Patent Application
20120155592
The present invention is directed towards systems and methods for detecting nuclear material and, specifically, for detecting nuclear material in cargo, trucks, containers, or other structures using multiple stimulating-radiation and fission signatures. More specifically, the present invention incorporates the detection of nuclear material by measuring prompt neutrons released from fission events induced in the nuclear material by radiation probes.
The threat of nuclear-material and nuclear-device smuggling requires a fast and reliable non-intrusive inspection of all types of conveyances, such as containers and cargo at sea and airports or trucks at land ports of entry. Detection of the spontaneous emission of radiation from nuclear material has known limitations, which can be overcome by using active interrogation. Active interrogation typically employs narrow or wide beams of penetrating probes such as neutrons or X-rays (stimulation radiation) to stimulate fissions in the nuclear material, if present.
Conventionally, nuclear material is detected by exposing a container to radiation, such as X-ray radiation or neutrons, and inducing fission by interaction of the radiation with the nuclear material, referred to as photo-fission or neutron fission, respectively. The fission process causes the nuclear material to emit multiple penetrating signatures such as Prompt Neutrons, Delayed Neutrons, Prompt gamma rays and Delayed-gamma rays. In the past, most system were designed to detect fission events by detecting the delayed-neutron signature using detector arrays positioned external to the irradiated container. The detection of fission-related delayed neutrons is a very strong indication that nuclear material is present. Delayed neutrons, however, while a unique indicator of the fission occurrence, are very few and of low energy, thereby severely reducing the efficacy of the inspection system especially for hydrogenous cargo. In some instances, the sole fission signature measured is that due to the delayed-gamma rays. This signature can be highly attenuated in metallic cargos. In these cases, it is much more desirable to detect fission prompt neutrons, which are more abundant and penetrating. However, the fission prompt neutrons are produced at virtually the same time as the far more numerous probing radiation types (stimulation radiation) incident on the nuclear material blinding all detectors. Generally, by the time the detectors recover, no prompt-neutron signature exists.
Accordingly, there is need for improved methods and systems for detecting nuclear material that do not depend solely on one signature, but on multiple signatures where the vulnerabilities of one signature are mitigated by the strength of the others. Similarly, multiple probing radiation types, such as X-rays and neutrons used separately or in tandem make the inspection system far more sensitive to concealed nuclear material over the wide range of cargoes encountered in commerce.
The present specification discloses a system for measuring a plurality of fission signatures, comprising a radiation source, wherein said radiation source is configured to produce radiation and direct said radiation towards an object under inspection, wherein said radiation induces fission in any nuclear material that may be present in the object; a first detector wherein said first detector type is at least one of a threshold-activation detector, a plastic scintillator detector, a moderated He-3 detector, or a He-3 equivalent replacement detector; and a second detector wherein said second detector type is at least one of a threshold-activation detector, a plastic scintillator detector, a moderated He-3 detector, or a He-3 equivalent replacement detector.
Optionally, the first detector and said second detector are positioned on opposing sides of the object. One of said first detector or said second detector is positioned to detect radiation that is emitted from the object at back angles relative to the radiation produced by the radiation source. The first detector is positioned above the object. The second detector is positioned on a side of the object. The first detector is positioned below the object. The second detector is positioned on a side of the object.
Optionally, the radiation source is a linac having an energy in the range of 6 MeV to 9 MeV. The system further comprises a neutron-inducing material positioned between the radiation source and the object, wherein said neutron-inducing material converts some of the radiation emanating from the radiation source into a neutron source. The neutron source has an energy of approximately 2.5 MeV. The neutron-inducing material is heavy water or beryllium.
The radiation is at least one of X-rays, monoenergetic gamma rays, or gamma rays with a narrow energy band. The threshold-activation detector comprises at least one neutron threshold-activation material for detecting higher energy prompt fission neutrons. At least one neutron threshold-activation material includes at least one fluorine-containing compound. The plurality of fission signatures includes at least one of fission prompt neutrons, delayed-gamma rays, or delayed neutrons. The system further comprises an X-ray radiation source and transmission detectors for interrogating the object with X-rays and detecting X-rays transmitted through the object.
In another embodiment, the inspection system measures a plurality of fission signatures using a radiation source, wherein said radiation source produces radiation directed towards an object under inspection and induces fission in any nuclear material that may be present in the object; and a threshold-activation detector, comprising at least one threshold-activation material wherein said threshold-activation material is also a scintillant and at least one detector for detecting beta radiation resulting from activation of the threshold-activation material by prompt fission neutrons.
Optionally, the threshold-activation material or scintillant comprises fluorine-containing compounds. The fluorine-containing compounds comprise a liquid fluorocarbon. The threshold-activation detector comprises a plurality of detector tiles, wherein said plurality of detector tiles is configured in combination to function as a large area detector. The threshold-activation detector comprises one large area detector.
The aforementioned and other embodiments of the present shall be described in greater depth in the drawings and detailed description provided below.
These and other features and advantages of the present invention will be further appreciated, as they become better understood by reference to the detailed description when considered in connection with the accompanying drawings:
The present invention is directed towards multiple embodiments of system to detect nuclear material based on at least one source of probing radiation and a plurality of radiation signatures. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
As shown in
In another embodiment, a fast-neutron probe and X-ray probe are integrated to improve detection sensitivity across all types of cargo. X-rays penetrate very well into organic or hydrogenous material (e.g., food, wood, and plastic etc.) and less well into dense metallic cargo. On the other hand, fast neutrons penetrate well into metallic cargo but less well into hydrogenous cargo. Therefore, by combining these two different probing sources, fast neutrons and X-rays, and simultaneously or sequentially inspecting the same conveyance using these two different sources, the inspection system of the present invention can achieve high sensitivity across all types of cargos.
Persons of ordinary skill in the art should appreciate that neutrons can be generated by the well-known photo-nuclear reaction of high energy X-rays with materials having a low energy threshold for the photo-neutron reaction, such as heavy water (which is water where the hydrogen is replaced by its naturally occurring isotope, deuterium), beryllium, or any other suitable materials known in the art.
Referring back to
The X-ray system 100 further comprises detector arrays 102, 103, 104, which in one embodiment are located around three sides (in front, behind, or on top) of the inspected object 107. In one embodiment, detector arrays 102 are positioned proximate to radiation source 101 and detect any radiation that is emitted from the object at back angles relative to the probing beam. In one embodiment, detectors 102 are located adjacent to and on lateral sides of radiation source 101. In one embodiment, detector arrays 103 are positioned above or below the object under inspection. In one embodiment, detector arrays 104 are positioned on the opposite side of the object as the source and serve to detect radiation emitted in the forward angles relative to the probing beam direction. Therefore, once radiation 105 is directed towards container 107, if nuclear materials are present, then the resultant fission prompt neutrons, delayed-gamma rays and delayed neutrons 106 pass through and can be detected by the detector arrays 102, 103, and 104.
In one embodiment, the detector arrays 102, 103, 104 comprise neutron threshold-activation materials that are used to detect the higher energy, more penetrating neutrons, which are much more prolific than delayed-neutron fission signatures well after the fission process and any overload resulting from the blinding X-ray source.
In one embodiment, neutron threshold-activation materials include, but are not limited to fluorine-containing compounds, which are activated by fission neutrons (with an energy above 3 MeV in the case of fluorine) producing a short-lived radioactive material (nitrogen-16, an isotope of nitrogen with a half life of 7.1 s in the case of fluorine) that decays by emitting beta particles every time and often gamma rays (as is the case with fluorine) which can be detected by an appropriate detector, such as the unique threshold-activation detector described in U.S. Provisional Patent Application No. 61/313,200, by the Applicant of the present invention, which is herein incorporated by reference in its entirety. The fluorocarbon threshold-activation detector employed in the preferred embodiment is also an efficient gamma ray detector and thus, also detects the fission delayed-gamma rays.
In another embodiment, the detector arrays 102, 103, and 104 optionally include some lower cost plastic scintillator detectors which detect only fission delayed-gamma rays.
In another embodiment, the detector arrays 102, 103, and 104 optionally include moderated He-3 detectors or an equivalent replacement. One such embodiment of a suitable detector is described in co-pending U.S. patent application Ser. No. 12/976,861, which is also assigned to the Application of the present invention and herein incorporated by reference in its entirety.
It should be noted herein that the detector arrays 102, 103, 104 comprise at least one of and in some embodiments, a combination of, plastic scintillator detectors, fluorocarbon detectors, moderated He-3 detectors or He-3 equivalent replacements or any other detector suitable for the present invention depending upon the source and detection requirements.
The use of a threshold-activation detector (TAD) is advantageous because of its ability to detect prompt neutrons, via the activation, well after the source's blinding radiation has stopped. A typical pulsed source (e.g. of X-ray or neutrons) can be from a few microseconds to milliseconds wide, having a repetition rate of a few times per second to hundred or thousand times per second. The times between pulses afford enough time for the detectors to recover from the overload that may occur during the pulse and to collect an ample activation signal. The activation materials of the TAD are selected to have half lives ranging from a second to tens of seconds. The activation material is also selected to have a higher energy threshold so they will be activated only by the neutrons of interest (e.g. the higher energy fission prompt neutrons) and not by the numerous lower energy neutrons that originate directly or indirectly from the source. The TAD system allows the measurement to be conducted in situ or to quickly transfer the activated material to a location where the background is very low further increasing the sensitivity of detection.
In one embodiment the threshold-activation detector (TAD) is an activating substance (such as, but not limited to Teflon) with a separate gamma ray detector (such as NaI scintillation detectors which is usually employed to detect, between beam pulses, the thermal-neutron-capture gamma rays) for detecting the gamma-rays emitted by the activated material. Referring now to
In another embodiment, the activating substance is also a scintillant, such as, but not limited to certain liquid fluorocarbons (C6F6), BaF2, CaF2, thus allowing the detection of the beta activity with close to 100% efficiency and also the gamma-rays, but with lower efficiency, depending on the size of the detector. Referring now to
Referring back to
The exponentially declining energy spectrum of the delayed fission gamma rays is also shown, 262. It is measured along with the 19F(n,α) activation between the 9 MV linac pulses. In this mode of operation, the linac is pulsed 20 to 100 pulses per second; each pulse is typically 2-4 as wide. The fission delayed-gamma rays and the fluorine activation of the fluorocarbon detector itself is collected between the pulses.
The combined, observed spectrum from fission in uranium in fluorocarbon is shown by curve 263, and it is the sum of the delayed-gamma ray spectrum, shown as curve 262 and the prompt-neutron activation spectrum, shown as curve 261. Curve 263 is an exponentially declining spectrum with a broad bulge commencing at about 5.5. MeV and ending at about 10.4 MeV. When the “double-bulge” 252Cf spectrum is normalized to the former spectrum above 7 MeV and subtracted from the latter, the difference is the exponentially declining spectrum representing typical fission delayed-gamma ray spectrum 262. The fraction of the delayed-gamma ray spectrum above 5.5 MeV is very small (approximately 0.4%), whereas that of prompt-neutron fluorine activation is quite substantial. The spectrum above 5 MeV is virtually dominated by the prompt-neutron activation allowing for the determination of the two signatures employing one detector type.
It should be appreciated by those of ordinary skill in the art that detection of multiple fission signatures using the unique detector system ensures that the high complementarity (in terms of penetration) of the irradiation probes (fast neutrons and high energy X-rays) is fully utilized, in accordance with an aspect of the present invention. It should further be appreciated that applying fast neutrons, if appropriately pulsed, affords the use of the extremely high sensitivity of the Differential Die Away Analysis (DDAA)—or the detection of prompt neutrons resulting from thermal-neutron fission only in fissile isotopes (e.g., U-235, Pu-239, U-233). DDAA can be employed with a very high efficiency when fissions are induced by thermalized source neutrons in the SNM. The thermalization process is fast, where the resultant thermal neutrons very efficiently stimulate fissions for hundreds of microseconds, only in SNM if they are present. The high energy fission neutrons produced by the thermal neutrons can be detected with high efficiency cadmium-covered detectors containing 3He proportional counters or other alternative detectors based on 16B or 6Li.
In a second embodiment, shown in row 302, monoenergetic gamma rays are employed as the probing radiation to stimulate fissions and allow for the measurement of prompt neutrons, delayed neutrons and delayed-gamma rays.
In a third embodiment, shown in row 303, fast neutrons are generated from a charged-particle accelerator accelerating, for example deuterons impinging on deuterium target, generating high-energy neutrons to stimulate fission events. The fission events are detected via the resultant prompt neutrons from fast neutron fissions, thermal-neutron fissions, delayed-gamma rays and delayed neutrons also emitted as a result of the fission process.
In a fourth embodiment, shown in row 304, an x-ray source, such as a 9 MeV linac is employed to generate x-rays as well as neutrons via photoneutron conversion. Both radiation probes stimulate fission events in fissionable material, if present. The fission events are detected via the following signatures: prompt neutrons resulting from fast and thermal-neutron fissions, delayed-gamma rays and delayed neutrons. All emitted as a result of the fission process. The x-rays can serve also to provide high resolution x-ray radiography of the cargo.
Referring to both
System 400 is a two-tier inspection system, wherein for the first “basic scan”, the container is completely and rapidly inspected using two independent co-linear radiography arrays, the primary array and the Z-spec transmission spectroscopy array. The primary array uses, in one embodiment, 544 cadmium tungstate (CdWO4) detectors with conventional current mode readouts using photodiodes providing images with very high spatial resolution. The Z-spec array uses about one-fourth of that number of fast plastic scintillators with spectroscopic readouts using fast photomultiplier tubes.
Each of the independent radiography arrays are used to locate high-Z objects in the image, such as lead, tungsten and uranium, which would be potential shielding materials as well as the nuclear material itself. The methods of Z-spec transmission spectroscopy are described in co-pending U.S. patent application Ser. No. 13/033,590, also assigned to Applicant of the present invention, filed on Feb. 23, 2011 and entitled “A High-Energy Spectroscopy-Based Inspection System and Methods to Determine the Atomic Number of Materials”, which is herein incorporated by reference in its entirety.
The high-Z detection techniques cannot however, distinguish potential shielding materials such as lead or tungsten from nuclear material. Therefore, in one embodiment, the system and method of the present invention provides a second stage screening technique for discrimination of nuclear material. The second step of the inspection process is to inspect the location identified by the automated X-ray system as a possible alarm. This is done by a longer stationary direct scan of that location. In the direct scan, areas of the container image that were identified as high-Z are re-inspected by precisely repositioning the container to the location of the high-Z object and doing a stationary irradiation of the area with the X-ray beam. Since the X-ray beam has a continuous spectrum of X-rays with an endpoint of 9 MeV, some of the X-rays are above the energy required to cause photo-fission (approximately 6 MeV). SNM threats, as well as all fissionable materials will fission and produce fissions while in the X-ray beam. The system looks for one or more types of fission signatures to identify that fission is taking place. These can be prompt neutrons from the direct fission process and delayed-gamma rays from the decay of the fission products, as described in detail above. In one embodiment, system 400 uses two types of detectors in array 405: plastic scintillators and fluorine-based threshold-activation detectors in the form of fluorocarbon liquid scintillators. The plastic scintillators can detect delayed-gamma rays only. The fluorocarbon detectors can detect both delayed-gamma rays and prompt neutrons via the reaction 19F(n,α)16N, which has an effective threshold of 3 MeV, and is thus insensitive to most source photoneutrons. Exceptions are beryllium and deuterium which can produce higher than 3 MeV neutrons. Beryllium is a rare cargo that will cause neutron alarms but will have no accompanying delayed-gamma ray signature. The background from deuterium in normal hydrogenous materials is small.
As described above, the isotope 16N beta decays with a 7.1 second half-life. There are two major beta decay modes: 10.4 MeV endpoint (26%) and 4.3 MeV (68%). The detection of the prompt fission neutrons by the fluorocarbon detector is achieved by the close to 100% efficiency of the high-energy beta decay rather than the alpha particle in the (n, α) reaction which occurs during the X-ray pulse. The major advantage of detecting delayed-gamma rays and prompt neutrons using the fluorocarbon detector is that signals are delayed, relative to the fission event and the X-ray pulse. The X-ray pulse temporarily blinds the detectors but they recover between the pulses.
In another embodiment, x-ray photo-fission multiple signature-based system of the present invention is employed in a combined and simultaneous photo-neutron/x-ray inspection system, as shown in
System 500 comprises a source 505, which, in one embodiment, is a 9 MV linac. Neutrons are produced simultaneously with X-rays by the photonuclear interaction of the x-ray beam with a suitable converter. In one embodiment, a suitable converter is a D2O converter built around the x-ray source, which uses a tungsten target. A total neutron yield on the order of 1011 n/s is achieved with an average electron beam current of 100 μA. It should be noted herein that source 505 is movable. The prompt neutrons resulting from fission are detected in two independent detector systems: high efficiency DDAA detectors 510 and by direct detection of neutrons with energy >3 MeV using fluorine-based threshold-activation detectors 515, as described above. The delayed gamma-ray signals are measured with high efficiency using the same TAD and with additional lower cost plastic scintillators 525. Several NaI spectroscopic detectors are employed to detect the cargo neutron capture gamma ray spectra 520. The linac and array of additional plastic scintillator can be moved vertically 530 to scan or to aim better at the threat location.
In operation of system 500, a cargo container (or truck) is moved through a portal 525 using a conveyor mechanism 535. The X-ray source is pulsed typically at 30 to 300 pulses per second and produces high-energy x-rays. Photoneutron beams are generated by the same x-rays in a heavy water converter. If fissionable material is present in the cargo, fission is induced by the high-energy X-ray beam as well as by the photoneutrons. Typically, the photofission process dominates in hydrogenous cargos, while the neutron fission process dominates in dense metallic cargos.
The above examples are merely illustrative of the many applications of the system of present invention. Although only a few embodiments of the present invention have been described herein, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.
The present specification relies on U.S. Provisional Patent Application No. 61/308,254, filed on Feb. 25, 2010, for priority, and is hereby incorporated by reference in its entirety. Further, the present specification also relies on U.S. Provisional Patent Application No. 61/313,200, filed on Mar. 12, 2010, for priority, and is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61308254 | Feb 2010 | US | |
| 61313200 | Mar 2010 | US |
