Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
  • G01V5/20—Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects
  • G01V5/281—Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects detecting special nuclear material [SNM], e.g. Uranium-235, Uranium-233 or Plutonium-239
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T3/00—Measuring neutron radiation
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
  • G01V5/20—Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects
  • G01V5/22—Active interrogation, i.e. by irradiating objects or goods using external radiation sources, e.g. using gamma rays or cosmic rays
  • G01V5/234—Measuring induced radiation, e.g. thermal neutron activation analysis

Definitions

• the present invention relates to a method and apparatus for detecting and indentifying fissionable material, in which method the intensity of the neutron radiation emitted by the fissionable material is measured by means of a neutron detector.
• Fissionable materials such as plutonium, uranium and thorium, can be used in a fission reaction, where the heavy nucleus of an atom is split into two or more parts, generally by bombarding the nucleus with neutrons. In general, fission takes place in connection with a neutrons or gamma rays emission.
• An uncontrolled and illegal distribution of fissionable materials is extremely dangerous, because an incorrect use of the materials can, owing to the immense powers contained in the fission reaction, lead to remarkable destruction in the surroundings.
• An illegal transportation of fissionable material from one country to another is usually carried out in large containers, where small amounts of fissionable material are easily hidden. Such containers are generally inspected visually, in which case small amounts of fissionable material are difficult to detect.
• bremmstrahlung generated by a linear particle accelerator such as an electron accelerator
• a target and a target inspection device for moving in between a target and a target inspection device in order to form an image of the objects contained in the container.
• the object of the present invention is to eliminate some of the drawbacks of the prior art and to realize a method and apparatus suited for the detection and identification of fissionable materials, such as plutonium, uranium and thorium, utilizing bremmstrahlung generated by a linear particle accelerator, and a delayed neutrons emission from the fissionable material created by said bremmstrahlung.
• fissionable materials such as plutonium, uranium and thorium
• a high-energy bremmstrahlung source operated within the range of 7 - 10 MeV; the radiation created thereby is directed to the target to be inspected, such as a transport container.
• the bremmstrahlung hits fissionable material, between the radiation and the material there occur interactive reactions of the photon and the nucleus, followed by a neutrons emission.
• interactive reactions of the photon and the nucleus there occur photoneutron and photofission reactions, which are characterized by a prompt neutrons emission.
• this prompt neutrons emission per one interactive reaction there is emitted one neutron from the photoneutron reaction, and 2.5 neutrons from the photofission reaction.
• Photofission further results in the creation of radioactive fission fragments, part of which contain beta-decay energy that exceeds the neutron binding energy in the nucleus.
• the beta-decay energy further leads to the emission of delayed neutrons.
• the fission fragment emitting delayed neutrons is called a precursor.
• the delayed neutrons emission emitted from the fissionable material and passing through the bremmstrahlung irradiation zone is detected with at least one neutron detector.
• the neutron detector used according to the invention is advantageously for instance a helium-3 filled or a borium-10 filled neutron detector or a neutron-sensitive scintillation detector.
• the neutron detector is arranged, by means of a collimator, in a shadowed position so that a primary irradiation of the detector with bremmstrahlung is essentially prevented.
• the neutron detector particularly detects delayed neutrons, because delayed neutrons are natural only in fissionable materials.
• the yield of delayed neutrons is about 1 % of the yield of prompt neutrons, but the sensitivity of detection is high enough due to a much longer exposition and a practically absent neutrons background.
• a linear particle accelerator used according to the invention is advantageously operated in pulse mode.
• a bremmstrahlung burst can be made to appear for instance within the frequency range of 50 - 500 Hz, while the duration of the bremmstrahlung is about 1.5 microseconds.
• the interval between two successive bursts is 2.000 - 20.000 microseconds.
• a time selection unit that blocks the operation of the detection system and reopens the time window when the bremmstrahlung burst pulse is finished.
• the duration of the time window is within the range of 150 - 200 microseconds.
• the whole bulk of material to be inspected such as a transport container, is irradiated with bremmstrahlung. If an essential change is detected in the flux of delayed neutrons, the location of the material corresponding to this change is determined. After irradiating the whole bulk of material, the material is returned to the location corresponding to the neutron change, and this location is re-irradiated with bremmstrahlung in order to identify the material in question, essentially for a long period of time, said period being 30 - 60 seconds.
• the bremmstrahlung is first interrupted, and in this case the flux of delayed neutrons is measured for at least one minute after interrupting the bremmstrahlung.
• figure 1 illustrates a preferred embodiment of the invention, seen in a partial side ⁇ view cross-section
• figure 2 is a block diagram of the operation ofthe time selection unit ofthe neutron detection system connected to the embodiment of figure 1
• figure 3 illustrates the principle of operation of the time selection unit of figure 2 by means of time-area coordinates.
• a linear particle accelerator 1 generates a bremmstrahlung beam 2, passing through a chink provided in a collimator 3, and irradiates a container 4.
• a point scintillation detector 5, together with an image processor, is used for creating an image of the contents of the container 4. If the container 4 contains fissionable material 6, the fissionable material 6 begins to generate a prompt neutrons emission, when the fissionable material falls within the irradiation zone of the bremmstrahlung 2.
• the neutrons are detected with helium-filled detectors 7, the molar mass of helium being 3.
• the detectors 7 are immersed in a hydrogen-containing medium 8, for example water or paraffin, in order to soften the spectrum created by the neutrons, which ensures a high detection efficiency.
• the collimator 3 is installed in between the neutron detector 7 and the particle accelerator 1 , so that a primary irradiation of the detector with bremmstrahlung 2 is prevented.
• the collimator 3 is made of steel in order to ensure a sufficient threshold energy for the neutrons emission.
• the container 4 moves within the scanning bremmstrahlung field of the particle accelerator 1.
• the neutron detectors 7 register a certain amount of background neutrons emission. If, during the motion of the container, the neutron flux is suddenly increased, a special inspection unit marks the exact spot within the container 4 where the suspicious object is located. Now, after the inspection of the whole container 4, the container 4 is returned to the marked spot, and the material that emits photoneutrons is identified. For identification, the suspicious object 6 is irradiated with bremmstrahlung for 30 - 60 seconds.
• the object 6 contains fissionable material
• the amount of fission fragments accumulating close to the saturation level during the irradiation, causes the delayed neutrons to be detected at 1 - 2 minutes after the accelerator 1 is switched off. If the delayed neutrons flux is negligible, the suspicious object contains an element with a low photoneutron reaction threshold energy, i.e. an element that is not fissionable material.
• the neutron detector 7 is supplied from a high-voltage unit 12.
• the pulses received from the detector 7 pass through an amplifier 13 and a discriminator 14, whereafter the pulses enter the time selection unit 15.
• the time selection unit 15 is controlled by the particle accelerator control system 16.
• the pulses that arrive while the time window is open are registered by the counter 17.
• Figure 3 illustrates a bremmstrahlung burst as a function of time.
• a bremmstrahlung burst is generated, causing a pulse 21.
• the curve 22 represents the time distribution of the prompt neutrons flux.
• the time selection unit according to figure 2 is open for the passing pulses 23, which cause a neutrons capture in the detector.

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• Physics & Mathematics (AREA)
• Life Sciences & Earth Sciences (AREA)
• General Physics & Mathematics (AREA)
• High Energy & Nuclear Physics (AREA)
• General Life Sciences & Earth Sciences (AREA)
• Geophysics (AREA)
• Health & Medical Sciences (AREA)
• Molecular Biology (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Measurement Of Radiation (AREA)
• Analysing Materials By The Use Of Radiation (AREA)

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• 1995
  • 1995-09-18 FI FI954381A patent/FI954381A/fi unknown
• 1996
  • 1996-09-11 JP JP9512412A patent/JPH11512528A/ja active Pending
  • 1996-09-11 WO PCT/FI1996/000480 patent/WO1997011388A1/en not_active Application Discontinuation
  • 1996-09-11 CA CA002232039A patent/CA2232039A1/en not_active Abandoned
  • 1996-09-11 CN CN96197052A patent/CN1196797A/zh active Pending
  • 1996-09-11 TR TR1998/00440T patent/TR199800440T1/xx unknown
  • 1996-09-11 IL IL12363396A patent/IL123633A0/xx unknown
  • 1996-09-11 EP EP96930183A patent/EP0877953A1/de not_active Withdrawn

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