Abstract:
By using a scintillation surveymeter with good calibration performance evaluation for a secondary standard radiation field, and a working standard part obtaining an ambient dose equivalent rate, in cooperation with a portable irradiator, and an irradiator lifter, a laser range finder and a laser locator of a relevant radiation source, in-situ calibration is capable of being performed on fixed, or large-scale, or continuous monitoring type radiation monitors to be calibrated stationed in nuclear power plants, nuclear medical departments, and other nuclear facility operating institutions. Moreover, a time-efficient and effective in-situ calibration method is further provided, which can be performed based upon a standard calibration field that is achieved using a portable  137 Cs radiation source. The in-situ calibration method is capable of saving the trouble of delivering large-scale monitors, or monitors difficult to move, or monitors requiring continuous monitoring to calibration laboratories for scheduled calibration.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to an in-situ calibration system and method for radiation monitors, and in particular, to a calibration system and method for ambient dose monitors and area monitors provided with an in-situ calibration mode. 
         [0003]    2. Related Art 
         [0004]    It is well-known that, radiation monitors need to be delivered to qualified laboratories for calibration. However, fixed outdoor type nuclear facility environmental monitors and indoor type nuclear facility control area monitors require continuous monitoring; therefore, when calibration is required, facility operators need to prepare at least one standby instrument to replace an instrument to be delivered for calibration, which virtually increases costs of the facility operators. In addition, monitors are delivered back and forth between workplaces and calibration laboratories, which is time-consuming and increases a risk of accidents. As a result, the aforesaid method results in inconvenience and annoyance to nuclear facility institutions during actual operation. 
       SUMMARY OF THE INVENTION 
       [0005]    An objective of the present invention is to provide a method of directly using a portable  137 Cs standard radiation field to perform in-situ calibration, to save laboratory calibration time and time for delivering monitors to be calibrated back and forth between institutions and laboratories. Another objective of the present invention lies in the result of an in-situ calibration dose rate, whose accuracy conforms to requirements of the American National Standards Institute ANSI-N323A (1997) on radiation protection instrumentation calibration and test specifications. Another objective of the present invention is that facility operators do not need standby area monitors or standby environmental monitors to replace monitors to be calibrated that are delivered to laboratories for calibration, and thus instruments may execute routine radiation protection monitoring tasks without interruption. Another objective of the present invention is that the aforesaid method applies to calibration of various environmental monitors, such as a high-pressure ionization chamber (HPIC) and a multi-Geiger-Mueller tube (Multi-GM tube), and various area monitors, such as an ionization chamber (ion-chamber), a scintillation, and a Geiger-Mueller tube (GM tube). Another objective of the present invention is to overcome the problem of fixed outdoor type environmental monitors and indoor type area monitors installed for various nuclear facilities, because these monitors execute routine, continuous and real-time radiation dose monitoring tasks of environments or workplaces, and are generally not easy or not allowed to be moved away from sites and delivered to laboratories for scheduled calibration; the present invention is capable of providing in-situ calibration to maintain instrument quality and dose measurement accuracy. Another objective of the present invention is to provide an in-situ calibration mode to obtain a calibration result on site, because ambient dose monitors are widely located, so as to be capable of saving time for delivering instruments to and from laboratories, reducing delivery risks, and dispensing with costs of standby environmental monitors. Besides a dose rate calibration accuracy requirement (&lt;20%) that conforms to standards, the present invention uses correction factors of a natural nuclide Ra-226 and an artificial nucleus Cs-137 for correction, so as to be capable of increasing measurement accuracy of an ambient dose equivalent rate H*(10). Another objective of the present invention is that, measurement results of ambient dose monitors are one of the bases for decision making in case of nuclear facility accidents, nuclear explosion fallout pollution precautions and emergency measures for residents to take refuge and evacuate; while area monitors serve as warning equipment when radiation environment conditions are abnormal, and provide one of the dose bases for keeping maintenance staff and workplaces safe and secure. As a result, ambient dose monitors, area monitors, and other dose monitors require scheduled calibration, to maintain accuracy and reliability of dose data. 
         [0006]    An embodiment of the present invention provides an in-situ calibration system for radiation monitors, comprising a monitor to be calibrated, where a surveymeter is placed inside, so as to monitor an environmental or area radiation dose; a portable irradiator, having a radiation source inside; a laser locator, close to the portable irradiator during locating, so as to confirm a central location of the surveymeter of the monitor to be calibrated; an irradiator lifter, where an upper end of the irradiator lifter is fixed to a pig of the portable irradiator, so as to set a distance and a height between the portable irradiator and the monitor to be calibrated, so that the radiation source provides the monitor to be calibrated with a radiation dose for detecting the radiation source corresponding to the distance and the height; a laser range finder, used for measuring a distance between the radiation source of the portable irradiator and the monitor to be calibrated; and a portable surveymeter of a site background, used for measuring a radiation quantity of the background during in-situ calibration. 
         [0007]    Another embodiment of the present invention provides an in-situ calibration method for radiation monitors, comprising steps of: providing a portable irradiator, a laser locator, a working standard part, an irradiator lifter, a laser range finder, a monitor to be calibrated and a portable surveymeter of a site background; setting performance evaluations of the portable irradiator, comprising the effect field, beam uniform, and back scattering; setting performance evaluations of the working standard part, comprising the accuracy, stability, response time, energy depend, and angle depend; establishing a working standard radiation field of the portable irradiator to be used during in-situ calibration, and generating a table of relative relationships between a distance and an ambient dose equivalent rate H*(10) through calculation; analyzing an uncertainty of an in-situ calibration system, comprising the photon standard radiation dose, beam uniform, measurement distance and measurement time, with an expand uncertain (k=2) of 6%; and verifying the secondary standard radiation field and the working standard radiation field. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention will become more fully understood from the detailed description given herein below for illustration only, and thus are not limitative of the present invention, and wherein: 
           [0009]      FIG. 1  shows instruments of an in-situ calibration system for radiation monitors; 
           [0010]      FIG. 2  shows an in-situ calibration method for radiation monitors; 
           [0011]      FIG. 3  shows the energy depend of a working standard part; 
           [0012]      FIG. 4  shows relative relationships between a distance of a working standard part and an ambient dose equivalent rate; 
           [0013]      FIG. 5  shows a beam uniform test of a portable irradiator in a horizontal direction; and 
           [0014]      FIG. 6  shows a beam uniform test of a portable irradiator in a vertical direction. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention uses a spherical ionization chamber that dates back to an air kerma-rate primary standard to establish a secondary standard radiation field, uses a scintillation surveymeter with good calibration performance evaluations in the secondary standard radiation field, and a working standard part obtaining the ambient dose equivalent rate, in cooperation with a portable irradiator and an irradiator lifter and a laser range finder of a relevant radiation source, and is capable of directly performing in-situ calibration on fixed, or large-scale, or continuous monitoring-type radiation monitors to be calibrated that are stationed in nuclear power plants, nuclear medical departments, and other nuclear facility operating institutions; results of a calibration dose rate indicate that the accuracy conforms to requirements of the American National Standards Institute (American National Standards Institute, ANSI) N323A specifications. 
         [0016]    An embodiment of the present invention, as shown in  FIG. 1 , provides an in-situ calibration system and method for radiation monitors, where the system provides a set of instruments relevant to the in-situ calibration, including: a portable irradiator  23 , a laser locator  24 , a working standard part  61 , an irradiator lifter  31 , a laser range finder  51 , a monitor  41  to be calibrated that includes an ambient dose monitor and an area monitor, and a portable surveymeter of a site background, as shown in table 1. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Instruments relevant to the in-situ calibration 
               
             
          
           
               
                 Instrument 
                 Specification 
               
               
                   
               
               
                 Portable irradiator 23 and laser 
                   137 Cs (2.1GBq); collimator diameter: 
               
               
                 locator 24 
                 6 cm 
               
               
                 Pig 21 
                 Diameter: 15 cm; height: 22 cm; 
               
               
                   
                 weight: 15 kg 
               
               
                 Working standard part 61 
                 Scintillation surveymeter (Atomtex- 
               
               
                   
                 AT1121) 
               
               
                 Radiation source lifter31 
                 Height: 1 m-2 m; an oil hydraulic pump 
               
               
                 Laser range finder 51 
                 Distance: 1 cm-10 m (Bosch DLE-50) 
               
               
                 Environmental monitor 
                 A thermometer or a hygrometer 
               
               
                   
                 (TES-1364) 
               
               
                 Portable surveymeter of site 
                 Portable radiation surveymeter 
               
               
                 background 
                 (Automess- AD4) 
               
               
                   
               
             
          
         
       
     
         [0017]    The portable irradiator  23 , in this embodiment, as shown in  FIG. 1 , has a radiation source inside, and the radiation source is cesium  137  ( 137 Cs). To avoid leak of the radiation source when there is no operation, the portable irradiator  23  is placed in the pig  21 , where the pig  21  has a hole at a front end, and the radiation source of the portable irradiator  23  radiates via the hole; when the portable irradiator  23  is not used, a removable lead plug  22  is used to seal the hole.  FIG. 5  shows a beam uniform test of the irradiator in a horizontal direction, where a horizontal axis indicates a horizontal distance, and a vertical axis indicates a distance relevant to a central point.  FIG. 6  shows a beam uniform test of the portable irradiator in a vertical direction, where a horizontal axis indicates a vertical distance, and a vertical axis indicates a distance relevant to the central point. The beams are concentrated near the central point. 
         [0018]    The monitor  41  to be calibrated, in this embodiment, as shown in  FIG. 1 , has a surveymeter placed inside, so as to monitor an ambient or area radiation dose; and includes an ambient dose monitor, disposed in an open environment, so as to monitor an ambient radiation dose, or an area monitor, disposed in an indoor area, so as to monitor an area radiation dose. The ambient dose monitor is a high-pressure ionization chamber monitor, or a multi-Geiger-Mueller tube monitor. The area monitor is an ionization chamber monitor, a scintillation monitor, or a Geiger-Mueller tube monitor. 
         [0019]    An upper end of the irradiator lifter  31 , in this embodiment, as shown in  FIG. 1 , is fixed to the pig  21  in which the portable irradiator  23  is placed, so as to set a distance and a height between the portable irradiator  23  and the monitor  41  to be calibrated, so that the radiation source provides the monitor  41  to be calibrated with a radiation dose for detecting the radiation source corresponding to the distance and the height. 
         [0020]    The laser range finder  51 , in this embodiment, as shown in  FIG. 1 , is used for measuring a distance between the radiation source of the portable irradiator  23  and the monitor  41  to be calibrated. 
         [0021]    The portable surveymeter of a site background, in this embodiment, is used for measuring a radiation quantity of the background during in-situ calibration; a radiation quantity measured by the monitor  41  to be calibrated, minus the radiation quantity of the background measured by the portable surveymeter of a site background, is a measurement value of a radiation quantity of a real ambient or area dose rate. 
         [0022]    This embodiment further includes an environmental monitor, where the environmental monitor is a thermometer or a hygrometer. 
         [0023]    The working standard part  61 , in this embodiment, as shown in  FIG. 1 , is a scintillation surveymeter calibrated according to the secondary standard radiation field, so as to detect a radiation dose of the radiation source of the portable irradiator  23  under the conditions of the distance and the height. The secondary standard radiation field is calibrated according to the air kerma-rate primary standard, and the air kerma-rate primary standard is a spherical ionization chamber. As a result, the calibrated working standard part  61  may be regarded as a tertiary standard radiation field, and then the working standard part  61  performs performance evaluations on the portable irradiator  23 , as shown in table 2. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Results of performance evaluations performed by the 
               
               
                 scintillation surveymeter of the working part 
               
             
          
           
               
                   
                 Item 
                 ANSI N323A Requirement 
                 Working Part 
               
               
                   
                   
               
               
                   
                 Stability 
                   10% 
                  3% 
               
               
                   
                 Accuracy 
                 ±15% 
                  5% 
               
               
                   
                 Energy depend 
                 &lt;20% (0.8 MeV-1.2 MeV) 
                 20% 
               
               
                   
                 Angle depend 
                 &lt;20% (0°-45°) 
                 15% 
               
               
                   
                 Response time 
                 &lt;5 s (0.1 μSv/h−1 mSv/h) 
                 4 s 
               
               
                   
                   
               
             
          
         
       
     
         [0024]    The working standard part  61 , in this embodiment, as shown in  FIG. 3 , has an energy depend view, where a horizontal axis indicates radiation energy, and a vertical axis indicates relative sensitivity, which falls into an energy range of environmental radioactivity and nuclear accidents. 
         [0025]    The working standard part  61 , in this embodiment, as shown in  FIG. 4 , has a table of relative relationships between the working standard part  61  and the ambient dose equivalent rate, where a horizontal axis indicates a distance with the unit of centimeter (cm), and a vertical axis indicates a dose rate with the unit of μSv/h; the larger the distance is, the lower the ambient dose equivalent rate is. 
         [0026]    The portable irradiator  23 , in this embodiment, calibrates the ambient dose equivalent rate H*(10) of a laboratory radiation field, as shown in table 3. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Calibration of the ambient dose equivalent rate 
               
               
                 H*(10) of a laboratory radiation field 
               
             
          
           
               
                   
                 Dose Rate (μSv/h) 
                 Distance (cm) 
               
               
                   
                   
               
             
          
           
               
                   
                 90 
                 102.3 
               
               
                   
                 80 
                 108.4 
               
               
                   
                 70 
                 115.9 
               
               
                   
                 60 
                 125.1 
               
               
                   
                 50 
                 136.9 
               
               
                   
                 40 
                 152.9 
               
               
                   
                 30 
                 176.4 
               
               
                   
                 20 
                 215.6 
               
               
                   
                 10 
                 304.1 
               
               
                   
                 9 
                 320.4 
               
               
                   
                 8 
                 339.7 
               
               
                   
                 7 
                 362.9 
               
               
                   
                 6 
                 391.8 
               
               
                   
                   
               
             
          
         
       
     
         [0027]    The portable irradiator  23 , in this embodiment, performs in-situ calibration on the ambient dose equivalent rate H*(10) of the radiation field, as shown in table 4. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 In-situ calibration of the ambient dose equivalent rate 
               
               
                 H*(10) of the radiation field 
               
             
          
           
               
                   
                 Dose Rate (μSv/h) 
                 Distance (cm) 
               
               
                   
                   
               
             
          
           
               
                   
                 554 
                 50 
               
               
                   
                 148 
                 100 
               
               
                   
                 63 
                 150 
               
               
                   
                 37 
                 200 
               
               
                   
                 23 
                 250 
               
               
                   
                 15 
                 300 
               
               
                   
                 8 
                 400 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    The working standard part  61  dates back to correction factors of the ambient dose equivalent rate of the secondary standard, as shown in table 5. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Dating back to correction factors of the ambient dose 
               
               
                 equivalent rate of the secondary standard 
               
             
          
           
               
                   
                 Ambient Dose Equivalent Rate (μSv/h) 
                   
               
             
          
           
               
                 Secondary Standard 
                 Working Standard 
                 Correction Factor 
               
               
                   
               
             
          
           
               
                 500 
                 500 
                 1.00 
               
               
                 400 
                 400 
                 1.00 
               
               
                 300 
                 298 
                 1.01 
               
               
                 200 
                 199 
                 1.01 
               
               
                 100 
                 100 
                 1.00 
               
               
                  50 
                 50 
                 1.00 
               
               
                  40 
                 40 
                 1.00 
               
               
                  30 
                 30 
                 1.00 
               
               
                  20 
                 20.2 
                 0.99 
               
               
                  10 
                 10.3 
                 0.97 
               
               
                 Average 
                   
                 1.00 
               
               
                   
               
             
          
         
       
     
         [0029]    Another embodiment of the present invention, as shown in  FIG. 2 , provides an in-situ calibration method for radiation monitors, including the following steps: step  11 : providing a portable irradiator  23 , a laser locator  24 , a working standard part  61 , an irradiator lifter  31 , a laser range finder  51 , a monitor  41  to be calibrated and a portable surveymeter of a site background, as shown in  FIG. 1  and table 1; step  12 : setting performance evaluations of the portable irradiator  23 , as shown in table 2, including the effect field, beam uniform, and back scattering; step  13 : setting performance evaluations of the working standard part  61 , including the accuracy, stability, response time, energy depend, and angle depend; step  14 : establishing a working standard radiation field of the portable irradiator  23  to be used for calibration, as shown in table 4, and generating a table of relative relationships between a distance and an ambient dose equivalent rate H*(10) through calculation; step  15 : analyzing an uncertainty of an in-situ calibration system, as shown in table 6, including the photon standard radiation dose, beam uniform, measurement distance and measurement time, with the expand uncertain (k=2) of 6%; and step  16 : verifying the secondary standard radiation field and the working standard radiation field, as shown in table 5, where the monitor  41  to be calibrated includes an ambient dose monitor, disposed in an open environment, so as to monitor an ambient radiation dose; and an area monitor, disposed in an indoor area, so as to monitor an area radiation dose. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Uncertainty analysis of the in-situ calibration system 
               
             
          
           
               
                   
                 Standard 
                   
               
               
                   
                 Uncertainty 
               
               
                   
                 (%) 
               
             
          
           
               
                   
                 Analysis Item 
                 Type A 
                 Type B 
               
               
                   
                   
               
             
          
           
               
                   
                 Photon standard radiation dose 
                   
                 1.5 
               
               
                   
                 Beam uniform 
                 2.3 
               
               
                   
                 Beam stability 
                 0.35 
               
               
                   
                 Measurement distance 
                   
                 0.1 
               
               
                   
                 Measurement time 
                 0.001 
               
             
          
           
               
                   
                 Combination uncertainty 
                 2.77 
                   
               
               
                   
                 Expand uncertain (k = 2) 
                 5.54 
               
               
                   
                   
               
             
          
         
       
     
         [0030]    In this embodiment, when the performance of the effect field of the portable irradiator  23  is evaluated, a Geiger-Mueller tube with a diameter of 2 cm is used to measure a range of the effect field in positions that are 15 cm left, right, up and down from a beam center of the portable irradiator  23 , obtaining a maximum difference of about 4% from the beam center. When the performance of the beam uniform of the portable irradiator  23  is evaluated, a high-sensitivity lithium fluoride (LiF) thermoluminescent dosimeter (Harshow/100H) is used on a hollow acryl sheet and in a position that is 4 m away from the radiation source of the portable irradiator  23 , to perform measurement at a beam central point of the portable irradiator  23  and in four other positions that are 5 cm up, down, left, and right from the beam central point, thereby obtaining an average difference of smaller than 2.8%, which indicates good beam uniform in the position that is 4 m away. When the performance of the back scattering of the portable irradiator  23  is evaluated, the portable irradiator  23  is placed in a position on a concrete wall with a height of 1 m and a thickness of 0.1 m on site, so as to obtain from measurement that the largest back scattering influence of the in-situ calibration area monitor is about 10%. 
         [0031]    In this embodiment, the working standard part  61  is a scintillation surveymeter, whose test results of performance evaluations, including the accuracy, stability, response time, energy depend, and angle depend, conform to requirements of the American National Standards Institute ANSI-N42.17A specifications. The dose rate accuracy is 1 μSv/h to 100 mSv/h with a range difference smaller than 5%. The stability (standard deviation) of 20 successive measurement values is 3%. The response time is smaller than 1 second. The energy depend is smaller than 20%. The angle depend is smaller than 15%. 
         [0032]    In this embodiment, as shown in table 7, the verifying the secondary standard radiation field and the working standard radiation field includes comparing laboratory calibration and in-situ calibration dose rate results of a nuclear facility ambient dose monitor (such as an HPIC), with a laboratory calibration correction factor of 0.9-1.0, and an in-situ calibration correction factor of 0.9-1.1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Comparison of laboratory calibration and in-situ calibration results of a nuclear 
               
               
                 facility ambient dose monitor (such as an HPIC) 
               
             
          
           
               
                   
                   
                 (mR/h ) 
                   
                   
                 (μSv/h) 
                   
               
               
                   
                 Laboratory 
                 Instrument  
                   
                 In-Situ Calibration 
                 Instrument 
                   
               
               
                   
                 Standard 
                 Indication 
                 Correction 
                 Standard 
                 Indication 
                 Correction 
               
               
                 Location 
                 Value 
                 Value 
                 Factor 
                 Value 
                 Value 
                 Factor 
               
               
                   
               
             
          
           
               
                 Neutron 
                 0.4 
                 0.4215 
                 0.9 
                 148 
                 141.8750 
                 1.0 
               
               
                 house 
                 0.8 
                 0.8142 
                 1.0 
                 37 
                 35.9700 
                 1.0 
               
               
                   
                 2 
                 1.9652 
                 1.0 
                 15 
                 16.8667 
                 0.9 
               
               
                 Lungmen 
                 0.4 
                 0.4261 
                 0.9 
                 148 
                 148.6000 
                 1.0 
               
               
                   
                 0.8 
                 0.9400 
                 0.9 
                 37 
                 37.7025 
                 1.0 
               
               
                   
                 2 
                 2.0557 
                 1.0 
                 15 
                 17.0900 
                 0.9 
               
               
                 011 house 
                 0.4 
                 0.4246 
                 0.9 
                 148 
                 149.3500 
                 1.0 
               
               
                   
                 0.8 
                 0.8944 
                 0.9 
                 37 
                 38.1500 
                 1.0 
               
               
                   
                 2 
                 2.0310 
                 1.0 
                 15 
                 17.3875 
                 0.9 
               
               
                 Ashing 
                 0.4 
                 0.4240 
                 0.9 
                 148 
                 140.8000 
                 1.1 
               
               
                 room 
                 0.8 
                 0.8988 
                 0.9 
                 37 
                 36.2950 
                 1.0 
               
               
                   
                 2 
                 2.0317 
                 1.0 
                 15 
                 16.6450 
                 0.9 
               
               
                   
               
             
          
         
       
     
         [0033]    In this embodiment, as shown in table 8, the verifying the secondary standard radiation field and the working standard radiation field includes comparing the laboratory calibration and in-situ calibration dose rate results of an area monitor of an indoor area, with a maximum laboratory calibration difference of smaller than 16%, and a maximum in-situ calibration difference of smaller than 13%. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 In-situ calibration result of an area monitor in a 
               
               
                 nuclear medical department 
               
             
          
           
               
                   
                   
                 Standard 
                   
                 Standard 
                   
               
               
                   
                   
                 Value 
                 Difference 
                 Value 
                 Difference 
               
               
                 Surveymeter 
                 Brand 
                 (38 μSv/h) 
                 (%) 
                 (148 μSv/h) 
                 (%) 
               
               
                   
               
             
          
           
               
                 Ionization 
                 H 
                 34 
                 −10.5 
                 146 
                 −1.4 
               
               
                 chamber 
               
               
                   
                   
                 33 
                 −13.2 
                 139 
                 −6.1 
               
               
                 Ionization 
                 F 
                 40 
                 5.3 
                 146 
                 −1.4 
               
               
                 chamber 
               
               
                   
                   
                 38 
                 0 
                 155 
                 4.7 
               
               
                 Geiger- 
                 T 
                 38 
                 0 
                 141 
                 −4.7 
               
               
                 Mueller 
               
               
                 tube 
               
               
                   
                   
                 39 
                 2.6 
                 143 
                 −3.4 
               
               
                 Geiger- 
                 B 
                 39 
                 2.6 
                 151 
                 2.0 
               
               
                 Mueller 
               
               
                 tube 
               
               
                   
                   
                 30 
                 0 
                 123 
                 1.4 
               
               
                 Scintillation 
                 E 
                 40 
                 5.3 
                 139 
                 −6.1 
               
               
                   
                   
                 40 
                 5.3 
                 148 
                 0 
               
               
                   
               
             
          
         
       
     
         [0034]    In this embodiment, the ambient dose monitor of the monitor  41  to be calibrated is a high-pressure ionization chamber monitor or a multi-Geiger-Mueller tube monitor; the area monitor of the monitor  41  to be calibrated is an ionization chamber monitor, a Geiger-Mueller tube monitor, or a scintillation monitor. 
         [0035]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.