JP6282435B2 - Muon trajectory detector and muon trajectory detection method - Google Patents

Description

  The present embodiment relates to a technique for detecting a flight trajectory of a cosmic ray muon.

  In structures that are difficult to enter due to high radiation, such as nuclear power plants where severe accidents have occurred due to earthquakes and tsunamis, it is important to understand the inside.

  As a conventional method for grasping the inside of a structure, a technique for observing a cosmic ray muon reaching the ground surface and seeing through the inside is known. This technique has been suitably used for large-sized things that are difficult to enter, such as volcanoes or pyramids.

  As a method of observing the muon and seeing through the internal state of the structure, a transmission method for measuring the attenuation of the muon particle bundle and a scattering method for measuring the Coulomb multiple scattering angle of the muon are known. As a scattering method, a displacement method for measuring a shift of a locus due to Coulomb multiple scattering is also known.

  It has been proposed to apply the technique of observing the muon and seeing through the internal state to a structure under a high radiation environment (for example, Non-Patent Document 1). By applying Muon's fluoroscopy technology to a nuclear power plant where a severe accident has occurred, specific information such as the fuel rod assembly in the pressure vessel and the shape or mass of the molten fuel can be accurately grasped from outside the building. It is expected. By accurately grasping such specific information, it is possible to significantly shorten the reactor dismantling period, and it is possible to reduce the dismantling cost and the exposure amount of workers.

  In the muon fluoroscopy technique, a muon trajectory detector is externally provided on a structure to be fluoroscopically targeted. The detector detects the flight trajectory of muon and analyzes the trajectory, thereby imaging the inside of the structure.

The muon locus detector has a multi-layered arrangement of drift tubes filled with drift gas.
The drift tube has an anode wire at its center, and the drift gas enclosed by the passage of the muon is ionized to generate electrons. The passage of muons is detected when the generated electrons reach the anode wire. The muon passage position can be obtained from the drift time until the electrons reach the anode wire.

The muon trajectory detector can detect the flight trajectory of the muon from each passing position of the drift tube through which the muon has passed.
In the case of a muon trajectory detector composed of an aluminum drift tube, the spatial resolution and the angular resolution are typically about 0.5 mm and 2 mrad (full width half value), and the muon detection efficiency is close to 100%.

H. Miyada, et al. , AIP Advances 3,052133 (2013).

  However, in a high radiation environment such as a nuclear power plant where a severe accident occurred, a high gamma dose increases false detection due to Compton scattering between atoms and gamma rays on the drift tube wall. This is because electrons generated by Compton scattering with gamma rays reach the anode wire and are detected as muon passage.

  Originally, the drift tube has low sensitivity to gamma rays, and generation of electrons derived from gamma rays can be ignored. However, when the gamma dose is high, the number of occurrences of Compton scattering between gamma rays and atoms on the wall of the drift tube also increases, and detection due to gamma rays increases.

  For this reason, in a high radiation environment, the amount of data to be analyzed increases due to the detection caused by the gamma rays of the drift tube, making it difficult to operate the muon locus detector stably.

  The present invention has been made in consideration of such circumstances, and an object of the present invention is to provide a muon locus detector and a muon locus detection method that realizes stable operation in a high radiation environment.

The muon trajectory detector of the present embodiment includes a drift tube detector in which drift tubes that output an electrical signal when passage of a cosmic ray muon is detected are arranged and arranged in at least three layers, and a straight line stored in advance. Using the detection pattern of the drift tube, the drift tube from which the electrical signal is output within a predetermined time is excluded from the drift tube that is not on a straight line, while the one that is on a straight line is selected. Drift tube sorting means; passage position calculating means for calculating the passage position of the cosmic ray muon in the drift tube based on the rise time of the electrical signal corresponding to each of the sorted drift tubes; Muon locus deriving means for deriving a flight locus of the cosmic ray muon using the passing position. , Characterized in that it comprises a.

  ADVANTAGE OF THE INVENTION According to this invention, the muon locus detector and the muon locus detection method which implement | achieve stable operation | movement under a high radiation environment are provided.

The block diagram of the muon locus detector which concerns on this embodiment. Sectional drawing of the drift tube applied to this embodiment. (A), (B) The figure which shows the example of arrangement | positioning of a drift tube. The figure which shows the example of arrangement | positioning of the drift tube in the case of crossing the direction of an arrangement | sequence. (A) The circuit diagram of the multi-threshold discriminator applied to this embodiment, (B) The figure which shows the pulse conversion of the electric signal from which the pulse height by a multi-threshold discriminator differs. The figure explaining the method of deriving | leading-out a muon locus | trajectory except the drift tube from which the electrical signal was output by the gamma ray origin. The figure explaining the method of calculating a propagation delay by the positional information on the drift tube which crosses. The flowchart which shows operation | movement of the muon locus detector which concerns on this embodiment. The figure which shows an example at the time of applying the muon locus detector which concerns on this embodiment to a nuclear reactor.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
A muon trajectory detector 10 according to the embodiment shown in FIG. 1 includes a drift tube detector 12 in which drift tubes 11 that output an electrical signal when passage of a cosmic ray muon is detected are arranged and arranged in at least three layers. And drift tube selecting means 15 for selecting the one located on a straight line from the drift tubes 11 from which the electric signal was output within a predetermined time, and the rise time of the electric signal corresponding to each selected drift tube 11 The passage position calculation means 19 for calculating the passage position of the cosmic ray muon in the drift tube 11 based on the above, and the muon locus derivation means 20 for deriving the flight locus of the cosmic ray muon using the calculated passage position. .

  The muon locus detector 10 includes a drift tube detector 12 in which the arranged drift tubes 11 are formed in multiple layers, a PAD board 21 (electronic circuit), an FPGA board 22 (electronic circuit), and an analysis computer 23.

FIG. 2 shows a cross-sectional view of the drift tube 11.
The drift tube 11 is a cylindrical tube made of aluminum, and an anode wire 24 to which a high voltage is applied is stretched at the center of the cylindrical tube. A drift gas 25 containing a rare gas as a main component is sealed in the drift tube 11.

  When the muon passes through the drift tube 11, the drift gas 25 is ionized and separated into ions and electrons. When the electrons reach the anode wire 24, an electrical signal is output from the drift tube 11, and muon passage in the drift tube 11 is detected.

  The muon passage position (distance from the anode wire 24) in the drift tube 11 can be obtained from the drift time until the electrons reach the anode wire 24. This electron drift time indicates the rise time of the electrical signal output from the drift tube 11.

Under a high radiation environment, the position resolution deteriorates because the drift time of electrons changes due to the space electrification effect produced by the ions ionized in the drift tube 11.
The degradation of the position resolution due to the space electrification effect can be suppressed by making the drift gas 25 enclosed in the drift tube 11 a linear gas whose electron drift time does not depend on the electric field.

  The linear gas can be generated by adding two or more of nitrogen, methane, ethane, carbon dioxide, and tetrafluoromethane to a rare gas that is a main component. In particular, it is desirable to add a non-hydrocarbon gas in order to prevent deterioration over time in a high radiation environment.

  3A and 3B show an arrangement example of the drift tube 11. In the drift tube detector 12, the drift tube 11 is arranged in at least three layers in order to derive the muon trajectory from the muon passage position in the drift tube 11.

In FIG. 3A, a plurality of drift tubes 11 arranged in the same direction so as to be in contact with each other are arranged in three layers in parallel.
In FIG. 3B, the two layers of drift tubes 11 that are shifted from each other in the center position are set as one set, and the three sets are arranged in parallel, and the six layers of drift tubes 11 are arranged. As a result, muon detection omission due to failure of the drift tube 11 or the like is prevented.

FIG. 4 shows an arrangement example when the drift tubes 11 are crossed.
Two layers of drift tubes 11 whose arrangement directions are orthogonal to each other are alternately stacked, and a total of 12 layers of drift tubes 11 are arranged. As a result, the muon trajectory is derived three-dimensionally.

Returning to FIG. 1, the description will be continued.
The PAD board 21 is directly connected to the drift tube 11 and includes a preamplifier 13 and a multi-threshold discriminator 14.

  The preamplifier 13 amplifies the electrical signal output from the drift tube 11 and outputs it to the multi-threshold discriminator 14.

  The multi-threshold discriminator 14 converts the electric signal amplified through the preamplifier 13 into a pulse signal using at least two set threshold values.

FIG. 5A shows a multi-threshold discriminator 14 applied to this embodiment.
In this multi-threshold discriminator 14, three threshold values Th1 to Th3 are set. The output of the logic circuit is integrated into one signal line, and the electric signal s is input and converted into one pulse signal. By sequentially reading a series of rise and fall times of the pulse signal, it is possible to specify the time that passes through each of the threshold values Th1 to Th3.

  Usually, in a multi-threshold discriminator, the number of transmissions increases in correspondence with the number of threshold values to be set. Thus, since the number of transmission lines does not increase by integrating the outputs into one by the logic circuit, the device configuration of the muon locus detector 10 can be simplified and the manufacturing cost can be suppressed.

FIG. 5B shows a case where electrical signals s (s1, s2) having different wave heights are converted into pulse signals by the multi-threshold discriminator 14. The electric signals s1, s2 is the rise time t ₀ of the electrical signal s is identical, it is assumed that the difference in height is out due to differences such as muon incident angle to the drift tube 11.

In the electric signal s1, the time passing the threshold Th1~Th3 becomes _t 1 ~t ₃ respectively. Similarly, the electrical signals s2, time to pass through the threshold Th1~Th3 becomes _{t 1} '~t _3', respectively.

When the radiation count rate becomes high under a high radiation environment, the baseline of the electric signal s output from the drift tube 11 becomes unstable, and fluctuations occur in the rising time t ₀ of the electric signal s. Further, in the electric signals s1 and s2 having different wave heights, a time walk (reading error) due to the difference in wave heights occurs. Therefore, it becomes difficult to read the rise time t ₀ of the direct electrical signal s.

  Therefore, the correction means 17 (FIG. 1) of the analysis computer 23 corrects the rise time of the electric signal based on the time information when the pulse signal converted by the multi-threshold discriminator 14 passes each of the threshold values.

Specifically, for the electrical signals s1, running waveform fitting such as the least squares method using the time information t ₁ ~t ₃ passing through the threshold Th1～Th3, corrects the rise time t ₀ of the electric signal . Similarly, for the electrical signals s2, corrects the rise time _{t 0} of the electric signal with a time through the threshold Th1~Th3 information _{t 1} '~t _3'.

  Thus, by performing correction based on the time information of the pulse signal, the time resolution can be improved to several ns level, and the rise time of the electric signal can be obtained with high accuracy. It is also possible to correct the time walk due to the difference in wave height. Thereby, the muon passage position in the drift tube 11 can be calculated with high accuracy.

  Further, the time information of the pulse signal can be used for estimating the wave height and waveform. By estimating the wave height over a long period of time, it is possible to use it for the time-dependent change of the gain of each drift tube 11 and for the estimation of the muon passage position by waveform estimation.

Returning to FIG. 1, the description will be continued.
The FPGA board 22 includes a drift tube selection unit 15 and a time digital conversion unit 16.

  The drift tube sorting means 15 sorts out the drift tubes 11 that output an electrical signal within a predetermined time and that are positioned on a straight line.

  Gamma rays hit the drift tube 11 in a single shot, whereas muons penetrate the multilayer drift tube 11. The background caused by gamma rays is removed by utilizing the difference between these two properties.

Specifically, the positional relationship is examined for a number of drift tubes 11 for which electrical signals have been output within a predetermined time, and only the drift tubes 11 that are in a straight line are selected. And the drift tube 11 which is not on a straight line is excluded as the drift tube 11 from which the electric signal was output by the cause of a gamma ray.
As a method for selecting the drift tubes 11 on a straight line, a method of previously storing all the patterns of the drift tubes 11 that are in a straight line and searching for the drift tubes 11 that match the pattern can be considered.

  In this way, by removing the background caused by gamma rays, the amount of data to be subjected to trajectory analysis can be reduced, so that the muon trajectory detector 10 can be operated stably.

  Furthermore, in consideration of dead time and detection efficiency of the drift tube 11, for example, by imposing a relaxed condition that four or more of the six layers are aligned in a certain time, the detection efficiency of the muon is not lowered. In addition, the background caused by gamma rays can be removed.

In addition, for a certain time, in other words, the time for performing coincidence counting, a time walk (reading error) is set to the maximum drift time (the drift time of electrons from the wall surface to the anode wire 24, typically 10 ⁻⁶ (s)). The added time is optimal.

  The time digital conversion means 16 digitizes the time information of the pulse signal of the drift tube 11 selected by the drift tube selection means 15 and transfers it to the analysis computer 23.

  The analysis computer 23 includes correction means 17, propagation delay correction means 18, passage position calculation means 19, and muon locus derivation means 20. The propagation delay correcting unit 18 will be described later.

  The correction means 17 inputs time information of the selected pulse signal of the drift tube 11. And about each drift tube 11, the rise time of an electric signal is correct | amended based on the time information when the pulse signal passed each of the threshold value.

  The passage position calculation means 19 inputs the rise time of the electrical signal corresponding to the selected drift tube 11. And about each drift tube 11, the passage position of the muon in the drift tube 11 is calculated from the rise time of an electric signal.

  The muon trajectory derivation means 20 derives the flight trajectory of the cosmic ray muon using the passage position calculated for each drift tube 11.

  FIG. 6 is a diagram for explaining a method for deriving a muon locus by removing a background caused by gamma rays.

  The drift tubes 11 that are in a straight line are selected from the drift tubes 11 from which electrical signals are output during coincidence counting. The drift tube 11 that is not in a straight line is excluded as a background due to gamma rays.

  For each drift tube 11 on a straight line, the muon passage position (distance from the anode wire 24) is calculated from the rise time of the electrical signal. A muon's flight trajectory can be derived by drawing a common tangent of the passing position in each drift tube 11.

  Next, the propagation delay correcting means 18 (FIG. 1) will be described. The propagation delay correcting means 18 is applied when the drift tubes 11 whose arrangement directions are crossed are alternately stacked and arranged in multiple layers as shown in FIG.

In order to see through a large building such as a nuclear reactor with muons, a large muon trajectory detector 10 with a scale of several tens of m ² is required to earn statistics. In this case, since the length of 3 m or more is required for the drift tube 11, the propagation delay of the electric signal transmitted through the anode wire 24 (FIG. 2) cannot be ignored. Since this propagation delay is added to the rise time of the electric signal, it affects the calculation of the muon passage position.

  The propagation delay correcting means 18 calculates the delay of the propagation time of the electric signal using the position information of the drift tubes 11 intersecting each other, and corrects the rise time of the electric signal.

A method of calculating the propagation delay will be specifically described with reference to FIG.
Consider a case where a muon passes through the drift tube 11a perpendicular to the x-plane.
At this time, since the muon passes through a deep position of the drift tube 11a, a propagation delay of the electric signal occurs.

  The drift tube 11b perpendicular to the y plane intersects the drift tube 11a, and a muon passes immediately before the drift tube 11a. The position where the muon has passed through the drift tube 11a is determined from the position information of the drift tube 11b.

  The propagation delay correction means 18 calculates the delay of the propagation time of the electric signal using the position information of the drift tube 11b, and corrects the rise time of the electric signal. In addition, you may correct | amend using the positional information on the drift tube 11c. Thereby, the propagation delay can be corrected with the accuracy of the propagation time of the drift tube diameter (typically 0.3 ns or less).

FIG. 8 is a flowchart showing the operation of the muon locus detector 10 (see FIG. 1 as appropriate).
First, the electrical signal generated from the drift tube 11 is amplified via the preamplifier 13 and input to the multi-threshold discriminator 14 (S10).
Then, the multi-threshold discriminator 14 converts the electric signal into a pulse signal (S11).

  The drift tube sorting means 15 sorts the drift tubes 11 that have been output on the straight line from the drift tubes 11 from which the electrical signals have been output within a certain time (when simultaneously counting) (S12). The drift tube 11 that is not in a straight line is removed as an event caused by gamma rays (S15).

  The time digital conversion means 16 digitally converts the time information of the selected pulse signal of the drift tube 11 and transfers the data to the analysis computer 23 (S13, S14).

The analysis computer 23 groups the x-plane and y-plane events of the drift tube 11 based on the time information (S16).
And the correction | amendment means 17 correct | amends the rise time of an electrical signal about the selected drift tube 11 based on the time information when the pulse signal passed each of a threshold value (S17).

  Further, the propagation delay correcting means 18 calculates the delay of the propagation time of the electric signal using the position information of the drift tubes 11 intersecting each other, and corrects the rising time of the electric signal (S18).

  The passing position calculating means 19 calculates the muon passing position in the drift tube 11 based on the corrected rise time of the electrical signal for each of the selected drift tubes 11 (S19).

  The muon trajectory deriving means 20 derives the flight trajectory of the cosmic ray muon using the calculated passing position (S20). Finally, the analysis computer 23 images the derived muon flight trajectory (S21).

  According to the muon trajectory detector described above, it is stable in a high radiation environment by providing a drift tube selection means for selecting a drift tube that has been output in an electrical signal within a certain time and that is positioned on a straight line. Can be realized.

FIG. 9 shows an example in which the muon locus detector 10 of the present embodiment is applied to the nuclear reactor 26.
Muon trajectory detectors 10a and 10b are installed in the reactor building front and in the turbine building adjacent to the reactor building, respectively.

  In this way, by sandwiching the nuclear reactor 26 between the two muon trajectory detectors 10 (10a, 10b), the muon flux attenuation when passing through the nuclear reactor 26, the muon scattering angle, and the displacement of the muon trajectory are as follows. Therefore, it is possible to accurately determine the internal structure of the nuclear reactor 26. Note that the transmission method using the attenuation of the muon flux can be performed by only one muon locus detector 10, but in that case, the spatial resolution is deteriorated by one digit.

  In addition, the muon locus detector 10 of the present embodiment is an interior of a composite structure composed of at least two kinds of heterogeneous materials such as a dry cask, a vehicle, a container, a ship, a bridge pier, and an elevated road even under a normal environment. It can be used for structure estimation.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

10 (10a, 10b) Muon locus detector 11 (11a, 11b, 11c) Drift tube 12 Drift tube detector 13 Preamplifier 14 Multi-threshold discriminator 15 Drift tube selection means 16 Time digital conversion means 17 Correction means 18 Propagation Delay correction means 19 Passing position calculation means 20 Muon trajectory derivation means 21 PAD board 22 FPGA board 23 Analysis computer 24 Anode wire 25 Drift gas 26 Reactor s (s1, s2) Electrical signal

Claims (6)

A drift tube detector in which a drift tube that outputs an electrical signal when passage of a cosmic ray muon is detected is arranged and arranged in at least three layers;
Using the drift tube detection pattern that has been stored in advance, the drift tubes that have been output within the predetermined time are excluded from those drift tubes that are not located on a straight line, while those that are located on a straight line are excluded. Drift tube sorting means for sorting what to do,
Passage position calculating means for calculating the passage position of the cosmic ray muon in the drift tube based on the rise time of the electrical signal corresponding to each of the selected drift tubes;
Muon trajectory deriving means for deriving a flight trajectory of the cosmic ray muon using the calculated passing position, a muon trajectory detector. A multi-threshold discriminator that converts the electrical signal output from the drift tube into a pulse signal and outputs the pulse signal using at least two set threshold values;
Correction means for correcting the rise time of the electrical signal based on the time when the pulse signal has passed each of the threshold values;
The muon locus detector according to claim 1, further comprising:   The multi-threshold discriminator outputs a pulse signal whose output passes through each of the thresholds by specifying the output by integrating the output into a single signal line with a logic circuit and sequentially reading a series of rise and fall times. The muon trajectory detector according to claim 2, wherein the muon trajectory detector is output. The drift tube detector is arranged in multiple layers by alternately stacking the drift tubes crossing the direction of the array,
The apparatus further comprises propagation delay correcting means for calculating a delay in the propagation time of the electric signal using position information of the drift tubes intersecting each other and correcting the rising time of the electric signal. Item 4. The muon locus detector according to any one of Items 3 to 3.   The drift gas sealed inside the drift tube is a linear gas in which a rare gas is a main component and at least two kinds of gases selected from nitrogen, methane, ethane, carbon dioxide, and tetrafluoromethane are added. The muon locus detector according to any one of claims 1 to 4. When drift tubes that output electrical signals when passage of cosmic-ray muons is detected are arranged, a drift tube detector arranged in at least three layers is used.
Using the drift tube detection pattern that has been stored in advance, the drift tubes that have been output within the predetermined time are excluded from those drift tubes that are not located on a straight line, while those that are located on a straight line are excluded. Selecting what to do,
Calculating a passage position of the cosmic ray muon in the drift tube based on a rise time of the electrical signal corresponding to each selected drift tube;
Deriving a flight trajectory of the cosmic ray muon using the calculated passing position, and a muon trajectory detection method.

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