JP2002263962A - Metal mesh formed of small gauge wire and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metal mesh formed of small gauge wire suitable for an extraction electrode for high strength and high quality charged particle beam, an acceleration electrode, and a moderator, being high in conversion efficiency, for producing slow position beam of high strength. SOLUTION: This metal mesh formed by machining a single wire is thinned to a 1-25 μm average wire size by electrolytic grinding. A mesh-like material produced by machining a metal sheet is thinned by electrolytic grinding to have 60% or more opening rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin metal mesh and a method for manufacturing the same. The thinned metal mesh is suitable for use as a charged particle beam extraction electrode, an acceleration control electrode, or a moderator for low-speed positron generation.

[0002]

2. Description of the Related Art A metal mesh is manufactured by processing a wire into a mesh. Further, the plate material is alternately cut by pressing, and the plate material which is stretched in a direction perpendicular to the cut (called expanded metal) is further smoothed by flat roll processing to produce a flat mesh. The overlapping portions of the mesh lines are eliminated, and the mesh is integrated. There is also a mesh-shaped product (such as as mesh) in which a plate material is pressed to form a hole.

[0003] A metal mesh is used as a control electrode for a charged particle beam, such as an electrode for extracting electrons from the filament and accelerating them, a grid electrode for accelerating positrons emitted from a moderator, and an electrode for a charged particle acceleration tube. It's being used. Further, a mesh is used to extract electrons emitted from the filament and positrons emitted from the moderator in the beam direction. The acceleration / deceleration control of the charged particle beam is performed by changing the potential of the electrostatic lens. However, at the end of the electrostatic lens, a convergence and divergence of the charged particles occurs because a gradient is generated in the in-plane potential. By forming a mesh on the end of the electrostatic lens to prevent the convergence and divergence, a beam with good emittance can be obtained. Although the shape and the like of the mesh differ depending on the purpose, the wire diameter of the mesh used as the electrode of the slow positron beam is about 30 μm. Here, 30 mesh means that there are 30 meshes in one inch in length.

The positron annihilation method has been used to study the interaction between positrons and atomic molecules, detect lattice defects and impurities in solids, study the free volume of plastics, and study the Fermi surface of metals. In this case, the β of the radioisotope RI
⁺ Positrons having a continuous energy distribution (end point energy of about 1 MeV) emitted by decay are used. When this high-speed positron is incident on some kind of material, some positrons lose kinetic energy until they reach equilibrium with the surrounding temperature in the material (hereinafter referred to as thermalization), and then reach the material surface by diffusion. Is released outside. This is because the positron is more energetically stable outside than in the substance. Therefore,
A substance whose work for extracting a positron is negative is used as a moderator. By accelerating positrons emitted from the moderator with a kinetic energy corresponding to a negative work function in an electric field, a positron beam with variable energy and a slow positron beam can be obtained.

[0005] When a slow positron beam is used, studies on the solid surface and the selective electronic state of the interface, which do not include information on the bulk as noise, and the dependence of the distribution of impurities and vacancies in the solid bulk on the depth from the surface. Can conduct research. Also,
The use of the slow positron beam enables measurement under severe high temperature conditions where the positron beam source cannot be placed beside the measurement sample and the positron beam source diffuses into the sample. As the source of the slow positron beam, positrons emitted by electron-positron pair generation using an electron linear accelerator (LINAC) are used in addition to β ⁺ decay RI.

The moderator is roughly classified into a transmission type and a reflection type. For transmission moderator, a certain distance (penetration length)
Only positrons that penetrate into the moderator, heat, diffuse to the surface, and are emitted. Therefore, the entrance surface and the emission surface are different. On the other hand, in the reflection type, the incident surface and the emission surface are the same. Normally, tungsten foil is used for the moderator, and in order to obtain higher conversion efficiency,
Noble gas solids such as neon and argon are used.

[0007]

It is difficult to process an extremely fine wire having a thickness of about several μm to about 10 μm, as the thinner the wire, the lower the mechanical strength. It is difficult to mesh a metal wire of about 10 to several μm as a single wire. Flat mesh, as-mesh, etc. are limited to a line width of about 0.1 mm due to press working, and the aperture ratio is limited to about 50%. Further, the wire diameter and the wire width are determined by the diameter of the wire or the pressing conditions. In addition, as an example, the wire diameter of the finest tungsten mesh currently available as a commercial product is 30 μm.

[0008] When a metal mesh is used for the extraction electrode and the acceleration electrode of the charged particle beam, if the charged particle beam collides with a metal wire, the charged particle is lost and the energy distribution width of the beam is widened. For this reason, it is desirable to make the wire diameter of the metal mesh as small as possible. It is desirable to use a mesh having a wire diameter of several μm in order to prevent a reduction in beam transport efficiency due to collision between the metal wire and the charged particles and a deterioration in beam quality due to the spread of energy distribution.

On the other hand, in order to generate a high-intensity slow positron beam, a moderator with high efficiency for converting a fast positron into a slow positron is required. The conversion efficiency of the most frequently used polycrystalline tungsten foil is 2.6 × 10 ⁻⁴ , and the expensive and difficult to handle single crystal tungsten foil (1 μm) is 5.9.
× 10 ^-4, refrigerator, high conversion efficiency for the vacuum pump requires a large gimmick solid noble gases moderator for differential pumping is reported to 7 × 10 ^-3. In addition, the conversion efficiency of the rare gas solid moderator lacks stability, and the positrons emitted without being sufficiently heated are included in the beam, so that the energy distribution of the beam becomes wider. In general, the intensity of ²² Na used as a source of a slow positron beam is 100 mCi at the maximum. Even when the maximum intensity of ²² Na is used as the radiation source, the beam intensity of the slow positron beam is about 10 ⁶ counts per second. Much smaller than the electron beam intensity.

In view of the problems of the prior art, the present invention is directed to an extraction electrode for a high-intensity, high-quality charged particle beam, an acceleration electrode, or a method for generating a high-intensity slow positron beam. It is an object of the present invention to provide a thinned metal mesh suitable for a moderator having a high conversion efficiency. Another object of the present invention is to provide a method capable of easily producing a thinned metal mesh.

[0011]

Even if a single wire having a small mechanical strength and a small wire diameter is processed into a mesh, the strength becomes large and handling becomes easy. Therefore, by processing a metal wire having sufficient mechanical strength into a mesh and then reducing the wire diameter, it is possible to solve the difficulty of manufacturing a metal mesh using a single wire having a wire diameter with insufficient mechanical strength. In the present invention, in order to reduce the wire diameter of the metal mesh, electropolishing is performed by using a mesh as an electrode in an electrolytic solution (acid such as dilute sulfuric acid or alkali such as sodium hydroxide) as an electrode. By adjusting the electrolytic polishing time or the electrolytic current, a mesh having an arbitrary wire diameter can be easily manufactured. This method can also be used to reduce the line width of a mesh-like material (a flat mesh, an as-mesh, etc.) obtained by processing a plate material, and to improve the aperture ratio.

That is, the metal mesh according to the present invention comprises:
A metal mesh formed by processing a single wire is thinned to an average wire diameter of 1 to 25 μm by electrolytic polishing. In addition, the metal mesh according to the present invention is obtained by thinning a mesh-like material formed by processing a metal plate by electrolytic polishing, so that the aperture ratio is 60%.
% Or more.

The method for producing a thinned metal mesh according to the present invention is characterized in that a metal mesh material having a wire diameter in a range of 4 μm or more and 40 μm or less is electrolytically polished in an electrolytic solution to obtain an average wire diameter of 29 μm or less. And a metal mesh having a wire diameter that is smaller than the wire diameter of the metal mesh material and that is at least 1/3 of the wire diameter of the original metal mesh material.

A method of manufacturing a metal flat mesh according to the present invention is characterized in that a mesh material formed by processing a metal plate is thinned by electrolytic polishing to have an aperture ratio of 60% or more. According to this method, a flat mesh having an aperture ratio of up to about 85% can be obtained. These metal meshes are useful as electrodes for extracting a charged particle beam or controlling acceleration, or those in which a plurality of these metal meshes are fixed and fixed are useful as moderators for generating a slow positron beam.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of an electrolytic polishing apparatus. 1N sodium hydroxide solution 10 is used as an electrolyte,
A copper pipe 11 was attached to the cathode, and a tungsten mesh 12 for electropolishing was attached to the anode. Tungsten mesh 1
2 was fixed to an acrylic 4 cm square frame 13. An ammeter and a voltmeter were installed, and the current and voltage were measured. The tungsten mesh 12 having a mesh size of 100 mesh / inch, a wire diameter of about 25 μm, and a wire diameter of 30 μm was thinned by electrolytic polishing.

As for the attachment of the tungsten mesh of the anode, in addition to the square frame 13 shown in FIG. 1, annular acrylic frames 15a and 15b shown in FIG. 2 and Teflon (registered trademark) shown in FIG. Three examples of struts 16 made of steel were tried. FIG. 2 shows an electrode formed by sandwiching a 0.5 mm tungsten wire (referred to as a conductive wire) and a tungsten mesh 12 between acrylic annular frames 15 a and 15 b having a diameter of about 3 cm, and FIG. 3 shows a Teflon circle. A tungsten mesh (5 cm × 5 cm) 12 is wound around a columnar support 16, a conductive wire is wound thereon, and then fixed with a plating tape 17 to form an electrode. In addition to these three examples, a method of fixing the mesh to a polymer resin plate is also conceivable.

The voltage is 1 to 6 V, and the current value is 0.1 to 5.5.
A. The temperature of the electropolishing was room temperature (about 23 ° C.). Every time the electropolishing was performed for 3 to 180 seconds, the process of removing the mesh from the electrolytic solution and washing with water was repeated. The polishing time was lengthened while the wire diameter was large, and the polishing was advanced and the wire diameter was reduced and the time was shortened. The total polishing time was about 30 seconds to 12 minutes. When a sample of 5 cm square was formed into the shape shown in FIG. 3 and a voltage of 6 V was applied at room temperature, a current of 5.5 A flowed. Under these conditions, the electrolysis time and the number of times were each 10 seconds 3
In this case, the wire diameter of the tungsten mesh could be reduced to about 1/3 in about 3 seconds, 3 times, and about 1 second.

With a tungsten mesh having a wire diameter of 25 μm, the average wire diameter could be reduced to about 8 μm, and with a mesh having a wire diameter of 30 μm, the wire could be reduced to an average wire diameter of about 10 μm. FIG. 4 is a photograph of a raw material mesh having a wire diameter of 30 μm, FIG. 5 is a photograph of a mesh obtained by electropolishing and thinning the wire to an average wire diameter of about 15 μm, and FIG. It is a photograph of the transformed mesh. FIG.
7 is an enlarged photograph of FIG. 4 to 6 are 245 times magnification,
FIG. 7 shows a magnification of 100 times.

Comparing the three experiments, the example of FIG.
It seems somewhat superior in that the sample is polished uniformly. However, it is considered that this result also depends on the shape of the cathode. In the case of electropolishing a large mesh, as shown in FIG. 8, the other electrode (cathode) 21 is also made to have a large size using a corrosion-resistant metal plate or mesh so that a uniform electric field is generated. There is a way to do it. Further, as shown in FIG. 9, a corrosion-resistant metal mesh (one of which may be a metal plate) 22,
A method of sandwiching the metal mesh 12 to be subjected to electrolytic polishing by 23 to reduce the potential difference on the metal mesh surface and make the wire diameter uniform can be considered.

As the electrolysis proceeds, a part of the mesh melts and disappears. This is presumably because, for some reason, a portion having a higher potential than the surroundings is formed on the metal mesh surface, and electrolytic polishing rapidly proceeds in this portion. Therefore, as shown in FIG.
It is considered that a method of sandwiching a corrosion-resistant metal plate and a metal mesh or two metal meshes to make the potential in the surface of the sample metal mesh equal to these metal plate meshes is effective. In the examples, electrolytic polishing was performed by using a sample having a small area, and all the components were used. However, a method of performing electrolytic polishing on a sample mesh having a large area and cutting out a portion having a desired wire diameter is also effective. is there.

It was confirmed that when the electrolytic solution was cooled, the speed of the electropolishing was reduced. For the same sample, when a voltage of 1.2 V is applied at a liquid temperature of 23 ° C., 500 m
A current flowed. Next, when a voltage of 1.4 V was applied at a liquid temperature of 3 ° C., a current of 100 mA flowed. from this result,
It is considered that the cooling reduces the rate of electropolishing to about 1/5. In addition, the larger the current value is several A, 1
The surface tended to be smoother than the case of less than A. Therefore, the polishing rate can be easily adjusted by performing electropolishing at a relatively large current value and at a low temperature.

Next, the mesh material to be electropolished was a nickel flat mesh having a plate thickness of 0.2 mm and a line width of 0.1 mm, and electropolishing was performed to reduce the line width of the mesh and increase the aperture ratio. . A nickel flat mesh (6 cm × 5 cm) was sandwiched between stainless steel plate frames, and graphite was used as a cathode. As the electrolytic solution, a solution obtained by diluting concentrated sulfuric acid twice was used. A voltage of about 3 V was applied, and a current of 7 A was allowed to flow for 30 seconds to 2 minutes. FIG. 10 is a photograph showing the result at this time. Where the line width is narrow, 30 μm
It is about. FIG. 11 is a photograph of a flash mesh of a material.

Although the aperture ratio of the original flat mesh material shown in FIG. 11 was 50%, the aperture ratio after electrolytic polishing was 75%.
%Became. This flat mesh can be thinned to a line width of about 15 μm, and the aperture ratio in that case is about 85%. A flat mesh having an increased aperture ratio as described above is useful as a conductive sheet in a glass plate or a synthetic resin plate because the flat mesh does not have overlapping portions of the mesh lines and has a uniform thickness.

From an experimental example using a tungsten mesh and a nickel flat mesh, it is considered that there is no particular limitation on the mesh density and the wire diameter. However, when the wire diameter becomes about 1/3 of the wire diameter of the mesh of the material, the unevenness of the wire diameter progresses, and a phenomenon is observed in which a part of the wire melts and disappears. For this reason, it is desirable to use a material having a wire diameter within three times the target wire diameter. Gold other than tungsten and nickel,
By selecting an appropriate electrolytic solution for metals such as molybdenum, copper, and aluminum, the wire diameter of the mesh can be reduced by the method of the present invention. Hereinafter, an example of using the thinned mesh manufactured by the present invention will be described. FIG. 12 is a schematic diagram of a slow positron beam device. FIG. 13 is a conceptual diagram of the positron moderator and the extraction electrode, and FIG. 14 is a schematic diagram of the acceleration electrode provided in the acceleration tube.

Positrons emitted from a positron beam source such as ²² Na are decelerated by a moderator arranged in front of the positron beam source, extracted in the beam direction by an extraction electrode arranged in front of the moderator, and accelerated. The sample is irradiated by being accelerated by the accelerating electrode of the tube. The E × B filter is a filter for extracting only low-speed positrons slowed down by the moderator among positrons emitted from the positron beam source.
Positrons not decelerated by the moderator travel straight without being deflected by the E × B filter, enter the γ-ray shield, and disappear therein. Γ generated when the positron disappears
The rays are absorbed by the γ-ray shield and do not go outside. Of the positrons emitted from the positron source, those emitted in directions other than the beam forming direction disappear in the γ-ray shield surrounding the positron source.

First, the moderator will be described. ²² Na
The kinetic energy of the positron beam emitted from the
It is continuously distributed from 4 MeV to 0. When a positron beam is incident on tungsten, the average penetration distance is about 10 μm. Some of the positrons that have lost their kinetic energy reach the tungsten surface by diffusion due to thermal motion,
Further, it is emitted to the outside with kinetic energy corresponding to the work function.

The penetration distance increases with the energy of the positron, but the diffusion distance is constant. When a positron enters a tungsten wire, the smaller the wire diameter, the greater the probability of emission to the outside. However, since a positron that has been heated in tungsten can be used as a slow positron beam, the thickness of tungsten that can be used to heat the positron must be considered. In a tungsten mesh with a small wire diameter, the positron is sufficiently thermalized by being incident on a plurality of tungsten wires. The positron that has entered the tungsten wire and decelerated and passed through without being sufficiently heated is again incident on another tungsten wire, where the rate of thermalization increases and the penetration distance of the re-injection decreases. The conversion efficiency is expected to increase. However, the positron emitted with kinetic energy equivalent to the work function is
It is estimated that about 80% to 90% will be lost if it again collides with the tungsten wire. Therefore, the optimum value of the number of superimposed meshes is determined by these two effects. Assuming that the positron collides with the mesh wire twice or so, in the case of a wire diameter of 5 μm and 100 mesh, it is considered necessary to overlap about 50 sheets.

Here, as an example, a 100 mesh tungsten mesh is electrolytically polished to have an average wire diameter of 8 μm.
After thinning to about m, 12 pieces cut out to about 18 × 7 mm were stacked. This was wrapped in a two-folded tungsten foil (20 μm), fixed to an electrode in a vacuum chamber of an annealing furnace, and evacuated to 10 ⁻⁷ Torr.
Annealing treatment was performed by heating at 2000 ° C. for 2 minutes under a slight vacuum. After cooling to room temperature, the pressure was returned to atmospheric pressure, and it was taken out of the vacuum chamber. A 12-layer thinned mesh removed from the tungsten foil was used for an inner diameter of 10 m.
m and fixed to a ring-shaped stainless steel plate.

The moderator was placed at a position 1 mm in front of the positron beam source. This is because the moderator is placed at a position having a large solid angle from the source and positrons emitted from the source are efficiently incident on the moderator. The potential of the moderator is about 10 V lower than the positron source, but higher than the extraction electrode. By doing so, the positrons (kinetic energy of about 2 eV) which have been heated in the moderator and emitted in the direction of the source can also be used by accelerating them in the beam axis direction with an electric field. This is one of the advantages of the mesh moderator.

The condition of the moderator having a high conversion efficiency is as follows: [1] The rate of thermalization of the incident positron in the moderator is large;
[2] The rate of release of the heated positron from the moderator is large. When a metal mesh having a small wire diameter is used as the moderator, the ratio of the surface area to the volume increases, and [2]
Well suited for the conditions. In addition, the condition [1] can be satisfied by stacking metal meshes.

From the fast positron emitted from ²² Na, several e
It has been reported that the conversion efficiency of V to a slow positron is a maximum of 5.9 × 10 ^{−4 for} a 2 μm tungsten single crystal foil and 2.6 × 10 ^{−4 for} a polycrystalline tungsten. Single crystal tungsten is more efficient than polycrystalline tungsten, but it is not only difficult to handle, but also requires periodic annealing. The conversion efficiency is the number of positrons emitted from the source (product of the radiation intensity of the source and the probability of β ⁺ decay)
Is the intensity of the positron beam with respect to The intensity of the positron beam is measured by injecting a beam into a substance (eg, graphite) having a positive positron work function and measuring the annihilation γ-ray of the positron with a scintillation measuring device or a semiconductor detector.

Therefore, in the present invention, as a candidate for a moderator which has a higher conversion efficiency than single-crystal tungsten, is easy to handle, and is stable for a long period of time, it is much cheaper and easier to obtain than single-crystal tungsten foil. Attention was paid to the method of stacking and using multiple tungsten meshes. The emission efficiency increases as the ratio of the area of the surface from which the positrons having lost energy in the moderator are released to the volume increases. Therefore, a smaller mesh is expected to have a higher conversion efficiency than a larger mesh.

100 mesh / inch, average wire diameter 25 μ
A conversion efficiency of 6.4 × 10 ⁻⁴ was obtained by using a moderator whose average wire diameter was reduced to about 1/3 of the original diameter and about 8 μm by electropolishing the tungsten mesh of m. This value is
The conversion efficiency is not only higher than 4.5 × 10 ⁻⁴ when a tungsten mesh having an average wire diameter of 25 μm is used as it is, but also exceeds the value of single crystal tungsten. The conversion efficiency was stable for a long time.

Next, the extraction electrode will be described. The lead electrode has a diameter of 10 mm using tungsten 30 mesh. A mesh whose average wire diameter was reduced to 8 μm by electrolytic polishing was used. 150V for moderator
The potential is lowered by about 350 V from the positron, and the positron emitted from the moderator is extracted in the direction of the central axis to be a beam. It is clear that as the wire diameter of the mesh becomes smaller, the cross-sectional area where these charged particles collide with the electrode becomes smaller. Therefore, the probability of collision is reduced, the loss of charged particles due to collision is reduced, and a decrease in transport efficiency can be prevented. Also,
The spread of the beam energy distribution due to the change in the kinetic energy of the colliding charged particles can be reduced.

As the accelerating electrode of the accelerating tube, a 30-mesh and 25-mm-diameter tungsten mesh whose average wire diameter was reduced to 8 μm by electrolytic polishing was used as in the case of the extraction electrode. By using a metal mesh as the acceleration electrode, a planar equipotential surface can be easily formed. At this time, by using a mesh having a small wire diameter, the probability of collision with charged particles is reduced, and a decrease in transport efficiency can be prevented. Further, the spread of the beam energy distribution due to the change in the kinetic energy of the charged particles colliding with the mesh can be reduced.

[0036]

According to the present invention, a single wire can easily produce a metal mesh having a wire diameter which is difficult to be processed because of its low mechanical strength. Further, by adjusting the time of the electrolytic polishing, a mesh having an arbitrary wire diameter independent of the diameter of the metal wire of the material can be obtained. By drawing out the charged particles and using the metal mesh as an accelerating electrode, it is possible to reduce the transport efficiency of the charged particle beam and reduce the spread of the beam energy distribution. In addition, by using this metal mesh as a moderator for slow positron beam generation, high conversion efficiency can be stably obtained for a long period of time.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an electrolytic polishing apparatus.

FIG. 2 is a schematic diagram of an electrode in which a conductive wire and a mesh are sandwiched between acrylic frames.

FIG. 3 is a schematic view of an electrode in which a tungsten mesh is wound around a Teflon support.

FIG. 4 is a photograph of a raw material mesh having a wire diameter of 30 μm.

FIG. 5 is a photograph of a mesh thinned to an average wire diameter of about 15 μm by electrolytic polishing.

FIG. 6 is a photograph of a mesh thinned to an average wire diameter of about 10 μm by electrolytic polishing.

FIG. 7 is an enlarged photograph of FIG. 6;

FIG. 8 is an explanatory diagram of an example of an electrode when a large-area mesh is electropolished.

FIG. 9 is an explanatory diagram of a method for reducing a potential difference on a metal mesh surface.

FIG. 10 is a photograph of a nickel flat mesh after electrolytic polishing.

FIG. 11 is a photograph of a nickel flat mesh before electrolytic polishing.

FIG. 12 is a schematic diagram of a slow positron beam apparatus.

FIG. 13 is a conceptual diagram of a positron moderator and an extraction electrode.

FIG. 14 is a schematic diagram of an acceleration electrode provided in an acceleration tube.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Electrolyte solution, 11 ... Cathode (copper pipe), 12 ... Tungsten mesh, 13 ... Square frame, 15a, 15b ... Annular frame, 16 ... Post, 17 ... Plating tape, 21 ...
Cathode, 22, 23 ... metal mesh

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) G21K 5/04 G21K 5/04 Z H01J 37/04 H01J 37/04 Z // B23H 3/00 B23H 3/00 (72 Inventor Shunichi Kurihara 1-1-103-101, Kasuga 1-1, Tsukuba-shi, Ibaraki Pref. (72) Inventor Yasushige Yano 2-1 Hirosawa, Wako-shi, Saitama Prefecture Inside RIKEN (72) Inventor Masayuki Kase Hirosawa, Wako-shi, Saitama Pref. No. 1 in RIKEN (72) Akira Goto, inventor 2-1 Hirosawa, Wako-shi, Saitama Pref. Inventor Yuzo Shinoka 1-242 Nisshin-cho, Omiya-shi, Saitama F-term (reference) 3C059 AA02 AB01 HA15 5C030 BB09

Claims (6)

[Claims] 1. A metal mesh obtained by processing a single wire and thinning the metal mesh to an average wire diameter of 1 to 25 μm by electrolytic polishing. 2. A metal flat mesh, wherein a mesh-like material formed by processing a metal plate is thinned by electrolytic polishing to have an aperture ratio of 60% or more. 3. A metal mesh material having a wire diameter of 4 μm or more and 40 μm or less is electrolytically polished in an electrolytic solution to have an average wire diameter of 29 μm or less and smaller than the wire diameter of the original metal mesh material. Characterized in that the metal mesh has a wire diameter of 1/3 or more of the wire diameter of the metal mesh material. 4. A method for producing a metal flat mesh having an increased aperture ratio, wherein a flat mesh formed by processing a metal plate is thinned by electrolytic polishing to have an aperture ratio of 60% or more. 5. An electrode for extracting a charged particle beam or controlling acceleration, wherein the metal mesh according to claim 1 or 2 is used. 6. A moderator for generating a low-speed positron beam, wherein a plurality of metal meshes according to claim 1 or 2 are stacked and fixed.