Abstract:
Provided is a measurement unit and measurement method for reducing attenuation due to optical fiber length and SN degradation due to background in a dosage rate monitor that uses optical fiber. This system comprises: a radiation detector for detecting radiation dosage; a light source for irradiating stimulating light on the radiation detector; a photodetector for detecting light generated by the radiation detector; an optical fiber for connecting the photodetector and the radiation detector and light source, and transmitting light from the light source and light from the radiation detector; a measurement unit for counting the pulses outputted from the photodetector; and an analysis unit for extracting the luminous energy originating from the radiation detector, from time information, wave height information, and the count value, which are measurement results obtained by the measurement unit, and converting the luminous energy to a dosage and dosage rate.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a dosage rate measurement system and a dosage rate measurement method, and in particular, to dosage rate measurement systems and dosage rate measurement methods for suitably measuring a dosage and a dosage rate in a radiation detector by counting pulses radiated from the radiation detector and analyzing a light quantity on the basis of time and pulse height information in the radiation detector. 
       BACKGROUND ART 
       [0002]    As a radiation detector for measuring a dosage based on radiation, a detector such as an ionization chamber, a GM (Geiger-Muller) counter tube, a scintillation detector or a semiconductor detector is widely used as a dosage monitor such as an area monitor, a process monitor or a survey meter. A radiophotoluminescence glass dosimeter, a thermoluminescence dosimeter, or an optically stimulated luminescent dosimeter is used as an individual radiation exposure management or imaging plate. These dosage monitor, individual dosimeter and imaging plate are used mainly in atomic energy facilities, medical facilities using radioactive isotope, or research accelerator facilities. 
         [0003]    In a conventional method using a dosage monitor, it is general that an application voltage of several volts to thousands of volts is supplied to the aforementioned detector to operate the detector an electrical signal associated with radiation is sent from the detector via a coaxial cable or the like to a measurement device provided at a downstream stage to analyze the signal. As an example, the detector applied to the dosage monitor used in atomic energy facilities is one of a silicon semiconductor detector, an NaI (TI) scintillation detector and an ionization chamber, any of which requires electricity supply thereto. 
         [0004]    In the aforementioned various facilities, dosage monitoring based on these detectors can be achieved without any troubles in a usual operating situation. However, when power supply cannot be achieved due to a power failure or the like, it becomes difficult to measure a dosage. 
         [0005]    As a radiation measurement means which eliminates the need of power supply to the detector, a dosage monitor using optical fiber is suggested. The arrangement is that scintillation element is connected at a tip end of an optical fiber, light applied to the element is converted to an electrical signal by an optical detector provided at a downstream stage of the fiber for analysis thereof. As a position measurement means, an optical detector is provided at each of both ends of an optical fiber and a detection position is derived by using a time difference between light detections of the detectors. 
         [0006]    However, even in any of the above systems, the amount of scintillation light is very small and it is difficult to prolong an optical fiber length from the viewpoint of SN ratio. Accordingly, such a system cannot be applied to such atomic energy facilities as to require long wiring of the cables. In order to achieve dosage monitoring using optical fiber in such facilities, a measurement system and a measurement method for reducing attenuation due to optical fiber length and SN degradation due to background are required. 
         [0007]    Typical fluorescent materials for converting a dosage to a light quantity include NaI (TI), BGO, and GSO, and light irradiated immediately after occurrence of an interaction with radiation is used to analyze a dosage, a radiation energy and so on. An optically stimulated luminescent (OSL) element, utilizing a proportional relationship between the formation amount of a color center and an accumulated dosage, analyzes an accumulated dosage from OSL light irradiated when optically stimulated light from a light source is transited from an excited state to a base state. 
         [0008]    PATENT LITERATURE 1 is characterized in that an OSL element is connected at a tip end of an optical fiber, stimulated light is irradiated from a laser light source thereto to thereby generate scintillation light originated from the OSL, and the light is measured by an optical detector provided at a stage downstream thereof. 
         [0009]    PATENT LITERATURE 2 is characterized in that a device for analyzing the fluorescent dosage of a fluorescent material contained in a sample includes a means for increasing a multiplication factor of an optical detector only in a time range where the fluorescent dosage of a fluorescent material is dominant on the basis of a pulse light generation timing signal of a light source. 
       CITATION LIST 
     Patent Literatures 
       [0010]    PATENT LITERATURE 1: JP-A-11-237479 
         [0011]    PATENT LITERATURE 2: JP-A-9-159527 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0012]    In facilities requiring long wiring of a cable, in order to attain a dosage monitor using optical fiber, a measurement system and a measurement method for reducing attenuation due to optical fiber length and SN degradation due to background becomes necessary. 
         [0013]    In PATENT LITERATURE 1, attenuation of OSL light and laser light is suppressed by connecting two optical fibers to an OSL element (with use of a twin fiber system). However, there is no description of a device arrangement and its algorithm for reducing SN degradation in the measurement system and arithmetic device and so on therein. 
         [0014]    In PATENT LITERATURE 2, an SN ratio is improved by providing a means for increasing a multiplication factor of an optical detector only in a time range where the fluorescent dosage of a fluorescent material is dominant. However, use of the technique of PATENT LITERATURE 2 has a restriction, because a very long time is provided to a laser to which light from a fluorescent material is directed and such a means can be applied only when the fluorescent light is sufficiently sustained even after the fluorescent light directed to the laser is stopped. For example, in order to carry out dosage measurement using OSL light, it becomes necessary to generate OSL light by irradiation of a laser or the like and to extinguish it. However, the irradiation time of the OSL light is sufficiently short with respect to the laser irradiation time. Thus application of the means of PATENT LITERATURE 2 becomes difficult. 
         [0015]    When attenuation due to optical fiber length and SN degradation due to background can be reduced, the dosage monitor can be made accurate and measurement can be made with an accuracy equivalent to that of various conventional monitors requiring electricity supply thereto. An object of the present invention is to solve the above problem by providing a dosage rate measurement system and a dosage rate measurement method for reducing attenuation due to optical fiber length and SN degradation due to background. 
       Solution to Problem 
       [0016]    A dosage rate measurement system of the invention of claim  1  for attain the above object is characterized in that the system includes a radiation detector, a light source for irradiating stimulated light on the radiation detector via an optical fiber, an optical detector for detecting light generated in the radiation detector and transmitted via the optical fiber, a measurement unit connected to the radiation detector, the light source and the optical detector for counting pulses outputted from the optical detector and also for obtaining attendant information, and an analysis unit for extracting a dosage originating from the radiation detector and converting the dosage and a dosage rate on the basis of time information, pulse height information and the counted value as measurement results obtained in the measurement unit. 
         [0017]    A dosage rate measurement system of the invention of claim  2  is characterized by including a wavelength analysis unit provided in an optical detector part. 
         [0018]    A dosage rate measurement system of the invention of claim  3  is characterized in that the system measures a detected time and a detected pulse peak value for each of pulses inputted to the measurement unit. 
         [0019]    A dosage rate measurement system of the invention of claim  4  is characterized in that the system includes an analysis unit for analyzing a measures result as a net value with use of time information, pulse height information and a counted value in the background. 
         [0020]    A dosage rate measurement system of the invention of claim  5  is characterized in that the system includes an OSL element provided in a radiation detector. 
         [0021]    A dosage rate measurement system of the invention of claim  6  is characterized in that a dosage rate is derived from the irradiation interval of a light source and a measured result of an analysis unit. 
         [0022]    A dosage rate measurement system of the invention of claim  7  is characterized in that the system includes a condensing lens for collecting OSL light at a radiation detector and inputting the light to an optical fiber. 
         [0023]    A dosage rate measurement system of the invention of claim  8  is characterized in that the system includes an optical fiber for the light source and a detecting optical fiber for connecting the radiation detector and the optical detector. 
         [0024]    A dosage rate measurement system of the invention of claim  9  is characterized in that an optical coupler is provided between the light source, the optical detector and the radiation detector and a single optical fiber is provided between the optical coupler and the radiation detector. 
         [0025]    A dosage rate measurement system of the invention of claim  10  is characterized in that an optical switch is provided between the light source, the optical detector and a plurality of radiation detectors, the plurality of radiation detectors are connected via the optical switch to the single optical detector and to the light source, and dosages and dosage rates in the plurality of radiation detectors are measured by a single measurement unit and by a single analysis unit. 
         [0026]    A dosage rate measurement system of the invention of claim  11  is characterized in that an analysis result is displayed in the form of a three-dimensional distribution from analysis results obtained from time information, pulse height information and a counted value as measured results obtained in the measurement unit. 
         [0027]    A dosage rate measurement system of the invention of claim  12  is characterized in that an analysis method in the analysis unit is carried out by specifying a time range and a pulse height range and by analyzing a dosage and a dosage rate. 
         [0028]    A dosage rate measurement method of the invention of claim  13  is characterized in that stimulated light is irradiated on a radiation detector via an optical fiber, light generated in the radiation detector and transmitted via the optical fiber is detected, pulses outputted from the detected light are counted as a counted value, a light amount originating from the radiation detector is extracted on the basis of time information, pulse height information and the counted values as measured results obtained by the pulse counting, and is converted to a dosage and a dosage rate 
       Advantageous Effects of Invention 
       [0029]    In accordance with the inventions of claims  1  and  13 , the dosage monitor can be used with a high accuracy. 
         [0030]    In a more concrete example, when a counted pulse result is analyzed on a time basis; an environmental background stationarily present without any correlation to stimulated light irradiation, a light source background generated by stimulated light irradiation and stationarily present, and OSL light generated by the stimulated light irradiation and attenuated in a manner of exponential function can be discriminated and analyzed on a time basis. Further, highly frequent areas of the environmental background and light source background can be discriminated by analyzing in an area where a counted value for the OSL light is superior to a counted value due to the background as pulse height value. As a result, even when it is required in various facilities to route or wire an optical fiber in a wide range, power supply to a detector can be eliminated, and thus a dosage and a dosage rate can be measured with a high accuracy and sensitivity. 
         [0031]    In accordance with the invention of claim  2 , when a measuring range is specified to the wavelength area of OSL light, the background due to light source wavelength can be effectively reduced, and thus a dosage and a dosage rate can be measured with a high accuracy and sensitivity. 
         [0032]    In accordance with the invention of claim  3 , when time pulse height information for each pulse are used, a time analysis and pulse height analysis ranges in the analysis unit can be arbitrarily changed, and thus a dosage and a dosage rate can be analyzed with an optimum accuracy in various measurement environments. 
         [0033]    In accordance with the invention of claim  4 , a net value is measured by subtracting an environmental background present at the time of measuring an OSL light output and a background due to light source scatted light from measured data on the basis of time, pulse height and counted value then a dosage and a dosage rate can be measured with a high accuracy and sensitivity. 
         [0034]    In accordance with the invention of claim  5 , when an OSL element is used as a radiation detector, a radiation amount can be accumulated; and when scintillation light is irradiated based on the stimulated light of the light source, a dosage and a dosage rate can be measured at an arbitrary time. Further, when the OSL element is arranged so as to sufficiently accumulate the dosage, a high SN ratio can be measured. 
         [0035]    In accordance with the invention of claim  6 , the irradiation interval of the light source becomes a dosage accumulation time of the radiation detector. Thus when the irradiation interval time and a measured dosage are used, dosage rate can be measured. 
         [0036]    In accordance with the invention of claim  7 , when the OSL element is provided with a lens for collecting light at a tip end of an optical fiber, OSL light irradiated in a 4π direction can be efficiently collected and a dosage and a dosage rate can be measured with a high SN ratio. 
         [0037]    In accordance with the invention of claim  8 , when an optical fiber is connected between the light source and the radiation detector and between the radiation detector and the optical detector respectively, attenuation on an optical path can be minimized and a dosage and a dosage rate can be attained with a high SN ratio. 
         [0038]    In accordance with the invention of claim  9 , when an optical coupler is provided, optical fiber wiring to the radiation detectors can be attained with a single optical fiber, and thus the system of the invention ca be easily installed in various facilities. 
         [0039]    In accordance with the invention of claim  10 , when an optical switch is provided, the radiation detectors can be easily used in a multichannel form, and dosages and dosage rates at the plural parts can be measured with use of a single measurement unit and a single analysis unit. 
         [0040]    In accordance with the invention of claim  11 , when the acquired time information, pulse height information and counted value are set for any of X, Y and Z axes as a three-dimensional distribution and measured results are displayed; the range of high OSL light output level or the range of the background can be easily confirmed. Thus, a time and a pulse height range with a high SN ratio can be specified and a dosage and a dosage rate can be analyzed with an optimum accuracy. 
         [0041]    In accordance with the invention of claim  12 , when a time range and a pulse height range are specified, a range where the background id dominant can be eliminated and thus, a dosage and a dosage rate can be measured with a high SN ratio. Other objects, features and advantages of the present invention will become clear from the following description of embodiments of the invention by referring to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0042]      FIG. 1  shows an arrangement of a dosage rate measurement unit in an embodiment 1 of the present invention. 
           [0043]      FIG. 2  shows an arrangement of a radiation detector in the embodiment 1. 
           [0044]      FIG. 3  shows an example of an output of an optical detector in the embodiment 1. 
           [0045]      FIG. 4  shows an example of analysis data based on time pulse height and counted value in the embodiment 1. 
           [0046]      FIG. 5  shows a relationship between a measured value and a dosage in the embodiment 1. 
           [0047]      FIG. 6  shows an arrangement of a dosage rate measurement unit in an embodiment 2. 
           [0048]      FIG. 7  shows distributions of OSL light and light source wavelength and also shows a wavelength analysis range in the embodiment 2. 
           [0049]      FIG. 8  shows an arrangement of a radiation detector having a condensing lens in an embodiment 3. 
           [0050]      FIG. 9  shows an arrangement of a dosage rate measurement unit in an embodiment 4. 
           [0051]      FIG. 10  shows an arrangement of a dosage rate measurement unit in an embodiment 5. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0052]    The inventors of the present invention have, in various manners, examined and found a measurement unit and a measurement method for reducing attenuation due to optical fiber length and SN degradation due to background in a dosage monitor using optical fiber on the basis of novel consideration. 
         [0053]    In this consideration, when a counted pulse value result of an optical detector output is analyzed on a time basis, an environmental background stationarily present without any correlation to stimulated light irradiation, a light source background generated by stimulated light irradiation and stationarily present during the irradiation, and OSL light generated by the stimulated light irradiation and attenuated in a manner of exponential function can be discriminated and analyzed. Further, highly frequent areas of the environmental background and light source background can be discriminated by analyzing in an area where a counted value for the OSL light is superior to a counted value due to the background as pulse height value. Thus, even when it is required in various facilities to install an optical fiber in a wide range, the need of power supply to a detector can be eliminated and therefore a dosage and a dosage rate can be measured with a high accuracy and a high sensitivity. 
         [0054]    Suitable embodiments of a dosage rate measurement system and a measurement method thereof in accordance with the present invention, that is, the contents of the novel consideration will be explained in detail with reference to the attached drawings. 
       Embodiment 1 
       [0055]    A dosage rate measurement system and a measurement method thereof in an embodiment 1 as one of suitable embodiments of the present invention will be explained by using  FIGS. 1 to 5 . The dosage rate measurement system of the embodiment 1 includes a radiation detector  1 , an optical coupler  2 , a light source  3 , a light source control device  4 , an optical fiber  5 , an optical detector  6 , a measurement unit  7 , an analysis unit  8 , and a display device  9 . A control measurement unit  11  includes the optical coupler  2 , the light source  3 , the light source control device  4 , the optical fiber  5 , the optical detector  6 , the measurement unit  7 , the analysis unit  8 , and the display device  9 . The radiation detector  1  is connected via the optical fiber  5  to the optical coupler  2 . The optical coupler  2  is connected to the light source  3  and to the optical detector  6  via two of the optical fibers  5 . The light source  3  is connected to the light source control device  4 . The optical detector  6  is connected to the measurement unit  7 . A light-generation timing cable  10 , connected to the measurement unit  7  and the light source control device  4 , is connected to the analysis unit  8 . The analysis unit  8  is connected to the display device  9 . 
         [0056]    The radiation detector  1  has a function of detecting a radiation  12 . 
         [0057]      FIG. 2  shows a structure of the radiation detector  1 . An optically stimulated luminescent (OSL) element  13  for accumulating a radiation amount is installed within the radiation detector  1 . Examples of such OSL elements will be enumerated here. Typical ones of such OSL elements are BaFBr:Eu for use in an imaging plate and Al2O3:C for use as an individual dosimeter. Other OSL elements include BaFl:Eu, NaCl:Cu, KCl:Eu, KBr:Eu, RbBr:Tl, SrS:Eu, Sm, CsS:Eu,Sm, CaS:Ce,sm, MgS:Eu,Sm, MgS:Ce,Sm, MgO:Fe, Zn2SiO4:Mn, Ba6(PO4)3O:Eu, 25Na2O and 75B2O3:Eu. When stimulated light generated from the light source  3  is irradiated on the OSL element, OSL light  14  is generated from the OSL element. When the OSL light  14  is directed into a tip end of the optical fiber  5 , the OSL light is transmitted via the optical coupler  2  to the optical detector  6 . 
         [0058]    As the light source  3 , a light source for generating light having a range of such wavelengths as to generate the OSL light  14  is used. The light emission of the light source  3  is controlled with use of the light source control device  4 . Light for stimulating the OSL light may be light of a continuous or pulse type. The light source control device  4  has a function of externally issuing its output at timing equivalent to light generation, and the output is transmitted via the light-generation timing cable  10  to the analysis unit  8 . 
         [0059]    The optical detector  6  detects the OSL light  14  transmitted via the optical coupler  2 , a light source background originating from the light source  3 , and an environmental background. Typical examples of such optical detectors  6  include a photomultiplier tube and a semiconductor detector. 
         [0060]    The measurement unit  7  has a function of measuring a time of output of the optical detector and a pulse height thereof.  FIG. 3  shows an example of the output of the optical detector. When light is inputted to the optical detector  6 , a pulse output  15  as the output of the optical detector is outputted from the optical detector  6  and measured by the measurement unit  7 . A pulse height value  18  of the pulse output  15  is a voltage level with respect to a base line as a reference, and the pulse height value  18  is measured for each pulse. As an example of a method of acquiring a detection timing of the pulse output  15 , a timing exceeding a pulse height discrimination level  16  can be set at a detection timing  17 . There are other methods which include a method of setting a peak of the pulse output  15  as a detection timing and a method of setting zero-cross point obtained by a constant fluction timing system as a detection timing. The detection timing  17  acquired by any of the methods is measured for each pulse by the measurement unit  7 . 
         [0061]    The analysis unit  8  is used to extract a signal component based on the OSL light  14  from time and pulse height information for each pulse measured by the measurement unit  7 .  FIG. 4  is an example of analysis data analyzed by the analysis unit  8 . In this drawing, OSL light measurement data  25  is denoted by a solid line, and a background  26  is denoted by a dotted line. A stimulated light irradiation timing  21  is determined on the basis of an output transmitted from the light-generation timing cable  10 . 
         [0062]    With respect to the OSL light measurement data  25 , measurement data measured prior to the stimulated light irradiation timing  21  in a time axis is data based on an environmental background  19 . Measurement data measured later than the stimulated light irradiation timing  21  is data including data based on the OSL light, light source and an environmental background  20 . Data measured after the light output based on the OSL light  14  is sufficiently reduced and after generation of stimulated light from the light source  3  is stopped at an arbitrary time is data based on the environmental background  19 . Even in a pulse height axis, a change in the pulse height distribution later than the stimulated light irradiation timing  21  is observed. 
         [0063]    The background  26  indicates measurement data in such an environment as to have no signal component based on the OSL light  14 . The background based on the light source  3  is measured later than the stimulated light irradiation timing  21 . When compared with the OSL light measurement data  25 , a change in the output of the background data appears later than the stimulated light irradiation timing  21 . 
         [0064]    In order to analyze a light quantity of the OSL light  14  with use of the OSL light measurement data  25 , a time specification range  22  and a pulse height specification range  23  are selected, and a counted value in a time/pulse height specification range  24  satisfying both of the first-mentioned specification ranges is used as a measurement value. 
         [0065]    When the influence of the background  26  to the OSL light measurement data  25  is great and this influences a measurement accuracy, it is also possible to subtract the background  26  therefrom and derive the measurement value as a net value. 
         [0066]      FIG. 5  shows a relationship between a measurement value and a dosage. The measurement value acquired from the OSL light measurement data  25  is derived as a dosage from a relationship  27  prepared before measurement. Since a light quantity based on the OSL element and an accumulated dosage have a substantially proportional relationship, the relationship  27  between the measurement value and dosage is set as a first-order or second-order function. The dosage derived in  FIG. 5  can be converted to a dosage rate with use of an accumulation time. As the accumulation time, for example, an irradiation interval time of the stimulated light irradiation timing  21  is used. 
         [0067]    When the radiation detector  1 , optical coupler  2 , light source  3 , light source control device  4 , optical fiber  5 , optical detector  6 , measurement unit  7 , analysis unit  8  and display device  9  included in the above embodiment are used, a dosage rate measurement system and a measurement method thereof can be attained, attenuation due to optical fiber length and SN degradation due to background can be reduced, the need of power supply to the detector can be eliminated, and a dosage and a dosage rate can be measured with a high accuracy and a high sensitivity. 
       Embodiment 2 
       [0068]    Explanation will be made as to a dosage rate measurement system and a measurement method thereof as an embodiment 2 as another embodiment of the present invention. In the embodiment 2, an SN ratio can be effectively increased by adding a wavelength analysis unit. 
         [0069]      FIG. 6  shows a dosage rate measurement unit. The arrangement of this embodiment is substantially the same as the arrangement of the embodiment 1, except that the wavelength analysis unit  28  is connected to the optical coupler  2  via the optical fiber  5  and also connected to the optical detector  6  provided at a downstream stage. 
         [0070]      FIG. 7  shows OSL light and a light source wavelength distribution. The wavelength analysis unit  28  measures an OSL light wavelength  30  and a light source wavelength  31 . When a wavelength analysis range  32  where the OSL light wavelength  30  and the light source wavelength  31  can be effectively discriminated is specified and when the optical detector  6 , measurement unit  7  and analysis unit  8  provided at a downstream stage perform measuring operation only in the specified range; measurement with a high SN ratio can be attained. Accordingly, when the present embodiment is applied, a dosage and a dosage rate can be measured with a high accuracy and a high sensitivity. 
       Embodiment 3 
       [0071]    Explanation will be made as to a dosage rate measurement system and a measurement method thereof as an embodiment 3 as another embodiment of the present invention. In the embodiment 3, an SN ratio can be effectively increased by adding a condensing lens to the radiation detector. 
         [0072]      FIG. 8  shows a radiation detector provided with a condensing lens. A condensing lens  33  is provided between the optical fiber  5  and the OSL element  13 . It is assumed that the focus point of the condensing lens  33  is set at the wavelength of the OSL light and the position of the focus point is set at a tip end of the optical fiber  5 . The condensing lens  33 , the OSL element  13  and the tip end of the optical fiber  5  are located within a radiation detector  34 . The control measurement unit  11  provided at a stage downstream of the optical fiber  5  is substantially the same as that in the embodiment 1. When the present embodiment is applied, since OSL light can be effectively collected into the optical fiber by condensing, a dosage and a dosage rate can be measured with a high SN ratio. 
       Embodiment 4 
       [0073]    Explanation will be made as to a dosage rate measurement system and a measurement method thereof as an embodiment 4 as another embodiment of the present invention. In the embodiment 4, an optical fiber is connected between a light source and a radiation detector and between a radiation detector and an optical detector respectively, attenuation on an optical path can be minimized. 
         [0074]      FIG. 9  shows a dosage rate measurement unit. This embodiment has a substantially the same arrangement as the embodiment 1, except that two of an optical fiber  35  and an optical fiber  41  are connected between the light source  3  and the radiation detector  1  and between the optical detector  6  and the radiation detector  1  respectively. The optical fiber  35  connected between the light source  3  and the radiation detector  1  is provided to irradiate stimulated light generated from the light source  3  on the OSL element  13  provided in the radiation detector  1 . The optical fiber  41  connected between the optical detector  6  and the radiation detector  1  is provided to transmit the OSL light  14  to the optical detector  6 . Since the optical fiber  41  does not transmit directly stimulated light generated from the light source  3 , the light source background level can be reduced. Further, since the optical fiber  41  is connected directly to the radiation detector  1  and to the optical detector  6  to eliminate the arrangement of attenuating a light quantity of the optical coupler  2  and so on, a signal component based on the OSL light  14  can be efficiently detected. Accordingly, when the present embodiment is applied, a dosage and a dosage rate can be measured with a high SN ratio. 
       Embodiment 5 
       [0075]    Explanation will be made as to a dosage rate measurement system and a measurement method thereof in accordance with an embodiment 5 as another embodiment of the present invention. In the embodiment 5, since an optical switch is provided, the radiation detector can be easily made in the form of a multichannel. 
         [0076]      FIG. 10  shows a dosage rate measurement unit. This embodiment has a substantially the same arrangement as the arrangement of the embodiment 1, except that an optical switch  37  is provided in a control measurement unit  39  and radiation detectors  40  corresponding in number to the number of multiple channels of the optical switch  37  are provided. The optical switch  37  is connected to the optical coupler  2  and also to the plurality of radiation detectors  40 . The optical switch  37  can select arbitrary one of the connected radiation detectors  40 , and units or devices connected downstream of the optical detector  6  can allow measurement of a dosage and a dosage rate in each of the radiation detectors  40 . When the present embodiment is applied, the plurality of radiation detectors  40  can easily perform its measuring operation with a low cost with use of the single light source  3 , light source control device  4 , optical detector  6 , measurement unit  7 , analysis unit  8  and display device  9 . 
       Embodiment 6 
       [0077]    A dosage rate measurement system and a measurement method thereof in accordance with an embodiment 6 as another embodiment of the present invention, when the aforementioned embodiments 1 to 5 are suitably combined, can measure a dosage and a dosage rate with a high accuracy and a high sensitivity even in various facilities requiring a wide range of laying of optical fibers. The above description has been made in connection with the embodiments. However, it will be clear to those skilled in the art that the present invention is not limited to the embodiments, but may be changed or modified in various ways within the scope of the attached claims. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1 ,  34 ,  40  radiation detectors 
           2 ,  38  optical couplers 
           3  light source 
           4  light source control device 
           5 ,  35  optical fibers 
           6 : optical detector 
           7 : measurement unit 
           8 : analysis unit 
           9 : display device 
           10  light-generation timing cable 
           11 ,  29 ,  36 ,  39  control measurement units 
           12  radiation 
           13  OSL element 
           14  OSL light 
           15  pulse output 
           16  pulse height discrimination level 
           17  detection timing 
           18  pulse height value 
           19  environmental background 
           20  OSL light, light source and environmental backgrounds 
           21  stimulated light irradiation timing 
           22  time specification range 
           23  pulse height specification range 
           24  time/pulse height specification range 
           25  OSL light measurement data 
           26  background 
           27  relationship 
           28  wavelength analysis unit 
           30  OSL light wavelength 
           31  light source wavelength 
           32  wavelength analysis range 
           33  condensing lens 
           37  optical switch