Abstract:
A shielding ( 11 ) for reducing the amount of radiation passing through the shielding comprises a first part ( 111 ) and a second part ( 112 ), wherein the first part is arranged for being withdrawn from the second part and wherein said first and second parts comprise abutments. At least one pair of corresponding abutments of said first and second parts has a transverse section which is curvilinearly shaped along a portion of at least half of said transverse section.

Description:
FIELD OF THE INVENTION 
     The present invention is related to a shielding for ionizing radiation. More particularly, the present invention is related to a shielding with at least one movable part, said part arranged for opening said shielding. 
     STATE OF THE ART 
     Radiation emitting sources, such as particle accelerators, targets, radioactive sources or wastes, emit unwanted ionising radiations, such as protons, neutrons, electrons and photons. In order to protect personnel from irradiation diseases, these radiation sources are generally placed in a shielding. The shielding must absorb the majority of the emitted radiations, such that transmission through the shield is below a threshold level specified by law or by company specifications. 
     A basic solution for shielding is achieved by encapsulating said radiation sources, e.g. a cyclotron, into walls of concrete and/or other compounds. Such a configuration is known from document GB 2358415. The document discloses the use of building blocks to construct shielding walls. These blocks are provided with male and female-type sides that snugly fit into each other. The male-type sides have a tongue, bordered by coplanar shoulders. The shoulders occupy at least 20% of the total width of the blocks. However, this solution has a drawback as follows: when the installation of such walls around a radiation source is completed, the radiation source is no more accessible, unless one or more blocks are removed from the walls. This operation can be relatively long and complex due to blocks weight or numbers. 
     Another solution is described in document US 2005/0218347, wherein one or more doors are provided for selectively access a targeting assembly of a particle accelerator. The side of the doors, which abut in the wall, have a staircase shape to reduce the transmission of radiation. However, additional shielding is often required in order to reduce transmission through the door clearances. 
     AIMS OF THE INVENTION 
     The present invention aims to provide a shielding comprising at least one part that can be opened and closed, which is more efficient than the prior art shieldings in preventing or limiting the entrance of radiation into the shielding and/or the exit of radiation from said shielding. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a shielding for reducing the amount of radiation passing through the shielding. The shielding comprises a first part and a second part, wherein the first part is arranged for being withdrawn from the second part and wherein said first and second parts comprise abutments. At least one pair of corresponding abutments of said first and second parts has a transverse section which is curvilinearly shaped along a portion of at least a part and preferably half of said transverse section. 
     In normal operating conditions the first and second part of the shielding are positioned in face of each other and may contact each other. When a person wants to access what is covered by the shielding, at least the first part is arranged for being withdrawn from the second part, in order to open the shielding and gaining access to what is covered by the shielding. 
     The term curvilinear in the present invention has the meaning of a line having in all its points a finite radius of curvature, wherein the term finite does not comprise zero. The curvilinearly shaped portion of the transverse section may extend along 50, 60, 70, 80, 90, or even 100 percent of the length of said transverse section. Preferably, the curvilinear section may have the shape of a C or an S. Other curvilinear sections may equally be employed, as long as the totality of curvilinear portions is substantially larger than the totality of rectilinear portions. More preferably, the curvilinear section may have a constant radius of curvature. Preferably, the curvilinear portions of corresponding abutments match. Preferably, at least a portion of said transverse section shows a value for the inverse of the radius of curvature different from zero. 
     The present invention is useful for shielding radiation produced by a radiation source, such as a particle accelerator, a target, a radioactive source or radioactive waste. 
     Advantageously, the radiation source is a cyclotron. 
     Advantageously, the shielding comprises a shell that can be filled with radiation absorbing material. 
     More advantageously, said shell comprises an outer region that can be filled with a high Z compound and an inner region that can be filled with a low Z compound. 
     Preferably, said high Z compound comprises lead or iron. 
     Preferably, said low Z compound comprises a polyethylene and/or a paraffin compound. 
     Preferably, when the invention is used for shielding radiation produced by a cyclotron comprising a target, the cyclotron comprises an additional high Z material shield in front of said target. 
     Advantageously, the shielding comprises wheels for displacing said first part. More advantageously, the shielding comprises wheels for also displacing said second part. 
     Advantageously, the shielding comprises a lifting mechanism for said wheels. 
     In an embodiment of the present invention, the second part is a container for limiting the exit of radiations from the radiation source to the outside. Such a container could be used, for example, for transporting and/or shielding radioactive sources, radioactive wastes, or the like. 
     In another, more preferred embodiment of the present invention, said first part is a lid or a door adapted for fitting in an opening of said second part. Without any limitation, said opening could refer to a ceiling wall of a chamber, or a shielding vault door. 
     According to a second aspect of the present invention, there is provided a method for reducing the amount of radiation passing through a shielding, the method comprising the steps of: providing a shielding comprising a first part and a second part, said first part and said second part comprising abutments and shaping corresponding abutments of the first and second part curvilinearly along a major portion of a transverse section of said abutments. The method prevents or limits the entrance of radiation into and/or the exit of radiation out of a shielding. 
     Preferably, the method, according to the invention, comprises the step of providing wheels for moving said first part and said second part. 
     Optionally, the method, according to the invention, comprises the step of providing a lifting mechanism for lifting up and down said first part and said second part such that they respectively move or rest. 
     Preferably, the method according to the invention comprises the step of providing a shell filled with radiation absorbing material. 
     More preferably, according to the second aspect of the invention, said shell comprises an outer region that can be filled with a high Z compound and an inner region that can be filled with a low Z compound. 
     Advantageously, according to the second aspect of the invention, said high Z compound comprises lead or iron. 
     Advantageously, according to the second aspect of the invention, said low Z compound comprises a polyethylene and/or a paraffin compound. 
     Preferably, according to the second aspect of the invention, said radiation is produced by a radiation source. 
     More preferably, according to the second aspect of the invention, said radiation source is a cyclotron. 
     Advantageously, the method according to the invention, wherein said cyclotron comprises a target, comprises the step of providing an additional high Z material shield in front of said target. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a cyclotron encapsulated in a shielding according to the invention. A cross-sectional view of the shielding is provided in  FIG. 1 . 
         FIG. 2  represents a cross-sectional view C-C as defined in  FIG. 1 . The cyclotron is not sectioned. 
         FIG. 3  represents a cross-sectional view B-B as defined in  FIG. 1 . The cyclotron is not sectioned. 
         FIG. 4  represents the shielding opened. 
         FIG. 5  represents the shielding closed. 
         FIG. 6  represents an S-shaped clearance. 
         FIG. 7  represents a lateral view of the shielding in closed state. 
         FIG. 8  represents a lateral view of the shielding in opened state. 
         FIG. 9  represents a top view of the shielding in opened state. 
         FIG. 10  represents a schematic cross-section of a shielding without any clearance used for Monte Carlo simulations. 
         FIG. 11  represents a schematic cross-section of a shielding with a rectilinear clearance  32   a  used for Monte Carlo simulations. 
         FIG. 12  represents a schematic cross-section of a shielding with a staircase rectilinear clearance  32   b  used for Monte Carlo simulations. 
         FIG. 13  represents a schematic cross-section of a shielding with a C-shaped clearance  32   c  used for Monte Carlo simulations. 
         FIG. 14  represents Monte Carlo simulated transmission doses for the configuration of  FIG. 10 . 
         FIG. 15  represents Monte Carlo simulated transmission doses for the configuration of  FIG. 11 . 
         FIG. 16  represents Monte Carlo simulated transmission doses for the configuration of  FIG. 12 . 
         FIG. 17  represents Monte Carlo simulated transmission doses for the configuration of  FIG. 13 . 
         FIG. 18   a  represents a preferred embodiment according to the invention. 
         FIG. 18   b  represents another preferred embodiment according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a radiation source  10 , in the following embodied by a cyclotron, enclosed in a shielding  11 . The cyclotron  10  rests on feet  12  mounted on a concrete floor  13 . Pipes that lead to the cyclotron may be embedded in the floor  13 . The floor level  131  on which the cyclotron is mounted is at a lower level with reference to the level  132  on which the shielding  11  rests. Shielding  11  comprises a shell  113 , preferably made out of steel. This shell may be filled with radiation absorbing materials. Currently, suitable materials are e.g. lead, iron, polyethylene or a paraffin compound. Lead is provided in an outer region  114  of the shielding  11  in order to stop primary and secondary gamma rays. The inner region  115  of the shielding  11  may comprise a neutron absorbing material such as polyethylene or a paraffin compound. Preferably, an additional lead shield  116  is provided in front of each target of the cyclotron in order to slow or stop photons emitted from the source. Such an additional lead filter  116  permits to reduce the thickness of the shielding  11  at these locations for a specified required transmission dose. 
     The shielding  11  comprises two parts, a male part  111 , and a female part  112 , both of which are provided with wheels  14 . Hence, male part  111  and female part  112  are movable in order to open and close the shielding  11 .  FIG. 4  shows the shielding  11  in opened state. In this state, the cyclotron can be accessed. 
     Preferably, each of moving parts  111  and  112  rest on three wheels. As the mass of such a shielding may exceed ten tons, wheels are designed such as to be able to bear the heavy load. Wheels  14  slide on rail tracks  15 . A clearance between the floor and the moving shielding parts  111  and  112  has to be provided for said parts to move. In a closed configuration, such as depicted in  FIG. 5 , this clearance would constitute a bottom Leakage path for the radiation emitted by the cyclotron. 
     A method of reducing the transmission of radiation along this leakage path comprises the step of providing a lifting mechanism for the wheels. When the moving parts  111  and  112  are to be moved, this mechanism lifts the parts up so that they may travel. When the shielding is closed, the mechanism may lift said moving parts down such that they rest on the floor without any clearance. This method is, however, cumbersome, particularly in view of the large mass of the shielding. Moreover, deformation in the structure of the shielding, due to the large mass, may cause the clearance not to vanish everywhere. 
     An alternative method comprises the step of placing the cyclotron on a lower floor level  131  with respect to the level  132  on which the moving parts of the shielding are placed, as shown in  FIG. 1 . The clearance  133  between shielding  11  and floor  13  can then be sealed by providing a strip  16  of radiation absorbing material at the inside of the shielding. In this way, radiation that enters the clearance must first pass the absorbing material before entering the clearance. Strip  16  covers the inlet of clearance  133  and may consist of polyethylene or paraffin compounds. An additional step may be to further reduce the transmission of radiation along the clearance by providing a strip  17  of absorbing material at the underside of moving parts  111  and  112 . 
     When the shielding  11  is closed, as depicted in  FIGS. 1 ,  2 ,  3  and  5 , clearances occur wherever one of the moving parts  111  and  112  abuts against the other. In the particular embodiment as presently outlined and referring to  FIG. 4 , this occurs in between lateral abutments  18  and  19  (i.e. the points where two structures or objects meet) of respectively male part  111  and female part  112 , and in between the upper abutments  20  and  21 , respectively of the male and female part. In the more general case, a clearance (i.e. the amount of clear space or distance between two objects) will occur between any two moving parts and between any moving and fixed part of the shielding. 
     Clearances have to be kept as small as possible, but can not be avoided. They constitute a mechanical tolerance limit. In fact, the large mass of the shielding would deform the shielding structures, and a clearance has to be specified in order for one part to abut as snugly as possible against another part. However, the occurrence of these clearances notwithstanding, the transmission of radiation through such clearances can be significantly reduced by an appropriate design of the abutments  18 ,  19 ,  20  and  21  and without the need of providing additional shielding to cover the clearances. 
     Abutments  18  and  20  are of a male type and are arranged for fitting into the female type abutments  19  and  21 . The transverse section of these abutments is curvilinearly shaped along a substantial portion of the section. Referring to  FIG. 3 , abutments  18  and  19  are entirely curvilinearly shaped. The transverse section of both abutments  18  and  19  has a constant radius. The radius of abutment  19  is slightly larger than the radius of abutment  18  in order to keep the design clearance constant. Referring to  FIG. 1 , upper abutments  20  and  21 , feature a transverse section which is curvilinearly shaped along a substantial portion of the section. 
       FIGS. 10 to 17  present Monte Carlo simulation results of the transmission of radiation for different clearance configurations.  FIG. 10  represents the case of a totally closed shielding, with no clearances.  FIG. 11  represents the case of a shielding with one rectilinear clearance  32   a .  FIG. 12  represents the case of a shielding with a stair-cased clearance  32   b .  FIG. 13  represents the case of a shielding with a C-shaped clearance  32   c . At a number of regularly spaced locations, within the shielding and along the outside of the shielding, the incident radiation, emitted from the target  31 , was measured by a virtual dosimeter in terms of neutron and photon doses. These locations are indicated by hollow circles on  FIGS. 10-13 . 
     The fact that the clearance follows a curvilinear path along a substantial portion of its length, causes the radiation (photons, neutrons, . . . ) travelling through the clearance to be reflected a much larger number of times with reference to a clearance having large rectilinear portions. As only a fraction of the incident radiation is reflected, the former kind of clearances provides a reduced transmission of radiation.  FIGS. 1 to 5  present abutments featuring an essentially C-shaped transverse section. Other curvilinear sections are equally effective, as long as the totality of curvilinear portions is substantially larger than the totality of the rectilinear ones.  FIG. 6  depicts, for example, an S-shaped clearance. 
     Furthermore, referring to  FIG. 13 , one can observe that the total thickness of the shielding that radiations encounter, when travelling through the shielding, is approximately the thickness of the shielding minus two times the thickness of the gap in the clearance  32   c , independently from the direction of the radiations emitted from the target  31 . By contrast, referring to  FIG. 11  or  12 , one can observe that said total thickness value depends somehow on the direction of the radiations. In the latter case, one can also easily realize that some directions are privileged since they make the total thickness value met by radiations much lower than the one according to the case of  FIG. 13 . 
     The results of these Monte Carlo simulations for the cases depicted in  FIGS. 10-13  are presented in  FIGS. 14-17 .  FIG. 14  presents the simulated incident doses for the case of  FIG. 10 . The graphs on the left hand show the doses along the rectilinear path in the shielding. On the horizontal axis, 0 cm refers to the inner border of the shielding, and 60 cm to the outer border. The dashed vertical line marks the limit between the polyethylene or paraffin compound and the lead or iron. The doses are normalised with reference to the first calculated value. The graphs on the right hand show the doses along an arc (virtual dosimeter)  30 , outside the shielding. On the horizontal axis, 0 cm refers to the centre of the arc. The doses are normalised with reference to the first calculated value (leftmost value on the graphs). Likewise,  FIGS. 15-17  present simulation results for the cases depicted respectively in  FIGS. 11-13 . For the case of the rectilinear clearance of  FIG. 11 , a very large dose is transmitted through the clearance  32   a , as shown in  FIG. 15 . For the case of the stair-cased clearance of  FIG. 12 , at the arc centre a peak value in relative dose is 50 for neutrons and 20 for photons, as shown in  FIG. 16 . These peak values are significantly reduced by the use of the C-shaped clearance of  FIG. 13 , as shown in  FIG. 17 . These peak values reduce to 2.3 and 2.2 respectively. The location of occurrence of the peaks is also displaced along the arc (not in the centre any more). Comparing the results of  FIG. 17  with the results of  FIG. 14  it is clear that the values with the C-shaped clearance are of the same order of magnitude as the values for the case of a totally closed shielding. Additional shielding is therefore not necessary. 
     In a preferred embodiment according to the present invention, the shielding  11  comprises a steel shell  113 . The total thickness of the shielding is 850 mm around the cyclotron and 600 mm above it. The outer diameter of the shielding is 3.3 m. The gap between cyclotron and shielding in closed state is about 5 cm. Abutments in this preferred embodiment have a transverse section essentially of C or S shape, and abut against each other, each of said abutments having a complementary shape with respect to another. 
     In another preferred embodiment according to the present invention, a part  182 , as shown in  FIG. 18   a , is a container. When the part  181  and the part  182  are in a closed configuration, the C-shape of the abutments  18  and  19  limits the exit of radiations from the radiation source  10  to the outside. Such a container could be used, for example, for transporting and/or shielding a radioactive source, radioactive wastes, or the like. 
     In another preferred embodiment according to the present invention, represented in  FIG. 18   b , a part  184 , having C-shaped abutments  19 , has an opening  9  which can be closed with the moveable part  183 , also having C-shaped abutments  18 . Without any limitation, the part  184  can be a ceiling wall of a chamber, or simply a shielding vault door.