering explosive phase changes in
metastable liquids are laser heating, nuclear particle (e.g.,
neutrons) knock-on collisions and acoustic energy. The following
discussion will focus on triggering by means of neutron-nuclei
knock-on collisions. Explosive phase changes can thus be initiated
mechanically or via nuclear particles or photons from a laser. The
rapid, pulsed energy deposition resulting from knock-on collisions
between high energy particles, specifically neutrons, and
individual nuclei of liquid molecules can cause nanoscale
triggering and explosive phase change. The pulsed energy deposition
of recoils from knock-on collisions is in the form of thermal
energy and is deposited over about a few nanometers causing a vapor
nucleus to form. The range in which the energy is deposited depends
on the stopping power of the recoil ion in the liquid. If the
thermal energy deposition rate is high enough to cause a vapor
nucleus larger than the critical size the nucleus will continue to
grow into a macroscopic vapor bubble. Critical radii are generally
in the nanometer range and are reached in nanoseconds. Photons from
a laser source can also be used to trigger explosive phase changes
though more are needed because individual photons of visible light
have a relatively small amount of energy (about 1 eV) and less
linear energy transfer (LET) compared to fast (MeV) neutrons.
[0095] For example, an approximately 4 MeV neutron colliding with a
carbon atom in acetone will, on average, transfer about 0.72 MeV to
the carbon nucleus. This gives an energy density, where the volume
is defined by the critical radius of an acetone vapor bubble (about
30 nm), of about 36.4 MJ/kg. In comparison, the latent heat of
vaporization for acetone is about 0.534 MJ/kg. A single photon from
a blue laser (about 400 nm) with an energy of about 2.48 eV has an
energy density of about 9.6.times.10.sup.-7 MJ/kg. In the case of
light photons the volume is defined by the wavelength of the light
photon. Thus, it takes interactions from about 1.3.times.10.sup.9
blue light (UV) photons to equal the energy density of one neutron
knock-on collision.
[0096] The detection of nuclear particles from tensioned metastable
states requires the induction of appropriate levels of negative
pressures. This is distinct from that for the famous
"bubble-chamber" as used for the superheated droplet detector (SOD)
where the liquid is put above its boiling point. In the system
according to embodiments of the present disclosure, the liquid
remains at room temperature. The principle of detection for fluids
in tension metastability is based on an analogy with stretching of
structures. The greater the degree of tension the easier it becomes
to tear the bonds holding the material together. In an analogous
manner, the greater the degree of negative pressure imparted to the
molecules and atoms of the working fluid, the easier it becomes to
tear the bonds holding the molecules together (i.e., to then cause
localized bubbles to form which can grow from nanometers to
relatively large, multi-mm size pockets before redissolving on
implosion). The sensitivity of detection is based on the degree of
imparted tension and the value of spatial energy deposition from a
given incident nuclear particle, or, dE/dx.
ATMFD Design
[0097] Another embodiment of the ATMFD system, shown schematically
in FIGS. 11 and 12, is a resonant acoustic system comprised of a
(approximately 60 mm OD, 150 mm long) cylindrical glass, preferably
Pyrex glass, resonant chamber powered by a concentrically affixed
ring shaped piezoelectric transducer. A sinusoidal signal amplified
by a linear amplifier drives the piezoelectric transducer.
Reflectors placed at the top and bottom of the chamber aid in
energy focusing by the formation of a standing pressure wave. In
this embodiment four (approximately 7 mm OD) disc-shaped
piezoelectric transducers were fixed to the outer wall of the
cylindrical portion of the chamber and used to detect the shock
wave spectra generated by radiation induced cavitations occurring
in the sensitive volume of the detector.
[0098] The sensitive volume of the ATMFD is defined as the region
in which the magnitude of the tension (negative) pressure is below
a certain threshold value for which critical size vapor nuclei can
be formed via energy deposition by incident nuclear particles
colliding with the metastable state molecules.
Modeling and Simulation
[0099] Two simulation tools can be used in the characterization of
the ATMFD system: COMSOL Multiphysics.TM. (referred to from here on
as COMSOL.TM.)--a finite element multiphysics program and MCNP5--a
nuclear particle transport code. COMSOL.TM. allows for the coupling
of acoustic, fluid, and structural models of the resonant acoustic
system. MCNP can be utilized to evaluate the combined spatial and
energy dependent physical aspects effecting the neutron flux and
energy spectrum over the sensitive volume of the ATMFD.
Finite Element Simulation
[0100] A numerical model using COMSOL.TM. which is based on finite
element methods, can be developed for frequency domain analyses and
the results from the model can be compared with experimental data.
The multiphysics model set here utilizes COMSOL.TM.'s structure
mechanics module including stress-strain and piezoelectric effects
analysis and the acoustic wave transport module together with
modeling of electromagnetic coupling.
[0101] We assume that the system is axi-symmetric. In an embodiment
the detector liquid is chosen to be pure acetone at about
25.degree. C. The values for various properties of acetone are
listed in Table 1.
TABLE-US-00001 TABLE 1 PROPERTIES OF ACETONE AT 25.degree. C.
Density (kg/m.sup.3) 0.786 .times. 10.sup.-3 Viscosity (Pa s) 0.308
.times. 10.sup.-3 Sound velocity (m/s) 1174 Bulk viscosity (Pa s)
about 1.5 .times. 10.sup.-3
[0102] A similar model, which uses ethylene glycol as the liquid,
has been benchmarked against experimental data for the pressure
distribution and frequency spectra response. In the present
embodiment, we keep the same physical domain settings and boundary
conditions of the benchmarked model; changing the properties of the
liquid and the structure of the system and introducing structural
fluid damping.
[0103] Due to the large variations of dimensions between structure
and liquid, the maximum relative size of mesh finite elements in
liquid and solid regions are about 0.003 and about 0.017,
respectively. The meshed structure embodied a total of about 5237
elements. Numerical convergence has been checked by use of finer
meshing (about 20948 elements).
[0104] In order to visualize the relation and accompanying
variations between the power driving the system and the sensitive
volume in the chamber, the oscillating pressure distributions are
plotted in FIGS. 11A and 11B at the resonant frequency of about
18.85 kHz.
[0105] As shown in FIGS. 11A and 11B, the sensitive volume of the
ATMFD can be varied by varying the drive voltage. Various other
options also become feasible (e.g. using higher modes or
superposition). Such a modeling approach can be utilized for
designing and devising an as-needed ATMFD with desired levels of
detection sensitivity along with ability to derive directionality
information.
Monte Carlo Simulation
[0106] A system model can be developed for nuclear particle
transport assessments using the MCNP5 code and shown in FIG. 12. It
consists of the ATMFD's resonant chamber and a PuBe neutron source
(emitting about 2.times.10.sup.6 n/s). The chamber is substantially
axial-symmetric. All structural material including the reflectors
can be quartz glass, the piezoelectric transducers can be lead
zirconate titanate (PZT), and the representative detection fluid
can be acetone (C.sub.3H.sub.60). The portion above the top
reflector, inside the top and bottom reflectors and outside the
chamber is modeled as air.
[0107] Two regional sensitive volumes are defined in the detector
fluid as seen in FIG. 12. The cylinder (r=about 1.5 cm and h=about
4 cm) defining the entire sensitive volume is divided into
substantially two halves; one half facing the source (V1) and one
half facing away from the source (V2). The two half cylinders form
the neutron tally volumes.
[0108] The neutron energy spectrum for a bare PuBe source is used
and all cross sections are evaluated at about 300.degree. K. The
source is placed about 35.5 cm from the central axis of the chamber
to be consistent with the experimental configuration.
[0109] Results of MCNP5 simulations demonstrate about a 25%
increased probability of neutron interactions in the direction of
the incident neutron source. This result confirmed estimates from
first principle estimates.
[0110] By comparing the solid angles subtended by the two sensitive
volumes to the PuBe source, one can see that the spatial effect for
neutron flux amounts to about a 15% higher flux in V1 than V2. In
comparison, down scattering accounts for about a 10% higher fast
neutron flux in V1. This indicates that even if the source is
further away, effectively negating the solid angle dependence of
the neutron flux, detection in the ATMFD (as presently disclosed)
will still be preferential and discernable as favoring to the side
nearest to the source.
Automation
[0111] The collapse of the imploding vapor cavity formed by a
nuclear particle interaction causes an audible click that can be
heard several feet away from the chamber. The audible clicks from
the collapsing cavities can be readily recorded using the
piezoelectric transducers which can be affixed to the outside of
the chamber. Recording the time that the shock wave reaches each
transducer then allows the time difference of arrival (TDOA) to be
calculated. The TDOA between transducers can be used in conjunction
with a hyperbolic positioning algorithm to calculate the actual
position of the bubble cavitation events.
[0112] The electrical signals from these cavitation events can
first be sent through a third order Butterworth high pass filter to
eliminate the dominate drive frequency therefore isolating the high
frequency components. The signals from the filter can then be sent
to an oscilloscope, such as an Agilent.TM. 100 MHz digital storage
oscilloscope, for display, storage and further signal processing. A
screen shot of the actual signals used in the experiment is shown
in FIG. 13.
[0113] The first channel shown in FIG. 13 is the unfiltered
transducer signal. The second channel is the corresponding signal
after the high pass filter. The peaks in the signal are the
unmistakable results of a recorded cavitation pulse. A LabVIEW.TM.
program was created as a graphical user interface with the
oscilloscope. Using the LabVIEW.TM. program the oscilloscope was
set to run until the analog signal on the triggering channel
crossed a predetermined threshold level. From the screenshot of the
oscilloscope, several measurements are acquired; XatMaxY (time in
.mu.s of the highest peak relative to the trigger point), XatMinY
(time in .mu.s of the lowest peak relative to the trigger point),
and Maximum (maximum voltage recorded in the screenshot). Actual
screenshots of typical signals used are shown in FIG. 14.
[0114] The measurements XatMaxY and XatMinY serve two purposes.
These measurements allow for the calculation of the TDOAs between
signals and for an estimation of the frequency of the cavitation
pulse as recorded by each transducer. The maximum, voltage
measurement ensures that the height of the cavitation signals on
all four channels is larger than the triggering level. The values
of the TDOAs, frequency, and maximum voltage of each cavitation
pulse as recorded by the transducer were used as constraints to
determine whether the signal analyzed is that of a cavitation
pulse.
Data Constraints
[0115] The TDOA constraint was set using a numerical analysis of a
hyperbolic positioning algorithm. A LabVIEW.TM. computer program
can be used to generate a random sample of cavitation events inside
the chamber. The cavitation positions are then used to calculate
the TDOA that each transducer would record. The TDOAs are then
analyzed with a hyperbolic positioning algorithm. Upper constraints
are set on the TDOAs used in the data set to investigate what TDOAs
would result in a cavitation position mapped outside of the modeled
sensitive volume of the chamber as mentioned earlier. The results
are shown in Table 2. Therefore the upper constraint for TDOAs of
about 20 .mu.s resulted in cavitations within about 2 cm of the
central axis of the chamber, and is consistent with experimental
findings.
TABLE-US-00002 TABLE 2 DATA CONSTRAINTS USED FOR TDOA CALCULATIONS
TDOA Constraint Maximum Radius of Zone 15 .mu.s 1.57 cm 20 .mu.s
1.97 cm 40 .mu.s 2.95 cm
[0116] The dominant frequency of the largest peaks in the
cavitation pulse can be determined using the XatMaxY and XatMinY
measurements. As seen in FIG. 14, the XatMaxY and XatMinY
measurements should occur at the peaks of the cavitation pulse with
the greatest magnitude. The frequency constraint can be
experimentally investigated using LabVIEW.TM. software and the
oscilloscope. A LabVIEW.TM. program can be designed to record
substantially the entire analog waveform of a cavitation pulse. An
experimental data set of about 100 cavitation based neutron
detection events can be recorded for analysis. Fast Fourier
Transforms are then done on the cavitation waveforms. It was found
that the dominant frequency of the largest peaks in the cavitation
pulse is about 300 kHz. A lower constraint of about 200 kHz can be
used to determine whether or not the signal recorded contained a
cavitation pulse. A large range of frequencies are accepted because
the recorded cavitation frequencies vary according to the
cavitation strength, the distance to the recording transducer, the
frequency response of the transducer (due to manufacturing), and
the level of dampening in the chamber (due to scattering centers
e.g. vapor or gas bubbles).
[0117] The maximum voltage measurement allowed for a minimum
voltage constraint in order to eliminate triggering bias was
discovered in initial experiments done by hand calculations and is
explained later. The minimum voltage constraint can be set using
experimental data. The maximum voltage identified from about 450
cavitations was recorded by all four transducers using a PuBe
neutron source. In order to eliminate any maximum voltage bias due
to source position, four sets of data are taken with the source
about +35.5 cm and -35.5 cm away from center of the chamber on the
X-axis with transducers 1 and 3 and on the Y axis with transducers
2 and 4. The average maximum voltages of the cavitations recorded
are shown in Table 3.
TABLE-US-00003 TABLE 3 RECORDED AVERAGE MAXIMUM VOLTAGE MICROPHONE
RESPONSE TO CAVITATION SHOCK EVENTS Mic 1 Mic 2 Mic 3 Mic 4 Source
1 716 mV 692 mV 703 mV 667 mV Source 3 697 mV 702 mV 716 mV 661 mV
Average 707 mV 697 mV 710 mV 664 mV
[0118] The average maximum voltage of a cavitation pulse can be
used to set trigger levels for each transducer. Only cavitations
that have maximum voltages larger than the trigger levels for all
four transducers are recorded for analysis. This method allows the
oscilloscope to essentially trigger on all four signals at once,
therefore eliminating any trigger bias. The initial results
indicate that the trigger levels are within about 6% of each other,
therefore the same trigger levels could be used for each
transducer. Early experimental results indicate that the triggering
levels used influence the accuracy of the results. To investigate
the effect, a LabVIEW.TM. program can be designed to record the
rate of cavitations as recorded by each transducer as the trigger
level is varied from about 5 mV to 195 mV. The results are shown in
FIG. 15.
[0119] The experimental results indicate that triggering rate was
unstable for small triggering voltages up to about 100 mV and
stabilized around about 200 mV. Therefore a triggering level of
about 200 mV can be used as a baseline for this embodiment. Due to
variations in manufacturing tolerances and variability in use of
epoxy or some other material to attach the pill microphones, the
precise trigger level can now be developed using the method just
described.
Directionality Determination Experimentation and Results
[0120] The experimental setup utilized a quartz ATMFD chamber with
diameter of about 6.9 cm, as described in MCNP and COMSOL.TM.
models. The liquid used in the chamber was pure acetone at about
25.degree. C. and under about 20 in. Hg of vacuum. The chamber was
operated with a wave-form generator and a linear amplifier. The
resonant frequency was found at about 18.3 kHz, and the drive
voltage used was about 100 V. Data were taken utilizing an
oscilloscope which recorded the shock traces. The LabVIEW.TM.
program mentioned earlier controlled the operation of the
oscilloscope and collected the data. Communication with the
oscilloscope can be accomplished via a GPIB interface. Four
piezoelectric transducers were placed at right angles to each other
on the same XY plane, with the exception of the fourth transducer
which was placed with a positive Z component to allow for 3-D
positioning. The setup is shown in FIG. 16.
[0121] Data were first taken utilizing an oscilloscope which
recorded the cavitation pulses. The TDOAs were recorded manually
with the oscilloscope cursors. The TDOAs were then analyzed with a
hyperbolic positioning algorithm as mentioned earlier. The
preliminary data (shown in FIG. 17), with the PuBe source about 13
cm away from the center of the chamber on the X axis, provided
evidence that the location of the cavitation events in the chamber
of the detector was convincingly biased towards the direction of
the radioactive source.
[0122] The chamber can also be divided into two equally sized
sections. The section closest to the source comprises about 65%
(84/124) of the cavitation events, and about 32% (40/124) of the
cavitation events occurred in the section furthest away from the
source. This resulted in a ratio of about 2.1:1 (2.1). It was also
discovered that a bias towards the triggering signal occurs. This
triggering bias occurs because the data set was taken while
triggering on only one signal. The triggering bias occurs because
the cavitations closer to the triggering transducer have greater
recorded shock signal amplitudes; therefore the cavitations that
occur close to the triggering transducer may be preferentially
biased. However, this trigger bias can be eliminated if all four
transducers are used to trigger a cavitation event. This method of
recording cavitation positions is inefficient, and taking data by
hand only allowed for approximately 2 Sa/min to be recorded.
Therefore, an automation system that allows for processing of
significant amounts of data was designed and used thereafter.
However, the manually obtained data serves as confirmation of the
ability to offer directional information and also serves as a
benchmark.
[0123] The data taken using the automated system are analyzed with
the hyperbolic positioning algorithm mentioned earlier as well as
with a LabVIEW.TM. program designed to keep track of how many times
each transducer recorded the cavitation pulse first. A count of how
many times two of the transducers recorded the cavitations first is
listed in Table 4.
TABLE-US-00004 TABLE 4 RESULTS TAKEN WITH PuBe SOURCE PLACED 35.5
cm FROM CENTER AXIS OF CHAMBER ON-AXIS WITH MIC 1 AND MIC 3 Mic 1
Mic 3 Source at 1 88 65 Source at 3 71 82
[0124] The chamber again can be divided into two equally sized
sections. The section closest to the radioactive source contains
about 56% (170/306) of the cavitation events, and about 44%
(136/306) of the cavitation events occur in the section furthest
away from the source. The resulting ratio of cavitation events is
given as about 5:4 (1.25). These results correlated with the
results previously taken by hand, and with the theoretical value
given by the MCNP model (about 1.24). The difference between the
counts taken by the computer and by hand can be attributed to the
elimination of the trigger bias by setting a lower constraint for
the maximum voltage of the cavitations. The TDOAs recorded are also
used to calculate the positions of the cavitations and are in FIGS.
18 and 19.
[0125] The graphs are also divided into two substantially equally
sized sections. The section closest to the radioactive source
contains about 56% (170/306) of the cavitations, and about 44%
(136/306) of the cavitation events occur in the section furthest
away from the cavitation source. The resulting ratio of cavitation
events is given as about 5:4 (1.25). It is noted that the data
using the first transducer of arrival method and the hyperbolic
position method correlate with about 100% accuracy, and also
correlate to the theoretical model estimates (i.e., MCNP5 and
COMSOL.TM.) to within about 2%.
[0126] A graph was also prepared which included all cavitation
events recorded, with the radioactive source about -35.5 cm and
35.5 cm away from the center of the chamber on the X axis with Mics
1 and 3. The graph of the cavitation events as seen in the XZ plane
shows that the sensitive volume of the chamber was similar in size
and shape to the developed COMSOL.TM. mode (FIG. 20). It can be
seen that all neutron detection (cavitation events) occur within a
radius of about 1.5 cm from the centerline of the ATMFD, which
corresponds very well with the predictions of the COMSOL.TM. model.
Therefore, the value of about 1.5 cm is used for MCNP
calculations.
[0127] The radial (spatial) distribution of the cavitation
locations was also analyzed. The cavitation events are distributed
into substantially two equal parts; the half of the chamber closest
to the source, and the half of the chamber furthest away from the
source. The radial positions were then tabulated in a histogram,
which resulted in the number of cavitations in opposing concentric
arcs. The results are shown in FIGS. 21 A and 21 B.
[0128] The center section of the chamber, which has the greatest
tension (negative) pressure, was eliminated from this count,
because this location is where cavitation events preferentially
occur, and the error in the hyperbolic positioning algorithm is the
greatest. Therefore the cavitations that occurred in this section
of the chamber contribute the least to directional information. The
resultant counts show that about 56% (154/274) of the cavitation
events occurred in the section closest to the source, while about
44% (120/274) occurred in the section furthest from the source,
equivalent to a ratio of 5:4 (1.28). This method of data analysis
provides for improved and better capability for deriving
directional information, when compared to the simpler first
transducer of arrival method.
[0129] A related means for determining directionality of incoming
radiation is possible from visual inspection of bubble shapes. It
is found that radiation from a source such as a Pu--Be source of
neutrons delivers energy from the radiation to the nuclei of atoms
in the direction of the source to give rise to nanoscale bubbles
which grow to macroscopic visible sizes. In an oscillating
acoustically-driven field, the tiny bubbles grow to macroscopic
sizes in the multi mm range in the ATMFD systems discussed earlier
and then elongate themselves in elliptic shape transporting
themselves radially outward towards the glass walls via acoustic
pressure gradient prior to dissolving and disappearing. This
feature is shown in FIG. 22 where the major axis of the elongated
bubble cluster (formed in an ATMFD system):from neutron induced
collision with nuclei of acetone is pointing towards and is in line
with the neutron source. Observation of movie clips taken with a
1,000 fps camera as well as with a conventional 30 fps video camera
indicates that approximately 8 of every 10 of the bubble clusters
point in this preferential direction. Some of the incoming neutrons
striking the tensioned liquid molecules can be expected to come as
reflected neutrons from other angles, or also to strike the nuclei
of target atoms of the fluid at grazing angles, and hence, may be
expected to give rise to elliptically transported bubbles in
various other directions away from the true source of radiation.
Nevertheless, this finding gives rise to the possibility for
determining directionality on a relatively instant (within seconds)
reliable basis by direct visual image monitoring and analysis
inspection of the transient bubble clusters. Such a system would
also become extremely valuable in situations involving very low
intensity radiation arriving at the detector (e.g., from
well-shielded nuclear materials) whereby, use of the TDOA based
technique becomes impractical for real-time monitoring of
directionality.
SUMMARY AND CONCLUSIONS
[0130] In both the first transducer of arrival and the hyperbolic
positioning methods used, the neutron detection as evidenced by
location of cavitation events (in an ATMFD with at about 70 mm OD)
preferentially occurs on the side of the detector nearest the
source both with a ratio of about 1.25:1. Downscattering events in
chambers of this size plays an important role in permitting
reliable discerning of directionality; larger ATMFDs can lead to
even higher confidence levels and in less time. Therefore, it is
discernible that the addition of the hyperbolic positioning
algorithm, which allows for mapping of the cavitation events in 3D,
does not increase the error involved. The ability to map the
cavitation events in three dimensions signifies that there is not
only the ability to detect 2D directionality, but also 3D
directionality information.
[0131] Cavitation events are found to occur preferentially on the
side of the detector nearest to the source with a ratio of about
1.25:1 compared with predictions from the multiphysics based
simulations (about 1.24:1). These ratios are for a source to
detector distance of about 35.5 cm. Calculations confirm that for
this distance the increase in solid angle from the sensitive volume
nearest the source to furthest from the source accounts for about a
15% reduction in neutron flux. This means that down scattering of
the neutrons through the acetone (even for an approximately 6 cm OD
ATMFD system) accounts for a very significant (approximately 10%)
effect. Larger ODs will allow for greater ability for
directionality. Therefore, directional information may be obtained
even when the source is far enough away such that solid angle
effects on the magnitude of the neutron flux are negligible.
[0132] Our COMSOL.TM. coupled physics simulation shows the ability
to scale the sensitive volume of the detector by increasing the
drive power and yielding increased confidence directional
information in less time than the baseline case.
[0133] ATMFD may be insensitive to gamma radiation and by changing
the liquid to be composed of Cl or B nuclei (Freon-113 or trimethyl
borate) the ATMFD can also be simultaneously used to detect
neutrons/radiation with directionality with energies from thermal
to fast and approximately 100% intrinsic efficiency has also been
demonstrated for TMFD systems.
[0134] Radiation collisions with nuclei of TMFD systems deliver
energy in preferential directions that coincide to a large extent
with the direction of arrival on to the nuclei of atoms of liquid
molecules of the TMFD system. Tension pressures amplify the bubbles
from nanoscale to the multi-mm scales in such way that bubbles can
deform to elongated and approximately cylindrical comet-like shapes
with the major axis substantially pointing in line with the
direction of incoming radiation.
[0135] While the present disclosure has been described with
reference to certain embodiments, other features may be included
without departing from the spirit and scope of the present
invention. It is therefore intended that the foregoing detailed
description be regarded as illustrative rather than limiting, and
that it be understood that it is the following claims, including
all equivalents, that are intended to define the spirit and scope
of this invention.
* * * * *