This invention relates to the field of long range active interrogation of special nuclear materials, such as highly enriched uranium (HEU). In particular, the invention relates to inducing fission in targets by high energy particles or photons and identifying fission product signature(s) for the special nuclear materials based on thermal neutron multiplication.
Long range detection and identification of special nuclear materials (SNM) is an important goal for nuclear nonproliferation, national security and nuclear safeguards. As defined by Title I of the Atomic Energy Act of 1954, special nuclear materials include plutonium, uranium-233, or uranium-238 enriched with uranium-233 or uranium-235 isotopes. In concentrated form the special nuclear materials can be the primary ingredients of nuclear explosives. In fact, the uranium-235 content of natural uranium can be concentrated, i.e., enriched, to make highly enriched uranium (HEU), which is the primary ingredient of an atomic bomb.
Historically, many of the tools used to safeguard SNMs have relied on measuring one or more of the inherent nuclear attributes or signatures of these SNMs. Some of these attributes or signatures include (i) spontaneous fission, (ii) susceptibility to undergo fission following neutron absorption, (iii) the emission of high energy alpha (α) particles, which can generate neutrons through (α,n) reactions, and (iv) the emission of both low and higher energy gamma (γ) rays. The presence of interferences, such as shielding and other high-Z profile materials, however, may influence these attributes or signatures to the degree that many of the historical tools and techniques used in nuclear safeguards must be modified or redesigned to be able to provide specificity in the presence of interference.
Active interrogation techniques, which use external radiation sources to excite nuclear processes in materials, may help in addressing these challenges. In particular, active interrogation amplifies inherently weak signatures. By using pulsed radiation fields to stimulate fissionable material, active interrogation can, furthermore, investigate time-correlated signatures, which passive signature techniques generally cannot by monitoring the induced radiation with appropriate nuclear detectors. Active interrogation, as a tool for nuclear safeguards and process monitoring in the nuclear fuel manufacturing and recycling, has been the subject of many research projects summarized in Gozani, T., “Active Nondestructive Assay of Nuclear Materials,” Report NUREG/CR-0602,” U.S. Nuclear Regulatory Commission, Washington, D.C., (1981); which is incorporated herein by reference in its entirety.
Active interrogation monitoring techniques, however, have not seen widespread application in today's fuel reprocessing facilities, because alternate technologies have been found to achieve the same goals. Moreover, active interrogation techniques are typically more complicated and more expensive than comparable passive techniques. Yet, because active interrogation measurements can be performed without contacting SNMs, active interrogation is an ideal solution for performing on-line monitoring in areas where the SNMs may be hazardous or difficult to contain, and where the consequences of spills and contamination are high. Also, since active interrogation measurements can be made without contacting SNMs directly, instrumentation maintenance and repair can be performed without the need to break into the SNM containers or impact subsequent operations.
While a number of active interrogation techniques are available, such as the irradiated fuel assay, leached cladding hulls analysis, and process monitoring, the available techniques take an unacceptably long time to obtain reliable data, which can be on the order of several minutes. (D. L. Chichester & E. H. Seabury, “Active Interrogation Using Electronic Neutron Generators for Nuclear Safeguards Applications,” INL/CON-08-14196, 2008; incorporated herein by reference in its entirety). Since in almost all scenarios of interest the target containing the SNMs is moving, for active interrogation to be useful, all the data must be obtained either in few or perhaps in only one interrogation pulse.
One of the techniques for performing active interrogation is to use neutron activation to generate high intensity radiation probes. While, the special nuclear materials, such as uranium-233, uranium-235, and plutonium-239, are only mildly radioactive, neutron activation can induce radioactivity by fission in the target containing special nuclear materials by exciting atomic nuclei within the target. The excited nuclei are subsequently relaxed by emitting subatomic particles and γ-radiation. Appropriate detection of the neutrons and γ-radiation and analysis of the detected spectrum facilitates the identification of a particular substance within the designated location of the target material.
Scintillation detectors such as a sodium iodide (NaI) scintillator coupled with a photomultiplier tube (PMT) have been proposed and used for detecting the γ-radiation induced from the target material by the neutron activation. However, despite the availability of such technique, it suffers from numerous drawbacks. While scintillation detectors are known to have fast response times, which can be on the order of nanoseconds with relatively simple structure, they exhibit poor energy resolution of y-radiation. Such poor energy resolution of y-radiation causes, inter alia, a loss of information. That is because γ emissions from different isotopes of the target material that have similar γ energy spectrums cannot be properly separated. For example, the γ energy spectrum for a relatively benign fission material 238U is very similar to the highly enriched fission material 235U/238U. Moreover, in applying the high intensity radiation probes, the data rate required to analyze the nuclear return signal in the prompt region defined by 100 μs-10 ms, i.e., is beyond the capability of standard analytical tools.
Thus, it is desirable to have an active interrogation system that can separate the signal of the special nuclear materials from relatively benign fission material with high-Z value when the induced fission is used. It is also desirable to have a system with the data rate sufficient to analyze the nuclear return signal in the prompt region defined by 100 μs-10 ms.
A method for reliably detecting and identifying special nuclear materials is provided. The method relies both on the emission of fast neutrons that create nuclear reactions in bulk media and on the emission of delayed neutrons and gamma rays present in the decay of fission products as unique signature(s) for the special nuclear materials. The emission of delayed neutrons is detected by radiation measurements preceding the β-decay of the delayed neutron precursor fission products that begins 100 μs after the generation of the fission-inducing radiation pulse. Preferably, the disclosed method for detecting and identifying special nuclear materials has the steps of (i) exposing a target to a high intensity particle or photon beam pulse to induce fission; (ii) detecting emission of prompt and delayed radiation from the target using a detection system including fast and thermal neutrons; (iii) recording fast neutrons induced by thermal neutron fission and gamma-rays from thermal neutron induced fission; (iv) analyzing a die away in the fast neutron yield induced by thermal neutron fission of the target; and (iv) comparing the fast neutron yield from the target to the fast neutron yield characteristic for a special nuclear material; where a close correlation of the fast neutron die away indicates the presence of the special nuclear material within the target. The die away yield of fast neutrons are recorded in the 100-500 μs time region after the induced fission, and preferably in the 150-450 μs time region after the induced fission. A detection system to carry out the disclosed method is also provided. The system affords a pulse height data analysis for both neutrons and gamma rays at a much higher data rate than in traditional pulse processing systems
It is within the bounds of the disclosed embodiments that the SNMs can be scanned, monitored or identified either within a short- or long-range distance between the target and the detection system. In the preferred embodiment, however, the method is used in the long-range active interrogation of highly enriched uranium (235/238U) The long-range implies the distance between the target and the detection system of 10 m to 200 m, and preferably, 20 m to 50 m.
The method includes exposing a target to a short high intensity particle beam pulse to induce neutron fission. The pulse can come from high energy proton sources, photon (bremsstrahlung) sources, or any other similar sources known to induce neutron fission. If the pulse comes from the high energy proton source, the pulse is, preferably of <10−7 sec in duration and intensity of 1012 for about 100 m interrogation distance and 1010 protons for about 10 m distance. If the pulse comes from photon (bremsstrahlung) source, the duration is in the range of <10-7 sec and the radiation dose at ˜8 m distance is ˜20 rad.
The method further includes detecting emission of neutrons generated by the reactions in the target and separated by time of flight (TOF) measurement. The signatures of reaction neutrons can occur in the 100 ns-2 μs region. The detection can be facilitated, but not limited to, a detection system having a sufficiently fast organic or inorganic scintillators optically coupled to a sufficiently fast photomultiplier tube operating in a low-gain mode capable of a very high count rate recording, and a recording system operably linked to the photomultiplier. The recording system is a high-bandwidth digitizer (e.g., digital oscilloscope).
The recording system of the disclosed detection system is designed to measure target reaction neutron output by time-of-flight and to determine the neutron and γ energy spectra by pulse height analysis. The recording bandwidth of the recording system is greater than 100 MHz, which is considerably faster than standard pulse analysis systems of the prior art. In the preferred embodiment, the recording bandwidth is between 100 MHz and 500 MHz. The present system allows γ ray analysis of the isomeric states of the reactant products to further differentiate bombarded targets in addition to the delayed neutron measurements.
The method for reliably detecting and identifying special nuclear materials further includes monitoring for thermal neutron induced fast neutron production that occurs hundreds of microseconds after the interrogating pulse by pulse height discrimination, and comparing the fast neutron yield from the target to the fast neutron yield characteristic for a special nuclear material. Based on this comparison, a die-away characteristics of the fast neutron yield indicates the presence of the special nuclear material within the target. An off-line, software based pulse shape discrimination can be used to separate the fast neutron yield from recorded mixed neutron and γ-ray pulses (G. F. Knoll, “Radiation Detection and Measurement” (3rd ed.), John Wiley & Sons Inc., New York (2000) pp. 679-680; incorporated herein by reference in its entirety). Typically, the system operates in the time regime of 100-5000 μs, which is associated with thermal neutrons produced in the target. The thermal neutron fission within the time regime of 100-500 μs provides a unique signature of special nuclear materials such as the highly enriched uranium.
These and other characteristics of the method for reliably detecting and identifying special nuclear materials and the system to carry out the method will become more apparent from the following description and illustrative embodiments which are described in detail with reference to the accompanying drawings. Similar elements in each figure are designated by like reference numbers and, hence, subsequent detailed descriptions thereof may be omitted for brevity.
A method for reliably detecting and identifying special nuclear materials and a detection system to carry out this active interrogation are provided. A fundamental problem in identifying special nuclear material (SNM) is that induced fission may occur in high-Z targets that are not special nuclear materials, for instance depleted uranium (238U). In contrast, all special nuclear materials have high neutron fission cross sections as opposed to other heavy elements (238U, 232Th, etc.) The method measures induced fission produced fast neutrons, thermal neutrons, isomeric states of fission fragments and gamma-ray emission from neutrons interacting in bulk media to reliably detect and identify special nuclear materials even in presence with other high-Z materials. The method enables neutron/gamma ray spectral analysis from active single pulse interrogation sources in the early time range of about 10 μs to about 1 s. The thermal neutron fission within the time regime of 100-500 μs provides a unique signature of special nuclear materials such as the highly enriched uranium.
Application of pulsed beams is superior to direct current, because the time-of-flight (TOF) of the particles contains valuable information on the nature and energy of the radiation. Application of a single energetic pulse has preferable peak to background conditions, but suffers from the fact that the intensity of the induced radiation quickly falls, thus resulting in poor statistics in the pulse counting measurement. In contrast, in the first millisecond the high intensity of the radiations (˜103 neutron counts in 100 ns, i.e. 1010 Hz) blinds the detectors and saturates the counting electronics. In this first millisecond time, however, there is a large amount of valuable information, which is normally lost, because the high intensity saturates the detector. The neutron TOF spectra in this first millisecond, however, can identify neutron sources and their distance. If there is a moderator around the fissionable target material, the moderator will create thermal neutrons, which are multiplied in the special nuclear material (SNM), bearing a unique signature in case of Highly Enriched Uranium (HEU) or Depleted Uranium (DU).
The gamma ray spectrum contains significant radiation lines that can identify target materials e.g. 2.2 MeV photons from thermal neutron capture from hydrogen. The die away of the fast neutrons (and consequently the hydrogen line) may differentiate HEU from DU. In principle the die away of the neutrons and gamma rays, and their relative intensities can be used to determine the multiplication constant of SNM carrying targets.
Preferably, the method includes exposing a target to a short high intensity particle beam pulse to induce neutron fission. The pulse can come from high energy proton sources, photon (bremsstrahlung) sources, or any other similar sources known to induce neutron fission, as long as the pulse has the required parameters, preferably of <10−7 sec in duration and intensity of 1012 for 100 m interrogation distance and 1010 protons for ˜10 m distance. For bremsstrahlung pulses the duration is in the range of <10-7 sec and the radiation dose at ˜8 m distance is ˜20 rad.
A high energy proton pulse will also produce spallation and fission in a target (although the fission probability becomes very low for Z<80). Fast neutrons produced by the proton flash undergo fast multiplication as they traverse fissionable material in the target. This occurs in all materials whose fission threshold is above 1.5 MeV (e.g. U-238). A fraction of the neutrons is typically thermalized in surrounding media. Thermal neutrons can cause thermal neutron fission in SNM. Fast neutrons from thermal fission have a fast multiplication as well as the thermal fission multiplication. Times of interest are that fast multiplication takes place in couple of μs, thermal neutron fission occurs in 100's of μs, delayed neutron half-lives are from 0.1 s-1 min.
When considering a multiplying media with NT neutrons produced by proton bombardment, a fast neutron multiplication of Mf, a probability of thermal fission Pt, a fraction of the total neutrons thermalized entering the media of fth, a thermal neutron multiplication factor of Mth, a number of delayed neutrons from delayed neutron precursors of Nd, and then Nft and Nfd are the number of fast neutrons that can be detected:
N
ft
=N
T
f
th
P
t(Mf+Mth)Thermal neutron production(50-1000 usec)
N
fd
=N
d(Mf)+NdfthPt(Mf+Mth)Time>1 sec
where:
In principle, these factors give an overwhelming advantage for the thermal neutron production approach for fth>0.1. This advantage grows when NT may include contributions of all materials by spallation, whereas Nd only depends on production of the long lived delayed neutron precursors. These advantages are limited by the fact that the scintillator responds to both gamma rays and fast neutrons. The gamma ray population may have a 2:1 advantage for fission, have a contribution from the targets isomeric states of fission and spallation products, and have a contribution from thermal neutron capture by all materials in the target. Additionally, the interaction probability for 2 MeV neutrons in the scintillator may be 3 times higher than for 2 MeV photons. A detector shield of 5 cm lead would preferentially transmit neutrons by a factor of 3.
In a preferred embodiment, the short high intensity particle beam pulse is a single proton pulse of duration less than 100 ns that contains at least 1011 protons. While the method is not limited to a short- or long-range detection and can be successfully applied to either one, in the preferred embodiment, however, the method is used in the long-range active interrogation of highly enriched uranium (235/238U) The long-range implies the distance between the target and the detection system of 10 m to 200 m, and preferably, 20 m to 50 m.
The disclosed method further includes detecting emission of neutrons by neutron interaction with bulk media from the fission decay of the target. Neutrons created by the flash can undergo elastic and inelastic scattering, neutron capture, fission (resulting multiplication), and many other reactions until they leave the target. The process of neutrons leaving the target is referred to as die-away and in a target of tens of cm in size, fast neutron die-away has mean times of the order of microseconds. Those neutrons that come to thermal equilibrium in the target will flow out by diffusion in time over 100 microseconds. Thermal neutrons in the target may undergo capture or fission (all thermal neutron fissionable materials are of high interest). Table 1 illustrates minimum bandwidth (Hz) required to record die-away reactions for gamma ray and neutron detectors.
Using the disclosed method, the signatures of delayed neutrons occur in the 0.1-600 μs time region. The detection can be facilitated by a detection system having a scintillator optically coupled to photomultiplier operating in a low-gain mode. The scintillator can be a sufficiently fast organic or inorganic scintillator, capable to resolve pulses at ˜100 MHz data rate. The photomultiplier is not specifically limited in the described method as long as it is a sufficiently fast photomultiplier capable of a very high (˜100 MHz) count rate recording. The photomultiplier is subsequently connected to a recording system that includes a high-bandwidth digitizer to record the signal.
The scintillator used in the disclosed detection system must be sufficiently fast to convert neutrons and gamma-rays into photons that are detected by the photomultiplier. In particular, the scintillator should have a rise time of less than 1 ns and an exponential decay constant of 2-20 ns, preferably 5 ns. In addition, besides the efficiency of conversion of neutrons and gamma-rays into photons, the response of the scintillator depends on the transit time of the neutrons and gamma-rays across the thickness of the scintillator. In a preferred embodiment, the scintillator is a liquid organic scintillator, e.g., Bicron® BC-501A, intended for neutron detection in the presence of gamma radiation. The liquid scintillators have good particle discrimination capability. In an organic scintillator the corresponding scintillation pulse may be described by a sum of two exponential time-dependent components. One component is termed “fast” and the other component “slow”. These two components are weighted differently depending on the excitation induced by particles of different types. While, neutrons interact with the scintillating medium through the process of proton recoil, gamma-rays are primarily detected through Compton electron recoil. The scintillation response for electron ionization is faster than the response for proton ionization. This type of scintillation response is commonly used as a means of suppressing gamma-ray background in neutron detection systems.
The photomultiplier employed in the disclosed detection system is optically coupled to the scintillator. Typically, a head-on type photomultiplier, such as Bicron, Model 5MAB-1F2BC501A/2-X or 2MAB-1F1BC501A/2-X, can be used with low-level light sources. The radiation pulse time response of these fast photomultiplier/scintillators makes it possible to record high count rates, 107 counts/second. However, these devices are normally count rate limited by the rate of charge replenishment through the bleeder string that supplies the photo multiplier (PM) dynode structure. Initially, the detector output signal, as measured at the anode of the photomultiplier, reaches the saturation voltage or the highest voltage the anode circuit can deliver. Moreover, the high current in all the later dynodes draws charge from its capacitance storage at a rate not replaceable by the bleeder string. This causes a gain reduction that is fixed until charge replacement occurs from the high voltage supply through the bleeder string. In contrast, the photomultiplier can be operated at low gain, where the charge per particle may be a factor between 1/100 to 1/10000, although preferably with 1/1000, or lower (see
While energized, photomultipliers must be shielded from background neutron radiation to prevent their destruction through overexcitation. If used in a location with strong magnetic fields, photomultipliers can be shielded by a layer of mu-metal, i.e., nickel-iron alloy, lead, cadmium or a combination thereof. The strong magnetic field can curve electron paths, steer the electrons away from the dynodes and cause loss of gain. Such magnetic shielding is often maintained at cathode potential. Preferably, the external shield is electrically insulated because of the high voltage applied to the shield. While the photomultiplier used in the disclosed system can be used without shielding, photomultipliers with large distances between the photocathode and the first dynode are especially sensitive to magnetic fields and the shielding would typically be required.
If the neutrons travel to the photomultiplier/scintillators without collisions, the neutron spectra can be measured using the time-of-flight technique. The arrival time at the detector corresponds to the energy of the neutron and typically follows the proton flash after about 0.1-1.0 μs. The photomultiplier gain during the neutron time-of-flight signal can be estimated from the photomultiplier recovery signal. The neutron time-of-flight signal is in agreement with the neutron spectra derived from the cross-section measurement data using the estimated gain.
After gain recovery, analysis of the recorded data can be performed by counting pulses within a chosen time and amplitude window. Pulse height can be converted into deposited gamma-ray energy based on 60Co calibration.
The recording system of the disclosed detection system is designed to measure target prompt neutron output by time-of-flight and to determine the neutron and γ energy spectra by pulse height analysis. The recording system has a high-bandwidth digitizer. The recording bandwidth of the digitizer is >100 MHz, which is considerably faster than standard pulse analysis systems of the prior art. Preferably, the bandwidth ranges between about 100 MHz and about 600 MHz. More preferably, the bandwidth ranges between about 100 MHz and about 400 MHz. The disclosed system allows γ ray analysis of the isomeric states of the reactant products to differentiate bombarded targets when added to the delayed neutron measurements. The system operates in the time regime associated with thermal neutrons produced in the target, 100-500 μs and measures any thermal neutron fission by pulse height discrimination.
Thus, the disclosed method for reliably detecting and identifying special nuclear materials, further includes monitoring for a fast neutron yield after the induced fission; and comparing the fast neutron yield from the target to the fast neutron yield characteristic for a special nuclear material. Based on this comparison, a close correlation of the fast neutron yield indicates the presence of the special nuclear material within the target. In a preferred embodiment, in order to separate the fast neutron yield from recorded mixed neutron and y-ray pulses, an off-line pulse shape software analysis is used.
The examples set forth below also serve to provide further appreciation of the invention but are not meant in any way to restrict the scope of the invention.
A series of targets were exposed to single 4 GeV proton pulse of duration <100 ns containing 1011 protons at Brookhaven National Laboratory's (BNL) Alternate Gradient Synchrotron Facility (AGS). The prompt gamma and neutron radiation (see
Gain can be determined as a function of time after the proton flash from the data. For U-235, the gain recovery is given in
The U-235 TOF data can be reduced using equation (1) when the number of individual detection pulses is large. The detected current Id(tof) as a function of neutron time-of-flight is given by:
I
d(tof)dt=So(tof)dt T(D)X(En)q(En/2)G(Q), (1)
where for rapid neutron production in the target, So(tof) is the number of source neutrons/sec at distance D, T(D) is the fraction that reach the detector at distance D, X(En) is the scintillator interaction probability for a neutron of energy En, q(En/2) is the charge at the photocathode for the average proton energy deposited in the scintillator, and G(Qt) is the electrical gain measured from photocathode to the PM output and is set by the inter-dynode charge Qt. At later times, the gain of the PM recovers from charge depletion and individual pulses are recorded as illustrated in
It may appear as though individual pulses in
This example illustrates gamma-ray spectroscopic measurement by pulse height analysis of different targets with thickness of about 18 g/cm2 that were irradiated by 4 GeV proton pulses. The detector was placed at 12 m distance upstream from the target. After the proton burst, the first large observable peak in the detector was mainly from prompt gamma radiation. After the prompt peak, the fast neutrons arrived at the detector. The fast neutrons were initially formed by either spallation, fission or multiplied chain reactions. After the prompt gamma radiation, the fast neutrons were detected within the <1 μs region. In the following 1 μs-0.2 s region, the gamma-ray emission from the isomeric states of the spallation/fission products dominated the spectrum. Although, the recorded time limit is 0.2 s, it is expected that β-decay of the products would be observable after that.
The isomeric transition region gamma-ray energy spectra of different targets was analyzed by placing 100 μs wide time windows and extracting the pulse height distribution within each window. An example of spectra where the 100 μs window was started 350 μs after the prompt burst is shown in
This example tests the ability of using the prompt recording technique, described in Example 3, to identify the depleted uranium targets in various shielding scenarios. The target containing a 20 kg cube of depleted uranium in various shielding configurations was placed at 10 or 20 m from the disclosed detector. The target was located in a tunnel, near the tunnel wall, where the beam passed 2 m from the detector on its way to the target assembly.
In this configuration the bulk target was surrounded by different neutron and gamma-ray shielding materials such as polyethylene, polyethylene with boron, steel and aluminum. The targets were located 20 meters from the detector, and data analysis was from measurement of the baseline shift. As illustrated in
This example provides Monte Carlo calculations for fast and thermal neutron yields, as a function of time, in the shielded highly enriched uranium. In the initial model calculations the time evolution of the system after a fission event was investigated for 2 MeV fission neutrons. To realize this scenario a 2 MeV neutron source was placed in 20 kg 235/238U and the resulting neutron and gamma-flux were tallied. The yield of fast and thermal neutrons for depleted uranium and highly enriched uranium with and without polyethylene shield were calculated in 100 μs steps. As illustrated in
As provided in Examples 1-5, the radiation output from a single pulse of 4 GeV protons incident was measured on various targets. The spallation and fission processes produced gamma emitting nuclides indicative of isomeric transitions. The presence of these nuclides was slowly varying with atomic mass, e.g., for iron and nickel, in agreement with many high energy proton measurements (P. Kozma and J. Kliman, J. Phys. G: Nucl. Part. Phys. 16 (1990), p. 45; incorporated herein by reference in its entirety). Typically, in high energy proton measurements, the charge distribution of spallation products was found to have a maximum close to the unchanged charge distribution quantity of the nuclear collision, (Z+1)/(A+1). However, 235U and 238U products had the highest energy photon production of the nuclides in the tested target set.
Furthermore, a large gamma-ray signal was detected in the presence of an efficient thermalizing material, e.g., polyethylene. Monte Carlo calculations suggest that the large gamma-ray signal yield is from thermal neutron capture. The data further suggest that thermal neutron fission in 235U would produce high energy neutrons that would unequivocally differentiate highly enriched uranium from the depleted uranium on the basis of spectra. Thus, useful signatures can be obtained from moderated highly enriched uranium in a time window close to the prompt signal of the excitation pulse using high energy proton irradiation.
All publications and patents mentioned in the above specification are herein incorporated by reference in their entireties. Various modifications and variations of the described materials and methods will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the disclosure has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, those skilled in the art will recognize, or be able to ascertain using the teaching herein and no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.
This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61/490,292 filed on May 26, 2011, the content of which is incorporated herein in its entirety.
The present invention was made with Government support under contract number DE-AC02-98CH10886, awarded by the U.S. Department of Energy, and contract number IACRO 09-46641 by the U.S. Department of Defense. The government has certain rights in the invention.
Number | Date | Country | |
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61490292 | May 2011 | US |