Patent Publication Number: US-8541763-B2

Title: Modifiable layered shield assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/509,602 entitled MODIFIABLE LAYERED SHIELD ASSEMBLY, filed on Jul. 20, 2011 which is incorporated herein by reference in its entirety and to which this application claims the benefit of priority. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to shields used with cyclotrons for shielding against radiation, and more particularly, to a shield assembly located within a movable shield wherein the shield assembly includes removable layered shield elements. 
     BACKGROUND OF THE INVENTION 
     Positron Emission Tomography (PET) is a procedure used for imaging and measuring physiologic processes within the human body. As part of the procedure, radioisotopes are injected into a patient to assist in diagnosing and assessing a disease. A cyclotron or particle accelerator is used to produce the radioisotopes. In a cyclotron, a particle beam is accelerated which then bombards a target material housed in a target system of the cyclotron. Referring to  FIG. 1   a , a general configuration for shielding a cyclotron  10  is shown. The cyclotron  10  is positioned between movable shields  12  and  14  (shown in an open position thus exposing the cyclotron  10 ) and stationary shields  16  and  18 . Referring to  FIG. 1   b , the cyclotron  10  generates a particle beam  20  that then bombards target material  22  located within target enclosure  24  to produce a radioactive isotope which then decays. The decay of the isotope as well as other interactions generates gamma and neutron radiation  26  that is attenuated by the shields  12 , 14 , 16 , 18  so as to protect personnel in the vicinity of the cyclotron against unsafe levels of radiation. 
     Typically, cyclotron shields are configured as large, monolithic blocks of shielding material that includes lead shot suspended in a polymer matrix, for example. However, this approach has disadvantages from the viewpoint of design, manufacturing, disposal and the environment. In particular, it is preferable that the material used for the shields is specifically adapted for shielding against the radiation that is being emitted, such as gamma and neutron radiation, so that each form of radiation is addressed separately. Further, an approach to shielding is needed which is easily configured to better suit the needs of specific cyclotron configurations, uses materials which minimizes potential environmental effects and is easier to assemble during manufacturing and disassemble for appropriate disposal when decommissioning a shield. 
     SUMMARY OF THE INVENTION 
     A shield for absorbing radiation emitted by a target during operation of a cyclotron is disclosed. The shield includes an inner box structure having a recess for receiving the target. In addition, the shield includes a first plurality of shield elements arranged in a layered configuration about the inner box structure. The shield also includes an outer box structure for receiving the inner box structure and the first plurality of shield elements. Further, a second plurality of shield elements is arranged in a layered configuration about the outer box structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  depicts a general configuration for shielding a cyclotron. 
         FIG. 1   b  depicts a cyclotron and generation of a radioisotope. 
         FIG. 2  is an assembled view of a layered shield assembly in accordance with the invention. 
         FIGS. 3   a  and  3   b  depict inner and outer box structures for the layered shield. 
         FIG. 4  depicts the layered shield in an exploded view. 
         FIG. 5  depicts the layered shield assembled in a movable shield. 
         FIG. 6  depicts a target portion of a cyclotron located within a recess of the layered shield assembly. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. In the description below, like reference numerals and labels are used to describe the same, similar or corresponding parts in the several views of  FIGS. 1-6 . 
     Referring to  FIG. 2 , an assembled view of a modifiable layered shield assembly  30  is shown. As will be described, the layered shield  30  is located in a movable shield  12 , 14  used in connection with a cyclotron. The layered shield  30  includes inner and outer box structures, a plurality of horizontally and vertically oriented planar shield elements and a cover  112  for providing access to a recess  46 . In accordance with the invention, the recess  46  receives a target portion of a cyclotron used for producing radioisotopes for Positron Emission Tomography (PET). 
     Referring to  FIGS. 3   a  and  3   b , inner  32  and outer  34  box structures, respectively, for the layered shield  30  are shown. The inner  32  and outer box  34  box structures are each fabricated from steel, although other suitable materials may be used. The inner box  32  includes right  36  and left  38  vertical inner surfaces and top  40  and bottom  42  horizontal inner surfaces. The inner box  32  also includes an inner vertical back surface  44  to form the recess  46 . The outer box  34  includes right  48  and left  50  vertical outer surfaces and top  52  and bottom  54  horizontal outer surfaces. The outer box  34  also includes an outer vertical back surface  56  to form an opening  58  for receiving the inner box  32 . 
     Referring to  FIG. 4 , the layered shield  30  is shown in an exploded view. The layered shield  30  includes a plurality of planar shield elements constructed of steel, borated polyethylene, high density lead and aluminum which serve to shield against gamma and neutron radiation generated during use of the cyclotron. The boron content of the borated polyethylene ranges from 5% to 30%, which is a commercially available range. In other embodiments of the invention, other boron content ranges may be utilized. In accordance with the invention, many smaller shielding layers are used in the invention, in contrast to one or two bulky layers of homogeneous materials as found in conventional, monolithic type radiation shielding. Further, during the shielding process and as radiation is reduced, secondary or lower forms of radiation may be created. The layered shield elements of the invention capture the secondary radiation that may escape a monolithic shield. 
     The layered shield  30  includes lower  60 , upper  62 , right  64 , left  66  and rear  68  sets of ordered shield elements that are arranged about the inner  32  and outer  34  boxes. Each shield element in each of the lower  60 , upper  62 , right  64 , left  66  and rear  68  sets is fabricated from shielding material or a combination of shielding materials and is configured to shield against radiation generated during operation of a cyclotron. In particular, each shield element incrementally reduces neutron and gamma radiation emitted during operation of a cyclotron. 
     In one embodiment, the lower set  60  includes first  70 , second  72 , third  74 , fourth  76 , fifth  78 , sixth  80 , seventh  82 , eighth  84  and ninth  86  horizontal shield elements. The inner box  32  is positioned within the opening  58  of the outer box  34  when the layered shield  30  is assembled. The first shield element  70  is oriented horizontally underneath and closest to the bottom inner surface  42  (see  FIG. 3   a ) of the inner box  32  followed by the remaining shield elements  72 , 74 , 76 , 78 , 80 , 82 , 84 , 86  which are each oriented horizontally to form a horizontally stacked, layered configuration underneath the bottom inner surface  42 . The first  70  and second  72  shield elements each include polyethylene material having 30% boron content (i.e. 30% borated polyethylene). The third  74 , fourth  76  and fifth  78  shield elements include lead, 5% borated polyethylene and lead material, respectively, arranged in an alternating configuration. The sixth  80  and seventh  82  shield elements each include 5% borated polyethylene material. The eighth  84  and ninth  86  shield elements include standard polyethylene material. The first through sixth shield elements  70 , 72 , 74 , 76 , 78 , 80  are located within the opening  58  (see  FIG. 3   b ) of the outer box  34  when the layered shield  30  is assembled. The bottom outer surface  54  of the outer box  34  is positioned between the sixth  80  and seventh  82  shield elements when the layered shield  30  is assembled and serves as an additional shield element. The seventh  82 , eighth  84  and ninth  86  elements are fastened to the bottom outer surface  54 . Feet members  88  are attached to the ninth shield element  86  which serve to support the layered shield  30 . 
     The upper set  62  includes a second set of the first through ninth shield elements  70 , 72 , 74 , 76 , 78 , 80 , 82 , 84 , 86 . The first shield element  70  of the upper set  62  is oriented horizontally above and closest to the top inner surface  40  of the inner box  32  followed by the remaining shield elements  72 , 74 , 76 , 78 , 80 , 82 , 84 , 86  which are each oriented horizontally to form a horizontally stacked, layered configuration above the top inner surface  40  to thus form a shield arrangement that mirrors the lower set  60 . The first through sixth shield elements  70 , 72 , 74 , 76 , 78 , 80  are located within the opening  58  of the outer box  34  when the layered shield  30  is assembled. The top outer surface  52  of the outer box  34  is positioned between the sixth  80  and seventh  82  shield elements when the layered shield  30  is assembled and serves as an additional shield element. The seventh  82 , eighth  84  and ninth  86  elements are fastened to the top outer surface  52 . 
     The right set  64  includes first  90 , second  92 , third  94 , fourth  96 , fifth  98 , sixth  100 , seventh  102 , eighth  104  and ninth  106  vertical shield elements. The first shield element  90  is oriented vertically adjacent and closest to the right inner surface  36  of the inner box  32  followed by the remaining shield elements  92 , 94 , 96 , 98 , 100 , 102 , 104 , 106  which are each oriented vertically to form a vertically stacked, layered configuration to the right of the right inner surface  36 . The first  90  and second  92  shield elements each include 30% borated polyethylene. The third  94 , fourth  96  and fifth  98  shield elements include lead, 5% borated polyethylene and lead material, respectively, arranged in an alternating configuration. The sixth  100  and seventh  102  shield elements each include 5% borated polyethylene material. The eighth  104  and ninth  106  shield elements each include standard polyethylene material. The first through sixth shield elements  90 , 92 , 94 , 96 , 98 , 100  are located within the opening  58  of the outer box  34  when the layered shield  34  is assembled. The right outer surface  48  of the outer box  34  is positioned between the sixth  100  and seventh  102  shield elements when the layered shield  30  is assembled and serves as an additional shield element. The seventh  102 , eighth  104  and ninth  106  elements are fastened to the right outer surface  48 . 
     The left set  66  includes a second set of the first through ninth shield elements  90 , 92 , 94 , 96 , 98 , 100 , 102 , 104 , 106 . The first shield element  90  is oriented vertically adjacent and closest to the left inner surface  38  of the inner box  32  followed by the remaining shield elements  92 , 94 , 96 , 98 , 100 , 102 , 104 , 106  which are each oriented vertically to form a vertically stacked, layered configuration to the left of the left inner surface  38  to thus form a shield arrangement that minors the right set  64 . The first through sixth shield elements  90 , 92 , 94 , 96 , 98 , 100  are located within the opening  58  of the outer box  34 . The left outer surface  50  of the outer box  34  is positioned between the sixth  100  and seventh  102  shield elements when the layered shield  30  is assembled and serves as an additional shield element. The seventh  102 , eighth  104  and ninth  106  elements are fastened to the left outer surface  50 . 
     The rear set  68  includes a third set of the first through ninth shield elements  90 , 92 , 94 , 96 , 98 , 100 , 102 , 104 , 106 . The first shield element  90  is oriented vertically adjacent and closest to the inner back surface  44  of the inner box  32  followed by the remaining shield elements  92 , 94 , 96 , 98 , 100 , 102 , 104 , 106  which are each oriented vertically to form a layered configuration to the rear of the inner back surface  44 . The first through sixth shield elements  90 , 92 , 94 , 96 , 98 , 100  are located within the opening  58  of the outer box  34  when the layered shield  30  is assembled. The outer back surface  56  of the outer box  34  is positioned between the sixth  100  and seventh  102  shield elements when the layered shield  30  is assembled and serves as an additional shield element. The seventh  102 , eighth  104  and ninth  106  elements are fastened to the outer back surface  56 . In addition, shims  101  are located between the sixth shield element  100  and the outer back surface  56  to provide support to the first through sixth shield elements  90 , 92 , 94 , 96 , 98 , 100 . 
     A front edge  108  of each of the first through ninth shield elements  70 , 72 , 74 , 76 , 78 , 80 , 82 , 84 , 86  in the lower set  60 , the top  40  and bottom  42  inner surfaces and the top  52  and bottom  54  outer surfaces has a curved shape for accommodating the curvature of a cyclotron. In addition, several front edges  108  of the first through ninth shield elements  70 , 72 , 74 , 76 , 78 , 80 , 82 , 84 , 86 , the bottom  42  inner surface, and the bottom  54  outer surface has a notch  110 . The layered shield  30  includes the cover  112  that includes a cover opening  114  for providing access to the recess  46 . The cover  112  is curved to correspond to the shape of the front edges  108  in order to also accommodate the curvature of a cyclotron. Further, the cover  112  includes a cutout  116  which corresponds to the notches  110  thus forming a channel  118  when the layered shield  30  is assembled. The cover  112  may be fabricated from aluminum or other suitable material. The channel  118  serves to accommodate and route cables and wires for the cyclotron. Although the lower  60 , upper  62 , right  64 , left  66  and rear  68  sets in the current embodiment are described as having nine shield elements, it is understood that additional or fewer shield elements may be used. 
     Referring to  FIG. 5 , the layered shield  30  is shown assembled in a movable shield  120 . The layered shield  30  is located in the movable shield  120  so that the recess  46  receives a target portion of a cyclotron. The layered shield  30  may be assembled in both moveable shields of a cyclotron system. Referring to  FIG. 6 , a target portion  122  of a cyclotron  124  is shown located within the recess  46  of the layered shield  30 . In accordance with the invention, positioning of the target portion  122  within the recess  46  enables the layered shield  30  to provide a primary or substantial portion of the shielding necessary to protect personnel in the vicinity of the cyclotron  124  against unsafe levels of radiation. The configuration described in relation to  FIG. 4  is sufficient for shielding against radiation emitted during operation of a cyclotron having an energy level of approximately 11 MeV such as a Siemens Eclipse™ cyclotron. 
     Each shield element  70 , 72 , 74 , 76 , 78 , 80 , 82 , 84 , 86  (i.e.  70 - 86 ) and  90 , 92 , 94 , 96 , 98 , 100 , 102 , 104 , 106  (i.e.  90 - 106 ) of the lower  60 , upper  62 , right  64 , left  66  and rear  68  sets serves as a shield layer. Due the horizontal and vertical stacking arrangement, each shield element  70 - 86  and  90 - 106  may be removed and replaced with another shield element  70 - 86  or  90 - 106  or with other shield elements having alternative shielding materials or alternative shielding characteristics. The removability of the shield elements  70 - 86  and  90 - 106  enables upgrading or reconfiguring of the layered shield  30  so as to provide sufficient shielding appropriate for cyclotrons having higher or lower radiation energies. In addition, a size of the shield elements  70 - 86  and  90 - 106  may be changed to accommodate different size inner  32  and outer  34  box structures that may be used when reconfiguring the layered shield  30 . This allows arrangements that are specifically designed for the radiation emitted from specific cyclotron configurations. In addition, providing interchangeable shielding elements  70 - 86  and  90 - 106  enables minimization of the amount of lead that is used thus minimizing the environmental impact of the shielding. Further, the order of the shield elements  70 - 86  and  90 - 106  may be rearranged as desired. 
     The removability of the shielding elements  70 - 86  and  90 - 106  also enables dismantling of the assembly and stacking is smaller spaces. Conventional shielding is bulky and requires demolition to process the shielding to a state that is disposable. The layered shield  30  may be disassembled and only the portions that are susceptible to radiation are required to be disposed. In an alternate embodiment, a frame having slots may be used to hold the shield elements  70 - 86  and  90 - 106  wherein the shield elements  70 - 86  and  90 - 106  are slid into and out of a corresponding slot. 
     While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations.