JPH0697697A - Composite magnetic shielding body - Google Patents

Abstract

PURPOSE:To improve magnetic shield from strong external magnetic fields by enclosing axially-joined oxide superconductors in a ferromagnetic shield body. CONSTITUTION:A magnetic shielding body 2 consisting of a divided superconductor is formed to a plurality of tubes or square housings whose both ends are opened or one end is closed or opened. The magnetic shielding body 2 is butted in its axial direction and a ferromagnetic shield body 1 is arranged outside the oxide superconductors. Thereby, an outside magnetic field of 100 gausses or more can be reduced up to at least one-tenth by the outside ferromagnetic shield body 1. Then, the superconductive magnetic shield body 2 can attenutes the magnetic field lowered to a level of 10 gausses up to one-hundred to one millionth. Thereby, a maximum outside shield magnetic field can be remarkably improved by using a composite magnetic shield body.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic shield body utilizing a superconducting phenomenon (excluding magnetic flux), and in particular, the superconductor comprises a sintered body of a high temperature oxide superconductor, a divided body of a thick film and a thin film, The present invention relates to a composite magnetic shield body in which a ferromagnetic body is arranged on the outside.

[0002]

2. Description of the Related Art Recently, studies have been conducted to apply an oxide superconductor to a magnetic shield container by utilizing the superconducting phenomenon of eliminating its magnetic flux. In this case, the superconducting shield does not take an external magnetic field into the superconductor and eliminates it toward the outside to prevent the magnetic field from penetrating into the internal space, and the magnetic shield effect is orders of magnitude greater than that of a ferromagnetic material. That is, the ferromagnetic shield has a limit due to the presence of remanent magnetization, and the magnetic shield in a region exceeding this limit must wait for the superconducting magnetic shield. However, for example, in Nb-based metal superconductors, liquid helium needs to be used as a coolant, so there is a cost barrier in magnetic shield construction, and superconducting shields have been put to practical use except for a small part. The reality is that there is none.

Recently, studies have been activated to measure the magnetic field emitted from the brain to elucidate the mechanism of the brain, elucidate headache, and examine the brain. Conventionally, MRI and positron C
Although the search for the inside of the brain such as T is practiced clinically,
There are restrictions on the resolution and the radiation used, and the intensity of the brain magnetic waves is an extremely weak magnetic field of 10 −9 Gauss, and the geomagnetism is 0.3 Gauss. It is in the area beyond the limit in.
In this case, a magnetic flux sensor called SQUID (superconducting quantum interference device) and a superconducting magnetic shield are essential.

The present inventors have previously clarified the effectiveness of a magnetic shield for biomagnetism measurement, which is a combination of an oxide high-temperature superconductor and a ferromagnetic magnetic shield (Japanese Patent Application No. Hei 3).
-242134). When measuring the human brain magnetic field, it must be clinically applicable to the target patient without fear, and as a result, the superconducting magnetic shield becomes large. For example, diameter 1m, length 3m
Etc. Although it is technically possible to manufacture such a superconductor integrally, it is too costly and not practical.

Further, if the divided superconductors are combined and magnetically shielded, the magnetic flux leaks from the divided portions and a sufficient magnetic shield effect cannot be obtained. In addition, when applied to biomagnetic measurement, it is usually said that it is sufficient to shield a weak magnetic field (1 Gauss or less) of the order of geomagnetism.
For example, it is conceivable that a magnetic field is suddenly generated from a commercial power source or the like and the magnetic shield of the superconductor is broken.

A superconducting magnetic shield has an external magnetic field of 1-10.
1 to 1 in 1,000,000 for Gauss's relatively weak magnetic field
It has a sufficient shielding effect. However, for a magnetic field exceeding, for example, 100 Gauss, the magnetic flux penetrates into the superconductor, and the shielding effect is lost. on the other hand,
The ferromagnetic shield has an external magnetic field of 100 Gauss or more 10
It is easy to reduce it to about Gauss, but it is extremely difficult to reduce it to, for example, 10 to 1,000,000. The present invention pays attention to the characteristics of both of them, and found that a stronger magnetic field can be effectively shielded by combining the both.

The present invention combines divided superconductors,
By arranging a ferromagnetic material on the outside, a composite magnetic shield body that does not deteriorate the magnetic shield characteristics is provided. By combining a superconducting magnetic shield body and a ferromagnetic magnetic shield body, a stronger external magnetic field can be obtained. An object of the present invention is to provide a composite magnetic shield body capable of shielding.

[0008]

According to the present invention, a plurality of magnetic shields made of a cylindrical or rectangular tube type superconductor having both end openings or one end closed / one end opening are butted in the axial direction, and these are oxidized. A composite magnetic shield body in which a ferromagnetic shield body is arranged outside a superconductor, and the length of the ferromagnetic shield body is preferably longer than the total length of the superconducting magnetic shield body. Is the solution.

As the superconductor in the present invention, a sintered body, a thick film or a thin film formed on a substrate can be used, and as a ferromagnetic material, permalloy or Ni-Fe.
Alloys, soft iron, etc. can be used, especially soft magnetic materials (Fe-C
(r-Si-Al alloy) is preferable because it does not need to be demagnetized and its characteristics do not deteriorate even at liquid nitrogen temperature.

In general, even if both ends of a monolithic superconducting magnetic shield are open, when a magnetic field tries to enter the openings, a shield current flows in the circumferential direction of the cylinder to prevent the magnetic field from entering. In this case, a path for a current through which the superconducting current flows can be easily formed in the circumferential direction, but if the container is divided in the circumferential direction, the flow of the superconducting current is blocked, so that the shielding effect is significantly reduced. From this point of view, in the present invention, a plurality of containers which are not divided in the circumferential direction are used by abutting in the axial direction thereof. FIG. 1 shows a case where two such containers are assembled by abutting their end faces. In this case, it is preferable that the gap between the container 1 and the container 2 is as narrow as possible.
~ 2% or less is good.

The ratio of the length Lf of the ferromagnetic shield to the total length Ls of the superconductor is L when the superconductor is a double-ended type.
f / Ls ≧ 1.4 is preferable, and in the case of one-end opening / one-end closing type, Lf / Ls ≧ 1.2 is preferable. If the ratio of Lf to Ls is less than this, the maximum magnetic field that can be shielded will suddenly decrease. Further, if the above ratio is 3 or more, the magnetic field that can be shielded also decreases, and the shield body becomes too large, which is not practical. On the other hand, in the case of a cylindrical container, the diameter (Df) of the ferromagnetic material and the diameter (Ds) of the superconductor are Df.
Can be increased from several percent to several tens of percent compared to Ds.

Now, the length L of the ferromagnetic shield having openings at both ends is L.
When f and the diameter are Df, it is regarded as a spheroid with an axial ratio p = Lf / Df, and from the calculation by the magnetic circuit, the shielding degree S (vertical) of the vertical magnetic field parallel to the cylindrical axis is S (vertical) = 4NS You can write (1). Here, when the ferromagnetic material is a single layer, S = μt / Df (2) where μ is the magnetic permeability and t is the thickness. Also, N is N = 1 / p (In2p-1) (p> 4, or 4
(3). Considering using these approximate expressions, when the length Lf of the ferromagnetic material and the length Ls of the superconductor are the above-mentioned ratios,
It turns out that it can shield efficiently.

The shield mechanism by the composite magnetic shield body of the present invention constructed as above will be described for better understanding of the present invention. Therefore, the following description does not limit the scope of the invention. In the composite magnetic shield body of the present invention, first, the outer ferromagnetic shield body is made of 10
An external magnetic field of 0 Gauss or more can be reduced to 1/10 or less. Next, the magnetic field lowered by the superconducting magnetic shield body to the level of 10 Gauss can be attenuated to 10 to 1,000,000. When the superconductor is divided, the shield effect is reduced by about one digit for a magnetic field in the direction parallel to the cylindrical axis, so the diameter / depth ratio is preferably 1 or less. It can be:
Further, since the outer ferromagnetic shield body is excellent in the shield effect against the magnetic field in the direction perpendicular to the cylindrical axis,
An external magnetic field of 00 Gauss can be attenuated to 1/100. Furthermore, the inner superconducting magnetic shield can attenuate this to 1/100 or less.

According to the composite magnetic shield of the present invention, 1
A strong magnetic field of 00 Gauss or more can be attenuated to 1 to 1,000,000th. Further, even if the inner superconductor is divided in the axial direction, since these are butted in the axial direction, the shielding effect is not deteriorated, and the diameter / depth ratio can be set to about 1, which is relatively small. A large space can be magnetically shielded by a superconductor.

The present invention is particularly suitable for biomagnetic measurement, but is not limited to this and can be applied to various fields such as physical experiments and device experiments requiring magnetic shielding.

[0016]

According to the present invention as described above, the following effects can be obtained. (1) The attenuation rate does not decrease even if the divided superconductors are used. (2) Since the superconductor can be divided, it is possible to magnetically shield a large space with a relatively small superconductor. (3) It can shield an external magnetic field of 100 Gauss or more, which is relatively difficult only with a superconductor. (4) It is possible to obtain an attenuation rate of 1 to 1,000,000 which is difficult with a ferromagnetic material alone.
(5) In the case of a thin superconductor such as a thick film, the maximum external shielding magnetic field is usually as low as 1 to 10 gauss, but the use of the composite magnetic shield of the present invention dramatically improves the maximum external shielding magnetic field. it can.

[0017]

Example 1 Bi: Pb: Sr: Ca: Cu = 1.8
Using an oxide superconductor powder having a composition of 4: 0.34: 1.91: 2.06: 3.06, an inner diameter of 10 mm and a length of 10
mm, a cylinder having a thickness of 1 mm and having both ends opened was molded by a cold isostatic press and fired to prepare a bulk sample. As shown in FIG. 2, two cylinders are abutted in the axial direction, and an inner diameter of 12 mm, a thickness of 1.5 mm, and a length Lf of the soft iron cylinder appropriately changed (Sanyo Special Steel QMS3.
L), the GaAs Hall element is placed in the center of the superconductor with two butts, the superconductor and the soft iron cylinder are held together at liquid nitrogen temperature, and a longitudinal magnetic field parallel to the cylinder axis is applied from the outside. The internal magnetic field was measured. The strength of the internal magnetic field when the external magnetic field is increased is shown in FIG. The length of Lf in this case was 45 mm. As can be seen from FIG. 3, the composite magnetic shield completely shields the external magnetic field up to 320 gauss. In FIG. 3, the strength of the external magnetic field generated by the internal magnetic field is defined as the maximum shield magnetic field Bs.
FIG. 4 shows changes in Bs when the length Lf of the ferromagnetic material is changed. As can be seen from this figure, Lf is 35 mm and shows a maximum shield magnetic field of 400 Gauss. Lf / in this case
The Ls was 1.75 at 35/20. As can be seen from FIGS. 3 and 4, when the composite magnetic shield body of the present invention is used, an external magnetic field of 100 Gauss or more can be attenuated to a sensitivity (0.01 Gauss) or less that the Hall element cannot detect. As a result, the composite magnetic shield of the present invention has a magnetic field attenuation rate of 1 / 10,000 or less. Next, length 40mm
The internal magnetic field was measured by applying a transverse magnetic field perpendicular to the cylinder axis from the outside using the soft iron cylinder of No. 1 in the same manner as above.
As a result, the maximum shield magnetic field was 350 gauss, and the magnetic field attenuation rate was 1 / 10,000 or less.

[0018]

Example 2 In the same manner as in Example 1, a superconducting magnetic shield container having a diameter of 10 mm at one end closed / an opening at one end and a depth of 10 mm and a cylindrical container having both ends opened as in Example 1 were butted. Then, a soft iron cylinder having a length of 40 mm, which is the same as that of the first embodiment, was placed thereon to form a composite magnetic shield. The magnetic field attenuation rate of this magnetic shield body was 1 / 10,000 or less in both the longitudinal magnetic field and the transverse magnetic field.

[0019]

[Comparative Example] A magnetic shield effect was measured by cooling the liquid cylinders with the same double-ended open cylinders as in Example 1 to liquid nitrogen temperature. The maximum shield magnetic field for this cylindrical magnetic shield is
The longitudinal magnetic field was 5 gauss and the transverse magnetic field was 0.6 gauss. The magnetic field attenuation factor of this magnetic shield is 1/50 in the longitudinal magnetic field.
It was 0 and the transverse magnetic field was 1/60.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a composite magnetic shield body of the present invention, in which (a) is an overall perspective view, (b) is a both-end opening type, and (c) is a one-end closed / one-end opening type. Shown respectively.

FIG. 2 is a schematic explanatory diagram of a magnetic field measurement experiment in Examples.

FIG. 3 is a relationship diagram between an internal magnetic field and an external magnetic field of the composite magnetic shield body of the present invention.

[Fig. 4] Length Lf of soft iron cylinder and maximum shielding magnetic field Bs
FIG.

[Explanation of symbols]

 1 Ferromagnetic shield 2 Superconducting magnetic shield 3 GaAs Hall element

Claims (2)

[Claims] 1. A magnetic shield body composed of a plurality of cylindrical or prismatic superconductors having both ends open or one end closed / one end open in the axial direction, and a ferromagnetic shield is provided outside the oxide superconductor. A composite magnetic shield that consists of a body. 2. The composite magnetic shield body according to claim 1, wherein the length of the ferromagnetic shield body is longer than the total length of the superconducting magnetic shield body.