U.S. patent application number 14/949445 was filed with the patent office on 2016-07-14 for particle detection system.
The applicant listed for this patent is Sagamore Adams Laboratories. Invention is credited to Rusi P. Taleyarkhan.
Application Number | 20160202360 14/949445 |
Document ID | / |
Family ID | 41379809 |
Filed Date | 2016-07-14 |
United States Patent
Application |
20160202360 |
Kind Code |
A1 |
Taleyarkhan; Rusi P. |
July 14, 2016 |
Particle Detection System
Abstract
A system to detect ionizing particles that includes an enclosure
which holds a fluid in a tensioned metastable state. The
interaction of a particle with the liquid creates a vapor pocket
that can be seen and recorded, and also results in a shock wave
that can be heard and recorded. The level of tension metastability
in combination with agents, such as Be and B atoms, and surfactants
that minimize evaporation losses is associated with a particular
type of particle.
Inventors: |
Taleyarkhan; Rusi P.;
(LaFayette, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sagamore Adams Laboratories |
Chicago |
IL |
US |
|
|
Family ID: |
41379809 |
Appl. No.: |
14/949445 |
Filed: |
November 23, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12462487 |
Aug 4, 2009 |
9194966 |
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14949445 |
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11337416 |
Jan 23, 2006 |
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12462487 |
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60646022 |
Jan 21, 2005 |
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Current U.S.
Class: |
376/153 |
Current CPC
Class: |
G01T 3/00 20130101; G01T
1/167 20130101; G01V 5/0008 20130101; G01T 3/008 20130101; G01V
5/0091 20130101 |
International
Class: |
G01T 1/167 20060101
G01T001/167; G01T 3/00 20060101 G01T003/00 |
Claims
1-21. (canceled)
22. A system for detecting particles comprising a liquid in a first
enclosure, wherein said liquid comprises at least one additive for
broadening the energy range of detectable neutrons having energies
in the range of from about 0.01 eV to about 10 MeV; said liquid
comprising at least one neutron absorbing additive for the
detection of photons; and a surface of said enclosure comprising a
coating for the detection of alpha or photon particles, wherein
said particles include at least one of: neutrons; protons, or
alphas in the MeV range; wherein local regions of the liquid are in
a metastable negative pressure state such that when said liquid is
in the presence of said photons or particles a detectable vapor
pocket forms and implodes to generate a detectable sonoluminescent
light flash and a shock wave.
23. The system of claim 22, further comprising a secondary
removable and rotatable enclosure surrounding the first enclosure,
the secondary enclosure having at least one open zero thickness
window to permit directional passage of at least one of neutrons,
gamma rays, and alpha particles thereby limiting said at least one
neutrons, gamma rays, and alpha particles reaching said first
enclosure to the direction of the source of the nuclear
particle.
24. The system of claim 22, wherein the liquid is treated by
compression of the enclosure and enclosed liquid; compression or
filtration of the liquid ahead of introduction into the detector
structure; neutron-induced acoustic agitation based degassing of
liquid within the enclosure or a combination thereof.
25. The system of claim 22, further comprising at least one
additive in the liquid comprising a material to absorb low energy
neutrons resulting in alpha or proton recoils which nucleate vapor
pockets.
26. The system of claim 22, further comprising at least one
additive in the liquid comprising at least one of: a chlorinated
compound; a nitrated compound; boron; trimethyl borate; U-235;
U-233; Pu-239; Pu-241; U-238; Th-232; Th-233; U-233; and
Np-236.
27. The system of claim 22, further comprising at least one
additive in the liquid comprising B-10; N-14; Li-6; and Cl-35.
28. The system of claim 22, wherein the detection of MeV energy
neutrons colliding at grazing angles with detector liquid molecules
occurs when the tension level of the liquid is at a tensioned
metastable state around and above a threshold level.
29. The system of claim 22, wherein said coating on said wall of
the enclosure surface is of a thickness ranging from about 0.01 mm
to 1 mm and comprises Be; B; C; F; Li for detecting alpha
particles.
30. The system of claim 22, wherein the detection of the vapor
pocket is by a light beam and a photo-switch; a photosensitive
detector; or an acoustic wave monitor comprising an acoustic
transducer/microphone.
31. The system of claim 22, wherein a waiting time for detection
can be varied.
32. The system of claim 22, wherein the detection waiting time to
detect MeV energy neutrons is in the range of tens of seconds for a
threshold tension metastable state corresponding to direct
front-end knock-on from incident nuclear particles with detector
liquid molecules of about -3 bar for liquids including Freon-113,
acetone, methanol, isopentane, and trimethyl borate.
33. The system of claim 22, wherein the waiting time for tensioned
metastable liquids including Freon-113, acetone, methanol, ethanol,
isopentane and trimethyl borate is less than about 1 millisecond
for a tension metastable state higher than the threshold tension
pressure for detecting angular grazing collisions with detector
liquid molecules.
34. The system of claim 22, wherein the liquid for detection of
photons comprises an additive having a high Z element bearing
soluble compound.
35. The system of claim 22, wherein the liquid for detection of
photons comprises an additive having a high Z element bearing
soluble compound comprising at least one of lead acetate; KI; and
Nal.
36. The system of claim 22, wherein the liquid further comprises at
least one of D atoms and Be atoms; and the coating of said walls
comprises at least one of D atoms and Be atoms.
37. The system of claim 22, wherein the liquid comprises a
surfactant.
38. The system of claim 22, comprising a generator coupled to the
enclosure for inducing acoustic pressure oscillations in the liquid
that cause a negative pressure metastable state in the liquid in
local regions, wherein the generator transfers said acoustic
pressure oscillations of above 0 psia during compression and below
0 psia through spinoidal limits during tension of the liquid,
thereby creating for transient periods of time, regions of high and
low (sub-zero) pressure.
39. The system of claim 22, further comprising a motor for spinning
the enclosure to tension the fluid into a metastable state.
40. The system of claim 22, further comprising a motor for spinning
the enclosure to tension the fluid into a metastable state, wherein
said spinning enclosure has a substantially vertical axis and
protruding arms that are inclined to the axis for introducing
tension into the liquid within a venturi sized with
contraction-expansion regions for establishing levels of tension
pressure in the venture throat region when centrifugal force is
applied to the enclosure as the spinning enclosure rotates, and
wherein each of said protruding arms comprises at least one liquid
ejection orifice at the end of its arm for ejecting liquid in
response to centrifugal force.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 12/462,487, now U.S. Pat. No. 9,194,966 which is a continuation
of U.S. application Ser. No. 11/337,416, filed Jan. 23, 2006 and
claims benefit of U.S. Application No. 60/646,022, filed Jan. 21,
2005, each of which is incorporated by reference in their
entireties.
BACKGROUND
[0002] The detection of neutrons, gamma rays and alpha particles is
of great importance in the global war against weapons of mass
destruction and terrorism, and in the fields of physics and
engineering. Conventionally, gamma ray and alpha particle detection
is performed using HPGe/NaI scintillation systems or by use of
other systems that cause gas ionization to take place under high
voltages, as in a Geiger-Muller (G-M) tube. The detection of
neutrons is more difficult, especially in a high gamma ray
background and is usually performed in liquid-scintillation (LS),
He-3 or BF.sub.3 type detectors in combination with accompanying
components which permit discrimination of neutrons from gamma
fields. Such systems, however, are expensive and require good
knowledge of nuclear physics/instrumentation and are usually
non-portable.
[0003] Specifically, conventional detectors for nuclear particle
detection (especially ones that can also detect and discriminate
neutrons) depend on the use of systems such as plastic
scintillators, liquid scintillators to fission chambers, G-M type
counters and superheated droplet detectors (SDDs). These devices
(with the exception of the SDDs) require extensive electronics in
the form of high-voltage power supplies, photomultiplier tubes,
preamplifiers, associated pulse shape discrimination and counting
logic systems. These systems rely on nuclear interactions that
result in ionization, light production and amplification, etc. Note
that these conventional systems are not implemented as a
comprehensive hand-portable system that can distinguish neutrons
from alpha particles and gamma ray sources with the benefits of
low-cost, high efficiency and simplicity of operation. Also, once
the SDD droplets are vaporized the droplets need to be visually
inspected (or counted in a special counter) and the system needs to
be taken off service and refilled or reset overnight since it is
not regenerative.
SUMMARY OF INVENTION
[0004] In general, various implementations of the present invention
provide a simple-to-use novel and unique, low cost system for
detection of neutrons and gamma rays from special nuclear materials
based on the physics of interaction of nuclear particles with
fluids in a tensioned metastable state. An optimized system for
field use is provided that represents a departure from present-day
approaches in terms of simplicity of set up, use, improved
efficiency, and at costs that are significantly less than that of
conventional present-day systems.
[0005] In a general aspect of the invention, a system for detecting
particles includes an enclosure which generates neutrons upon
interaction with alpha particles or gamma photons and which holds a
pretreated fluid in combination with an additive to detect thermal
energy level neutrons through absorption, and further includes a
motor coupled to the enclosure for spinning the enclosure to
pretension the fluid into a metastable state. The interaction of
the particle with the fluid in the metastable state creates a
respective vapor pocket, which creates a shock wave, the of which
is associated with a particular type of particle.
[0006] In another aspect of the invention, a system for detecting
nuclear particles includes an enclosure which holds a pretreated
fluid and a generator coupled to the enclosure to induce acoustic
pressure oscillations of the desired shape, amplitude and frequency
in the fluid to pretension the fluid into a metastable state.
[0007] Further features and advantages of this invention will
become readily apparent from the following description, and from
the claims.
BRIEF DESCRIPTION OF FIGURES
[0008] FIG. 1 is a schematic of a system for ionizing particle
detection in accordance with an embodiment of the invention.
[0009] FIG. 2. is an image of an enclosure of the system of FIG. 1
holding a metastable fluid before the interaction of an ionizing
particle with the fluid.
[0010] FIG. 3 is an image of the fluid showing a void space created
when an ionizing particle interacts with the fluid.
[0011] FIG. 4 is a plot showing negative pressure attained via
centrifugal force with variations in arm length and frequency of
rotation.
[0012] FIG. 5 is a composite detector-logger system for the system
shown in FIG. 1 in accordance with an embodiment of the
invention.
[0013] FIG. 6 is a schematic of a centrifugal force based system
that permits continuous recharging without stopping in accordance
with an embodiment of the invention.
[0014] FIG. 7 is a pictorial view of a resonant acoustic test cell,
in accordance with another embodiment of the invention.
[0015] FIG. 8 is a schematic view of the electronic train of
components for the resonant acoustic test cell operation in
accordance with an embodiment of the invention.
[0016] FIG. 9 is a schematic view of detector components and
electronic train of components for a conical design system resonant
acoustic test cell operation.
[0017] FIG. 10 is a schematic view of the detector enclosure with
test fluid or the test fluid in a container being subjected to
pretreatment to result in desired levels of amorphous states to
prevent false positives.
[0018] FIG. 11 is a schematic view of the venturi-based
continuously operating centrifugal tensioned metastable fluid
detector system.
[0019] FIG. 12 is a schematic view of the Venturi region of the
test cell shown in FIG. 10.
DETAILED DESCRIPTION OF INVENTION
[0020] In accordance with various embodiments of the invention, a
tensioned metastable fluid detection (TMFD) system is implemented
that reduces costs and advances the state-of-the-art in detection
of key particle spectral emanations (viz., neutrons, alphas and
gamma rays over a range of energies) from special materials such as
U, or Pu or other materials of strategic interest such as those
from "dirty" bombs using fission products. The system is based on
the nucleation and implosive collapse of critical size cavities on
the nano (for example, between about 10 and 100 nm) dimension scale
by collision interaction of particles with nuclei of metastable
fluids. Two effective means for induction of a tensioned metastable
state encompassing virtual superheat are: (1) to cause centrifugal
force-based pretensioning, and, (2) acoustically-induced pressure
oscillations in selected fluids (such as water, acetone, methanol,
trimethyl borate, C.sub.2Cl.sub.4, etc.). Calculations show that
"individual" neutrons, gamma rays, or alpha particles from
spontaneously emitting sources such as even numbered Pu isotopes or
Cf-252, U isotopes, and gamma emitters such as Cs or Co induce
growth of (10-100 nm) critical size vapor pockets if the fluid is
in a suitable metastable state. The resulting shock waves give rise
to the characteristic audible "click" of a nuclear particle count.
A simple electronic counter, and notebook PC based software then
gives an indication of the strength and type of source being
interrogated. The threshold metastability for nucleation by these
particles varies very strongly with the energy and type of nuclear
emission. By appropriately tailoring the degree of metastability of
the fluid, the system in FIG. 1 can be implemented as a uniquely
simple, hand-held, ultrasonic or centrifuge based nuclear particle
detector of size like a "Dremel" tool or ultrasonic toothbrush--one
with enough sensitivity to collectively detect and if desired
distinguish gamma rays, alpha particles and neutrons from one
another. The working fluid used for detection purposes is
appropriately pretreated and/or encompasses a surfactant like
substance to minimize evaporation at the gas-liquid interfaces and
to improve wetting. Proper pretreatment ensures that the system
will not give rise to spurious nucleation resulting in
false-positives. Also, such pretreatment then permits attainment of
the desired levels of tensioned metastability such that the system
becomes sensitive to the appropriate nuclear particle (for example,
neutrons of a given energy). Pretreatment schemes can involve (most
desirably) subjecting the test liquid and enclosure to
pre-compression (to pressures in the range of up to several kbars).
While doing so, the test system (shown schematically in FIG. 10) is
located in the fluid of the pressure chamber. In this mode of
pretreatment the liquid inside the test enclosure and outside
pressurizing fluid are both subject to the same extent of
pressurization such that the walls of the enclosure retain their
structural integrity. Such a means for pretreatment allows the
liquid to "Wet" all foreign objects within it creating a glued-like
environment such that weak points for spurious nucleation are
eliminated. The same holds true for creating a state of good-enough
wetting between the liquid and the walls of the detector cell
container--to then prevent spurious nucleation at wall-liquid
interfaces. Alternative means of pretreatment involve filtering the
test fluid via use of .about.1 mm filters prior to filling the test
detector cell or the removal of dissolved gases in the test liquid.
Dissolved gases in the test liquid can be removed via boiling of
the test liquid, or via acoustic agitation in the presence of
neutron sources. Pretreatment can also incorporate the use of
surfactants that form a monolayer type barrier at interfaces
between test liquid and atmosphere (for example, in FIGS. 1 and 6)
and also to promote wettability.
[0021] Therefore, a unique capability results, one that can be
utilized for identifying individual special materials and threats
from gamma emitting fission product based "dirty" bomb
packages.
[0022] In accordance with an embodiment of the invention, a
centrifugal metastable state-inducing system 10 shown in FIG. 1
detects neutrons of various energy levels in a high gamma ray
background. Metastability in this case is induced by introducing a
tension force on the molecules of the liquid 12 in the central bulb
region of an enclosure 14 at the base of the two arms 16, 18 of the
diamond-shaped spinner 20. The enclosure 14 is mounted to a holder
22, which in turn is coupled to a motor 24. As such, the motor 24
spins the enclosure 14 to create the tension force on the molecules
of the liquid 12. A feature of the centrifugal system 10 shown in
FIG. 1 is that once the liquid 12 in the central region of the
enclosure 14 nucleates a vapor pocket, the vapor pocket then grows
pushing the liquid outwards in lower portion of the arms 16, 18 and
then inwards in the upper portions of the respective arms. The
filling of the arms 16, 18 with the liquid raises a weight or
projectile 25 that sits on top of the arms 16, 18; the projectile
falls as the vapor pockets collapse. This becomes a convenient
means for ensuring that a nuclear particle has been detected and
counted in combination with a simple light-sensor switch 26 and a
pill-hydrophone 32 attached to the apparatus that records events as
low intensity shock waves.
[0023] A secondary removable and rotatable enclosure 29 surrounds
the aforementioned components. The enclosure 29 is provided with
windows 31 to permit directional passage of neutron (n), gamma
(.gamma.), and alpha (.alpha.) particles to decipher the direction
of a nuclear particle.
[0024] FIGS. 2 and 3 show schematics of the central bulb region in
the enclosure 14 illustrating a void space 30 caused when a neutron
is detected. Successful testing has been conducted with neutrons
from a spontaneous fission source (Cf-252) as well as from an alpha
emitting special nuclear material (Pu-239 and Be), and from 14 MeV
neutrons generated from a pulse neutron generator. Note that these
measurements were taken in a high gamma ray field (2.times.10.sup.6
gamma/second) and as such represents the ability to detect neutron
emitters in a high gamma background (as may be the case if neutron
emitting SNMs are encased in spent fuel--dirty bombs).
[0025] Thus, in particular implementations the system 10 includes
the enclosure 14 which generates neutrons upon interaction with
alpha particles and which holds the pretreated fluid 12 in
combination with an additive, such as B-10, to detect thermal
energy level neutrons through absorption. The enclosure 14 may
include a surfactant and valving system to minimize or eliminate
liquid loss via evaporation and to improve wettabilility. The motor
24 coupled to the enclosure 14 spins the enclosure 14 to pretension
the fluid into a metastable state. The interaction of the nuclear
particle with the fluid in the metastable state creates a
respective vapor pocket 30. The creation of the vapor pocket
results in a shock wave, the intensity of which is associated with
a particular type of nuclear particle.
[0026] In certain implementations the enclosure 14 is coated with
Be or B. The Be or B atoms can be then used to stop alpha particles
which after absorption release neutrons in the multi-MeV energy
range. The neutrons can then readily penetrate the structural walls
and enter the region of space occupied by tensioned metastable
liquid. Once there, they then cause nucleation of nanometer size
bubbles which quickly (within microseconds) grow to visible
multi-millimeter size (as in FIGS. 2 and 3). This approach permits
the same system to also permit the ability to detect not just
(penetrating radiation such as neutrons and gammas photons) but
also non-penetrating radiation such as alpha particles that get
released from special nuclear material atoms such as U-235, U-238,
Pu-239. The fluid 12 can be pretreated with pre-compression,
degassing or filtration or a combination thereof.
[0027] In certain embodiments the liquid chosen includes an atom
(for example, Li-6 or B-10) that preferentially absorbs thermal
neutrons which then gives rise to a nuclear reaction releasing
alpha particles, since eV energy neutrons are too low in energy to
cause bubble formation. However, alpha particles that get released
from B-10 or Li-6 like isotopes are in the multi-MeV range and can
readily, like MeV level neutrons, cause bubble nucleation in
tensioned metastable liquids. One such liquid that has been
successfully tested for the ability to detect both fast neutrons in
the MeV range as well as much lower energy neutrons in the eV range
is trimethyl borate which has been found experimentally to not only
display capability to detect high energy neutrons in the multi-MeV
range but also neutrons that are millions of times lower in energy
(that is, in the eV range). Thus, in accordance with the invention,
the same detection system can not only provide capability to detect
fast neutrons emitted by special nuclear materials but also lower
energy neutrons (which result if the special nuclear material is
heavily shielded). This capability permits in the same system
neutrons of vastly different energies (from MeV to eV and below).
Experiments have demonstrated that to detect thermal (eV energy
neutrons) and use of trimethyl borate as a model liquid required
tension metastable pressures of about -5 bar and more. However, for
fast neutrons in the MeV range can be detected with tension in the
-3.5 bar range.
[0028] The successful attainment of desired states of metastability
is achieved by preparation of the liquid and enclosure and upon
introduction of tensile states from centrifugal forces during
spinning about the central axis. Rotation is achieved by using the
motor 24, such as a commonly available Dremel-like tool. Various
levels of negative pressures are shown in FIG. 4. Since 5 MeV type
neutron energy based nucleation can be readily attained with about
-3 bar of molecular tension and metastability in a range of fluids
(for example, isopentane, R-113, trimethyl borate and other liquids
with similar physical properties of surface tension, vapor
pressure, density and viscosity), and the fact that simple drivers
like Dremel-tools can provide up to 50,000 rpm, a large range of
neutron energies can be readily accommodated. The detection of
particles is related to a combination of key parameters: degree of
tension metastability, temperature and the linear-energy transfer
(LET) of the incident nuclear particle. Greater the LET, less is
the required state of metastability, as a consequence of which
gamma ray detection requires greater states of mestability--but
still well within the purview of various embodiments of the
invention. In other implementations, the state of metastability
includes detection of gamma rays in combination with neutrons and
alpha particles. Using the kinetic theory for nucleation under
negative pressure conditions also permits the estimation of the
limits of tension pressure, P.sub.neg, using,
P neg = - [ 16 .pi. 3 .sigma. 3 kT ln ( NkTtlh - .DELTA. f ) ] 1 /
2 ( 1 ) ##EQU00001##
where k and h are the Boltzman and Planck constants, .sigma. is the
surface tension, t is the waiting time, .DELTA.f is assumed to be
the activation energy for viscosity and T is the liquid
temperature, respectively.
[0029] As seen from Eqn. (1), the threshold p.sub.neg pressure can
be expected to vary roughly with the surface tension (to the
exponent 1.5). Using data taken for a variety of liquids a
comparison was made to evaluate whether Eqn. (1) could be used to
predict experimentally observed trends and values of limiting
tension.
TABLE-US-00001 TABLE 1 Measured and predicted negative pressure
thresholds for various liquids. Negative pressure thresholds (bar)
at -20.degree. C. Highest Measured Liquid Value Predicted [Eqn.
(1)] Aniline -300 -654 Acetone Not available -272 Acetic acid -288
-334 Benzene -288 -360 Chloroform -317 -333 Trichloroethylene -274
-370 Water -1,400 -1,500 to -1,700
Interaction of Nuclear Particles with Tensioned Metastable
Liquids
[0030] Once a pre-determined state of tension metastability is
attained, the determination of how much energy can be transmitted
to a given set of molecules when the nuclei of the atoms are
subject to neutrons, alphas, fission products or gamma photons is
made.
[0031] The traverse of energetic ionizing particles in a medium
gives rise to energy transfer to that medium, for which a figure of
merit that is commonly used is linear-energy-transfer (LET). In
order to estimate the LET for various target atoms from interaction
with neutrons or other particles such as alphas and fission
products, the well-known Bethe approximation can be employed. The
stopping power, the rate of energy loss per unit path length
(dE/dx), for heavy particle of charge ze is given by,
- E x = 4 .pi.z 2 4 m e v 2 nZ ( ln 2 m e v 2 I - ln ( 1 - .beta. 2
) - .beta. 2 ) ( 2 ) ##EQU00002##
where, ze is the charge of the primary particle, Z is the atomic
number of the absorber atom, (.beta.=v/c, with v the particle
velocity, c the velocity of light, n is the number of atoms per
unit volume of absorber, and / is the mean ionization potential. At
the same velocity, alpha particles has a higher z value compared
with protons and lose energy four times as fast.
[0032] The above formula of Eqn. (2) for dE/dx provides useful
clues on energy deposition by looking at the term before the terms
in parenthesis. It is seen that as the density of the absorber
increases, the stopping power increases. The stopping power is also
linearly dependent on the value of nZ, which represents the
electron density of the absorber. Therefore, CI atoms with a Z of
35 stops 35 times faster than a proton for which Z=1. The
dependence with speed of motion, v, of the charged particle is even
more noteworthy. As the energy decreases, the energy deposition in
space increases. As mentioned previously, since the energy transfer
to high-Z nuclei is smaller, this results in the high-Z target
nucleus depositing it's energy over a smaller distance. The Bohr
formulation may be employed; however, this formulation does not
include the effect of energy of the charged particle and thus, for
the present is not used. In addition to stopping power and dE/dx,
every nuclear particle has a certain probability of interaction
based on energy of the incident particle and the various atoms in
the liquid molecules. It has been found, for example, that despite
the fact that methanol and acetone have similar physical
properties, the threshold for nucleation and detection are
significantly different (that is, methanol can be detected with
only about -5 bar of tension metastability, whereas, for acetone
about -7 bar may be needed). The interaction probability has been
verified experimentally and also confirmed by MCNP nuclear
transport code calculations. This provides the mechanism for
choosing the appropriate liquid for a targeted tension metastable
level and type and energy of the incident nuclear particle. The
waiting time for detection is controllable and found possible to
bring down from -20 s at the threshold tension metastable level to
virtually instantaneous (less than millisecond) by increasing the
tension metastable state by -50% (for example, if, for a given
neutron flux, R-113 with about -3 bar of tension metastable state
will detect fast neutrons within 20 seconds; however, at -4.5 bar
the detection is found to be instantaneous).
[0033] The stopping power for gamma ray photons takes into account
attenuation from photoelectric, Compton scattering and
pair-production. The typical formulation for transmission
of l photons from the incident value of l.sub.o is provided as,
I I o = - .mu. x , ##EQU00003##
where .mu. is the linear attenuation coefficient and x is the
distance over which attenuation takes place. The value of m is
usually tabulated for a variety of materials and liquids.
Alternately, to obtain LET, the above equation may be rewritten as
dl/dx=-.mu.l.
Determining Energy Required for Triggering Nucleation in Tensioned
Metastable Fluids
[0034] To nucleate bubbles in liquids, kinetic theory of nucleation
(see, for example, Fisher, 1948) provides that a critical radius be
reached beyond which only can a vapor bubble begins to grow;
otherwise, that bubble will go back to the liquid phase. Using the
thermal-spike theory (originally developed by Seitz (1958) to
predict events in a conventional bubble chamber) the energy
required (W) for an incidental particle collision can be estimated
as the sum of five terms as follows:
W.sub.1=Surface energy=4.pi.r.sub.c.sup.2.sigma. (3)
W.sub.2=pdV work done by bubble during
expansion=4/3.pi.r.sub.c.sup.3p.sub.ext (4)
W.sub.3=enthalpy of evaporation=4/3.pi.r.sub.c.sup.3p.sub.vh.sub.v
(5)
W.sub.4=kinetic energy given to the
liquid=32.pi.D.sup.2p.sub.lr.sub.c (6)
W.sub.5=viscous losses in moving liquid away=64.pi.vDr.sub.c
(7)
where r.sub.c is the critical radius
(=2.sigma./(p.sub.v-p.sub.ext)), D is the thermal diffusivity,
h.sub.v is the enthalpy of evaporation and v is the liquid
viscosity.
[0035] The available energy from alpha recoils over the length
corresponding to the size of the critical bubble diameter was
compared with the computed energy for vaporization of the critical
size bubble for both the positive pressure cases and for the
negative pressure cases. Results of the comparisons are summarized
below in Table 2. As is shown in Table 2, Seitz's (1958) thermal
spike theory gives results which match data taken at positive
pressures (in bubble chambers where the nuclear particles are alpha
recoils from dissolved emitters like Po) very well indeed. However,
the same theory, when directly applied to nucleation for liquids
under negative pressure (that is, tension metastability) indicates
that a factor of 5 less energy may be needed for nucleation. The
theoretical basis provides an estimate of the tensioned
metastability level (order of magnitude) required for detection of
nuclear particles. The actual computation of particle interaction
and nucleation energy may be provided by nuclear particle transport
calculations, such as well-established codes including MCNP and
supplemented with actual empirical evidence.
TABLE-US-00002 TABLE 2 Comparison of Nucleation Energy Thresholds
for Positive and Negative Pressure Liquids Test Liquid Freon-12
Freon-113 r.sub.c (A) 860 635 W.sub.1 4.9 5.6 W.sub.2 8.7 3.5
W.sub.3 94.7 2.9 W.sub.4 0.3 0.2 W.sub.5 1.8 3.2 Total W (keV)
110.4 15.4
[0036] In the embodiment shown in FIG. 1 it is not necessary that
the drive motor 24 be placed below the diamond-shaped spinner; the
drive motor (Dremel-like tool) may also hold the top of the
diamond-shaped spinner to introduce centrifugal force to the fluid
in the central bulb. In such a system once the nuclear particle is
detected a void/bubble forms as shown in FIG. 3. Thereafter, the
system is stopped to allow the gas bubble to escape from the aims
after which liquid re-enters the central bulb and the detector is
re-armed for detection again.
[0037] Alternatively, FIG. 6 shows an embodiment 100 in which the
spinning about the central axis creates a motive force to circulate
the liquid continuously through the use of connecting arms 102,
104. A nucleation event that causes a gas bubble to form in the
central bulb region 14 can then also continuously move the gas
bubble along with the liquid away from the central bulb continually
filling the central bulb such that it is not necessary to stop the
apparatus. The gas bubble moving upwards to the liquid-gas
interface lets gas escape but the liquid keeps recirculating.
Of-course, if the test liquid is thoroughly degassed to begin with,
only vapor bubbles can form which would re-condense back into the
liquid phase upon evacuation from the central bulb region.
[0038] As shown in FIG. 5, in accordance with the invention, a
portable TMFD system 50 for detection of fission spectrum range
neutrons from Cf-252 and Pu--Be sources, as well as 14 MeV
monoenergetic neutrons from a pulse neutron generator has a wide
range of neutron and gamma energy capabilities. Moreover, the
system 50 can be employed in applications with lower and higher
states of metastability for detection of alpha particles and gamma
rays characteristically emitted from special materials (like Pu and
U isotopes) and fission products like Cs-137 and Co-60, which
drives the tensioned states to much greater levels. The aspect of
distinct separations of metastability required for distinguishing
neutrons from alpha particles and gamma rays can be used in
software developed to identify SNMs on a portable PC platform as
depicted in FIG. 5. In certain implementations, the TMFD system 50
detects neutrons, gamma rays and alpha particles.
[0039] When compared to conventional detectors, various embodiments
of the invention may provide one or more of the following
advantages. The cost of a centrifugal portable TMFD system 50 can
be at least an order of magnitude less than that of a
liquid-scintillator system doing PSD (needing a HV supply, preamp,
amplifier, pulse shaping system, MCA/MCS, and the related
expertise) or for a BF.sub.3 detector system. Even an SDD-based
system may cost a factor of two or more than the cost of a
liquid-scintillator system.
[0040] The TMFD system 50 is transparent even for the non-physicist
type user. It is a simple mechanical system that without having
special training in nuclear physics or instrumentation. The
sensitive volume of the TMFD system can be made large relative to
conventional systems at little or no additional cost or
complication. Compared with SDD the sensitive space for nucleation
per cc is about 1000 times larger. The sensitivity level of the
TMFD system 50 can be varied at will rather than having a fixed
sensitivity as typically found in SDD systems. The TMFD system 50
can detect neutron emitting isotopes blanketed in a large mass of
gamma emitters (as in a dirty bomb) because of its fast neutron
detection capabilities in high gamma background. In various
implementations, depending on the tension level, the TMFD system
detection can be set for neutrons only of various energy levels
and/or for gamma ray detection of various energy levels. Since the
lifetime of the TMFD detector in certain implementations depends
largely on the rotating parts--parts that are simple and relatively
cheap to replace, the TMFD system 50 may have greater longevity as
compared to conventional systems.
[0041] The TMFD system 50 can be set to a desired detection level
on demand by varying the level of tension, as compared to SDD
systems which have to be recharged. When operated as a spinner
system, the TMFD system 50 can be operated in an intermittent mode,
and when implemented as an acoustic system (consisting of a simple
chamber driven only with a pulser and amplifier) the TMFD system
can be operated continuously.
[0042] The complete system enables a single, hand-portable detector
(the size and weight that is commonly associated with Dremel-tools
and ultrasonic toothbrushes) to detect neutrons, gamma rays and
alpha particles relevant to identification of special materials
like U or Pu coupled with gamma emissions from spent nuclear
fuel.
[0043] The TMFD system 50 implemented, for example, as a small
hand-held device, can be fixed in place, or carried in a suitcase
or pocket and activated at will to act as an as-desired detector
for selected nuclear particles. The freedom to be able to detect a
wide range of nuclear particles emanating from a range of special
radioactive materials with a portable TMFD system enables radiation
detection for interdicting smuggling and terrorist threats as well
as for inventory taking. In general, the TMFD system 50 can be
tailored for specific situations involving nonproliferation.
[0044] Other embodiments of the invention are also contemplated.
For example, as illustrated in FIG. 7, a system 200 includes an
enclosure 204 containing a liquid 206 acoustically driven by a
piezoelectric driver element 202. The piezoelectric driver element
202 induces the desired shape, amplitude and frequency to
pretension the liquid 206 into a metastable state. A bubble cluster
208 forms upon a nuclear particle strike. This bubble cluster grows
to visible sizes and implodes creating an audible shock wave that
is recordable by a microphone. The intensity of the shock wave is
associated with a particular type of nuclear particle, such as
neutrons, gamma rays, and alpha particles, as well as fission
products. As shown in FIG. 8, the system 200 may be configured as a
resonant acoustic test cell that continuously operates at, for
example, 20,000 Hz or higher, through the use of a PA amplifier 210
and a controller 212.
[0045] Similar to the arrangement for the system 10, the system 200
may include a secondary removable and rotatable enclosure that
surrounds the nuclear detector and includes windows to permit
directional passage of neutrons, gamma rays, and alpha particles to
decipher the direction of a nuclear particle. The liquid may be
pretreated with pre-compression, degassing, or filtration, or a
combination thereof. The fluid may include one or more additives
such as boron or trimethyl borate to absorb low energy neutrons (in
the eV range) resulting in alpha recoils which then result in
nucleation and detection. The additive may be B-10.
[0046] FIG. 9 illustrates yet another continuously operating
detector system 300, which is a conical design system resonant
acoustic test cell operation that includes appropriate electronic
components for detection of neutron, gamma rays, and alpha
particles, and fission products. Similar to the conical design, a
spherical design with the PZT element 308 strapped to its side on
the equator or as a set of disks around the circumference.
[0047] Specifically, the system 300 includes a conical enclosure
302 coupled to a reservoir 304. The enclosure 302 holds a liquid
306 that is in tension as the enclosure is acoustically driven by a
piezoelectric element 308. As bubbles 310 form by neutron strikes,
the liquid is pushed into the reservoir 304. The system also
includes a resonance controller 312, a wave generator 314, and an
amplifier 316 to drive the piezoelectric element 308, for example,
at or above 20,000 Hz. Also shown in FIG. 9 are the pressure
profiles 318 at various frequencies to predetermine the sensitive
zones (illustrated as stars) in the liquid 306.
[0048] In another embodiment of the invention shown as a detector
system 400 in FIG. 10, the detector enclosure with test fluid is
subjected to pretreatment to provide desired levels of amorphous
states to prevent false positives (that is, precompression in a
high-pressure chamber). Specifically, the system 400 includes a
detector enclosure 402 positioned within a high pressure chamber
404, which can be pressurized, for example, to about 30 ksi through
the use of a high pressure pump 406. There is open communication
between the detector chamber 402 and the high pressure chamber 404
to permit the liquid in the detector chamber to be pressurized
while the detector chamber experiences no net stress; that is, the
pressure on the inside and the outside of the detector chamber 402
remains the same.
[0049] The detection of low energy (eV) neutrons may be further
enabled with inclusion of N-14 into the test liquid. The
interaction of thermal energy neutrons with N-14 results in a (n,p)
reaction with a Q value of 0.626 MeV. This gives rise to an
energetic proton which can be readily detected within seconds in a
host tension metastable liquid such R-113 at tension levels of
around -3 bar. Introduction of N-14 (which is the naturally
occurring isotope of nitrogen) may be accomplished by using a
soluble N-bearing molecule such as a nitrate form which is readily
soluble in water and which then may be mixed with the host solute
such as acetone.
[0050] The detection of thermal neutrons may be further enabled by
inclusion of fissile atoms which have high cross-sections for
interaction. The process leads to energetic ionizing high-Z
products of reaction enabling ready detection. Salts of fissile
atoms such as uranyl nitrate or thorium nitrate or in acetate form
are preferable due to their ready solubility in the host liquids
such as acetone, etc. Particular isotopes of choice along with
their probabilities of interaction with thermal neutrons in barns
are: U-235 (531 b); U-233 (582 b); Pu-239 (742); Pu-241 (1009 b);
Np-236 (2500 b), Th-233 (15 b). Alternately, Th-232 may also be
employed due to the 7.4 b cross-section which produces energetic
alpha recoils.
[0051] The detection of fast (MeV) neutrons with fission assist may
be enabled with the addition of isotopes such as U-238 (for
energies above about 0.6 MeV) and also as a means for delineating
incoming neutron energies.
[0052] Earlier we have noted that external alpha particles may be
detected with 0.01 mm to 1 mm thick coatings of Be or B to the
outer walls of the enclosure holding the tensioned metastable
liquid. This function may be further attained with use of other
elements such as Li-7, C-13, and F-19 which exhibit "Q" values of
reaction of -2.79 MeV, 2.2 MeV, and -1.93 MeV, respectively. These
"Q" values are noted in comparison to the "Q" values for B-10,
B-11, and Be-9 viz., 1.07 MeV, 0.158 MeV, and 5.71 MeV,
respectively. The "Q" value correlates as a rough measure of the
efficiency of conversion of alpha interactions with the target
nucleus. As a consequence, for alpha energies in the 5 MeV range,
the conversion efficiency (in terms of # neutrons per million
incident alphas) may be computed as: Be (70); B-10 (13); C-13 (11);
F-19 (4.1) respectively. Such specific characteristic may further
be used to devise level of efficiency for detection on as-needed
basis. For example, in an extremely hot radioactive environment,
one may decide to use a coating of B (which is much less expensive)
than Be. Also to be noted is that the elements considered herein
allow detection of alphas of wide energy such as 3.18 MeV alphas
from Gd-148 to 6.6 MeV alphas from Es-253 (including alphas from
strategically important U-235, Pu-239 and Am-241 isotopes which are
4.4 MeV, 5.1 MeV and 5.48 MeV respectively).
[0053] The detection of external gamma photons may be further
enabled by use of D and Be elements either in the test liquid
itself or as a coating. Gamma photons of minimum energy greater
than 2.22 MeV and 1.6 MeV may then be possible to detect via
detection of the emitted neutrons that strike the tensioned liquid
as discussed earlier.
[0054] The detection of gamma photons may be further enabled by
including high "Z" elements into the test liquid itself. From Eq.
(2) we have noted that dE/dx is directly proportional to the Z
number of the target atom. Although gamma photons may be detected
without high Z elements by increasing the tension pressure by
roughly factor of 10 (i.e., about -35 bar in R-113 instead of -3.5
bar for fast neutrons), it would be desirable to not have to
tension the liquid to such high negative pressures for enabling a
less structurally robust system. For such cases, high "Z" elements
may be considered to be added to the test liquid. Example high "Z"
elements are Pb (Z=82) and I (Z=53). Lead acetate and molecular
forms of I such as KI and Nal are examples. Particularly Nal due to
its ready solubility in an array of potential host liquids (e.g.,
28 g/100 g of acetone; 63 g/100 g and 184 g/100 g of water at 25
C). Such concentrations offer the possibility of increasing dE/dx
by factors of up to 10 or more and hence the negative pressures may
for example be reduced from about -35 bar to a more practical level
of about -10 to -15 bar. Such detection mode also offers the
ability to see visible tracks in the multi-mm length scale so as to
simultaneously offer the ability to note directionality of the
incoming photons.
[0055] The ability to note detectability has been noted earlier to
rely on either mechanical shocks generated from nucleated bubbles,
the use of a photoswitch-light beam combination or the use of a
retractable stopper which receives the mechanical impulse of the
rapidly expanding bubble and then moves physically--an aspect which
may be recorded visually or via other means such as meter or video.
The retractable stopper as shown in FIG. 1 (item 25) is a generic
element which is to be presumed as available as a technique for
incorporation into the acoustic based systems of FIGS. 7, 8 and 9,
respectively. That is, any bubble nucleation event sends a
mechanical impulse in all directions such that if a mechanical
float (25) were placed at the liquid-air boundary, that object
would then move to extents commensurate with the energetics of the
situation (i.e., less the energy of the particle more the tension
level needed for detection and greater the size of the exploding
bubble and hence, greater is the motion of the retractable element
(25) as also the resulting shock waves monitored by the external
microphone (32).
[0056] An additional means to enable monitoring and recording of
the detection event (i.e., when bubbles are formed due to
particle-tension fluid interactions) is the generation of light
flashes from the process known as sonoluminescence (SL) as the
expanded bubble implodes and collapses. The SL flashes are
typically in the pico second duration range and the number of
emitted photons in the visible range are proportional to the
intensity of implosion, which is furthermore, a function of how
large the bubble gets before implosion, which is furthermore, an
indication of the energy of the particle being detected as
mentioned above. Therefore, a means can now be devised to record
the SL intensity with a system such as a photo-multiplier tube,
photo-diode or other such ordinary light detection apparatus. The
SL intensity, as with the mechanical shock intensity may now be
used readily to relate to the energy and type of the particles
being detected.
[0057] An alternate means for deriving a continuous centrifugally
tensioned metastable fluid detector (CCTMFD) is enabled as shown in
FIG. 11. The task for deriving a continuously operating CCTMFD
system with a reservoir (500) is addressed via demonstrating the
ability to use fluid passing through the rotating arms (501) of the
CCTMFD as a pump. FIG. 11 depicts such a system successfully tested
for ability to function as a pump when the Y-shaped system is
rotated with a Dremel-tool motor as in FIG. 1. Alternately, the
function may be devised by pressurizing the liquid in the master
reservoir itself. The pumping action of the rotating arms (as in
the CTMFD of FIG. 1) causes suction pressure that elevates liquid
from a reservoir. If the liquid were to now be passed through a
venturi (502), as shown in FIG. 12, or an orifice the desired
tension metastable states can readily be derived in a flowing
medium. If so, the bubble formation event upon neutron interaction
would not require system stoppage (as in the CTMFD of FIG. 1. We
now present the theoretical methodology for attaining negative
(sub-zero) pressures in CCTMFD systems with the venturi (or
orifice) principle. The basis is explained using two levels of
analyses. (or orifice) principle. The basis is explained using two
levels of analyses.
1-D, Incompressible, Inviscid Flow Modeling Based on the Well-Known
Bernoulli Law
[0058] First, the fundamental ability to attain desired levels of
negative pressures starting from a given pressure head (P1) and
then driving flow against ambient (atmospheric) pressure can be
simply depicted using the well-known Bernoulli Principle (BP) for
energy conservation. For a one-dimensional (1-D), inviscid,
incompressible, steady state flow through a venturi (FIG. 8b) the
BP states,
P/p+gh+v.sup.2/2=Constant (8)
Assume fluid of density "p" at velocity v1 is driven due to a head
pressure (P1) in a control volume with area, A1 into another
control volume of area, A2 at the same elevation. Equation 8
provides the relationship between P.sub.1, v.sub.1, P.sub.2, and
v.sub.2
P.sub.1-P.sub.2=0.5.times..rho..times.(v.sub.2.sup.2-v.sub.1.sup.2)
(9)
Eqn. (9) needs to be combined with the mass conservation
equation,
.rho.Av=Constant (10)
[0059] In order to attain a given pressure P.sub.2 the flow area,
A.sub.2, can be calculated as,
A.sub.2=(V.sub.1/V.sub.2)A.sub.1 (11)
For example, if working with a liquid of density=1,000 kg/m.sup.3,
if P.sub.1=3 bar, v.sub.1=1 m/s, P.sub.2=-3 bar, A.sub.1=1
cm.sup.2, .rho.=1000 kg/m.sup.3, we calculate the values for
v.sup.2 and A.sup.2 as 36 m/s and 0.277 cm.sup.2, respectively.
Velocities in the range of .about.1 m/s should be readily
attainable at the entrance of the venturi in the CCTMFD
configuration at modest rotational speeds.
Validation via 2-D Incompressible Computational Fluid Dynamics
(CFD) Simulation
[0060] A similar problem was modeled using the well-established
FLUENT CFD code system. For a venturi with a throat flow area
diameter of about 0.25'' and a pressure difference of 2 bar from
the drive chamber to atmospheric pressure at the exit end of the
venturi, the pressure and velocity profiles were obtained for water
as the test fluid (assuming negligible heat transfer effects).
Results of CFD simulations indicate that in the throat region the
pressure has turned negative to about -4.4 bar after which it
recovers to positive pressure values before exiting. The velocity
profiles indicate that the maximum velocity is in the throat
region, reaching values around 36 m/s. These results are similar in
magnitude and scope as those derived from Eqns. (8) through (11)
and thus, offer a means to devise a continuously operating CCTMFD
system.
[0061] Other embodiments are within the scope of the following
claims.
* * * * *