DETECTION OF FISSILE MATERIAL

Abstract: A system and method for detecting penetrating radiation emitted by concealed fissile material. A beam of penetrating radiation irradiates an object during all or part of an operating cycle, while penetrating radiation detected by a detector is distinguished as to whether it arises from fissile material within the object.

Description: DETECTION OF FISSILE MATERIAL Technical Field The present invention relates to a method and apparatus for simultaneously inspecting containers with penetrating radiation and searching for fissile material contained within the containers. Background of the Invention There is a need to find fissile material that is clandestinely transported across national boundaries. During recent years, the United States government has placed mobile vehicles at strategic areas in Europe with gamma ray detectors dedicated to the task of finding fissile material. Atomic explosives are made from 235 U, a rare, naturally occurring, isotope of uranium that lives almost 10 9 years, or 239 Pu, a reactor-made isotope that lives more than 10 4 years. 235 U decays with the emission of gamma ray photons (also referred to 'gammas'), principally at 186.7 keN and 205.3 keN. 239 Pu emits a number of gamma rays when it decays, the principal ones being at 375 keN and 413.7 keN. These gamma rays are unique signatures for the respective isotopes. But fissile material invariably contains other radioactive isotopes besides those essential for nuclear explosives. For example, weapons grade uranium may contain as little as 20% 235 U; the rest of the uranium consists of other isotopes. The other uranium and plutonium isotopes reveal their presence by gamma rays emitted by their daughters. For example, a daughter of U emits a high energy gamma ray at 1,001 keN; a daughter of U, an isotope present in fissile material made in the former USSR, emits a very penetrating gamma ray at 2,614 keN; and a daughter of 241 Pu emits gamma rays of 662.4 keN and 722.5 keN. Systems for searching luggage for contraband items such as weapons or drugs are now commonly employed both at airports and at border crossings. Airport installations typically employ lower energy (160 keN)) X-rays while high energy (> 450 keN) x-ray systems are becoming common at border crossings. Summary of the Invention In accordance with one aspect of the invention, in one of its embodiments, there is provided an inspection system for inspecting an object. The inspection system has a source of penetrating radiation for generating a beam and for irradiating the object and at least one detector for detecting the penetrating radiation after interaction with the object. Additionally, the inspection system has a processor for distinguishing penetrating radiation detected by the detector that arises from fissile material within the object. In accordance with alternate embodiments of the invention, the detector may have a segment having selective energy sensitivity. The source of penetrating radiation may be temporally gated, thereby allowing distinction of the radiation arising from within the object. The detector may have two serial scintillators, and an x-ray absorbing wall may be interposed between the scintillators. One of the scintillators may contain a heavy fluorescing material such as bismuth. In accordance with other alternate embodiments of the invention, the instantaneous energy spectrum of the source may be selected to excite characteristic emission lines of fissile elements, and the beam of penetrating radiation may be a pencil beam. Brief Description of the Drawings The foregoing features of the invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings: FIG. 1 provides a top view of a cargo container being examined by two backscatter x-rays systems, one on either side of the container, and two orthogonal transmission systems, one horizontal, the other vertical, as an example of an inspection system that may be employed also for detection of fissile material in accordance with a preferred embodiment of the present invention; FIG. 2 depicts a passive method for fissile detection, in accordance with an embodiment of the invention, wherein, when no x-ray beam is present in the inspection tunnel, the detector is switched to pulse mode; FIG. 3 depicts a passive method for detection of radioactivity in a mode in which the detector is sensitive mainly to Compton x-rays; FIG. 4 depicts an active method of radiation detection using a two chamber detector to measure the emission of fissile material; and FIG. 5 is a schematic view of a two-scintillation chamber detector configuration for detection of fissile material in accordance with an embodiment of the present invention. Detailed Description of Preferred Embodiments This invention describes two independent ways in which x-ray inspection systems, currently in use for detection of contraband materials, may additionally be used for finding fissionable material in the containers they examine. The first method is passive; i.e., the gamma rays from the fissile materials are the signatures for an alert. Several ways of carrying out such passive measurements are described. The second method is active; i.e., the x-rays inspecting a container excite fluorescence of the fissile material and the characteristic x-rays of uranium or plutonium produce an alert signal. Preferred embodiments of the invention make use of systems in which a beam of x- rays is swept through a plane of a container. X-rays transmitted through the container are detected in large area transmission detectors while x-ray backscattered from the container and its contents are detected in large area backscatter detectors. In the discussion that follows, illustrative calculations make use only of the backscatter detectors. Inspection systems that may be used for practice of the present invention are of the variety described and shown in copending U.S. Patent Application, Serial No. 09/238,686, which is herein incorporated by reference. Other inspection systems that may be used for practice of the present invention are of particular utility for the inspection of large cargo containers such as trucks or sea/air containers in that they employ mobile platforms that may be driven past the inspected container during the course of the inspection. Such systems are described in U.S. Patent no. 5,764,683, which is incorporated herein by reference. Referring now to Fig. 1, a top view is shown of a cargo container 10 being examined by two backscatter x-rays systems 12 and 14, one on either side of container 10, and two orthogonal transmission systems, one horizontal 16, the other vertical 18. These inspection systems are shown by way of example, and single inspection systems or different combinations of systems may be used within the scope of the present invention. One or more generators of penetrating radiation may be used for each of the transmission and scatter modalities. Describing, first, backscatter x-rays systems 12 and 14, x-ray beam 20 is emitted by an x-ray source 22 of one of various sorts known to persons skilled in the art. Beam 20 may also be comprised of other forms of penetrating radiation and may be monoenergetic or multienergetic, or, additionally, of varying spectral characteristics. Backscatter x-ray beam 20 is typically generated by a DC voltage applied to the anode of an x-ray tube 22 so that beam 20 is typically continuous. However, a beam 20 of other temporal characteristics is within the scope of the invention. Beam 20 has a prescribed cross sectional profile, typically that of a flying spot or pencil beam. Beam 20 will be referred to in the present description, without limitation, as an x-ray beam. Penetrating radiation scattered by an object 27 within enclosure 10 is detected by one or more x-ray detectors 26 and 28. X-ray detectors 28 may be disposed at varying distances from x-ray beam 20 for differential sensitivity to near-field objects 30 and far-field objects 27, as described, for example, in Patent Application Serial No. 09/7238,686. In order to obtain greater spatial resolution of the source of scattered radiation, collimators 32 may be employed, as known to persons skilled in the x-ray art, for narrowing the field of view of segments of detector 28. Transmission system 16 employs an x-ray beam 34 produced by source 36 which is typically a high energy source of penetrating radiation such as a linear accelerator (Linac) for example. X-ray emission from a linear accelerator is inherently pulsed, with typical pulse rates in the range between 100 and 400 pulses per second. The portion of transmission beam 34 which traverses enclosure 10 and objects 30 and 38 contained within the enclosure is detected by transmission detector 40. The electrical output signals produced by detectors 26, 28, and 40 are processed by processor 42 to derive characteristics such as the geometry, position, density, mass, and effective atomic number of the contents from the scatter signals and transmission signals using algorithms known to persons skilled in the art of x-ray inspection. In particular, images of the contents of enclosure 10 may be produced by an image generator. As used in this description and in the appended claims, the term "image" refers to an ordered representation of detector signals corresponding to spatial positions. For example, the image may be an array of values within an electronic memory, or, alternatively, a visual image may be formed on a display device 44 such as a video screen or printer. The use of algorithms, as known in the art of x-ray inspection, for identifying suspect regions within the enclosure, and identification of the presence of a specified condition by means of an alarm or otherwise, is within the scope of the present invention. In many applications, it is desirable that enclosure 10 be inspected in a single pass of the enclosure through the x-ray inspection system. Enclosure 10 may move through the system in a direction indicated by arrow 46, either by means of self-propulsion or by any means of mechanical conveyance of the enclosure with respect to the system. Detectors 26, 28, and 40, used in systems for inspection of the contents of baggage or cargo containers are typically operated in a current integration mode rather than in a mode of counting individual x-ray pulse by virtue of count rates that are typically too high to permit counting and processing individual x-ray pulses. Images of the distributions in the currents produced by the transmitted and backscattered x-rays are typically built up as the container passes through the plane of x-rays. Passive Method I. Gated Detectors: Referring to Fig. 2, the x-ray beams 20 in x-ray inspection systems typically sweep, as by rotation of chopper wheel 70, through the inspection volume during a large fraction of the operating time. During the remaining fraction of each sweep cycle there are essentially no source x-rays striking the target container. Thus, during the time of source quiescence, the detectors are only counting background. In a preferred embodiment, particularly useful for lower energy (140 keN- 160 keN) x- ray systems, the output from backscatter detectors 28 are switched to a pulse counting circuit 72 during the fraction of the operating cycle during which the source of x-ray irradiation is off. During this period, individual gamma rays 74 can be detected and analyzed. The efficiency of the backscatter detectors of an x-ray inspection system for detecting the 185.7 keN gamma ray may be estimated to be -0.4% on the basis of the following assumptions: the target container is in the range of the backscatter detectors for about 4 seconds so that the detectors are pulse counting for 0.05 x 4 = 0.2 seconds; the intrinsic efficiency of the backscatter detectors for 186 keN gamma rays is -10%; and the geometrical efficiency is -20% for material near the center of the inspection volume. The 186 keN gamma rays are emitted in 53% of the decays of 235 U (shown as object 76) but only a thin layer of the bulk uranium is accessible since the mean free path of 186 keN gammas in uranium is only 0.36mm. Still, every square centimeter of 10% enriched uranium will emit - two thousand 186 keN gamma photons per second, giving rise to a count of 2,000 x 0.004 = 8 counts for every square centimeter of surface area of uranium that faces the detectors. A 1" cube of uranium (weighing - 3 Λ pounds) would signal its presence with ~ 50 counts in the 0.2 second off-period of the inspection. A signal of this magnitude is easily discriminated. The signal strength is further increased by increasing detection efficiency, enlarging the detectors, and increasing the off-time of the sweeping x-ray beam. A luggage security system such as shown in Fig. 1 may advantageously continue to operate normally while the fissile detection system operates efficiently in the backgraound, as described. Passive Method II. Continuous detection: The minimum energy of the gamma rays from fissile material is 187 keN. The maximum energy of x-rays detected in the backscatter counters is given by: where E mcιdenl is the energy of an incident photon, E scattered is the maximum energy of a scattered photon, m e c is the rest energy of an electron, and θ is the scattering angle. In the backward direction, E scattered is typically only 100 keN for 160 keN x-ray generators and 170 keN for 450 keN generators. The preponderance of detected x-rays, thus, in either passive or active inspection modality, have energies well below 100 keN. It is therefore feasible to count continuously (that is, during the inspection itself) with a detector that has a threshold at say 160 keN, a straightforward task if the radiation is detected with a pulse counter. As shown in Fig. 3, the radiation detectors of certain x-ray inspection systems, measure current; i.e. the charge integrated over specific times, as measured by a current- integrating circuit 80. A current mode counter 80 sensitive to the fission material gamma rays can be implemented by the following method. Referring now to Fig. 4, a two-chamber backscatter detector is used. A front chamber 50, through which the fluorescing and fissile material radiation 52 and 54 pass through first, has very good efficiency for detecting the radiations below about 100 keN; i.e., the bulk of the Compton scattered radiation. A rear chamber 56, with thicker, higher-Z scintillators, has very good efficiency for detecting radiation up to 200 keN. An opaque wall 58 between the front and rear chambers may be an absorber properly chosen to further reduce the lower energy radiations while passing the higher energies that signify the presence of gamma rays emitted from container 10. The ratio of the current pulse in back detector 60 to that in front detector 62 is a good measure of the presence or absence of higher energy gamma rays. When there is no radioactive source in the inspected container, the ratio will have a range of values that will always be lower than the range of values when the gamma rays from uranium and plutonium are present. To verify the reality of an alarm, as determined by a high ratio, the x-ray beam is switched off while the container is still in the inspection volume. Active Detection: Photoelectric interaction takes place in uranium and plutonium when the elements are bombarded with x-rays greater than 115.6 keN and 121.72 keN, respectively. The excited atoms then decay back to their ground states by isotropically emitting their characteristic K α and K β x-rays. The energies of the K α x-rays of U are 94.6 keN and 98.4 keN, while those of Pu are 99.4 keN and 103.7 keN. The energies of the Kβ x-rays of U are 111.3 keN and 114.5 keV, while those of Pu are 117.1 keN and 114.6 keN. These are uniquely high-energy characteristic x-rays; the characteristic K x-rays of lead, the heaviest element that might be found in any quantity in a container, span the much lower range from 72.8 keN to 87 keN. The x-ray generators used for inspecting luggage and smaller containers have maximum energies of 140 keN to 160 keN. The components of the x-ray spectrum above 115.6 keN and 121.72 keN (the K-electron binding energies of uranium and plutonium respectively) interact with the fissile elements through the photoelectric effect. The result is that the entire energy spectrum above the binding energies is effectively converted into the characteristic x-rays of the elements. High-energy x-rays are readily detected with detectors operating in the pulse-counting mode, as known in the art. When the detectors are operated in a current integrating mode, it is necessary to use unorthodox methods. The use of the simple two-chamber method described above in the context of passive measurements is typically not preferred here because the characteristic x-ray emission of the fissile material is at considerably lower energy than the gamma rays emitted by the fissile materials. In accordance with a preferred embodiment, the two-scintillation chamber method described above is modified in the manner now discussed. Referring further to Fig. 4, front chamber 50 is, as before, especially sensitive to energies below 100 keN, while back chamber 56 is specifically sensitive to energies up to 130 keN. The unusual feature of the back scintillation chamber, described now in reference to Fig. 5, is the inclusion a layer of bismuth 58, approximately a mean free path thick, for energies above its K edge. X-rays below the K binding energy see the bismuth as a relatively thin absorber and the photoelectrically induced L x-rays have too low an energy to be effectively detected. Alternatively, other heavy materials such as lead or gold may be used in place of bismuth. X-rays above 90.5 keN, however, are converted about 60% of the time into x-rays of 74.8 keN and 89.8 keN, which are detected in scintillation detectors of the back detector. When no fissile signal is present, the ratio of the current integrated signals in the back detector to that of the front detector is lower than the ratio when fissile material is present. Thus the ratio can be used to automatically alarm on the presence of fissile material. Sensitive to 100 gram amounts of fissile material is readily achieved. A simple calculation, referring to Fig. 5, shows the effectiveness of a two-chamber method of detection. Ray 52 represents backscattered radiation from a container 10 that has negligible fissionable material or any very heavy element such as lead or gold. The spectrum of backscatter 52, generated from an x-ray generator with a maximum electron energy of 160 keN, has few x-rays above 100 keN. Typically, for every 100 x-ray photons with energy in the range of 60 keN to 75 keN, there will be less than 5 x-ray photons above 100 keN. Ray 54 represents backscatter and additionally K x-rays fluoresced from fissile material in container 10. Front chamber 50 has a scintillator 62 of, for example, 200 mg/cm 2 of GdOS, which has an efficiency of -70% for counting the 60 keN to 75 keN radiation, but only a 30% efficiency for counting the x-rays above 100 keN. Thus, the signal in chamber 50 will consist of 70 counts from the 60 keN to 75 keN x-rays and 1.5 counts from the x-rays above 100 keN. Passing out of chamber 50 into chamber 56 are 30 x-rays in the 60-75 keN range and 3.5 x-rays above 100 keN. The x-rays that enter chamber 56 pass through a bismuth absorber 58 which has a 70% efficiency for stopping the x-rays in the 1 0 keN to 120 keN range and a 50% efficiency for stopping the x-rays in the 60 keN to 75 keN range. Thus 15 of the 30 x-rays of 60-75 keN stop in the bismuth; the produced L x-rays have too low an energy to be counted in the scintillator 60. Scintillator 60 is similar to scintillator 62 and it therefore counts - 7 x-rays of 60-75 keN and less than 1 x-ray greater than 100 keN. The ratio of counts in chamber 56 to chamber 50 will be 7/70 =~0.1 ; the ratio of currents will be almost the same. The case when fissile material is present is now considered, with shielding around the fissile material neglected for clarity. A 140 keN x-ray produces approximately 500 characteristic x-rays 54 in the spectrum from every square centimeter of uranium or plutonium that is struck by the beam. Approximately 100 of those x-rays will enter chamber 50 and 30 of them will stop and be counted. The total counts in chamber 2 will then be 70 + 1.5 + 30 = 101.5. The 70 fissile-induced x-rays 52 that penetrate into chamber 56 will interact with the bismuth and 50 of them will stop and produce bismuth x-rays of 75 keN to 100 keN. These will be counted in chamber 56 with an efficiency of - 70% so that the bottom chamber will count s~35 x-rays over and above the count were fissile material to be absent. The ratio of counts, or current, between chamber 56 and 50 will rise from -0.1 to almost 0.5, a readily distinguished change. Use of a pencil beam for x-ray irradiation of the inspected enclosure allows determination of the outline and position of the fissile material, using standard x-ray inspection algorithms. Upon detection of fissile material, processor 42 (shown in Fig. 1) may then give rise to activation of an alarm 43, and, additionally, display the outlines of the fissile material on display 44 using highlighting coloring or other standard techniques. It is to be noted, also, that the methods described herein may advantageously also be employed in conjunction with x-ray inspection systems employing a fan beam, or otherwise shaped beams, such as standard transmission-imaging systems commonly employed for luggage scrutiny at airports. The preferred position, however, for the fission detectors is in the back direction, i.e., on the same side, with respect to the inspected object, as the x-ray generator. In that geometry, the energy of the x-rays Compton-scattered from material in the container will be lowest and furthest in energy from the high-energy characteristic x-rays, or gamma rays, emanating from the fissionable material. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.

Claims: I CLAIM: 1.An inspection system for inspecting an object, the inspection system comprising: a. a source of penetrating radiation for generating a beam and for irradiating the object, the so * urce characterized at each instant of time by an instantaneous energy spectrum and an intensity; b. at least one detector for detecting the penetrating radiation after interaction with the object; and c. a processor for distinguishing penetrating radiation detected by the detector that arises from fissile material within the object. 2. A system according to claim 1 , wherein the at least one detector includes a segment having selective energy sensitivity. 3. A system according to claim 1, wherein the source of penetrating radiation is temporally gated. 4. A system according to claim 3, wherein the detector is switched from a current- integrating mode to a pulse counting mode for a period during which the instantaneous intensity of the source is substantially zero. 5. A system according to claim 1, wherein the at least one detector comprises two serial scintillators. 6. A system according to claim 5, wherein an x-ray absorbing wall is interposed between the two serial scintillators. 7. A system according to claim 5, wherein one of the scintillators contains a heavy fluorescing material. 8. A system according to claim 7, wherein the heavy fluorescing material is bismuth. 9. A system according to claim 1, wherein the instantaneous energy spectrum of the source is selected such as to excite characteristic emission lines of fissile elements. 10. A system according to claim 1 , wherein the beam of penetrating radiation is a pencil beam. 11. A method for detecting fissile material within a container, the method comprising: a. illuminating the container with penetrating radiation; b. detecting penetrating radiation emanating from the container; and c. distinguishing between detected radiation due to scatter and detected radiation due to emission by fissile material. 12. A method according to claim 11, further comprising a step of activating an alarm upon detection of emission by fissile material. 13. A method according to claim 11, further comprising a step of displaying contours of the fissile material.

Inventors: GRODZINS LEE, GRODZINS, LEE
Applicants: AMERICAN SCIENCE & ENG INC, AMERICAN SCIENCE AND ENGINEERING, INC.
Application year: 2001
Main classification: G01N23
IPC codes: G01N23