Abstract:
A drum-type volume source calibration phantom is provided, which comprises a drum-type container; a plurality of plate groups stacking up inside the drum-type container, at least one slab of radioactive source, each of which is disposed between the adjacent plate groups and comprises a plurality of radionuclides. The present invention further provides a calibration method that starts by the step of providing a radioactivity test for each drum-type volume calibration phantom. Then, a calibration relationship of density vs. counting efficiency corresponding to the several different drum-type volume source calibration phantoms is performed in a waste curie monitor. Finally, a characteristic of photonic energy dependency is measured for a modification factor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is related to a calibration phantom. Especially it refers to a drum-type source calibration phantom with different densities to derive the calibration curve for density and counting efficiency. According to the counting efficiency from the multi-density source calibration phantom corresponding to the density of the sample of nuclear waste, it is a source calibration phantom to derive accurate measurement results when it measures total Gamma activity or specific activity for different nuclear waste materials. 
     2. Description of the Prior Art 
     Until now, the equipment for the total gamma activity measurement for bulk nuclear waste with extremely low activity is Waste Curie Monitor, a large-area plastic scintillation detector assembly. Its advantages include high radiation sensitivity, high counting efficiency, short counting time, unlimited waste volume and displayable weight for waste samples. 
     However, the calibration method for traditional activity monitor has the following shortcomings: (1) underestimate or overestimate for activity because usually it only considers weight but not material density and causes radiation self-absorption effect; (2) counting efficiency calibration is inapplicable because usually it uses single material in monitor shielding to establish density efficiency and ignores that the sample is not single component material; (3) sample measurement position is different because usually the sample is placed under the shielding, which is different from the geometric center for efficiency calibration; (4) sample volume is different; usually volume is not limited, so different distance to scintillation detector during efficiency calibration causes errors in activity analysis; it does not meet the requirement for accuracy in waste activity analysis by radiation safety administrator; (5) there is no correction for the calculation for the radiation energy for various radionuclides in radioactive waste and the Gamma activity for multiple radionuclides; it causes errors in total activity measurement. 
     In recent years, researchers further develop new calibration methods to replace the original method for point source efficiency at the geometric center of the activity monitor. They measure total Gamma activity for the wastes to determine classification of the wastes and non-radioactive wastes. Current efficiency calibration methods for plastic scintillation detector include: (1) US Themo-Eberline uses transmission factor to correct the self-absorption effect for different standard mass; the formula is TF=net counting with shielding source/net counting without shielding source; generally TF≦1.0; assuming no shielding source in compensation air for calibration efficiency in geometric center; let the transmission factor parameter for the established water phantom calibration efficiency be 1; enter weight to mass parameter (every 10 kg in one unit); different sample weight will be correlated to that with transmission factor=1 and obtain total Gamma activity after correction; (2) German RADOS uses single-material metal plate assembly for multi-density calibration efficiency; (3) Japan Nuclear Energy Safety Organization&#39;s metal pipe and metal plate assembly for calibration efficiency for multiple radionuclides; (4) US NE Technology uses assembly of multi-radionuclide point source and single-material Brazil logs (density=1) for multi-weight (0˜60 Kg) calibration efficiency. However, current correction methods still only consider approximate weight and geometry without sufficient correction for self-absorption effect and various factors in mass and energy reactions, so they fail to obtain accurate total Gamma activity. 
     In summary, it is necessary to have a drum-type volume source calibration phantom and measurement and calibration methods to solve the issues with traditional methods. 
     SUMMARY OF THE INVENTION 
     The invention provides a drum-type volume source calibration phantom, which uses drum-type volume source calibration phantom with different densities to measure the total Gamma activity or specific activity for different radioactive waste materials. 
     The invention provides a drum-type volume source calibration phantom, which uses drum-type container with same dimensions and volume to the drum-type calibration phantom to obtain accuracy in sample activity measurement under the same conditions of calibration efficiency. 
     In one embodiment, the invention provides a drum-type volume source calibration phantom in a detector with accommodating space that has several radioactivity detectors. The drum-type volume source calibration phantom includes a drum-type container; several plate assemblies, stacking inside the container; and at least one source plate, located between adjacent plate assemblies, with several sources on each source plate. 
     The above and other objects, features and advantages of the present invention will be apparent from the following detailed description taken with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration for the radioactive waste activity detector and the drum-type volume source calibration phantom for the invention. 
         FIG. 2A  is a three-dimensional diagram for the drum-type container of the invention. 
         FIG. 2B  is an illustration for stacking of plate assemblies and the source plates. 
         FIG. 3A  is top view for source plate in a preferred embodiment of the invention. 
         FIG. 3B  is cross sectional view for the source plate. 
         FIG. 4  is cross sectional view for the source calibration phantom of nine pieces of plate. 
         FIG. 5  is an illustration of various numbers of source plates horizontally arranged in a body calibration phantom. 
         FIG. 6  is the counting efficiency diagram for the calibration phantom at density 1.1 g/cm 3  and 2.0 g/cm 3  to have different number of large-area sources. 
         FIG. 7  is the testing diagram for activity accuracy for planar point sources. 
         FIG. 8  is the flow diagram for the calibration method for the drum-type volume source calibration phantom in the invention. 
         FIG. 9  is the layout of 44 sources on the source plate. 
       From  FIG. 10A  to  FIG. 10D  there are testing curves of activity uniformity for different planar point sources. 
         FIG. 11  is the correlation between the drum-type volume source calibration phantom and the radionuclide counting efficiency. 
         FIG. 12  is the correlation curve between the density and the ratio of counting efficiency relative to radionuclide  137 C. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Please refer to  FIG. 1  for the activity detector for radioactive wastes and the diagram for the drum-type volume source calibration phantom. For the radioactive waste activity detector  1 , there is a shielding  10 , which is a hexahedron assembly that is made of lead of the same thickness. Inside the shielding  10  there is a measurement space  100 , which inner wall has 10 units of the same large-area radioactivity detectors  11 . In the embodiment the radioactivity detector  11  is a plastic scintillation detector. In the measurement space  100  there is a weight meter  13  to measure the object weight. The activity detector has a microcomputer processor  12  that uses the built-in calculation programs and calibration parameters for functions like sample counting, radiation total activity (Bq) for the background radiation or the specific activity (Bq/g), efficiency calibration and the minimum equipment detectable activity as well as printing and displaying analytical results. 
     In the measurement space  100  there is a drum-type volume source calibration phantom  2  with consistent activity. Please refer to  FIG. 2A . The periphery for the drum-type volume source calibration phantom  2  is a drum-type container  20 . The drum-type container  20  has an inner space  21 . The drum-type container can be a 55-gallon container, but not be limited to this. Presently, 55-gallon drum-type container is the mainstream container for waste storage. The embodiment uses 55-gallon for explanation. Please refer to  FIG. 2B . The figure is an illustration of stacking of the plates and the source plates. The inner space  21  can accommodate stacking of a plurality of homogeneous plate assembly  22  and source plates  23 . The plate  22  materials can be metal or non-metal. If it is metal, it can be steel. If it is non-metal, it can be paper, wood, plastics, cement or glass et al., but not limited to these. 
     In the embodiment, different material is cut to a circular piece of homogeneous planar material of 40 cm diameter and 1 cm thickness, and then all pieces stack to form a homogeneous plate assembly  22 . It is then placed in a drum-type container of 56 cm diameter and 86 cm height, with weight capacity of 30 kg˜490 kg. With material weight and container volume (200 cm), the material average density for calibration phantom can be obtained, as examples, for paper board (density 0.15 g/cm 3 ), wood board (density 0.55 g/cm 3 ), plastics (density 1.13 g/cm 3 ), cement (density 1.80 g/cm 3 ) and glass (density 2.5 g/cm 3 ), as shown in Table 1. 
     As shown in  FIG. 2B , there is a source plate  23  (only one shown in the figure for illustration) between adjacent plate assemblies  22 . Please refer to  FIGS. 3A and 3B .  FIG. 3A  has a top view diagram for a preferred embodiment of the source plate in the invention.  FIG. 3B  has a crossectional view for the source plate. On the leak-proof filter  231  in 40 cm diameter  44  drops of 0.2 cc homogeneous liquid sources  230  are placed. Through a top cover  232  and a bottom cover  233  on the leak-proof filter  231 , a large area of circular homogeneous source plate  23  is formed. Every drop of liquid source expands to a circle in diameter less than 5 cm. The source is a γ source. The γ source can be  57 cobalt,  137 cesium,  54 manganese, or  60 cobalt or combination of either one. In the embodiment, each piece of the source has total activity of 49 kBq ( 57 cobalt), 35 kBq ( 137 cesium), 29 kBq ( 54 manganese) and 29 kBq ( 60 cobalt) respectively. With four radionuclides,  57 Co,  137 Cs,  54 Mn and  60 Co, drum-type volume source calibration phantoms with even activity and 4 different energies and previously mentioned 5 different densities can be formed. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Specification for Calibration Phantom of Different Materials 
               
             
          
           
               
                 Materials 
                 Volume (cm 3 ) 
                 Weight (Kg) 
                 Density (g/cm 3 ) 
               
               
                   
               
             
          
           
               
                 Paper board 
                 200,000 
                 30 
                 0.15 
               
               
                 Wood board 
                 200,000 
                 101 
                 0.49 
               
               
                 Plastics (PVC) 
                 200,000 
                 233 
                 1.13 
               
               
                 Cement 
                 200,000 
                 422 
                 1.97 
               
               
                 Glass 
                 200,000 
                 491 
                 2.37 
               
               
                   
               
             
          
         
       
     
     In a preferred embodiment, 9 pieces of large-area planar source plates  23  of each of the radionuclides,  57 Co,  137 Cs,  54 Mn and  60 Co, are horizontally placed in equal distance, 8.7 cm approximately, in five drum-type volume source calibration phantoms  2  of different densities. The crossectional view for the 9-piece drum-type volume source calibration phantom is shown in  FIG. 4 . Each drum-type volume source calibration phantom  2  is placed in the radioactive waste activity detector  1  (as shown in  FIG. 1 ). The 9-piece planar source plate  23  is 8.7 cm from the ten scintillation detectors at top, bottom, left, right, front and back, comprising the drum-type volume source calibration phantoms with the uniform activity and different energies and different densities. 
     The following is to assess the performance of the drum-type volume source calibration phantom. The assessment includes reasonable number of the planar source for calibration phantom, activity uniformity, source accuracy, counting efficiency, and energy dependence et al. The assessment method and the result are in the following: 
     1. Reasonable Number of Volume Source 
     First one piece of the nine completed  60 Co large-area homogeneous source  23  (as shown in  FIG. 4 ) is horizontally placed in the center of the drum-type calibration phantom, followed by symmetrically placing the second piece in the same distance and the remaining pieces in sequence. In the center of the calibration phantom there is a planar source plate  23  as shown in  FIG. 5 .  FIG. 5  ( a ) is a one-piece planar source plate  23 ;  FIG. 5  ( b ) is a two-piece planar source plate  23 ;  FIG. 5  ( c ) is a three-piece planar source plate  23 ;  FIG. 5  ( d ) is a four-piece planar source plate. In the calibration phantoms of densities at 1.1 g/cm 3  and 2.0 g/cm 3  there obtain the counting efficiencies for different number of pieces of large-area sources. As shown in  FIG. 6 , they are 17.9%˜18.3% and 9.3%˜10.7%, respectively. The largest difference in efficiency and average efficiency among the nine large-area sources happens in the calibration phantoms of densities at 1.1 g/cm 3  and 2.0 g/cm 3 , and is 1.7% and 10.7%, respectively. In the fitting curve of large-area source counting efficiency, the efficiency does not increase with number of pieces. There is no clear trend of increase. When the number of pieces is between 3 and 7, the efficiency levels off. When the number of pieces of sources increases to 9, although the source strength gradually increases, the counting efficiency only increases by 2.0%. Thus, it is reasonable to use 9 large-area pieces of sources for calibration phantom. 
     2. Activity Accuracy 
     In the shielding of 10 cm thickness in the iron chamber, five measurements are conducted for the point sources for radionuclides  57 Co,  137  Cs,  54 Mn and  60 Co at 25 cm from the pure germanium detector that has 40% counting efficiency of the sodium iodine (thallium) detector. According to the primary standard in national and weight method, the activity for each point source is 1107Bq, 803Bq, 657Bq and 657Bq, respectively. The results for the point source measurement for the four radionuclides with pure germanium detector are compared to the activity of production point sources and the largest different is found less than 10%, as shown in  FIG. 7 . 
     Please refer to  FIG. 8  for the flow diagram for the calibration method for the source calibration phantom in the invention. The method 3 includes the following steps: first proceed with step  30  to provide a plurality of drum-type volume source calibration phantoms, and each drum-type volume source calibration phantom has different density, and the drum-type volume source calibration phantom has a drum-type container, a plurality of plate assemblies and a plurality of source plates, and each source plate has a plurality of sources. The step uses a drum-type volume source calibration phantom made of materials of different densities as in previously mentioned Table 1 and assembled by source plates of different sources. For example, Table 1 has five materials, and the source plate has three sources of choice, including  54 Mn,  60 Co and  137 Cs. In this way there are 15 calibration phantoms of different materials and different sources. 
     Step  31  is following to conduct activity measurement for each drum-type volume source calibration phantom. The following describes the method to measure activity uniformity. 
     3. Activity Uniformity 
     A drum-type sodium iodine (thallium) detector in 3-inch diameter and 3-inch thickness is placed in a lead can in 12 cm diameter, 6 cm inner diameter and 50 cm height. The 3 cm thick lead can block the adjacent radioactive interference from background and the planar source. 5 cm from the detector surface, there place circular planar sources. In total, there are 44 point sources of circle in 3 cm diameter (as shown in  FIG. 9 ). The result of the average counting rate for the large-area sources for  57 Co,  137 Cs,  54 Mn and  60 Co are shown in the figure, and are 27603 cps, 4992 cps, 6082 cps and 12703 cps, respectively. The respective counting rate for the four radionuclides is larger than the background counting rate by seven times, with standard deviation of 2.3%, 4.4%, 4.0% and 10.3%, respectively. The point source counting rate in 44 circular points for the four radionuclides and the average are different by 11.1%, 13.2%, 8.6% and 20.0%, respectively. Relatively larger different from the average happens to those adjacent point sources to the center of the circular planar source and is attributed to the radiation of the adjacent sources. The relatively small difference from the average happens to those adjacent point sources at the periphery of the circular planar source and is attributed to the relatively small radiation of the adjacent sources. The results of the abnormal values [(individual counting rate—average counting rate)/3 times of standard deviation] for the 44 counting rates for another four radionuclides,  57 Co,  137 Cs,  54 Mn and  60 Co, are 0.7%, 0.7%, 0.7% and 0.9%, respectively. The difference between the counting rates of the 44 circular point sources for each of the radionuclides,  57 Co,  137  Cs,  54 Mn and  60 Co, and the average is shown from  FIG. 10A  to  FIG. 10D . 
     As shown in  FIG. 8 , the following is step  32  to use a plurality of drum-type volume source calibration phantoms in a detector (as shown in  FIG. 1 ) to obtain correlation between density and counting efficiency. The following describes the method of counting efficiency. 
     4. Counting Efficiency 
     Five source calibration phantoms of materials of different densities are placed in a detector shielding for measurement. Among all, cement and glass have natural radioactive substances. Their counting rates are 2.63 times and 1.19 times of the background counting rate (950 cps) respectively. The efficiency measurement results for the drum-type volume source calibration phantoms for the radionuclides of  54  Mn,  60  Co and  137 Cs are shown in Table 2. The counting efficiency is mainly related to the mass attenuation coefficient of the material and energy. Since radionuclide  57 Co energy is low (122 keV and 157 keV), the counting rate at density 0.15 g/cm 3  and 0.49 g/cm 3  is slightly higher than background value. The counting efficiency is lower than 1%. 
     When the density is larger than 0.49 g/cm 3 , the  57 Co energy is blocked by the material, and there is no significant counting rate. Within five material density range 0.15 g/cm 3 ˜2.4 g/cm 3 , the counting efficiency is 20.7%˜2.1% for radionuclide  54 Mn, 41.6%˜6.9% for  60 Co and 14.2%˜1.1% for  137 Cs. The fitting curve for the relationship between the density of the drum-type volume source calibration phantom and the radionuclide counting efficiency is shown in  FIG. 11 . For the trend of variation in density and radionuclide counting efficiency, radionuclide  60 Co is more significant, radionuclide  54 Mn is the next, and while  137 Cs has the least variation. All of another three radionuclides have high efficiency at low density, and low efficiency at high density. It indicates high-density material has large self-shielding effect. At low density,  60 Co and  137 Cs have very different efficiency; at high density,  60 Co and  137 Cs have small difference in efficiency. 
     When the density is the same, counting efficiency is proportional to radionuclide energy. High energy means high counting efficiency. High energy is easy to penetrate material and captured by the detector. Thus, the counting efficiency for radionuclide  60 Co (1250 keV) is higher than that for low energy radionuclide  54 Mn (834 keV) and radionuclide  137 Cs(662 keV). On the other hand, when the density is 1.1 g/cm 3 ˜2.4 g/cm 3 , the counting efficiency for radionuclides have small variation. The fitting curve gradually levels off. Radionuclide  54 Mn and radionuclide  137 Cs have similar energy, so they have similar trend. 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Radionuclide Efficiency Measurement Results for Drum-type volume 
               
               
                 source calibration phantom 
               
             
          
           
               
                   
                 Density 
                 Efficiency (%) 
               
             
          
           
               
                   
                 (g/cm 3 ) 
                   60  Co 
                   54 Mn 
                   137 Cs 
               
               
                   
                   
               
             
          
           
               
                   
                 0.15 
                 41.6 
                 20.7 
                 14.2 
               
               
                   
                 0.49 
                 29.9 
                 13.0 
                 8.0 
               
               
                   
                 1.13 
                 18.3 
                 6.6 
                 3.7 
               
               
                   
                 1.97 
                 10.0 
                 3.4 
                 1.8 
               
               
                   
                 2.37 
                 6.9 
                 2.1 
                 1.1 
               
               
                   
                   
               
             
          
         
       
     
     Back to  FIG. 8 , proceeds step  33  to measure energy dependence for different sources, as correction factor. The following describes energy dependence. 
     5. Energy Dependence 
     The ratios of the counting efficiency for radionuclides  54 Mn and  60 Co to that for radionuclide  137 Cs are shown in Table 3. When the density is 0.15 g/cm 3 , they are 1.46 and 2.94 respectively; when the density is 0.49 g/cm 3 , they are 1.62 and 3.74 respectively; when the density is 1.13 g/cm 3 , they are 1.77 and 4.94; when the density is 1.97 g/cm 3 , they are 1.91 and 5.69; when the density is 2.37 g/cm 3 , they are 1.99 and 6.48. It indicates that the one with higher energy relative to  137 Cs has higher ratio in radionuclide counting efficiency.  60 Co has average energy two times higher than  137 Cs. It has very clear efficiency ratio to  137 Cs. At low density 0.15 g/cm 3 , it is about 3 times, while at high density 2.37 g/cm 3  it is about 6.5 times. The  54 Mn energy is slightly higher than  137 Cs energy. Thus, at different density, the ratio of radionuclide counting efficiency to  137 Cs is similar and does not vary by more than two times. The ratio of counting efficiency of individual radionuclide  54 Mn and  60 Co to radionuclide  137 Cs increases with density. At different density, the fitting curve for the counting efficiency of radionuclides relative to radionuclide  137 Cs is shown in  FIG. 12 . 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Ration of Counting Efficiency of Radionuclides to  137 Cs 
               
             
          
           
               
                 Density 
                 Relative Ratio 
                   
               
             
          
           
               
                 (g/cm 3 ) 
                   54 Mn/ 137 Cs 
                   60 Co/ 137 Cs 
               
               
                   
               
               
                 0.15 
                 1.46 
                 2.94 
               
               
                 0.49 
                 1.62 
                 3.74 
               
               
                 1.13 
                 1.77 
                 4.94 
               
               
                 1.97 
                 1.91 
                 5.69 
               
               
                 2.37 
                 1.99 
                 6.48 
               
               
                   
               
             
          
         
       
     
     The above examples are only preferred embodiments of the invention, but not to limit the scope of the invention. Those with equivalent changes and modification with the principles of the invention shall be considered within the scope of the invention.