JP2006177798A - Nondestructive inspection method using positron annihilation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to easily inspect the inside of a material by nondestructive inspection not only on the surface but also inside the material without requiring a large-scale detection device.
A nondestructive inspection method in which defects inside a material are inspected by stopping muons inside the material and detecting the lifetime of positrons generated by the muon decay inside the material.
[Selection] Figure 1

Description

  The present invention relates to a nondestructive inspection method for microscopic defects caused by damage / destruction in a material constituting a structure.

  As a method for detecting a defect in a material, there is a nondestructive inspection method in which the state of the material is directly examined by ultrasonic waves. This method is suitable for detecting a defect on the order of the wavelength of an ultrasonic wave, but is not suitable for a microscopic defect (<μm). Particularly in fatigue fracture, microscopic defects gather and grow into macroscopic defects. In fatigue fracture and the like, the generation process of microscopic defects occupies the main time until destruction. By observing microscopic defects, the lifetime of the material can be estimated from the beginning. A nondestructive inspection method using positron annihilation is known for detecting microscopic defects.

The principle of the inspection method using positron annihilation is as follows. The positron incident on the material loses energy while repeating the interaction with atoms and electrons in the material, and annihilates with the electron and converts it into two gamma rays. The time from incidence to conversion to gamma rays follows an exponential distribution, and the average time is defined as the positron lifetime.
When there is a part with a high defect density in the material, the positron is attracted to the defect and bound. Since the electron density in that portion is smaller than the surrounding area, the positron lifetime is increased. Thus, the positron lifetime reflects the defect density.

  The positron used in the positron annihilation method is usually supplied by a radiation source (Na: 0.54 MeV, Ge: 1.9 MeV), but its kinetic energy is several MeV or less. When a positron is incident on a material to be inspected, such as iron, the positron stops within 1 mm from the surface, resulting in a nondestructive inspection of the surface only. Not suitable for depth inspection. There is a non-destructive inspection method in the deep part of a material that uses a gamma ray source with a high transmission power of 1.02 MeV or more to generate a pair of gamma rays in the material. However, since the probability of pair generation decreases exponentially as it advances in the depth direction, it is inefficient for nondestructive inspection of the deep part.

As a solution to this problem, it has been proposed to perform positron annihilation gamma-ray spectroscopy deep inside the material by generating positrons inside the material using a highly transparent laser inverse Compton high-energy X-ray.
However, the above-described method using high energy X-rays has a drawback that position detection in the depth direction in which X-rays generate positrons in the material is not simple. That is, the position detection in the depth direction requires a position detector in the depth direction separately, and there is a drawback that the position detection device for that purpose becomes large.
JP 2003-215251 A JP 2003-270176 A JP 2004-150851 A

  Accordingly, an object of the present invention is to provide a non-destructive inspection method that can easily inspect the inside of a material by a non-destructive inspection not only on the surface but also inside the material without requiring a large-scale position detection device.

  The present invention is a nondestructive inspection method for inspecting defects inside a material by stopping the muon inside the material and measuring the lifetime of the positron generated by the muon decay.

  The present invention also detects the gamma ray generated in the positron bremsstrahlung process in the material as the start time, detects the gamma ray generated by the annihilation of the positron with the electron and sets it as the stop time, and determines the positron from the time difference. This is a non-destructive inspection method for measuring the life of the steel.

  Furthermore, the present invention is a nondestructive inspection method for measuring a defect position and a defect distribution in a depth direction of a material by measuring the positron lifetime while changing the momentum of a muon.

  In the present invention, non-destructive inspection of the deep part of the material by the positron annihilation method is possible by using mu particles, and the shape of the defect distribution in the depth direction can be measured by simply changing the energy of the mu particles. It has the effect of becoming.

Embodiments of the present invention will be described in detail with reference to the drawings.
Mu particles have strong penetrating power, and can easily penetrate several centimeters of iron, etc., and can be stopped at any location inside the material by controlling the momentum. A positively charged muon decays into a positron and two neutrinos. Therefore, muons stop and decay inside the material, so that positrons can be generated in the deep part of the material, and non-destructive inspection by the positron annihilation method in the deep part of the material becomes possible. A schematic diagram of the nondestructive inspection method described above is shown in FIG.

Next, FIG. 2 shows a schematic diagram of a measuring apparatus for carrying out a nondestructive inspection method using positron pair annihilation using muons. In the measuring device, barium fluoride (BaF ₂ ) detectors with good time response were arranged before and after the material to be inspected. As the material to be inspected, a SUS316 stainless steel plate with a thickness of 3 cm was used, and it was assumed that defects were distributed in the depth direction due to metal fatigue. In FIG. 2, the upper diagram is a bird's-eye view of the measurement system, and the lower diagram is a sectional view thereof.

FIG. 3 shows the relationship between the momentum of the mu particle and the position of the mu particle stopped and collapsed in SUS316 stainless steel. In the figure, the vertical axis represents the position where the mu particle stopped inside the stainless steel, and the horizontal axis represents the momentum of the mu particle.
It can be seen from FIG. 3 that the stop position within the material can be determined by controlling the momentum of the muon. Positrons generated by muon decay move through the material and annihilate with electrons.

  For example, when the momentum of the mu particle is p = 115 MeV / c (distribution width Δp / p = 1%), the distribution of the stop / decay position of the mu particle and the distribution of the positron annihilation position are shown in FIG. Since detectors are arranged in the direction of muon travel and in the opposite direction, it is detected when positrons travel in the respective directions from the mu decay point, and the positron annihilation position distribution is also decomposed into two components. The search location in the depth direction can be changed by the momentum of the muon.

FIG. 5 shows the calculation results of the muon stop position and positron annihilation position for various muon momentums.
In order to measure the positron lifetime in a material, it is necessary to measure the time when positrons are generated (start time) and the time when positrons annihilate with electrons (stop time). The start time and stop time can be measured as follows.

Start time The energy of the positron generated by decay is 53 MeV at maximum, and the distribution is as shown in FIG. When the positron energy is greater than the critical energy of the material, the positron emits gamma rays in the material by bremsstrahlung. Since this process occurs within a time shorter than the positron lifetime, the start time can be known by detecting gamma rays emitted by bremsstrahlung. When the energy of gamma rays emitted by bremsstrahlung is greater than 1.02 MeV, an electron-positron pair is generated. In this case, the start time is measured by detecting the electron or positron.

Stop time The stop time is measured by detecting the gamma rays generated by the positron annihilation with the electron. The start time and stop time are measured, and the time difference distribution is obtained. The distribution is an exponential distribution, and the lifetime is determined by fitting using an exponential function. FIG. 7 shows the time difference distribution obtained by simulation in consideration of the time measurement error and background by the measuring instrument. In the figure, the simulation data is indicated by a circle and the results (signal and background component) obtained by fitting are indicated by lines.
Two or more positron annihilation may occur in one muon decay, and in that case, one of them is selected at random.

Next, a method for measuring the defect position and defect distribution in the depth direction will be described.
In general, the positron lifetime depends on the defect density. Consider a case where the defect density (or positron lifetime) changes in the depth direction according to a Gaussian distribution.
FIG. 8 shows an example of the distribution of positron pair annihilation positions and the defect density distribution when the momentum of the muon is 85 MeV / c. The magnitude of the defect density (or positron lifetime) can be expressed by the height of the Gaussian distribution, the distribution width of the defect density can be expressed by the width of the Gaussian distribution, and the position of the defect density can be expressed by the center position of the Gaussian distribution. If the material to be inspected has a defect density, the positron lifetime also has a distribution, so that it becomes a continuous value from the maximum value to the minimum value, but what can actually be measured is the effective value.
FIG. 9 shows an evaluation of how the life measurement is performed by changing the parameter of the defect distribution. Since the measurement life varies depending on the shape and position of the defect distribution, the defect density distribution can be measured.

  In particular, in order to measure the center position of the defect density distribution in the depth direction, the momentum of the mu particles is changed to search the depth direction of the material to be inspected. FIG. 10 shows the positron annihilation position distribution by changing the momentum of the mu particles when there is a defect density distribution in the center of the depth of stainless steel. The positron lifetime measurement at that time is as shown in FIG. The defect distribution position can be specified by the momentum. The measured lifetime is related to the lifetime obtained by averaging the lifetime distribution determined by the defect density distribution by the weight of the positron annihilation distribution. This is shown in FIG. Therefore, it can be seen that the measured lifetime depends on the defect density distribution. Therefore, the defect distribution inside the material can be determined from FIG. 11 and FIG.

  In order to prevent destruction due to accumulation of microscopic defects that occur in materials constituting a structure such as a power plant, soundness is regularly evaluated. According to the present invention, when the soundness is evaluated, the progress of internal microscopic defects can be observed, so that the material life can be estimated from the initial stage. In addition, even in a complicated shape material evaluation experiment, internal material defect information can be known without being decomposed.

Schematic diagram of positron nondestructive inspection method Schematic diagram of a measuring device for nondestructive inspection by positron annihilation using muons Relationship diagram between the momentum of muon and the position of muon stopped and collapsed in stainless steel Distribution of stop and decay positions of muons and positron annihilation positions Diagram showing muon stopping position and positron annihilation position with respect to muon momentum Kinetic energy distribution map of positrons generated by muon decay. The simulation result figure of a time difference distribution. Positron annihilation position distribution and defect distribution model diagram Lifetime measurement diagram when changing parameters of defect distribution model Defect distribution model in the depth direction and muon positron annihilation position distribution map when muon momentum is changed Lifetime measurement diagram when measuring the defect distribution in the depth direction by changing the momentum of the muon Diagram showing the relationship between average life and measured life due to various defect distributions

Claims (3)

A nondestructive inspection method in which defects inside the material are inspected by stopping the muons inside the material and detecting the lifetime of positrons generated by the muon decay inside the material. Detects gamma rays or electrons generated by positron bremsstrahlung process in material and uses as start time, detects gamma rays generated by annihilation of positron and electrons as stop time, and measures lifetime of positron from the time difference The nondestructive inspection method according to claim 1. The nondestructive inspection method according to claim 1 or 2, wherein the defect position and defect distribution in the depth direction of the material are measured by measuring the lifetime of the positron while changing the momentum of the muon.