JP2004150851A - METHOD FOR MEASURING POSITRON ANNIHILATION gamma-RAY SPECTROSCOPY BY PHOTON INDUCTION AND SHORT-LIVED ATOMIC NUCLEUS LEVEL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform positron annihilation γ-ray spectroscopy in the depth of the interior of a specimen by generating positrons from the interior of the specimen by using laser inverse-Compton high-energy X rays having high transmissivity. <P>SOLUTION: According to a positron annihilation γ-ray spectroscopic analysis method for a specimen using X rays by laser inverse-Compton, the X rays are pulsed into time widths shorter than the life of generated positrons or the life of short-lived nuclides and guided into the specimen, and the γ-ray spectroscopy is performed on elements in the specimen, thereby measuring the positron annihilation life of the elements in the specimen and/or the level of short-lived atomic nuclei. Or else, CT images on the invisible internal state of the specimen are acquired by positron emission tomography (PET) by utilizing the fact that the efficiency of electron pair generation caused by high-energy X rays generated by the inverse-Compton varies according to the composition of the specimen, and that the heavier an element is, the higher the efficiency is. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention basically combines positron-electron pair generation by laser inverse Compton high energy X-rays with positron annihilation γ-ray spectroscopy. Although it was impossible to introduce neutrons into the sample, positron annihilation γ-ray spectroscopy was performed deep inside the sample by generating positrons from the inside of the sample using laser-transverse Compton high-energy X-rays with high transparency. It is possible to do.
[0002]
As a result, there is a merit that non-destructive measurement inside the structure, which is impossible with conventional positron annihilation γ-ray spectroscopy, and even three-dimensional information inside can be obtained. In addition, easily and safely measure highly volatile samples, which were difficult with the positron beam method in which the sample is placed in a vacuum, and samples that need to be measured at high temperatures where there is a concern that the radioisotope may be scattered by evaporation. There is a merit that can be performed.
[0003]
Further, laser-inverse Compton high energy X-rays have excellent straightness and little beam spread. When an X-ray of about 10 MeV or more generated by a laser inverse Compton is irradiated on a sample, most of the X-ray scattered from the sample is generated by an electron pair. Therefore, the X-rays coming out of the beam diameter of the X-rays are scattered inside the sample, and the detection of the scattered X-rays indicates the generation of a pair inside the sample. By performing simultaneous measurement with this scattered X-ray, it can be used for improving the S / N ratio in positron annihilation γ-ray spectroscopy and measuring the positron annihilation lifetime. In other words, there is a merit that the information on the incidence of the positron into the sample can be detected by the detection of the scattered X-rays by the conventionally performed method.
[0004]
Also, due to the extreme straightness of the laser inverted Compton high energy X-rays, there is no unnecessary emission of X-rays, and there is no need for extensive shielding.
[0005]
[Prior art]
Conventional positron annihilation γ-ray spectroscopy uses positrons obtained directly from radioisotopes, or extracts, transports, or accumulates positrons obtained by the generation of electron pairs by radioisotopes or high-energy radiation. It was used as a beam. Each of these methods is a method of analyzing a sample by injecting a positron into the sample from outside the sample.
[0006]
However, the positron has poor permeability, making it impossible to measure the deep part of the sample, and it is impossible to analyze the inside of the structure in a non-destructive manner. The introduction of positrons into the sample also requires forming a positron beam in vacuum and injecting the positrons, or bringing the radioisotope into close contact with the sample surface and introducing the positrons directly into the sample. In these methods, it was necessary to place the sample in a vacuum or to raise the temperature of the sample and simultaneously raise the temperature of the radioisotope.
[0007]
Further, as a method of knowing the state of the invisible portion inside the structure, there is a method using a transmission image using high-energy X-rays. However, it is impossible to know a three-dimensional structure by this method.
[0008]
Conventionally, methods for obtaining high-energy X-rays for performing positron annihilation γ-ray spectroscopy in the deep part of a sample include the use of nuclides that emit high-energy γ-rays or damped X-rays by hitting an electron beam against a target. However, each of them has a wider solid angle at which high-energy X-rays and γ-rays are emitted than laser-inverse Compton high-energy X-rays, wastes many X-rays, and requires shielding. Further, the straightness is poor, so that measurement at a remote place is impossible, and it is difficult to obtain information on pair generation, particularly time information.
[0009]
[Problems to be solved by the invention]
It is difficult to perform positron annihilation gamma-ray spectroscopy deep inside the sample if positrons are introduced from the outside of the sample, and only analysis near the surface of the sample (about several mm) can be performed. It is impossible to perform positron annihilation gamma-ray spectroscopy at
[0010]
Further, in conventional positron annihilation γ-ray spectroscopy, it is necessary to use a radioisotope or a positron beam. When a radioisotope is used, it is necessary to place the radioisotope inside the sample. For example, at the time of measurement at a high temperature, there is a risk of scattering due to evaporation of the radioisotope, which cannot be easily performed. When using a positron beam, it is necessary to transport the positron in a vacuum, and therefore, the sample must be placed in a vacuum. As a result, it is difficult to measure a sample having high volatility, and there is a limit to the measurement at a high temperature.
[0011]
On the other hand, a two-dimensional transmission image of a structure has been conventionally obtained, but it has been difficult to use a method of obtaining a three-dimensional image.
[0012]
[Means for Solving the Problems]
In the present invention, since positrons are generated from the inside of the sample, and a laser inverted Compton high-energy X-ray having excellent transparency is used for the generation of the positrons, it is possible to measure deep inside the sample. It is possible to thin the laser Compton X-ray to 1 mm, (collimates the flux) annihilation γ-rays collimating used in positron spectroscopy by, to measure the fine portion of the sample inside of 1 mm ³ orders It is also possible. Similarly, it was difficult to perform positron annihilation γ-ray spectroscopy in-situ while applying a load to the sample, but it can be easily performed by this method.
[0013]
In addition, even if the sample is placed in the air or gas, laser reverse Compton X-rays can be guided to the sample without any problem, so that measurement can be performed easily.
On the other hand, the γ-rays emitted from the sample are almost 511 keV annihilation γ-rays emitted by the annihilation of two photon pairs of positrons and electrons, which conventionally extract three-dimensional information by positron emission tomography (PET). It is used for Positron / electron pair generation by laser inverse Compton X-ray inside the sample depends on the element at each site, and the efficiency tends to be higher for heavier elements, and it is possible to obtain three-dimensional information on the invisible part of the structure. It is possible.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
By placing the sample on the beam of the laser Compton light energy X-ray, a positron is generated inside. Other than that, it is possible to measure by the same method as ordinary positron annihilation γ-ray spectroscopy, such as the Doppler spread method and the simultaneous measurement Doppler spread method.
[0015]
Two methods can be considered for positron annihilation lifetime measurement. 1) Laser inverse Compton high-energy X-rays are pulsed to subpicoseconds, and the lifetime is measured by obtaining a pulse signal and time information of annihilation γ-rays. The time information is time information of γ-rays generated when the pulsed X-rays generate positrons and the positrons disappear. 2) Detects X-rays scattered during positron-electron pair generation outside the laser inverse Compton high-energy X-ray beam slightly deviated from its trajectory. The positron annihilation lifetime is obtained based on the time information and the annihilation γ-ray time information. Is measured.
[0016]
To measure the position dependence of positron annihilation gamma ray spectroscopy, it is possible to collimate the beam and collimate the annihilation gamma rays.
In order to obtain a three-dimensional image of the structure, the sample is placed on a laser inverse Compton high-energy X-ray beam, the site where the positrons have disappeared is determined by a normal PET method, and the information is accumulated to obtain the inside of the structure. Images can be obtained.
[0017]
The laser inverse Compton high energy X-rays, short-lived nuclear levels, electron pair generation, and two-photon annihilation of protons and electrons used in the present invention will be described.
(1) Laser Compton high-energy X-rays In Compton scattering, high-energy X-rays collide with electrons, causing the electrons to be repelled and the X-rays to be scattered. High-energy X-rays give a part of the energy they have to electrons, so that the energy after scattering is small. Therefore, the laser inverse Compton scattering is a phenomenon opposite to this phenomenon, in which a laser beam obtains energy from electrons moving at high speed and becomes high-energy X-rays. Its features are high brightness, variable energy, quasi-monochromatic, and high directivity, and are produced without intervening substances. As a result, the light source is an extremely clean light source without activation or the like, and since it has good directivity, there is no need for a large-scale shielding or the like.
[0018]
(2) Short-Lived Nuclear Level Generally, the state (frequency) of a nucleus changes by absorbing γ-rays. This state is an excited state, which is a nucleus-specific and discontinuous value. Therefore, this frequency is called a level, and is usually expressed in units of energy. A transition from one level to another level is called transition. The ground state is a level having the lowest energy, and the level generally indicates how much energy is higher than the ground state. Thus, the nucleus in the excited state naturally transitions to the ground state after a certain time. This is a stochastic phenomenon. Usually, the time that exists in the excited state is called “lifetime”, and “short life” means that this time is extremely short.
[0019]
(3) Positron-electron pair generation A process in which a positron-electron pair is formed. It is considered that when high-energy X-rays or γ-rays collide with a substance, the energy is changed to a substance. While the formula E = mc ² Einstein can be converted energy and mass, high energy X-rays, pairs from the energy of the electron and proton γ-rays or fast particles are produced in a state filled with storage amount of physical charge such .
[0020]
(3) Two-photon pair annihilation of protons and electrons A pair of positrons and electrons is generated in the process of converting energy into a substance according to the above Einstein's equation, but the substance is also converted into energy in the reverse process. Is done. Pair generation occurs from one photon (γ-ray or X-ray). At this time, no pair generation occurs unless momentum is given to an external nucleus or the like in order to conserve momentum. However, in the case of the opposite pair annihilation, a plurality of photons (annihilation gamma rays) can be emitted, and as a result, the annihilation can be performed while preserving the momentum. The smaller the number of photons (annihilation gamma rays), the higher the probability of annihilation (short lifetime). As a result, most of the material emits two photons (annihilation gamma rays) and the positron-electron annihilation occurs. . In this case, the emitted photons are emitted in almost the opposite direction in order to conserve momentum, and positron emission tomography (PET) provides information on the disappeared portion by observing these two photons. Can be. By making this three-dimensional, three-dimensional information of the sample can be obtained.
[0021]
【Example】
Doppler spread measurements were performed on stainless steel bolts. As shown in FIG. 3, a stainless steel bolt was placed on a laser inverted Compton high energy X-ray beam, and the energy of the positron annihilation γ-ray was measured by a semiconductor detector (Ge detector) from a perpendicular direction. The measurement was performed without any background around the semiconductor detector and without any shielding.
[0022]
The results are shown in FIG. 1 (measurement of positron annihilation γ-ray peak by laser Compton light energy X-ray). Until now, positron annihilation γ-ray spectroscopy could not measure the Doppler spread of positron annihilation γ-rays inside such parts.
[0023]
Also, in Figure 2 (measurement of peaks by simultaneous measurement of two positron annihilation gamma rays by laser Compton light energy X-rays), the S / N ratio is improved by measuring two emitted annihilation gamma rays simultaneously. An example in which this is done is also shown.
[0024]
FIG. 1 shows that positrons are formed and disappear inside the sample, and that there is no noise signal around the signal obtained without any special shielding around the detector. Is shown. This signal itself contains various information, and various things can be discussed from the form of this signal, and it is already practical.
[0025]
Fig. 2 shows the elemental analysis by measuring the energy of both two photons emitted when the positron annihilates, and making it two-dimensional, so that the background can be significantly reduced and the bottom of the signal can be seen. And so on.
[0026]
The S / N ratio (Signal-to-Noise ratio) indicates a ratio of a noise component to an effective signal component, and a larger value indicates a better signal with less noise.
[0027]
Regarding this S / N ratio, when an event to be actually observed occurs, that is, when electron pair generation or nuclear excitation occurs, X-rays are scattered off the optical axis. By detecting the line, it is known that an event to be observed has occurred, and the gate is controlled so that the signal that has entered the detector can be captured (open the gate) only at that time. On the other hand, when a signal (noise) that is not desired to be observed comes, the gate is controlled so as to close the gate and prevent the signal from being captured. As a result, the noise portion is reduced, which leads to an improvement in the SN ratio.
[0028]
【The invention's effect】
Conventionally, positron annihilation γ-ray spectroscopy performed with radioisotopes and positron beams is very sensitive to defects such as metals and semiconductors, voids, and the like, and has been used in many studies. However, it was difficult to introduce the positron deep inside the sample, and it was not possible to apply it to the structures actually used.
[0029]
When positrons are generated from inside the sample by the method invented this time, positron annihilation gamma ray spectroscopy inside the sample becomes possible. In addition, the laser inverted Compton high energy X-ray has excellent transparency and straightness, can be guided to a relatively distant place, has almost no scattering of the beam in the air, and does not require shielding. Therefore, it can be used for diagnosis of metal fatigue of an aircraft, for example.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of measurement of a positron annihilation γ-ray peak by laser inverse Compton light energy X-ray.
FIG. 2 is a diagram illustrating an example of peak measurement by simultaneous positron annihilation γ-ray two-photon measurement using laser inverse Compton light energy X-rays.
FIG. 3 is a diagram showing a measurement example of positron annihilation γ-ray energy inside a stainless steel bolt (a diagram viewed from above).

Claims (7)

A method of positron annihilation γ-ray spectroscopy of a sample using X-rays by laser inverse Compton, or a method of measuring a short-lived nuclear level, wherein the X-rays are irradiated more than the lifetime of a generated positron or the lifetime of a short-lived nuclide. The sample is pulsed into a short time width, introduced into the sample, and the positron annihilation γ-rays of the elements in the sample or γ-rays emitted from short-lived nuclei are analyzed. How to measure the position. Most of the X-rays of several MeV generated by laser inverse Compton apply energy to the sample by electron pair generation, detect X-rays scattered at that time, and obtain the time information when positrons are formed inside the sample. How to measure the positron annihilation lifetime by obtaining. By detecting the scattered X-rays and thereby controlling the gate when performing positron annihilation gamma ray spectroscopy or short-lived nuclear level measurement, the positron annihilation lifetime or short-lived nuclear level can be increased with a high S / N ratio. The method according to claim 1 or 2, wherein the measurement is performed. Electron pair generation caused by high-energy X-rays generated by laser inverse Compton differs in efficiency depending on the composition of the sample, and by utilizing the high efficiency of heavy elements, conventional positron emission tomography (PET) can be used. A method of obtaining a CT image of the state inside a sample by using the method together. The method according to claim 4, wherein a crack or the like inside the sample is detected. By generating positrons from the inside of the sample using a high-energy X-ray that is highly transmissive and highly rectilinear, the laser does not require a high vacuum unlike the method using a positron beam from the outside of the sample. A method comprising measuring a sample in the atmosphere and a sample having a high vapor pressure without requiring a large scale. A method of obtaining three-dimensional information of positron annihilation gamma ray spectroscopy of a sample by collimating incident X-rays and also collimating positron annihilation gamma rays in front of a detector.

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