JP2008111793A - Radioactivity conversion factor determination method, and detection limit determination method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radioactivity conversion factor determination method capable of obtaining a radioactivity conversion factor with small error. <P>SOLUTION: A measuring object in a clearance measuring device 1 is conveyed by a tray 36 whose center section is constituted by a network. Before radiations emitted from a measuring object are measured by detector units 10 and 12, a projection image is obtained by a fluorescent light 40 and an image camera installed at a position facing this, the height dimension is obtained by a height sensor 44, and a weight value is obtained by a lifting device 38 with a load cell. Scintillators of the detector units 10 and 12 are previously required to perform response according to the height level and width dimension in a reference radiation source. The response of the radioactivity conversion factor in converting the radioactivity from the count per second obtained by the scintillators of the detector units 10 and 12 is arranged and determined according to the density, height level, and width dimension of the measuring object, based on the projection area of the measuring object to the scintillators of the detector units 10 and 12. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for determining a radioactivity conversion coefficient and a method for determining a detection limit used in measuring radioactivity emitted from a radioactive substance.

Various substances carried out from a nuclear power plant or the like are disposed of by a predetermined method according to the level of radioactive contamination.
On the other hand, even if a substance is carried out from a nuclear power plant or the like, a clearance level is provided that it does not need to be handled as a radioactive substance if it is sufficiently small compared to the radiation level in nature. In order to determine whether or not the clearance level is exceeded, a radiation detection apparatus including a scintillator (radiation detector) is known.
The scintillator has a counting rate of radiation such as gamma rays (cps; count per
In order to evaluate the radioactivity (Bq), it is necessary to use the radioactivity conversion coefficient (Bq / cps).
The radioactivity conversion coefficient is determined as a response of the scintillator to a reference radiation source having a known radioactivity by simulation or experiment. However, even if it has the same radioactivity as the reference radiation source, the measurement condition for the scintillator is different for a measurement object with a shape and size different from that of the reference radiation source. Occurs. As a technique for minimizing such an error, a radioactivity measuring apparatus described in Patent Document 1 can be cited.
The radioactivity measuring apparatus described in Patent Document 1 determines a conversion coefficient suitable for a measurement object based on shape information and material information prepared in advance.

Japanese Patent Laying-Open No. 2005-140706 (FIGS. 4-1 and 4-2)

  However, various substances carried out from a nuclear power plant or the like have a wide variety of shapes, and there are cases where there is no appropriate information in the shape information prepared in advance. In such a case, the error of the conversion coefficient becomes large.

  On the other hand, in the radiation detection apparatus, a radioactivity detection limit that is a minimum limit at which significant radioactivity can be detected is set. The radioactivity detection limit is determined by various factors. One of the major factors is a background relative error that is radioactivity radiated from the surrounding environment. The background relative error is affected by a background count rate that varies for reasons other than statistical factors. Therefore, if the background count rate can be accurately grasped, the background relative error can be reduced and the detection limit can be reduced.

  The present invention has been made in view of such circumstances, and a radioactivity conversion coefficient determination method capable of obtaining a radioactivity conversion count with less error, and a detection limit by reducing the background relative error. It is an object of the present invention to provide a detection limit determination method capable of reducing the size of the detection limit.

In order to solve the above problems, the radioactivity conversion coefficient determination method and detection limit determination method of the present invention employ the following means.
That is, the radioactivity conversion count determination method according to the present invention is a radioactivity conversion coefficient that determines a radioactivity conversion coefficient for converting radioactivity from a count rate obtained by a radiation detector that measures radiation emitted from a radioactive substance. In the determination method, the radioactivity conversion coefficient is determined based on a projected area of the radioactive substance on the radiation detector.

  As a result of intensive studies by the present inventors, it has been found that the radioactivity conversion coefficient can be obtained with extremely high accuracy when arranged based on the projected area of the radiation substance on the radiation detector. Therefore, the radioactivity conversion coefficient is determined based on the projected area of the radioactive substance on the radiation detector.

  Furthermore, in the radioactivity conversion coefficient determination method of the present invention, the density of the radioactive substance, the height dimension of the radioactive substance in the direction toward the radiation detector, and the radiation in the extending direction of the radiation detector. According to the width dimension of the active substance, the radioactivity conversion coefficient is determined using an approximate expression or a database arranged based on the projected area.

  By considering the density of the radioactive substance, the height dimension of the radioactive substance, and the width dimension of the radioactive substance, the radioactivity conversion factor can be determined more accurately.

  The detection limit determination method of the present invention is a detection limit determination method for determining a detection limit of a radiation detector that measures radiation emitted from a radioactive substance using a background count rate. The background count rate is corrected based on the projected area of the radioactive substance.

There is a background relative error as an error in which the background count of radiation emitted from the surrounding environment varies due to causes other than statistical factors.
On the other hand, as a result of intensive studies by the present inventors, it was found that the background count rate depends on the projected area of the radiation substance on the radiation detector. Therefore, the background relative error can be minimized by correcting the background count rate based on the projected area of the radioactive substance on the radiation detector.

  Further, the radioactivity conversion coefficient determination program of the present invention is executed by a computer that determines a radioactivity conversion coefficient for converting radioactivity from a count rate obtained by a radiation detector that measures radiation radiated from a radioactive substance. In the radioactivity conversion coefficient determination program made possible, the radioactivity conversion coefficient is determined based on a projected area of the radioactive substance on the radiation detector.

  As a result of intensive studies by the present inventors, it has been found that the radioactivity conversion coefficient can be obtained with extremely high accuracy when arranged based on the projected area of the radiation substance on the radiation detector. Therefore, the radioactivity conversion coefficient is determined based on the projected area of the radioactive substance on the radiation detector.

  In addition, the detection limit determination program of the present invention uses a background count rate to determine the detection limit of a radiation detector that measures radiation emitted from a radioactive substance, and is a computer-executable detection limit. In the determination program, the background count rate is corrected based on a projected area of the radioactive substance on the radiation detector.

As a result of intensive studies by the present inventors, it has been found that the background count rate depends on the projected area of the radiation material on the radiation detector. Therefore, the background relative error can be minimized by correcting the background count rate based on the projected area of the radioactive substance on the radiation detector.

  Further, the radiation measuring apparatus of the present invention determines a radiation detector that measures radiation emitted from a radioactive substance, a radiation conversion coefficient that converts the radioactivity from a count rate obtained by the radiation detector, and The radiation measurement apparatus includes a calculation unit that calculates the radioactivity conversion coefficient based on a projected area of the radioactive substance on the radiation detector.

  As a result of intensive studies by the present inventors, it has been found that the radioactivity conversion coefficient can be obtained with extremely high accuracy when arranged based on the projected area of the radiation substance on the radiation detector. Therefore, the radioactivity conversion coefficient is determined based on the projected area of the radioactive substance on the radiation detector.

  The radiation detection apparatus of the present invention includes a radiation detector that measures radiation radiated from a radioactive substance, and an arithmetic unit that calculates radioactivity from a count rate obtained by the radiation detector. In the radiation measurement apparatus that calculates a detection limit of the radiation detector using a background count rate, the calculation unit is configured to calculate the background based on a projected area of the radioactive substance on the radiation detector. The counting rate is corrected.

As a result of intensive studies by the present inventors, it has been found that the background count rate depends on the projected area of the radiation material on the radiation detector. Therefore, the background relative error can be minimized by correcting the background count rate based on the projected area of the radioactive substance on the radiation detector.
In addition, the radiation detection apparatus of this invention which correct | amends a background count rate can be combined with the above-mentioned radiation detection apparatus which determines a radioactivity conversion coefficient.

Since the radioactivity conversion coefficient is determined based on the projected area of the radioactive substance on the radiation detector, the radioactivity conversion coefficient can be obtained with high accuracy. In addition, since the radioactivity concentration can be obtained with high accuracy in this way, it can be strictly determined whether or not the clearance level is exceeded.
Since the background count rate is corrected based on the projected area of the radioactive substance on the radiation detector, the background relative error can be minimized. In addition, since the background relative error can be set small, the detection limit can be reduced, the detectable radiation level can be lowered, and the accuracy with respect to the clearance level can be improved.

Embodiments according to the present invention will be described below with reference to the drawings.
[First Embodiment]
FIG. 1 shows a clearance measuring device (radiation detection device) 1.
The clearance measuring device 1 judges whether or not the measurement object (radioactive substance) exceeds the clearance level that it is not necessary to treat it as a radioactive substance if it is sufficiently small compared to the radiation level in nature. It is a device to do.
The clearance measuring device 1 includes an Fe tunnel type shield (hereinafter simply referred to as “shield”) 3, a transport device 5 that transports a measurement object into the shield 3, and a control that controls operations of various devices. A board 7 and a computer 9 for performing calculation of measurement data and the like are provided.

In the shield 3, an upper detector unit 10 for detecting radiation and a lower detector unit 12 are provided.
The upper detector unit 10 is provided above the transport conveyor 14 of the transport device 5 and can be moved up and down by a lifting device 16. The lifting device 16 is of a ball screw type, and the upper detector unit 10 screwed to the feed screw 16a moves up and down by rotating the feed screw 16a. The operation of the lifting device 16 is controlled by the computer 9 and the control panel 7.
The lower detector unit 12 is fixed below the transport conveyor 14 at a position facing the upper detector unit 10.
The outputs of the detector units 10 and 12 are transmitted to the computer 9.

FIG. 2 shows a radiation detection unit 18 provided in each detector unit 10, 12. The radiation detector 18 is provided with a scintillator 20 having a detector on the side facing the measurement object. As the scintillator, a plastic scintillator or a NaI (Tl) scintillator is used to detect γ rays emitted from the measurement object.
The scintillators 20 are provided in a state where three scintillators 20 are arranged in a direction orthogonal to the moving direction of the measurement object. Therefore, in FIG. 2, the direction perpendicular to the paper surface is the moving direction of the measurement object. That is, as shown in FIG. 3, the measurement object 37 placed on the tray 36 is conveyed in the direction of arrow A, and radiation is detected by the scintillators 20 of the upper detector unit 10 and the lower detector unit 12.
As shown in FIG. 2, a lead shielding cover 22 is provided around the scintillator 20 and covers the scintillator 20 with its detection surface exposed. Each scintillator 20 is provided with a photomultiplier tube 24 to amplify the light obtained from each scintillator 20.

As shown in FIG. 1, the transport device 5 includes a transport conveyor 14 that extends through the shield 3 in one direction. The conveyor 14 is configured by a plurality of rollers 30. A plurality of rollers 30 are provided on both sides in the transport direction. Each roller 30 is rotationally driven by the conveyance motor unit 34 by, for example, chain driving.
On the upper surface of the conveyor 14, a tray 36 for placing an object to be measured is installed. The tray 36 is, for example, a 1 m square plate having a substantially square shape. The tray 36 includes a frame that forms four sides, and a measurement object is placed on a central net surrounded by the frame. The frame body of the tray 36 travels on the rollers 30 provided on both sides of the above-described transport path. The rollers 30 are provided on both sides of the conveyance path, and the central portion of the tray 36 is formed of a net, so that light from a fluorescent lamp 40 described later can pass through the central portion of the tray 36.

  A lifting device 38 with a load cell is provided on the upstream side (left side in FIG. 1) of the conveyor 14. The lifting device with load cell 38 is raised when measuring the weight of the measurement object, and lifts the tray 36 and the measurement object. Thereby, the weight of the measurement object placed on the tray 36 is measured. The output of the load cell of the lifting device with load cell 38 is transmitted to the computer 9.

  A fluorescent lamp 40 is installed below the conveyor 14 and upstream of the shield 3 (left side in FIG. 1). An image camera 42 is installed at an upper position facing the fluorescent lamp 40. As the image camera 42, for example, a CCD camera having 240,000 pixels can be used. An image projected on the upper side of the measurement object is obtained by photographing the measurement object illuminated by the fluorescent lamp 40 from below with the image camera 42. The projected image is processed by the computer 9 to obtain the projected area of the projected image and the width of the measurement object.

  A height sensor 44 is provided on the downstream side (right side in FIG. 1) of the fluorescent lamp 40. The height sensor 44 is an optical type. The output of the height sensor 44 is transmitted to the computer 9, and the height dimension of the measurement object is calculated in the computer 9.

  The control panel 7 controls the raising and lowering of the upper detector unit 10 and the operation of the transport device 5. Power is supplied to the control panel 7 from a power source 46.

The computer 9 obtains outputs of the detector units 10 and 12, the image camera 42, the load cell of the lifting device with load cell 38, and the height sensor 44, and performs various calculations. Further, the computer 9 sends a command value necessary for control to the control panel 7.
The computer 9 includes a display 9a and displays measurement information and the like. The main body 9b of the computer 9 is provided with a storage unit and a calculation unit.
The storage unit is a non-volatile memory such as a hard disk device, a magneto-optical disk device, a flash memory, a readable memory such as a CD-ROM, a volatile memory such as a RAM (Random Access Memory), or these It can be realized by a combination.
As will be described later, the calculation unit calculates the radioactivity concentration (Bq / g) and the detection limit based on the data obtained from each detector unit 10, 12 or the like. In this calculation, the radioactivity conversion coefficient and the correction value of the background count rate are calculated. The calculation unit is composed of a memory and a CPU (central processing unit), and loads a program for calculating the radioactivity concentration and detection limit into the memory and executes the program.

  The storage unit of the computer 9 stores an approximate expression of the radioactivity conversion coefficient obtained by the method described below. When a program for obtaining the radioactivity concentration is executed by the calculation unit of the computer 9, the approximate expression is referred to by the program. The program for obtaining the radiation density includes a radiation density conversion coefficient program for determining the radiation density conversion coefficient, for example, as a subroutine.

The radioactivity conversion coefficient is used when obtaining the radioactivity concentration as shown in the following equation.
Radioactivity concentration (Bq / g)
= [Radioactivity conversion coefficient (Bq / cps) x Net counting rate (cps)] / Weight (g) (1)
Here, the net count rate is obtained by subtracting the background count rate from the total count rate measured by the detector units 10 and 12. The weight means the weight of the measurement object.

The radioactivity conversion coefficient is obtained as a response of the scintillator 20 to a reference radiation source having a known radioactivity by a numerical simulation or experiment using the Monte Carlo method. This radioactivity conversion coefficient is arranged based on the projected area of the measurement object on the scintillator 20 according to the density of the measurement object, the height dimension of the measurement object, and the width dimension of the measurement object.
Here, the height dimension of the measurement object means the maximum dimension in the direction toward the scintillator 20. The width dimension of the measurement object means the maximum dimension in the extending direction of the scintillator 20, that is, the longitudinal direction of the three scintillators 20 arranged in line.

4 and 5 show the radioactivity conversion coefficients obtained by the numerical simulation by the Monte Carlo method. 4 and 5, the horizontal axis represents the projected area (cm ² ) of the measurement object, and the vertical axis represents the radioactivity conversion coefficient (Bq / cps).
4A and 4B are graphs when the density of the measurement object is 7.9 g / cc. FIG. 4A is a graph when the width dimension of the measurement object is 10 cm, and FIG. 4B is a graph when the width dimension of the measurement object is 20 cm. 4 (a) and 4 (b) show three types of plot points and an approximate curve. A black circle and this approximate curve have a height dimension of 0.3 cm, and a white circle and this approximate curve have a height dimension. 1 cm, a triangle mark and this approximate curve mean a height dimension of 2 cm.
FIGS. 5A and 5B are graphs when the density of the measurement object is 4.5 g / cc. FIG. 5A is a graph when the width dimension of the measurement object is 10 cm, and FIG. 5B is a graph when the width dimension of the measurement object is 20 cm. FIGS. 5A and 5B show three types of plot points and approximate curves. A black circle and this approximate curve have a height dimension of 0.3 cm, and a white circle and this approximate curve have a height dimension. 1 cm, a triangle mark and this approximate curve mean a height dimension of 2 cm.

  The approximate expression obtained as shown in FIGS. 4 and 5 is stored in the storage unit of the computer 9 in the form of a polynomial, for example, and is referred to when the program is executed by the calculation unit of the computer 9.

Next, the operation of the clearance measuring apparatus having the above configuration will be described.
First, as shown in FIG. 1, a measurement object that may be radioactively contaminated is placed on the tray 36. And the weight before and behind installation of a measuring object is measured with the raising / lowering apparatus 38 with a load cell. These measurement outputs are sent to the computer 9, and the weight of the measurement object is obtained from each measurement output.

Next, the transport motor unit 34 of the transport device 5 is driven to rotate the rollers 30 of the transport conveyor 14. Thereby, the tray 36 is moved to the shield 3 side.
When the tray 36 is positioned on the fluorescent lamp 40, a projection image of the measurement object irradiated by the fluorescent lamp 40 is acquired by the image camera 42, and this projection image is sent to the computer 9. The computer 9 calculates the projected area of the measurement object by image processing. Further, the computer 9 calculates the width dimension of the measurement object by image processing. The maximum value is adopted as the width dimension of the measurement object.

  Thereafter, the tray 36 moves to a position below the height sensor 44, and the height dimension of the measurement object is measured at this position. The height sensor 44 irradiates the measurement object with a light beam, and sends the height output at the position where the measurement object does not exist and the height output at the position where the measurement object exists to the computer 9. The computer 9 calculates the height of the measurement object from the difference between the height output at the position where the measurement object does not exist and the height output at the position where the measurement object exists, and calculates the maximum dimension at this time as the measurement object. The height of the object.

  The tray 36 is guided into the shield 3 and is positioned between the upper detector unit 10 and the lower detector unit 12. At this time, the upper detector unit 10 is lowered by a command from the computer 9 and the control panel 7 and brought closer to the measurement object on the tray 36. The interval between the measurement object and the upper detector unit 10 is preferably as narrow as possible, and is set to, for example, about several mm. The tray 36 stops between the detector units 10 and 12, and the scintillator 20 detects the radiation. The detection output is sent to the computer 9. As shown in FIG. 2, the scintillator 20 extends one-dimensionally in the width direction orthogonal to the conveyance direction of the tray 36, so the tray 36 is stopped at three different positions in the conveyance direction, By measuring radiation at the position, it is possible to measure in two dimensions.

  When the detection of radiation is finished by each of the detection units 10 and 12, the tray 36 is conveyed to the outside of the shield 3 and the measurement is finished.

  The computer 9 calculates the net count rate by subtracting the background count rate from the count rates obtained from the upper detector unit 10 and the lower detector unit 12, and calculates the radioactivity concentration using equation (1). Then, it is determined whether or not the measured radioactivity concentration exceeds the clearance level, and the result, radioactivity concentration, and the like are displayed on the display 9a.

In the calculation part of the computer 9, the radioactivity conversion coefficient used for Formula (1) is determined by the approximate expression obtained using FIG.4 and FIG.5. Below, an example of the determination method of a radioactivity conversion coefficient is demonstrated.
When the weight obtained by the lifting device with load cell 38 is 10 kg, the height dimension obtained by the height sensor 44 is 1.8 cm, the projection area obtained by the image camera 42 is 980 cm ² and the width dimension is 15 cm. The radioactivity conversion coefficient is interpolated from the approximate curve as follows.

First, the density ρ of the measurement object is calculated by the computer 9, and the density ρ is 10 kg / (980 cm ² × 1.8 cm) = 5.669 g / cc.
Next, from the approximate curves obtained from FIG. 4 and FIG. 5, an approximate curve when the height dimension that sandwiches the height dimension of 1.8 cm is 1 cm and 2 cm is selected, and the radiation at 980 cm ² is obtained. The performance conversion coefficient CF is calculated. That is, when the radioactivity conversion coefficient CF is expressed by CF (ρ (density), H (height dimension), S (projected area), L (width dimension)), the following six radioactivity conversion coefficients CF are calculated. To do.
CF (4.5, 1, 980, 10) CF (7.9, 1, 980, 10)
CF (4.5, 1, 980, 20) CF (7.9, 1, 980, 20)
CF (4.5, 2, 980, 10) CF (7.9, 2, 980, 10)
CF (4.5, 2, 980, 20) CF (7.9, 2, 980, 20)

Then, these eight CFs are used to interpolate to a width dimension of 15 cm.
CF (4.5, 1, 980, 15)
= [CF (4.5, 1, 980, 20)-CF (4.5, 1, 980, 10)] / (20-10) x (15-10)
+ CF (4.5, 1, 980, 10)
CF (4.5, 2, 980, 15), CF (7.9, 1, 980, 15), and CF (7.9, 2, 980, 15) are similarly calculated by interpolation.

Then, the above four CFs are used to interpolate to a height of 1.8 cm.
CF (4.5, 1.8, 980, 15)
= [CF (4.5, 2, 980, 15)-CF (4.5, 1, 980, 15)] / (2-1) x (1.8-1)
+ CF (4.5, 1, 980, 15)
The same interpolation is performed for CF (7.9, 1.8, 980, 15).

Then, using the above two CFs, the density is interpolated to 5.669 g / cc.
CF (5.669, 1.8, 980, 15)
= [CF (7.9, 1.8, 980, 15)-CF (4.5, 1.8, 980, 15)] / (7.9-4.5) x (5.669-4.5)
+ CF (4.5, 1.8, 980, 15)
In this way, the radioactivity conversion coefficient CF corresponding to the density, height dimension, projected area, and width dimension of the measurement object can be finally determined.

According to the present embodiment, since the projection area of the measurement object is obtained by the image camera 42 and the radioactivity conversion coefficient is determined based on the projection area, the radioactivity conversion coefficient having no error is obtained. Accurate radioactivity concentration can be measured. Since an accurate radioactivity concentration can be obtained, it can be strictly determined whether or not the clearance level is exceeded.
In addition, since the radioactivity conversion coefficient is obtained using an approximate curve with the density, height dimension and width dimension of the measurement object as parameters, a radioactivity conversion coefficient without errors can be obtained more accurately. Radioactivity concentration can be measured.

FIG. 6 shows a comparison between the case where the projection area of the measurement object is used as in this embodiment and the case where the projection area is not used. In the graph shown in the figure, from the left, when the radioactivity concentration is obtained without measuring the projected area of the measurement object, the radioactivity concentration is obtained using the projection area as in this embodiment. And actual radioactivity concentration (true value) are shown. The measurement object has a density of 7.9 g / cc and a height dimension of 10 cm. Further, when the projection area of the measurement object is not used, it is assumed that the measurement object is the same size as the tray 36 and the measurement object is assumed to be 100 cm square. The height dimension was set to 10 cm as in the case where the projected area was used.
As can be seen from the graph shown in FIG. 6, when the projection area is used as in this embodiment, it can be seen that it is very close to the true value. On the other hand, when the projected area is not used, the radioactivity concentration is greatly evaluated, and even if the object to be measured does not exceed the clearance level, it is determined that the clearance level is exceeded. .

  In this embodiment, the approximate curve is used when obtaining the radioactivity conversion coefficient. However, the data shown in FIGS. 4 and 5 is stored in the storage unit of the computer 9 as a database, and this database is stored by a program. It may be referred to.

ここで、
A_LD：検出限界(Bq)，κ：定数（=3），ｔ_T：放射能濃度確認対象物の測定時間(s)，
ｎ_Ｂ：バックグラウンド（BG）計数率(s^-1)，ｔ_Ｂ：BG測定時間(s)，
ＣＦ：放射能換算係数(Bq／s^-1)，ｒ_１：BG変動に起因する相対誤差(−)，
ｒ_２：放射能換算係数の相対誤差(−)，
である。 [Second Embodiment]
Next, a second embodiment of the present invention will be described. This embodiment is characterized when calculating a radioactivity detection limit (hereinafter referred to as “detection limit”) that is the minimum limit at which significant radioactivity can be detected. The configuration of the clearance measuring apparatus 1 is the first embodiment. Since it is the same as that, its description is omitted.
As for the detection limit, the basic formula of the Japan Atomic Energy Society standard has been published and is as follows.

here,
A _LD : Detection limit (Bq), κ: Constant (= 3), t _T : Measurement time (s) of the target for radioactivity concentration confirmation,
n _B : Background (BG) counting rate (s ⁻¹ ), t _B : BG measurement time (s),
CF: Radioactivity conversion coefficient (Bq / s ⁻¹ ), r ₁ : Relative error (−) due to BG fluctuation,
r ₂ : Relative error (−) of radioactivity conversion coefficient,
It is.

The above formula is described in the detection limit determination program according to the present embodiment, and this program is executed by the calculation unit of the computer 9.
As shown in the above equation, r ₁ is a relative error caused by background fluctuation, and is determined by the background count rate n _B that fluctuates due to a cause other than a statistical factor. In the present embodiment, as illustrated in FIG. 7, the background count rate n _B is corrected based on the relationship between the projected area of the measurement object and the relative value of the background count rate, thereby causing the background fluctuation. Relative error r ₁ is minimized.

The horizontal axis in FIG. 7 indicates the projected area of the measurement object with respect to the upper detector unit 10 (see FIG. 1). This projected area indicates the number when 22500 (150 × 150) cm ² iron blocks are placed.
The vertical axis | shaft of FIG. 7 shows the relative value of a background count rate. The relative value of the background count rate is the background count rate when a measurement object having a predetermined projected area is placed on the tray 36, when the measurement object does not exist on the tray 36 (that is, the projected area is zero). In the case of ()) divided by the background count rate.
As shown in the figure, the relative value of the background count rate decreases as the projected area of the measurement object increases. As shown in FIG. 3, this is considered due to the fact that the background radiated from the shield 3 (see FIG. 1) is blocked by the measurement object 37.
The approximate expression or database shown in FIG. 7 is stored in the storage unit of the computer 9 and is referred to by a program executed by the calculation unit.

A method of correcting the background count rate n _B when calculating the detection limit is as follows.
As described in the first embodiment, the projection area of the measurement object is obtained from the image camera 42. The computing unit of the computer 9 acquires the relative value of the background count rate corresponding to the obtained projection area from the approximate expression or database stored in the storage unit. Then, the relative value, by multiplying the background count rate n _B, background corrected count rate n _B.

Thus, since it was decided to correct the background count rate n _B in accordance with the projected area of the measurement object, it is possible to adopt a value close to the true background count rate during the measurement. As a result, the relative error r ₁ caused by the background fluctuation can be set small, and as a result, the detection limit A _LD can be reduced.
According to the present embodiment, since the detection limit can be reduced, the detectable radiation level can be lowered, and the accuracy with respect to the clearance level can be improved.

  In the first embodiment and the second embodiment, the projection area is obtained using the image camera 42. However, the present invention is not limited to this, and is based on a photoelectric sensor, a laser system, and an X-ray (CT). It is good also as obtaining a projection area.

It is the schematic which showed the clearance measuring apparatus concerning one Embodiment of this invention. It is sectional drawing which showed the radiation detection part. It is the perspective view which showed the positional relationship of a measuring object and a scintillator. It is the graph which showed the radioactivity conversion coefficient when a density is 7.9 g / cc with respect to the projection area. It is the graph which showed the radioactivity conversion coefficient in case a density is 4.5 g / cc with respect to the projection area. It is the graph which showed the radioactivity density | concentration obtained using the projection area of a measuring object compared with the radioactivity density | concentration obtained without using a projection area. It is the graph which showed the relative value of the background count rate with respect to the projection area of a measuring object.

Explanation of symbols

1 Clearance measuring device (radiation detector)
3 Shield 5 Transport device 7 Control panel 9 Computer 10 Upper detector unit 12 Lower detector unit 14 Transport conveyor 16 Lifting device 18 Radiation detector 20 Scintillator 36 Tray 37 Measurement object (radioactive substance)
38 Lifting Device with Load Cell 40 Fluorescent Lamp 42 Image Camera 44 Height Sensor

Claims (7)

In the radioactivity conversion coefficient determination method for determining the radioactivity conversion coefficient for converting radioactivity from the count rate obtained by the radiation detector that measures the radiation emitted from the radioactive material,
The radioactivity conversion coefficient determination method, wherein the radioactivity conversion coefficient is determined based on a projected area of the radioactive substance on the radiation detector.   Depending on the density of the radioactive substance, the height dimension of the radioactive substance in the direction toward the radiation detector, and the width dimension of the radioactive substance in the extending direction of the radiation detector, the projected area The radioactivity conversion coefficient determination method according to claim 1, wherein the radioactivity conversion coefficient is determined using an approximate expression or database arranged based on the data. In a detection limit determination method for determining a detection limit of a radiation detector that measures radiation emitted from a radioactive substance using a background count rate,
A detection limit determination method, wherein the background count rate is corrected based on a projected area of the radioactive substance on the radiation detector. In a radioactivity conversion coefficient determination program that can be executed by a computer to determine a radioactivity conversion coefficient for converting radioactivity from a count rate obtained by a radiation detector that measures radiation emitted from a radioactive substance,
A radioactivity conversion coefficient determination program for determining the radioactivity conversion coefficient based on a projected area of the radioactive substance on the radiation detector. In a computer-executable detection limit determination program that uses a background count rate to determine the detection limit of a radiation detector that measures radiation emitted from a radioactive material,
A detection limit determination program, wherein the background count rate is corrected based on a projected area of the radioactive substance on the radiation detector. A radiation detector for measuring the radiation emitted from the radioactive material;
A radiation conversion coefficient for converting the radioactivity from the counting rate obtained by the radiation detector, and a calculation unit for calculating the radioactivity,
In a radiation measurement apparatus comprising:
The calculation unit determines the radioactivity conversion coefficient based on a projected area of the radioactive substance on the radiation detector. A radiation detector for measuring the radiation emitted from the radioactive material;
A calculation unit that calculates the radioactivity from the count rate obtained by the radiation detector,
In the radiation measuring apparatus that calculates the detection limit of the radiation detector using a background count rate,
The calculation unit corrects the background count rate based on a projected area of the radioactive substance on the radiation detector.

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