JPH01111400A - Magnetic shielding structure body - Google Patents

Abstract

PURPOSE:To enhance magnetic shielding performance against an external magnetic field and its change by a method wherein an inner layer composed of a paramagnetic substance, an intermediate layer composed of a substance whose maximum resistivity value is specified and an outer layer composed of the paramagnetic substance are provided. CONSTITUTION:This body is constituted by the following at least three layers: an inner layer 11 composed of a paramagnetic substance, an intermediate layer 12 composed of a substance whose maximum resistivity value is 100 or less; an outer layer 13 composed of the paramagnetic substance. That is, in order to make the inner magnetic flux independent of the outer magnetic flux and to avoid their mutual interference, a shielding layer composed of the paramagnetic substance is divided into two inner and outer layers 11, 13; in order to reduce their mutual interference of the magnetic flux between the two inner and outer layers 11, 13, the substance whose maximum resistivity value is 100 or less is used for the intermediate layer 12 sandwiched between the two inner and outer layers 11, 13. By this setup, an excellent magnetic shielding effect can be realized.

Description

[Detailed description of the invention] [Industrial applicability! ! ? ] The present invention provides a magnetic shield structure suitable for use in equipment that requires magnetic shielding from changes in external magnetic fields at the same time as preventing Fii field leakage, such as nuclear magnetic resonance tomography equipment (hereinafter referred to as NMR-CT). Regarding.

[Prior Art] In magnetic shielding for NMR-CT and the like, it is customary to surround equipment to be shielded with a magnetic shielding structure made of a single layer of high magnetic permeability material such as pure iron or silicon steel.

Conventionally, there have been attempts to improve the magnetic shielding performance of the above-mentioned magnetic shielding structure, for example, in JP-A-80-208418.
As described in the above publication, this method has been implemented by improving the manufacturing method of the shielding material.

[Problem to be Solved by the Invention 1] However, in a magnetic shielding structure made of a single layer, even if a shielding material with high magnetic permeability and excellent magnetic shielding properties is used, it is difficult to prevent even slight leakage of magnetic flux. This is unavoidable, and it is impossible to completely block out the influence on internal shielded equipment.

This means that, for example, in NMR-CT, minute vibrations! ! !
This has an extremely large effect on shielded equipment that generates the I magnetic field, leading to a decrease in the operational accuracy (measurement accuracy, etc.) of those equipment.

An object of the present invention is to reliably improve magnetic shielding properties against external magnetic fields and changes thereof.

[Means for Solving the Problems] The present invention provides a magnetic shield structure provided for shielding the influence of a magnetic field, which includes an inner layer made of a paramagnetic material and an intermediate layer made of a material having a maximum relative magnetic permeability of 100 or less. It has at least three layers: a magnetic layer and an outer layer made of a paramagnetic material.

[Function] According to the inventors' considerations, in a conventional single-layer magnetic shield structure, since the external magnetic flux and the internal magnetic flux flow through the same shield material, mutual interference occurs between the magnetic fluxes, and the magnetic shield The effect is not sufficient.

As a result of repeated experiments to prevent the mutual interference between the internal and external magnetic fluxes, the inventors discovered that the mutual interference can be reduced by improving the structure of the shield structure.

In other words, in order to ensure that the magnetic fluxes inside and outside are independent and do not interfere with each other, a shield layer made of paramagnetic material is placed between the inside and outside 2F.
In order to further reduce the mutual interference between the magnetic flux between the two layers, an excellent magnetic shield is achieved by providing a material with a maximum relative permeability ra IE of 100 or less in the intermediate layer sandwiched between the two layers. [Example] In order to confirm the magnetic shielding effect of the magnetic shielding structure consisting of the three-layer structure of the present invention and the appropriate magnetic permeability that should be provided for the intermediate layer material, the following experiment was conducted. .

The magnetic shielding structure 10 used as an experimental device is as shown in FIG.
2. Constructed of three layers: intermediate layer shield body 13.

The size of the magnetic shield structure 10 is approximately 2 m x 2 m, and pure iron is used as the shielding material for the inner and outer layers.
The maximum ratio 'r magnetic permeability of the shielding material forming the intermediate layer was varied.

In addition, an internal magnetic field generator (steady magnetic field) 14 and a magnetic flux measuring meter 15 are provided inside and outside the magnetic shielding structure 10, respectively.
, as well as external magnetic field generator 2! (steady/alternating magnetic field) and magnetic flux measuring meters 17 were installed in stages.

First, in order to investigate the leakage of external magnetic fields into the interior, an alternating magnetic field was generated externally and the influence on the interior was investigated. Here, the external magnetic field was set to three levels: 0.5, 2, and 10 Gauβ, and an alternating magnetic field (period: 1 second) of ±30% was generated. Furthermore, a steady magnetic field of 30 Gauβ was generated as an internal magnetic field. The results are shown in Figure 2. That is, Figure 2 shows the internal magnetic flux change rate (internal magnetic field 30 Gauβ) when an external alternating magnetic field is applied. In this case, magnetic shielding properties are significantly improved compared to a single layer structure. In addition, the magnetic shielding effect is such that the average of the alternating magnetic field is 0.5, 2, 10
The same applies to any of Gauβ. therefore,
In carrying out the present invention, a sufficient magnetic shielding effect can be obtained by simply providing a non-magnetic material or a gap as the shielding material for the intermediate layer.

The magnetic flux change rate on the vertical axis in FIG. 2 is a ratio when the magnetic flux change width in the case of a single layer structure made of pure iron is set to 100.

Next, the degree of leakage of the internal magnetic field to the outside is shown in FIG. 3, where a steady magnetic field of I Gauβ was generated as the external magnetic field. That is, Figure 3 shows the leakage magnetic flux (external magnetic field I Gauβ) outside when an internal magnetic field is applied.
According to the magnetic shield structure 10 of the present invention, it is recognized that magnetic shielding properties can be improved significantly compared to a single layer structure. Note that the leakage magnetic flux ratio on the vertical axis in FIG. 3 is the ratio when the leakage magnetic flux in the case of a single layer structure made of pure iron is set to 100.

Table 1 shows the degree of leakage of the external alternating magnetic field into the interior of the one-layer shield structure (conventional example and comparative example) and the three-layer shield structure (
These are the results of experiments for each of the invention examples). 3
M! According to the shield structure, it is recognized that a significantly superior magnetic shielding performance can be obtained compared to a 4-m single-layer shield structure.

Therefore, according to the present invention, it is possible to improve the measurement accuracy of devices such as NMR-CT, and to reduce magnetic considerations to the surrounding environment when installing these devices.

Note that the magnetic shielding structure of the present invention only needs to satisfy at least 3I5 of an inner layer made of a paramagnetic material, an intermediate layer made of a substance with a maximum relative magnetic permeability of 100 or less, and an outer layer made of a paramagnetic material. , even if it consists of four or more layers with other layers inside and outside each layer.

〔Effect of the invention〕

As described above, according to one aspect of the present invention, magnetic shielding properties against external magnetic fields and changes thereof can be reliably improved.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing an example of the device of the present invention, Figure 2 is a diagram showing the internal magnetic flux change rate when an external alternating magnetic field is applied, and Figure 3 is a diagram showing the external magnetic flux rate when an internal oil magnetic field is applied. It is a diagram showing leakage magnetic flux. 10...Magnetic shield structure. 11... Inner layer shield body. 12...Outer layer shield body. 13...Middle layer shield body. Agent Patent Attorney Osamu Shiokawa Figure 1 Figure 2 Intermediate layer, maximum relative permeability of oFt

Claims (1)

[Claims] (1) In a magnetic shield structure provided to block the influence of a magnetic field, at least an inner layer made of a paramagnetic material, an intermediate layer made of a substance with a maximum relative magnetic permeability of 100 or less, and an outer layer made of a paramagnetic material. A magnetic shielding structure comprising three layers.