JPH07151860A - Apparatus for detecting radioactive surface contamination - Google Patents

Abstract

PURPOSE:To reduce the limit of detection and to decrease the number of photomultiplier tubes in a large-area type radioactive-surface-contamination detecting apparatus using scintillation fibers. CONSTITUTION:A plurality of fiber units 12 to 18 sequentially form the right-end neighboring pairs at the right end part by every two units and sequentially form the left-end neighboring pairs at the left end by every two units so that the pairs are shifted by one unit to the right-end neighboring pairs. Photomultiplier tubes 22 and 24 are connected for every right-end neighboring pair, respectively. Photomultiplier tubes 21, 23 and 25 are connected for every left-end neighboring pair, respectively. The simultaneous counting is performed with the right and left photomultiplier tubes for the respective fiber units 12 to 18. In this constitution, the simultaneous counting for every fiber unit can be performed with approximately the same number of the photomultiplier tubes as the number of the fiber units.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for detecting radioactive surface contamination for measuring contamination by radioactive substances on the surface of an object.

[0002]

2. Description of the Related Art In nuclear power plants, radioactive material handling facilities, etc., radioactive surface contamination detectors (hereinafter referred to as
Device) is used.

As such a device, the present applicant has proposed a radioactive surface contamination detecting device using a scintillation fiber in Japanese Patent Application No. 5-179661. This device uses a fiber array formed by arranging a plurality of scintillation fibers in a plane shape as a radiation detection unit.The scintillation light generated in the fiber by the incidence of radiation is multiplied by a photoelectron multiplier connected to the end face of the fiber array. It is converted into an electric pulse signal by a tube and detected. Then, this pulse signal is counted by the counting circuit connected to the photomultiplier tube.

When the surface contamination is to be detected over a wide range, it is desirable to use a large-area type device in terms of work efficiency. In the device using the scintillation fiber described above, as a method of satisfying such a demand for increasing the area, a photomultiplier tube having a large light receiving area is used and a large area is achieved by connecting a large number of scintillation fibers. That method is possible.

However, in such a configuration, the background (hereinafter abbreviated as BG) count increases, so that there is a problem in that the performance of the apparatus deteriorates. That is, since the amount of incident BG increases in proportion to the area of the fiber array, when the area of the fiber array becomes large, the detection limit (Bq) = (BG) ^1/2 (cps) / counting efficiency (cps / Bq ) ... The detection limit represented by (1) becomes large. The limit of detection refers to the lowest amount of radioactivity that can be identified as contamination in the device. Therefore, an increase in the detection limit means that the detection ability for contamination with low radioactivity decreases.

Therefore, as a conventional method for constructing a large-area device without increasing the detection limit, a fiber array is divided into a plurality of small-area fiber units, and a photomultiplier tube is provided for each fiber unit. The method of connecting was adopted.

FIG. 6 shows the structure of such a conventional device. In the figure, the fiber array 50 is divided into fiber units 52, 54, 56 and 58. Photomultiplier tubes 61 and 62 are connected to both ends of the fiber unit 52. Similarly, photomultiplier tubes 63 to 68 are connected to both ends of the fiber units 54, 56 and 58, respectively. The fiber unit in this conventional device has, for example, 0.5 per side.
It is formed by densely aligning 200 scintillation fibers having a square section of mm in a plane shape, and the size thereof is, for example, 50 cm × 10 cm.

FIG. 7 is a block diagram for explaining signal processing in this conventional apparatus. As shown in the figure, the coincidence counting preamplifier (AND) 72 simultaneously counts the output signals of the photomultiplier tubes (PMT) 61 and 62, thereby counting the radiation incident on the fiber unit 52. Similarly, simultaneous counting preamplifiers 74-78
By performing simultaneous counting on the photomultiplier tubes 63 to 68, respectively.
Is counted. In this conventional apparatus, by performing simultaneous counting, noise such as thermal noise due to the photomultiplier tube itself is canceled to improve accuracy.

In this conventional device, since the detection of contamination is performed for each individual fiber unit, the detection limit of the fiber unit becomes the detection limit of the entire device, and the detection limit of the entire device can be lowered.

Further, in this conventional apparatus, since it is possible to know in which fiber unit the contamination is detected, it is possible to specify the contamination position for each fiber unit.

[0011]

In such a conventional apparatus, two photomultiplier tubes are required for each fiber unit, and a large number of expensive photomultiplier tubes are used, resulting in high production cost. was there.

Further, in the conventional apparatus, in order to reduce the detection limit and enhance the detection capability, it is necessary to divide the fiber array into more fiber units to reduce the area per unit. However, if the number of fiber units is increased, the number of photomultiplier tubes must be increased at a rate of twice the increase amount, which causes a problem of further increase in production cost.

The present invention has been made in view of the above conventional problems, and an object of the present invention is to provide a high-performance radiation surface contamination detecting apparatus which requires a small number of photomultiplier tubes and has a small detection limit. To do.

[0014]

In order to achieve the above object, a radioactive surface contamination detecting apparatus according to the present invention includes a fiber array in which a plurality of scintillation fibers are arranged in a plane, and the fiber array has a predetermined size. The fiber unit is divided into a plurality of fiber units each consisting of a number of fibers, and the plurality of fiber units sequentially form two right end adjacent pairs at the right end portion, and are sequentially shifted one unit from the right end adjacent pair at the left end portion. Two units each form a left end adjoining pair, a right end photomultiplier tube is connected to each of the right end adjoining pairs, a left end photomultiplier is connected to each of the left end adjoining pairs, and a left end and a right end of each fiber unit are connected. It is characterized in that simultaneous counting is performed by a photomultiplier tube.

[0015]

The present invention has the above-described structure, and each fiber unit forms two adjacent pairs at the right end and the left end, and each adjacent pair is connected to one photomultiplier tube. With such a configuration, the number of photomultiplier tubes required for the number of fiber units can be halved compared to the conventional device.

Further, in this structure, scintillation light from two fiber units is mixed and incident on each photomultiplier tube. However, since the adjacent pairs are shifted by one unit at the right end and the left end of the fiber array, two fiber units connected to one photomultiplier tube at one end are respectively connected to two photoelectrons at the other end. Since it is separately connected to the multiplier tube, the scintillation light generated in one fiber unit is obtained by simultaneously counting the photomultiplier tube at one end and one of the two photomultiplier tubes at the other end. Only can be taken out. Therefore, it is possible to detect each fiber unit. When looking at each photomultiplier tube, two units of BG are detected, but BG incident on the fiber unit other than the target of simultaneous counting is not counted due to the above simultaneous counting, and therefore one unit of BG is also counted. The detection limit of one fiber unit becomes the detection limit of the entire apparatus.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the arrangement of a radioactive surface contamination detecting apparatus according to the first embodiment of the present invention. In this device, a fiber array 10 formed by aligning a plurality of scintillation fibers in a planar shape is used as a fiber unit 1.
It is divided into 2, 14, 16 and 18. The fiber units 12 and 14 are connected to the photomultiplier tube 22 at the right end in an adjacent pair, and the fiber units 16 and 18 are also paired at the right end to the photomultiplier tube 24. . Further, the fiber units 14 and 16 form an adjacent pair at the left end, and the photomultiplier tube 23
It is connected to the. The fiber units 12 and 18
Since there is no pair because they hit the front end and the rear end of the array of fiber units, respectively, the unit 12 is at the left end to the photomultiplier tube 21, and the unit 18 is at the left end to the photomultiplier tube 25. Connected by. In this embodiment, the size of the fiber unit and the number of scintillation fibers forming each fiber unit are the same as those in the conventional device shown in FIG.

As described above, in this embodiment, each fiber unit forms an adjacent pair of two units at the right end and the left end, and each adjacent pair is connected to one photomultiplier tube. The device can be constructed with almost the same number of photomultiplier tubes as. That is, compared with the conventional device shown in FIG. 6, it is possible to configure the device with almost half of the photomultiplier tubes. Strictly speaking, when the number of fiber units is n, 2n photomultiplier tubes are required in the conventional device, whereas in the present embodiment, n + 1 photomultiplier tubes are required.
Only need one.

FIG. 2 is a block diagram showing the signal processing for the output signal of the photomultiplier tube in this embodiment. As shown in the figure, in the present embodiment, the output signal of the photomultiplier tube 21 and the output signal of the photomultiplier tube 22 are simultaneously counted by the coincidence counting preamplifier 32, so that the counting for the fiber unit 12 is performed. I do. And so on
Simultaneous counting of the output signals of the photomultiplier tubes 22 and 23, 23 and 24, 24 and 25 is performed by the coincidence counting preamplifiers 34 to 38, respectively, so that the fiber units 14 to 18 are counted. As described above, in the present embodiment, the right and left photomultiplier tubes are sequentially combined and shifted to perform coincidence counting (such coincidence counting is hereinafter referred to as “alternate sequential coincidence counting”). Seeking a count of.

Here, the above-mentioned staggered sequential coincidence counting will be described in more detail by taking the photomultiplier tubes 22 and 23 as an example.

Since the scintillation lights generated in the fiber units 12 and 14 are incident on the photomultiplier tube 22 together, the output pulse of the photomultiplier tube 22 is
Both the pulse due to the radiation incident on the fiber unit 12 and the pulse due to the radiation incident on the fiber unit 14 are included. Further, the output pulse of the photomultiplier tube 22 also includes noise caused by the photomultiplier tube 22 itself. Similarly, the output pulse of the photomultiplier tube 23 includes a pulse due to the radiation incident on the fiber unit 14, a pulse due to the radiation incident on the fiber unit 16, and noise caused by the photomultiplier tube 23 itself. There are two types of radiation incident on each fiber unit, one is due to contamination and the other is BG.

In the case of detecting contamination as in this embodiment, since the target of detection is generally weak radioactivity, the amount of radiation incident on the fiber unit is small, and the radiation is due to contamination. Also for BG,
Since the light is randomly incident on each fiber unit, the probability that scintillation light is simultaneously generated in different fiber units is extremely small. Further, since the noise due to the photomultiplier tube itself is randomly generated in each of the photomultiplier tubes 22 and 23, the probability that noise is simultaneously generated in both photomultiplier tubes is extremely small. Therefore, by simultaneously counting the output pulse of the photomultiplier tube 22 and the output pulse of the photomultiplier tube 23, the fiber unit 12
Most of the pulses of radiation incident on the fiber unit 16 regardless of whether they are due to radiation from contamination or due to BG are canceled and noise due to the photomultiplier tube itself is also canceled. Only the pulses due to the radiation incident on 14 will be counted.

As described above, according to the alternate sequential coincidence counting, the radiation detection for each fiber unit can be performed with high accuracy. Therefore, since BG is also counted for each fiber unit, the detection limit of the detection device as a whole becomes equal to the detection limit of the fiber unit, as in the conventional device. Further, the contamination position can be specified for each fiber unit as in the conventional device.

As described above, according to this embodiment,
With almost half the number of photomultiplier tubes as compared with the conventional device, a radioactive surface contamination detection device having the same detection limit and the same contamination position resolution as the conventional device can be obtained. Therefore, in this embodiment, the number of expensive photomultiplier tubes can be significantly reduced,
It can greatly contribute to the reduction of production cost.

The photomultiplier tube used in this embodiment is connected with twice as many scintillation fibers as the photomultiplier tube in the conventional apparatus shown in FIG. Does not necessarily mean that the diameter of the photomultiplier tube of this embodiment needs to be larger than that of the conventional apparatus.

That is, the size of each individual fiber unit, more strictly speaking, the area, is subject to the limit of detection required for the detection apparatus, and the sizes illustrated in the conventional apparatus and the present embodiment also satisfy this limitation. To do. For example, the sum of the cross-sectional areas of the fibers included in the fiber unit of this size is 0.5 mm × 0.5 mm × 200 = 0.5 cm ² , but a photomultiplier tube with a small area corresponding to this is manufactured. The cost is very high. Therefore, the photomultiplier tube connected to the fiber unit is not necessarily a photomultiplier tube having a size suitable for the cross-sectional area and the number of fibers, and is usually a larger photomultiplier tube with a lower manufacturing cost, such as a diameter. The thing of about 5 cm is used. Therefore, even if the number of fibers connected to the photomultiplier tube is doubled, it is often the case that there is no need to change the photomultiplier tube to a large diameter one. Therefore, the increase in the diameter of the photomultiplier tube due to the number of fibers connected to the photomultiplier tube being doubled as compared with the conventional device, and the accompanying change in the manufacturing cost are almost considered as practical problems. You don't have to.

In the above description, the present embodiment has been taken from the viewpoint of the effect of reducing the manufacturing cost due to the reduction in the number of photomultiplier tubes, as compared with the conventional apparatus of FIG.
Another way of thinking is possible. The present embodiment will be described below based on this different view.

In this way of thinking, the conventional device to be compared is shown in FIG. In the conventional device shown in FIG. 8, the number of scintillation fibers connected to one photomultiplier tube is only twice that in the device shown in FIG. 6, and the basic structure is shown in FIG. It is the same as the device shown. That is, the number of scintillation fibers connected to one photomultiplier tube in the conventional apparatus of FIG. 8 is the same as in the case of this embodiment (in this embodiment, the scintillation fibers are arranged at the front and rear ends of the array of fiber units). Except for the photomultiplier tube that is connected). Therefore, what is described here is the effect of this embodiment when the number of scintillation fibers connected to one photomultiplier tube is the same as that of the conventional device.

In the conventional device shown in FIG. 8, the fiber array 80 is divided into two fiber units 82 and 84. Photomultiplier tubes 91 and 92 are connected to both ends of the fiber unit 82, and photomultiplier tubes 93 and 94 are connected to both ends of the fiber unit 84.

Comparing this conventional device with the present embodiment shown in FIG. 1, the number of photomultiplier tubes in the present embodiment is increased by one as compared with the conventional device, but the area of the fiber unit is increased. Since it is half that of the device and BG is also half, the detection limit is as small as (1/2) ^1/2 (from equation (1) above), and the number of unit divisions is doubled. The contamination position detection resolution is doubled.

As described above, the present embodiment reduces the detection limit by increasing the number of scintillation fibers connected to the photomultiplier tube by one, as compared with the conventional apparatus shown in FIG. The resolution of the contamination position detection can be improved.

Now, let us consider a case where the position of contamination comes to the boundary between two adjacent fiber units in the surface contamination detection operation in the above embodiments. In this case, the radiation from that contamination will be incident on the two fiber units separately. Therefore, a part of the total amount of radiation from contamination is counted by one fiber unit and the rest is counted by the other fiber unit. The radiation counts for each of these two fiber units will be less than if all of the radiation from the contamination were incident on one fiber unit. In other words, in both fiber units, a count value smaller than the count value representing the actual contamination intensity can be obtained. Therefore, it can be considered that the sensitivity of the detection device is lowered at the boundary between adjacent fiber units. The arrangement direction of the fiber units (see FIG.
FIG. 4 shows the sensitivity distribution along the (A-B direction).

Therefore, a second embodiment in which such sensitivity deterioration is prevented will be described below.

In this embodiment, in the apparatus shown in FIG.
This is an improvement on the signal processing of the output signal of the photomultiplier tube. FIG. 3 is a block diagram showing the signal processing for the output signal of the photomultiplier tube in this embodiment.

As shown in FIG. 3, in the second embodiment, the above-mentioned staggered sequential coincidence counting is performed on the output signals of each photomultiplier tube, and the coincidence counting is performed on the adjacent fiber units by the thumb amplifier. Seeking the sum of outputs. That is, in this embodiment, the coincidence counting preamplifiers 32 and 34 are provided by the sum amplifier (OR) 42.
Seeking the sum of the outputs of. Similarly, the sum of the outputs of the coincidence counting preamplifiers 34 and 36 and the sum of the outputs of the coincidence counting preamplifiers 36 and 38 are obtained by the sum amplifiers 44 and 46, respectively.

Therefore, even when the boundary between two adjacent fiber units is contaminated and the radiation from the contamination is separately incident on the two fiber units, they are adjacent by the thumb amplifiers 42 to 46 in this embodiment. By taking the sum of the counts for the two fiber units, it is possible to obtain a count value representing the intensity of the contamination. Therefore, the arrangement direction of the fiber units (A- in FIG. 1)
As the sensitivity distribution along the B direction), a uniform sensitivity distribution as shown in FIG. 5 is obtained.

In the first and second embodiments, the case where the number of fiber units is four has been described, but the same effect can be obtained regardless of the number of fiber units. Also,
In this embodiment, the case where the fiber array has a planar shape has been described, but the same effect can be obtained even when the fiber array has a curved surface shape that matches the shape of the inspection object.

[0039]

As described above, according to the present invention,
With almost half the number of photomultiplier tubes as compared with the conventional device, a radioactive surface contamination detection device having the same performance as the conventional device can be obtained. Further, according to the present invention, the detection limit is small,
A radioactive surface contamination detection device having high contamination position resolution can be obtained.

[Brief description of drawings]

FIG. 1 is a top view showing a configuration of a radioactive surface contamination detection device according to first and second embodiments of the present invention.

FIG. 2 is a block diagram showing signal processing for an output signal of the photomultiplier tube according to the first embodiment of the present invention.

FIG. 3 is a block diagram showing signal processing for an output signal of a photomultiplier tube according to a second embodiment of the present invention.

FIG. 4 is a sensitivity distribution along the arrangement direction of the fiber units of the radioactive surface contamination detection device according to the first embodiment of the present invention.

FIG. 5 is a sensitivity distribution along the arrangement direction of the fiber units of the radioactive surface contamination detection device according to the second embodiment of the present invention.

FIG. 6 is a top view showing a configuration of a conventional radioactive surface contamination detection device.

FIG. 7 is a block diagram showing signal processing for an output signal of a photomultiplier tube in a conventional radioactive surface contamination detection device.

FIG. 8 is a top view showing another configuration of the conventional radioactive surface contamination detection device.

[Explanation of symbols]

 10 Fiber array 12, 14, 16, 18 Fiber unit 21, 22, 23, 24, 25 Photomultiplier tube 32, 34, 36, 38 Simultaneous counting preamplifier 42, 44, 46 Sum amplifier

Claims (1)

[Claims] 1. A fiber array comprising a plurality of scintillation fibers arranged in a plane, wherein the fiber array is divided into a plurality of fiber units consisting of a predetermined number of fibers, and the plurality of fiber units are sequentially arranged at a right end portion. Two
Each unit forms a right-end adjacent pair, and at the left end, one unit is displaced from the right-end adjacent pair to sequentially form two left-end adjacent pairs by two units, and a right-end photomultiplier tube is connected to each right-end adjacent pair. ,
A left-side photomultiplier tube is connected to each of the pairs adjacent to the left end, and simultaneous counting is performed by the photomultiplier tubes at the left and right ends for each fiber unit.