METHOD AND APPARATUS FOR DETECTING AND IDENTIFYING FISSIONABLE MATERIAL

Abstract: The invention relates to a method and apparatus for detecting and identifying fissionable materials from among other transported goods, in which method the material (4) under inspection is irradiated with bremmstrahlung (2) generated by a linear particle accelerator (1). According to the invention, a delayed neutrons flux emitted from the material (4) under inspection by means of bremmstrahlung (2) is detected with at least one neutron detector (7) after interrupting the bremmstrahlung (2), and if essential changes occur in the neutron flux, the position of the material (4) in relation to the detector is determined. After inspecting the whole bulk of goods, the material (4) is returned to the position determined according to the change in the neutron flux in relation to the detector (7), and in this determined position the material (4) is again irradiated with bremmstrahlung (2) in order to identify the material generating the neutron flux, and at least one neutron detector (7) is used for detecting the delayed neutrons emitted from the fissionable material, in order to identify the fissionable material (6) in question.

Description: METHOD AND APPARATUS FOR DETECTING AND IDENTIFYING FISSIONABLE MATERIAL 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. In order to automatically inspect containers and detect objects hidden therein, there are developed methods utilizing bremmstrahlung generated by a linear particle accelerator, such as an electron accelerator, for surveying the contents of a container, and further 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. The essential novel features of the invention are apparent from the appended patent claims. According to the invention, in order to detect and identify fissionable material, there is used 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. When 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. As interactive reactions of the photon and the nucleus, there occur photoneutron and photofission reactions, which are characterized by a prompt neutrons emission. In 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. Fissionable materials have the lowest threshold energy of photonuclear reactions with neutron emissions. The ranges of the photonuclear reactions and photofission reactions for the fissionable material are large; for instance with the energy E = 8 MeV, the value is about 100 and 30 millibarn respectively. According to the invention, 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. Advantageously 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. Thus 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. Now the interval between two successive bursts is 2.000 - 20.000 microseconds. Irrespective of the installation ofthe neutron detectors in the shadow of a collimator in order to be protected from bremmstrahlung, the bremmstrahlung bursts, scattered in the accelerator shielding chamber, produce ionization in the neutron detectors. As a result, high-amplitude pulses with a duration of about 10 microseconds appear in the detection system output. During high-amplitude pulses, neutron detection is impossible. Therefore there is used, according to the invention, 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. According to the invention 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. For the identification of fissionable material, 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. The invention is explained in more detail below, with reference to the appended drawing, where 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 , and figure 3 illustrates the principle of operation of the time selection unit of figure 2 by means of time-area coordinates. According to figure 1 , 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. Advantageously the collimator 3 is made of steel in order to ensure a sufficient threshold energy for the neutrons emission. During the inspection, the container 4 moves within the scanning bremmstrahlung field of the particle accelerator 1. In an ordinary case, 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. If 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. In figure 2, 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. At a point of time to, a bremmstrahlung burst is generated, causing a pulse 21. The curve 22 represents the time distribution of the prompt neutrons flux. Within the period t, - t 2 , i.e. during the time window, the time selection unit according to figure 2 is open for the passing pulses 23, which cause a neutrons capture in the detector.

Claims: CLAIMS 1. A method for detecting and identifying fissionable materials among other transported goods, in which method the material (4) to be inspected is irradiated with bremmstrahlung (2) generated by a linear particle accelerator (1), and with at least one neutron detector (7) and by means ofthe bremmstrahlung (2), a delayed neutrons flux emitted from the material (4) under inspection is detected, and the bremmstrahlung (2) is interrupted for the duration of the measuring of the delayed neutrons flux, characterized in that 1) when essential changes occur in the neutrons flux, the position of the material (4) in relation to the detector (7) is determined, 2) after the inspection of the whole bulk of material, the material (4) is returned to the position determined according to the change in the neutron flux in relation to the detector (7), 3) in this determined position, the material (4) is again irradiated with bremmstrahlung (2) in order to identify the material generating the neutrons flux, 4) the delayed neutrons emitted from the fissionable material are detected with at least one neutron detector (7) in order to identify the fissionable material (6). 2. A method according to claim 1 , characterized in that the energy level of the bremmstrahlung (2) is within the range of 7 - 10 MeV. 3. A method according to claim 1 or 2, characterized in that the bremmstrahlung (2) is generated by a pulse-operated particle accelerator (1). 4. A method according to claim 1 , 2 or 3, characterized in that the delayed neutrons flux is detected with a helium-3 filled detector (7). 5. A method according to any of the preceding claims 1 - 4, characterized in that the material (4) is irradiated with bremmstrahlung (2) in order to identify the material for the duration of at least 30 seconds. 6. A method according to any of the preceding claims 1 - 5, characterized in that the operation of the registering unit (17) of the neutron detectors is controlled by the control system (16) of the particle accelerator. 7. A method according to claim 6, characterized in that in the detection of the delayed neutrons flux after interrupting the bremmstrahlung (2), there is used a time window with a duration within the range of 150 - 200 microseconds. 8. An apparatus for realizing the method according to claim 1 , said apparatus comprising a linear particle accelerator (1), a collimator (3) and means for moving the material (4) under inspection to the zone of the bremmstrahlung emitted from the linear particle accelerator, as well as means (7) for detecting the radiation emitted from the material under inspection, characterized in that the collimator (3) is installed in between the neutron detectors (7) and the particle accelerator (1) immersed in a medium (8) in order to prevent a primary irradiation of the neutron detectors (7) with bremmstrahlung (2). 9. An apparatus according to claim 8, characterized in that the collimator (3) is made of steel. 10. An apparatus according to claim 8 or 9, characterized in that the neutron detector (7) is immersed in a medium (8) containing hydrogen. 11. An apparatus according to claim 8, 9 or 10, characterized in that the medium (8) is water. 12. An apparatus according to any of the preceding claims 8 - 11 , characterized in that the neutron detector (7) is filled with helium 3.

Inventors: SIPILAE HEIKKI, SIPILAE, HEIKKI
Applicants: METOREX INT OY, SIPILAE HEIKKI, METOREX INTERNATIONAL OY, SIPILAE, HEIKKI
Application year: 1996
Main classification: G01T1
IPC codes: G01T1, G01T3, G01V5