Abstract:
The present invention provides a cellular thermal neutron detector, comprising: a cellular structure comprised of one or more hollow regular hexagonal prism-shaped cellular units, wherein a neutron absorbing material is coated on an inner side of a pipe wall of each of the cellular units. Since the neutron-sensitive cellular structure is employed, the present invention can perform thermal neutron detection without using the scarce nuclide 3He, and effectively reduce the manufacture costs of the thermal neutron detector. Further, the present invention has characteristics such as a reduced or eliminated neutron detecting dead zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to pending Chinese Patent Application No. CN201110454387.4, filed Dec. 30, 2011, the contents of which are incorporated by reference its entirety. 
     FIELD OF THE INVENTION 
     The present invention generally relates to application of nuclear technologies, and particularly to neutron scattering and security detection technologies. More particularly, the present invention relates to a thermal neutron detector. 
     BACKGROUND OF THE INVENTION 
     In traditional safety detection technologies for nuclear materials, it is a common technology to use  3 He proportional counters for thermal neutron detection. However, due to scarcity of  3 He gas, the thermal neutron detector used for neutron scattering and security detection is confronted with a large problem in respect of costs of use. In order to solve the so-called “ 3 He supply crisis” problem, those skilled in the art are developing novel neutron detectors in place of traditional  3 He proportional counters. 
     As known in the art, the thermal neutron measurement is based on some particular nuclear reactions. Nuclear reaction for thermal neutron measurement usually should have the following two basic characteristics:
         there is a large nuclear reaction cross section to allow for a high thermal neutron absorbing efficiency P 1 ;   high-energy charged particles should be produced, and a production probability P 2  of the high-energy charged particles should be large, because only such high-energy charged particles can produce effective ionization and form useable signals in the subsequent procedure.       

     With regard to a final detecting efficiency P, a probability P 3  of the charged particles entering a signal-forming volume of the detector and forming the signal should be also considered. Finally, the detecting efficiency P can be determined by the following equation:
 
 P=P   1   ×P   2   ×P   3  
 
     wherein the thermal neutron absorbing efficiency P 1  and the high-energy charged particle producing probability P 2  depend on types of nuclides used by the detector, and the probability P 3  of the charged particles entering the signal-forming volume of the detector and forming the signal is decided by the design characteristics of the detector. Nuclides with larger P 1  and P 2  are generally  3 He,  6 Li,  10 B,  155 Gd and  157 Gd. In view of such issues as nuclide section and actual applicability,  10 B,  155 Gd and  157 Gd are popular nuclides in the art attracting more concerns. It is important issues in the art to effectively use these nuclides and design a reasonable detector structure to replace the traditional  3 He proportional counter. To this end, the prior art provides detectors with different structures, such as boron-coated straw tube detectors and multigrid detectors. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a novel technical solution for thermal neutron detection without using scarce nuclide  3 He, so as to reduce manufacturing costs. 
     A further object of the present invention is to enable the technical solution of the present invention to have characteristics such as reduced or eliminated neutron detection dead zone, simple processing process and/or a low γ sensitivity as compared with the prior art. 
     In particular, the present invention provides a thermal neutron detector, comprising: a cellular structure comprised of one or more hollow regular hexagonal prism-shaped cellular units, wherein a neutron absorbing material is coated on an inner side of a pipe wall of each of the cellular units. 
     Preferably, the pipe wall of each cellular unit is electrically grounded; an anode wire is provided along a central axis of each of the cellular units; and a chamber of each of the cellular units is filled with a proportional working gas. 
     Preferably, the thermal neutron detector further comprises: a supporting frame surrounding a radial outer side of the cellular structure; and a first template and a second template respectively having a central opening and being fixed on the supporting frame in a way of adjoining or being adjacent to either end of the cellular structure. 
     Preferably, the cellular structure is connected to the supporting frame via a plurality of groups of springs evenly distributed along a envelope circumferential line of the cellular structure, to maintain spatial stability of the cellular structure, wherein each group of springs are comprised of at least two springs spaced apart from each other along a longitudinal axis of the supporting frame. 
     Preferably, the central opening is comprised of one regular hexagonal hole or a plurality of regular hexagonal holes which are adjoined and communicated with one another, the anode wire extending along the central axis of each cellular unit passes through a center of the corresponding regular hexagonal hole of the central opening of the first template and the second template and is connected to an electrical connecting device fixed relative to the first template and the second template. 
     Preferably, a range of pressure of the proportional working gas is 0.1-10 atmospheric pressures. 
     Preferably, the pipe wall of the cellular unit is made of aluminum, copper or stainless steel. 
     Preferably, the thickness of the neutron absorbing material coated on the inner side of the pipe wall of the cellular unit is in a range of 0.1 μm-4 μm. 
     Preferably, the neutron absorbing material is a boron-containing or gadolinium-containing material. 
     Preferably, each of the cellular units has an axial length of 10 cm-100 cm and a side length of 2 mm-5 mm. 
     In the thermal neutron detector according to the present invention, the neutron-sensitive cellular structure is employed, so that the present invention can perform thermal neutron detection without using the scarce nuclide  3 He, and effectively reduce the manufacture costs of the thermal neutron detector. Further, the regular hexagonal prism-shaped cellular units can be seamlessly arranged together to constitute a complete cellular structure so that the present invention has characteristics such as a reduced or eliminated neutron detecting dead zone as compared with the prior art. 
     From the following detailed description of preferred embodiments of the present invention with reference to the drawings, those skilled in the art can better understand the above and other objects, advantages and features of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Preferred embodiments of the present invention will be described in detail hereafter with reference to the drawings by way of example, but not limitation. Like reference numbers throughout the drawings refer to the like or similar parts or portions. Those skilled in the art should appreciate that these drawings are not necessarily drawn to scale. In the drawings: 
         FIG. 1  is a schematic perspective view of a cellular thermal neutron detector according to a preferred embodiment of the present invention; 
         FIG. 2  is a schematic perspective view of a cellular unit in the cellular thermal neutron detector shown in  FIG. 1 ; 
         FIG. 3  is a schematic end view of the cellular unit shown in  FIG. 2 ; 
         FIG. 4  is a schematic side view of the cellular unit shown in  FIG. 2 ; 
         FIG. 5  schematically illustrates a procedure in which neutrons are subjected to a (m, α) reaction with  10 B adhered on a pipe wall when incident neutrons are incident to a cellular structure in the cellular thermal neutron detector shown in  FIG. 1 ; 
         FIG. 6  is a schematic partially-enlarged view of area A in  FIG. 5 ; 
         FIG. 7  schematically illustrates a supporting and fixing manner of the cellular structure according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic perspective view of a cellular thermal neutron detector  100  according to a preferred embodiment of the present invention. The thermal neutron detector  100  comprises a cellular structure  10  which is formed by arranging one or more (e.g., two, three, four, six, eight, sixteen or more) hollow regular hexagonal prism-shaped cellular units  11 . 
       FIG. 2  illustrates an individual cellular unit  11 . The axial length of the cellular unit  11  may be specifically selected according to needs in practical applications. As shown in  FIGS. 3-4 , a typical value of the axial length L of the cellular unit  11  may be 10 cm-100 cm, and a typical value of the side length W may be 2 mm-5 mm. 
     A pipe wall  12  of the cellular unit  11  may be constructed of a suitable meal such as aluminum foil, copper foil or stainless steel foil of a certain thickness. In a preferred embodiment of the present invention, the pipe wall  12  of the cellular unit  11  is made of aluminum foil. Regarding the aluminum foil for serving as the pipe wall of the cellular unit, the thickness of the aluminum foil is preferably as small as possible to reduce γ sensitivity on the premise of meeting the structural rigidity, and the typical thickness, for example, may be 25 μm-100 μm. 
     A neutron absorbing material (or called neutron-sensitive material) is coated on an inner side of the pipe wall  12  of each cellular unit  11 . In the embodiment shown in  FIGS. 5-6 , an aluminum foil substrate  13  constitutes the pipe wall  12  of the cellular unit; the neutron absorbing material  14  (which is a boron-containing material in this example) with a certain thickness is adhered on the aluminum foil substrate  13 . The thickness T of the neutron absorbing material  14  cannot be too small or too large, and 0.5 μm-2 μm is a preferable range of thickness. In the embodiment shown in  FIG. 5  in which  10 B in the boron-containing material is a neutron-sensitive nuclide, when the thermal neutrons are incident on the cellular structure  10 , the incident neutrons are subjected to a (m, α) reaction with  10 B adhered on the pipe wall  12 ; α particles or  7 Li obtained from the reaction is likely to penetrate through the boron-containing material into an internal chamber of each cellular unit  11  filled with a proportional working gas, wherein the proportional working gas may be for example Ar90%+CO 2 10% gas which pressure may preferably be set to be large enough to absorb all of the nuclear energy of α particles or  7 Li entering the gas. A range of gas pressure of the proportional working gas may be 0.1-10 atmospheric pressures, typically 0.5-2 atmospheric pressures. Besides the boron-containing material, a gadolinium-containing material (e.g., a gadolinium foil or a Gd 2 O 3  film) may be used as the neutron absorbing material in other embodiments of the present invention. 
     An anode wire  15  is provided along a central axis of each cellular unit  11 . In operation, these anode wires all are applied a positive DC high voltage which together with a ground potential applied on the pipe wall  12  forms a radial electric field within the cellular unit  11 . The positive DC high voltage applied on the anode wires may be selected so that a local space around each anode wire  15  forms a strong electric field sufficient for occurrence of an avalanche effect such that the detector is in a proportional working mode. As appreciated by those skilled in the art, a specific magnitude of the voltage applied to the anode wires is preferably determined by the side length W of the cellular unit  11 , the type of the filled proportional working gas and the magnitude of gas pressure. 
     According to a preferred embodiment of the present invention, during preparing the cellular structure  10 , firstly a metal foil coated with the neutron absorbing material  14 , e.g., an aluminum foil coated with B 4 C, is obtained by a process such as electron beam evaporation or magnetron sputtering; then the metal foil coated with the neutron absorbing material  14  is manufactured as a unitary cellular structure  10  including one or more cellular units  11  by a honeycomb forming process. Such processes for forming the cellular structure by using various foil materials or sheet materials are very mature in the prior art of forming technologies and can be easily implemented by those skilled in the art, and then this will not be unnecessarily described in detail herein. 
     After the cellular structure  10  sensitive to neutrons is prepared, considering that the pipe wall  12  (such as an aluminum foil) has a small thickness, the present invention particularly provides a template and a supporting frame to reinforce the rigidity of the whole neutron detector  100 . As shown in  FIG. 7 , a first template  21 , a second template  22  and a supporting frame  23  provide rigid support and protection for the whole cellular structure  10 . Specifically, the supporting frame  23  surrounds a radial outer side of the cellular structure  10 ; the first template  21  and the second template  22  having a central opening are respectively fixed on the supporting frame  23  in a way of adjoining or being adjacent to either end of the cellular structure  10 . The central openings of the first template  21  and the second template  22  are both comprised of one regular hexagonal hole or a plurality of regular hexagonal holes which are adjoined and communicated with one another. Preferably, the number of regular hexagonal holes constituting the central opening is identical with the number of the cellular units included by the cellular structure, and each regular hexagonal hole constituting the central opening is concentric with the central axis of the corresponding cellular unit. The anode wire  15  of each cellular unit passes through a center of the corresponding regular hexagonal hole of the central opening of the first template  21  and the second template  22  and is connected to an electrical connecting device (not shown) fixed relative to the first template  21  and the second template  22 . The electrical connecting device is for example an electrical connection terminal or electrical connection grille operably connected to an external DC high voltage power source, which is easily implemented by those skilled in the art and then will not be unnecessarily described in detail herein. Therefore, the anode wire  15  passing through the cellular unit  11  may be considered to be connected at the center of the corresponding regular hexagonal holes of the two templates. 
     In a preferred embodiment of the present invention, the cellular structure  10  is wholly connected to the supporting frame  23  by a plurality of groups of springs  24  evenly distributed along an envelope circumferential line of the cellular structure, to maintain spatial stability of the cellular structure  10 . Preferably, each group of springs may be comprised of at least two springs spaced apart from each other along a longitudinal axis of the supporting frame  23 . The pipe wall per se of each cellular unit  11  is at a ground potential and functions as a cathode. Specifically, the pipe wall  12  is connected to the supporting frame  23  via the springs  14  and then is electrically grounded. 
     A circuit portion (such as, a specific grounding method, an amplifier circuit, an external DC high voltage power source, and so on) and an air charging and discharging device and a sealing device of the detector  100  are similar to those of for example boron-coated straw tube neutron detector or a traditional single-wire proportion chamber and can be readily implemented by those skilled in the art, and then this will not be unnecessarily described in detail herein. 
     So far, those skilled in the art can appreciate that although exemplary preferred embodiments have been illustrated and described in detail, many other variations or modifications conforming to the principles of the present invention can be directly determined or derived from the disclosure of the present application without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention shall be understood and recognized as covering all of these other variations or modifications.