JPH0980156A - Radiation dose measuring method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure a surface radiation dose of a radioactive substance housing container accurately in a short time with a compact structure. SOLUTION: A long scintillation fiber 30 is wound spirally on the outer circumference of a container 4 for measurement for housing a container 2 holding a radioactive substance. When a radiation is admitted into the fiber, light is generated and is propagated to both ends of the fiber. A propagation time difference is measured by a time difference measuring circuit 38 to specify an incident location. Simultaneously, the frequency of incidence is counted to obtain a distribution of radiation dose. A small scale of a circuit system achieves a lower fault rate and the simplification of the structure, lower manufacturing costs. This makes possible measurement of the whole of a surface thereby assuring an accurate measurement without leakage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring a radiation dose, and more particularly to a method and an apparatus for measuring a surface radiation dose of a container containing a radioactive substance or the like. The present invention is applicable to, for example, nuclear power plants, nuclear fuel material handling facilities, R
I Implemented at handling facilities.

[0002]

2. Description of the Related Art When a radioactive substance or the like is transferred, the substance is stored in an optimal radioactive substance transport container (cask) according to the transfer mode, the radiation level and the like. For such containers (hereinafter simply referred to as "containers"), from the viewpoint of radiation control, the measurement of radiation dose on the container surface (hereinafter simply referred to as "surface measurement") is obligatory by law.

[Prior Art 1] FIG. 1 is a block diagram of a surface measuring apparatus according to prior art 1 for measuring a surface. In the figure, a container 2 which is a bucket or a drum made of paper or metal for containing radioactive waste is placed in the center of a measuring container 4. The diameter and height of the container are each several tens of cm to several m.

A radiation detector 6 including a plurality of GM counters is installed on the wall surface of the measuring container 4. The detector has a diameter of several cm and a length of several tens of cm. In the case of a GM counter tube, the gas enclosed inside the counter tube is ionized by radiation (eg, γ-rays), and this is taken out as an electric signal. The electric signal is applied to the counting circuit 8 as the output of the detector, and the γ particles are counted. A radiation dose display unit 10 is placed at the subsequent stage of the counting circuit 8, and the measurement result of each radiation detector 6 is displayed here. The normal maximum value is used as the legal measurement result.

The radiation detector 6 may be composed of a scintillator such as plastic in addition to the GM counter tube. At this time, the scintillator receives gamma rays to generate scintillation light, and photoelectrically converts this to
Perform the same measurement.

[Prior Art 2] FIG. 2 is a block diagram of a surface measuring apparatus according to prior art 2 for measuring a surface. 1 is different from FIG. 1 in that there is only one radiation detector 6 and that a rotation drive device 20 including a motor and the like is added to the lower portion of the measurement container 4.

In the structure shown in FIG. 1, surface measurement is performed while rotating the measuring container 4 as a turntable.
At this time, when the container is long in the axial direction, the radiation detector 6 is appropriately shifted in the axial direction, and the entire surface is measured without fail. The measurement results are sequentially displayed or recorded, and this maximum value is usually the legal measurement value.

The above is a conventional general surface measuring apparatus.
These devices are suitable for remote operation, but when it is known in advance that the radiation level is sufficiently low, a worker may manually perform surface measurement by a survey meter. Although the legal maximum measurement of the radiation dose has been described here, in addition to this, the distribution measurement of the surface radiation dose is frequently performed for the management of the radiation substance.

[0009]

[Problems to be Solved by the Invention]

[Prior Art 1] In order to cover the entire surface with the radiation detector 6, it is necessary to provide a considerable number of radiation detectors 6. Accordingly, the number of counting circuits 8 also increases. For this reason, not only the cost for manufacturing the device is increased, but also the circuit system is complicated, so that the frequency of occurrence of failures cannot be ignored. A certain amount of maintenance work is required for adjustment and repair. Furthermore, even with a certain number of radiation detectors 6, unless the surface is completely covered, there will be leaks in the measurement of the surface between the detectors.

[Prior Art 2] A rotary drive mechanism is required, and the apparatus becomes complicated and large. Also, it takes several minutes to measure the entire surface. Some nuclear fuel material handling facilities use about 10,000 bucket-sized containers annually, and the measurement time accounts for a large proportion of man-hours.

The above is the problem of each device. In addition, in the case of manual measurement, the work man-hour further increases.

[Purpose] An object of the present invention is to solve the above problems. Therefore, in the present invention, a thin line-shaped radiation detection material is provided so as to surround the object to be detected such as a container,
The signals appearing at both ends are analyzed.

[0013]

A method for measuring a radiation dose of a radiation dose is a method in which a thin-line radiation detecting material is three-dimensionally arranged around an object to be detected, and a radiation detection signal reaching both ends of the detecting material is detected. Based on the comparison result, the distribution of the surface radiation dose of the detected object is measured.

The term "three-dimensionally arranged" means that it is simply arranged in a straight line, and preferably, the vertical and horizontal heights of the respective parts of the detection material are preferably along the vicinity of the surface of the object to be detected. Laying or arranging so that

In this structure, when the radiation flying from the surface of the object to be detected is incident on the place where the radiation detecting material is present,
From that point, the detection signal propagates toward both ends of the detection material. When this signal reaches both ends, a difference occurs in intensity, arrival time, etc., so that the distribution of the surface radiation dose of the detected object can be measured based on this difference.

On the other hand, the radiation dose measuring apparatus of the present invention is connected to both ends of the radiation detecting means in the form of a thin wire which is arranged around the object to be detected and the radiation detecting means, and is transmitted to these both ends. Signal processing means for comparing the detection signals. The radiation detection means may be, for example, a scintillation fiber, where the detection signal is scintillation light due to the radiation incident on the fiber. An example of scintillation fiber is plastic scintillation fiber.

In another aspect, the radiation detecting means is formed by connecting a plurality of scintillation fibers in a band shape, and the signal processing means uses the plurality of scintillation fibers as one detection material at both ends thereof. May be connected. To be connected in a strip shape means to fix the fibers in a state of being arranged in parallel by adhesion or the like. In this case, the thickness of the strip-shaped detection material (hereinafter, referred to as "strip-shaped fiber") made of a plurality of fibers is equal to the fiber diameter. The signal processing means is connected to both ends of the strip-shaped fiber as a unit of the detection material. Therefore, it is not specified which of the plurality of scintillation fibers constituting the strip fiber the radiation has entered. When incident on either fiber, the detection signal propagates to both ends of the strip fiber. The signal processing means compares the detection signals at both ends of the strip fiber.

At this time, the radiation detecting means may be arranged in a state of spirally surrounding the object to be detected. For example, when the object to be detected has a cylindrical shape such as a drum can, the radiation detecting means may have a structure in which it gradually descends from one point at the upper end of the cylinder while spirally tracing the surface of the cylinder and reaches one point at the lower end. Consider the structure of the spiral spring or the solenoid.

When radiation enters the radiation detecting means in this state, the detection signal propagates up and down the spiral.
These are input to the signal processing means through both ends of the detection means, and predetermined signal processing is performed.

At this time, the radiation detecting means may be spirally arranged so as to surround the object to be detected without any gap and without overlapping. When the object to be detected has a cylindrical shape, the radiation detection means starting from a point on the upper end of the cylinder is wrapped around the cylinder with a slight inclination so that when it goes around the cylinder, it just shifts downward by its own width. It should be done. That is, the width of the radiation detecting means is w and the circumference of the cylinder is L.
Then, the winding inclination of the radiation detecting means may be set to w / L. If this inclination is maintained, the radiation detecting means will not intersect (that is, overlap).

According to this structure, the radiation detecting means covers the entire surface of the object to be detected. Therefore, omission of detection is avoided.
Further, since the radiation detecting means do not overlap with each other, the radiation incident position can be uniquely specified.

As one mode of the radiation dose measuring device, the signal processing means measures a difference in arrival time of detection signals transmitted to both ends of the radiation detecting means. The arrival time difference is, for example,
The time when the detection signal reaches one end is set to time zero, and the time when the detection signal reaches the other end is measured and obtained. Further, at this time, the signal processing means calculates the incident intensity and the incident point of the radiation based on the detection signals transmitted to both ends and the arrival time difference.

Here, for example, when the radiation detecting means is a scintillator, the primary detection signal is light. If this is converted into an electric signal and counted, the intensity of the radiation can be obtained. On the other hand, since the velocity of light propagating in the scintillator is known, the displacement of the incident point from the center of the radiation detecting means can be calculated from the product of the arrival time difference and this velocity. As a result, the incident point is specified.

One aspect of the radiation dose measuring apparatus of the present invention is a measuring container for accommodating an object to be detected, a scintillation fiber spirally arranged around the measuring container, and the scintillation. A photoelectric conversion unit connected to both ends of the fiber and converting a scintillation light signal transmitted to both ends into an electric signal for each end, and a time difference for calculating a time difference when the scintillation light reaches the both ends based on the TOF method. A calculation unit, an intensity calculation unit that calculates the intensity of the radiation incident on the scintillation fiber from the count value of the electric signal, and a distribution that derives the surface radiation distribution of the detection target based on the calculated time difference and intensity. And a derivation unit.

In this structure, the object to be detected is first installed in the measuring container. When the detected object emits radiation,
This radiation enters any part of the scintillation fiber and emits scintillation light which is a detection signal. This light propagates towards both ends of the scintillation fiber and is converted into an electrical signal when it reaches each end. The arrival time difference of scintillation light is TOF
It is derived by the method (Time Of Flight). From this time difference, the incident point is found. On the other hand, since the radiation intensity is known from the count value of the electric signal, the surface radiation distribution of the detected object can be derived from the position information and the intensity information.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG . FIG. 3 is a configuration diagram of the surface measuring apparatus according to the first embodiment for surface measurement. Here, the difference from the configuration of FIG. 1 will be mainly described.

In the figure, the container 2 is placed in the center of the measuring container 4. The material of the measuring container 4 is not particularly limited,
When performing precision measurement, a material with low density is used so as not to attenuate radiation. On the contrary, if you want to measure after weakening the radiation to some extent, such as when the level of the radiation emitted from the container 2 is too high, make it with a metal such as lead,
Adjust the wall thickness.

The inner diameter of the measuring container 4 is set to be slightly larger than that of the container 2. This is because the container 2 can be installed and taken out, and continuous measurement of a plurality of containers 2 is taken into consideration. Measuring container 4
A positioning mechanism for the container 2 may be provided inside the container.

A thin plastic scintillation fiber (hereinafter "PSF") is formed on the outer wall surface of the measuring container 4.
30) is wound in a spiral shape. As an example, the core part of the PSF 30 is made of polystyrene, and the clad part is made of polymethylmethacrylate (PMMA). In FIG. 3, the PSF 30 is drawn with a gap for ease of viewing, but in this embodiment, this gap is set to zero and the PSF 30 is tightly wrapped. Originally, PSF30 is not suitable for bending,
As in this embodiment, relatively gentle bending is possible.
For this bending work, the PSF 30 having a thickness of about 1 mm is adopted in this embodiment.

Both ends a and b of the PSF 30 are connected to the signal processing circuit 32. FIG. 4 is an internal configuration diagram of the signal processing circuit 32. As shown in the figure, photoelectric conversion elements 34a and 34b are attached to both ends a and b of the PSF 30, respectively. The photoelectric conversion elements 34a and 34b are photomultiplier tubes (PMT) or avalanche photodiodes (A).
PD) etc., and transmits the scintillation light to the electrical signal S.
a, converted to Sb. PSF 30 and photoelectric conversion element 34
The entire a and b are covered with a light shielding material (not shown).
The electric signals are amplified by the amplifier circuits 36a and 36b and input to the time difference measuring circuit 38 as S'a and S'b, respectively.

The time difference measuring circuit 38 uses the electric signal S'a as a reference signal to calculate the time difference by the TOF method. This circuit includes a time wave height converter (TAC) and a wave height analyzer (MCA) which are not shown, and counting of scintillation light is also performed here. Here, the time when S′a arrives is set to zero, and the arrival time of S′b is measured. S'a
S'b so that it always reaches the time difference measuring circuit 38 first.
A delay circuit 40 is inserted in the transmission path of. The minimum value of the delay time D in the delay circuit 40 is determined so that S′a arrives first even when the radiation is incident on the part of the PSF 30 closest to the end b. If the time for which the scintillation light propagates over the entire length of the PSF 30 is t _L , D> t _L is the condition of D.

Although the time difference is obtained in this way, the time difference measuring circuit 38 also measures the radiation intensity from the count value of the radiation. The time difference and the intensity are given to the radiation dose display unit 42. For example, when the one end a of the PSF 30 is the origin, the distance on the PSF 30 at the incident point is the horizontal axis, and the radiation intensity is plotted on the vertical axis. it can. At this time,
Since the corresponding point on the surface of the container 2 is found from the incident point, the radiation dose distribution on the surface becomes clear.

The above is the outline of the present embodiment. According to this surface measuring device, since the entire surface is covered by the PSF 30, there is no measurement omission. Further, it is not necessary to provide a large number of radiation detectors and counting circuits. No special mechanical system is required, and the total measurement time is short, so the measurement time is short. As a result, the problems of the prior arts 1 and 2 are solved.

The following applications or modifications can be considered for this embodiment.

(1) This device is used only for surface measurement, but if this device is used as a storage container for radioactive material, it is convenient for radiation control during storage. Similarly, an application to be incorporated in a shipping container is also conceivable.

(2) In this apparatus, the container 2 is to be carried in and installed at the time of measurement. However, this device may be made portable while the container 2 is fixed. For this purpose, it is desirable that the measuring container 4 wound with the PSF 30 is made of a lightweight material.

(3) When the apparatus is portable, the measuring container 4 itself may be omitted. In that case, the PSF 30 is formed so as to be self-supporting by an adhesive or the like having a certain strength, and the PSF 30 is used by covering it with the container 2 to be measured.

Embodiment 2. In the first embodiment, one PSF
30 was used as the detection material. In the second embodiment, band-shaped fibers are used.

FIG. 5 shows the structure of the strip fiber 50 used in the surface measuring apparatus of the second embodiment. As shown in the figure,
The belt-shaped fiber 50 is manufactured by connecting 100 PSFs in a belt shape, for example. In appearance, the image of the strip-shaped fiber is similar to that of a flat cable used in, for example, communication. If the thickness of one PSF is 1 mm, the total width of the strip fiber 50 is 10 cm. The thickness remains 1 mm.

The strip-shaped fiber 50 constitutes one detection material as a whole. Therefore, the signal processing circuit 32 equivalent to that of the first embodiment is connected to both ends of the strip fiber 50. The scintillation light is processed in units of 100 PSFs, and it does not matter which PSF emits the scintillation light. The strip-shaped fiber 50 is P of the first embodiment.
Similar to SF30, the measurement container 4 is wound without any gaps or overlaps.

With the above configuration, the device of the second embodiment has the following advantages.

(1) Easy fabrication of the device A method of using a plate-shaped PSF instead of a fiber can be considered for realizing a detection material having a width of 10 cm. However, in this case, the processing for winding around the measuring container 4 becomes difficult. In the present embodiment, the workability of each thin fiber is effectively utilized.

(2) Miniaturization of device is possible Since the belt-shaped fiber 50 has a thickness of only 1 mm, the entire device becomes compact. When a large number of these devices are arranged adjacent to each other, it is very advantageous as compared with the prior arts 1 and 2.

(3) Easy maintenance of detection sensitivity In the second embodiment, since 100 lines are used as a unit, the first embodiment
The theoretical total fiber length can be shortened to 1/100. Generally, PSF has a very high transmittance,
It is not necessary to consider the attenuation of light during propagation. However, if the width of the band-shaped fiber is set too large, the accuracy in specifying the incident point is reduced. The width may be determined in consideration of the detection sensitivity.

[0045]

According to the radiation dose measuring method and apparatus of the present invention, accurate and short-time surface measurement is possible. The device can be realized with a small circuit configuration, and there are few failures. Naturally, it requires less man-hours than manual measurement and is suitable for remote operation, which is advantageous in terms of safety management.

Scintillation as radiation detection means
When using a fiber, the device can be downsized by adopting a thin fiber.

When a strip-shaped fiber is used, it is easy to maintain the detection sensitivity because the total fiber length is short.

When the radiation detecting means is arranged in a state of spirally surrounding the object to be detected, it is easy to specify the radiation incident point. At this time, if the radiation detecting means is arranged so as to surround the object to be detected without a gap and without overlapping, measurement omission is eliminated and the accuracy of specifying the incident point is improved.

When measuring the arrival time difference of the detection signals transmitted to both ends of the radiation detecting means, the incident point can be easily and accurately specified by the TOF method or the like.

When the incident intensity of the radiation and the incident point are calculated based on the detection signals transmitted to both ends of the radiation detecting means and the arrival time difference, the surface intensity distribution of the radiation can be obtained.

When the scintillation fiber is spirally wound around the measuring container, the scintillation light is converted into an electric signal for each fiber end, and the arrival time difference of the scintillation light is calculated based on the TOF method, the incident point can be identified. It will be possible. Further, the intensity of radiation can be calculated by counting the electric signals. From these, the surface radiation distribution of the object to be detected can be derived.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a surface measuring apparatus according to a conventional technique 1 for measuring a surface.

FIG. 2 is a configuration diagram of a surface measuring device according to a conventional technique 2 for measuring a surface.

FIG. 3 is a configuration diagram of a surface measuring apparatus according to the first embodiment for surface measurement.

FIG. 4 is an internal configuration diagram of a signal processing circuit 32 according to the first embodiment.

FIG. 5 is a diagram showing a structure of a strip-shaped fiber 50 used in the surface measuring apparatus according to the second embodiment.

[Explanation of symbols]

4 measuring container, 30 PSF, 32 signal processing circuit,
34a, 34b photoelectric conversion element, 36a, 36b amplifier circuit, 38 time difference measuring circuit, 40 delay circuit, 42
Radiation dose display, 50 strips of fiber.

Claims (9)

[Claims] 1. A radiation detecting material in the form of a thin wire is three-dimensionally arranged around the object to be detected, and the surface radiation dose of the object to be detected is determined based on a comparison result of radiation detection signals reaching both ends of the object. A method for measuring radiation dose, which comprises measuring the distribution of. 2. A thin line-shaped radiation detecting means arranged around the object to be detected, and a signal processing means connected to both ends of the radiation detecting means and comparing detection signals transmitted to these both ends. And a radiation dose measuring device comprising: 3. The radiation dose measuring apparatus according to claim 2, wherein the radiation detecting means is a scintillation fiber, and the detection signal is scintillation light caused by radiation incident on the fiber. Radiation dose measuring device. 4. The radiation dose measuring device according to claim 2, wherein the radiation detecting means is formed by connecting a plurality of scintillation fibers in a band shape, and the signal processing means comprises: A radiation dose measuring device characterized in that a plurality of these scintillation fibers are used as one detection material and are connected to both ends thereof. 5. The radiation dose measuring apparatus according to claim 2, wherein the radiation detecting means is arranged in a state of spirally surrounding the object to be detected. Quantity measuring device. 6. The radiation dose measuring apparatus according to claim 5, wherein the radiation detecting means is arranged in a spiral shape so as to surround the object to be detected without a gap and without overlapping. Quantity measuring device. 7. The radiation dose measuring apparatus according to claim 2, wherein the signal processing unit measures a difference in arrival time of detection signals transmitted to both ends of the radiation detecting unit. Radiation dose measuring device. 8. The radiation dose measuring apparatus according to claim 7, wherein the signal processing unit further includes: a detection signal transmitted to both ends of the radiation detection unit and the arrival time difference.
A radiation dose measuring device, which calculates an incident intensity of radiation and an incident point. 9. A measuring container for accommodating an object to be detected, a scintillation fiber spirally arranged around the measuring container, and connected to both ends of the scintillation fiber,
A photoelectric conversion unit that converts a scintillation light signal transmitted to both ends into an electric signal for each end, a time difference calculation unit that calculates a time difference when the scintillation light reaches the both ends based on the TOF method, and An intensity calculation unit that calculates the intensity of the radiation incident on the scintillation fiber from the count value; and a distribution derivation unit that derives the surface radiation distribution of the detected object based on the calculated time difference and intensity. Characteristic radiation dose measuring device.