EP1269166A2 - Detection de matiere fissile - Google Patents

Detection de matiere fissile

Info

Publication number
EP1269166A2
EP1269166A2 EP01932513A EP01932513A EP1269166A2 EP 1269166 A2 EP1269166 A2 EP 1269166A2 EP 01932513 A EP01932513 A EP 01932513A EP 01932513 A EP01932513 A EP 01932513A EP 1269166 A2 EP1269166 A2 EP 1269166A2
Authority
EP
European Patent Office
Prior art keywords
ken
rays
penetrating radiation
detector
fissile material
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP01932513A
Other languages
German (de)
English (en)
Inventor
Lee Grodzins
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
American Science and Engineering Inc
Original Assignee
American Science and Engineering Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by American Science and Engineering Inc filed Critical American Science and Engineering Inc
Publication of EP1269166A2 publication Critical patent/EP1269166A2/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
    • G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
    • G01N23/04—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material

Definitions

  • 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.
  • 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.
  • fissile material invariably contains other radioactive isotopes besides those essential for nuclear explosives.
  • 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.
  • 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;
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
  • 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.
  • source 36 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.
  • images of the contents of enclosure 10 may be produced by an image generator.
  • image refers to an ordered representation of detector signals corresponding to spatial positions.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • E mc ⁇ denl 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
  • ⁇ is the scattering angle.
  • 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.
  • 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.
  • 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.
  • 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.
  • the x-ray beam is switched off while the container is still in the inspection volume.
  • 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.
  • detectors When the detectors are operated in a current integrating mode, it is necessary to use unorthodox methods.
  • front chamber 50 is, as before, especially sensitive to energies below 100 keN, while back chamber
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 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 +
  • 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.
  • processor 42 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.
  • 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.
  • 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.

Abstract

L'invention concerne un système et un procédé de détection des rayonnements pénétrants émis par une matière fissile dissimulée. Un faisceau de rayonnement pénétrant irradie un objet, pendant tout ou partie d'un cycle de fonctionnement, les rayonnements pénétrants détectés par un détecteur étant distingués selon qu'ils proviennent ou non de la matière fissile contenue dans l'objet.
EP01932513A 2000-03-28 2001-03-27 Detection de matiere fissile Withdrawn EP1269166A2 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US19242500P 2000-03-28 2000-03-28
US192425P 2000-03-28
PCT/US2001/009784 WO2001073415A2 (fr) 2000-03-28 2001-03-27 Detection de matiere fissile

Publications (1)

Publication Number Publication Date
EP1269166A2 true EP1269166A2 (fr) 2003-01-02

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EP01932513A Withdrawn EP1269166A2 (fr) 2000-03-28 2001-03-27 Detection de matiere fissile

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EP (1) EP1269166A2 (fr)
WO (1) WO2001073415A2 (fr)

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CN1779451B (zh) * 2004-11-26 2010-04-28 清华大学 一种用放射源对液体进行背散射安全检测的装置
US10670740B2 (en) 2012-02-14 2020-06-02 American Science And Engineering, Inc. Spectral discrimination using wavelength-shifting fiber-coupled scintillation detectors
CN103630947B (zh) * 2012-08-21 2016-09-28 同方威视技术股份有限公司 可监测放射性物质的背散射人体安检系统及其扫描方法
CN103901486B (zh) * 2012-12-27 2018-04-24 同方威视技术股份有限公司 人体背散射安检系统及其方法
CN104865281B (zh) 2014-02-24 2017-12-12 清华大学 人体背散射检查方法和系统
CN107615052A (zh) 2015-03-20 2018-01-19 拉皮斯坎系统股份有限公司 手持式便携反向散射检查系统
GB2558238A (en) * 2016-12-22 2018-07-11 Smiths Heimann Sas Method and apparatus
WO2019245636A1 (fr) 2018-06-20 2019-12-26 American Science And Engineering, Inc. Détecteurs de scintillation couplés à une feuille à décalage de longueur d'onde
US11175245B1 (en) 2020-06-15 2021-11-16 American Science And Engineering, Inc. Scatter X-ray imaging with adaptive scanning beam intensity
US11340361B1 (en) 2020-11-23 2022-05-24 American Science And Engineering, Inc. Wireless transmission detector panel for an X-ray scanner

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Also Published As

Publication number Publication date
WO2001073415A2 (fr) 2001-10-04
WO2001073415A3 (fr) 2002-06-27

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