JPH03248069A - Squid probe - Google Patents

Abstract

PURPOSE:To actuate a SQUID device in a low magnetic field by arranging a superconductive shield body to be freely slidable and shiftable to a supporting body of a probe, thereby to make the magnetic field within the shield body nearly completely 0. CONSTITUTION:A supporting body 3 is provided with a superconductive magnetic shield body 4 and a superconductor block 5 along the inner peripheral surface of the block 5 along the inner peripheral surface of the shield body 4. The shield body 4 is slidable along the supporting body 3 and is selectable between two states wherein it covers a SQUID device 1 and it covers the block 5. When the shield body 4 covers the block 5 and cooled so that both are turned in the superconductive state, the magnetic field inside the shield body 4 is almost completely removed. Even if the shield body 4 is moved and the block 5 is removed in this state, no magnetic flux enters the shield body 4 because of the effect of the complete diamagnetism in the superconductive state. Therefore, if the SQUID device 1 is covered in this state, the SQUID device 1 is covered without almost any magnetic flux present in the surroundings.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a SQUID probe used for biomagnetism measurement, geomagnetism measurement, or general micromagnetic field measurement.

<Prior Art> In SQUID (superconducting quantum interference needle), generally, the magnetic flux to be measured is not directly picked up by the SQUID element, but an input circuit called a magnetic flux transformer method or the like is used. This input circuit is a superconducting closed circuit consisting of a pickup coil that picks up the magnetic flux to be measured and an input coil, and the input coil is magnetically coupled to the SQUID element. In this case, the SQUID element itself is shielded from external magnetism, and element noise is reduced.

FIG. 2 shows the structure of a conventional SQUID probe using such an input circuit.

The SQUID element 21 and the pickup coil 22 are each supported by a support 23, and the SQUID element 21 is connected to an external measurement circuit via wiring (not shown) provided on the support 23.

A cylindrical shield body 24 made of a superconductor is disposed around the SQUID element 21.
This is a measure to reduce magnetic field fluctuations that directly act on the QtJID element 21.

By the way, in such a structure, earth's magnetism etc. remain inside the shield body 24 during the process until the superconducting magnetic shield body 24 is cooled and reaches a superconducting state. Shield body 24
If a static magnetic field exists inside the shield body 24, vibrations and thermal fluctuations cause magnetic noise that directly affects the SQUID element 21. Therefore, it is necessary to suppress the static magnetic field trapped inside the shield body 24 as much as possible.

For this purpose, a superconducting shield body having a structure as shown in FIG. 3 has been devised.

This shield body utilizes thermal expansion, and has a structure in which it is folded as shown in Figure 3 (a) before cooling, and opens as shown in Figure 3 (b) when cooled. . That is, by expanding the volume within the shield in a superconducting state, the internal magnetic flux density is reduced.

<Problems to be Solved by the Invention> However, although the shield body having the structure shown in FIG. 3 has the effect of reducing magnetic flux density, it is not only structurally complicated but also has problems in terms of durability.

Furthermore, since this structure is folded with the SQUID element inside, it is impossible to reduce the internal volume to O even in the folded state, and there is naturally a limit to the reduction of internal magnetic flux density.

The present invention was made in view of these points, and it is possible to make the magnetic field inside the shield almost completely O with a simple structure, and it is possible to operate the SQUID element in an extremely low magnetic field environment. The purpose is to provide SQUID probes.

<Means for Solving the Problems> In order to achieve the above object, in the present invention, as shown in FIG. 4 and a superconductor block 5 having a shape that follows the inner peripheral surface of the superconducting shield body 4, the superconducting shield body 4 is slidable along the support body 3, and covers the SQUID element 1 and the superconductor block 5. The configuration is such that the state in which the block 5 is covered can be selected.

Effect> The magnetic field inside the superconducting shield 4 is almost completely eliminated by placing the superconducting shield 4 in a state covering the superconducting block 5 and cooling it until both become superconducting.

Even if the superconducting shield 4 is moved in this state and the superconductor block 5 inside is removed, magnetic flux will not enter the inside of the shield 4 due to the complete diamagnetic effect in the superconducting state.
Therefore, if the SQUID element 1 is covered in that state, the SQUID
The D element 1 is covered with the superconducting shield body 4 in a state in which almost no magnetic flux enters around the D element 1.

<Example> FIG. 1 is a front view showing the structure of an embodiment of the present invention.

The SQUID element 1 is supported at the midpoint of a rod-shaped support 3,
A pickup coil 2 is supported at the tip of the support 3. The relationship between the pickup coil 2 and the SQUID element 1 is based on the magnetic flux transformer method described above (equivalent to the conventional method).

A shield moving column 6 is arranged in parallel with the support body 3, and a cylindrical superconducting shield body 4, for example, is fixed to the tip of this shield moving column 6. and,
This shield moving column 6 has a structure that allows it to move parallel to the support body 3 in its longitudinal direction along a guide (not shown).

Moreover, the space between the SQUID element 1 and the pickup coil 2 on the support body 3 has an outer shape that is almost the same as the inner circumferential surface of the superconducting shield body 4, and is smaller than the inner circumferential dimension of the superconducting shield body 4. A superconductor block 5 having slightly smaller external dimensions is also fixed.

Then, the superconducting shield body 4 is moved by a shield moving column 6.
By moving, at least two states can be selected: a state in which the periphery of the superconductor block 5 is covered and a state in which the SQUID element 1 is covered.

Next, the function of the embodiment of the present invention will be described together with its usage method.

First, the superconducting shield body 4 covers the superconducting block 5, and is slowly immersed in a coolant such as liquid helium.
Cool the whole thing. After the shield body 4 and the block 5 are sufficiently cooled and both become superconducting, the shield moving column 6 is pulled up so that the superconducting shield body 4 covers the 5QLJID element 1 as shown by the two-dot chain line in the figure. .

When the superconducting shield body 4 covers the superconducting block 5 and both become superconducting, the magnetic flux inside the shield body 4 is almost eliminated, and the magnetic flux is distributed between the inner circumferential surface of the shield body 4 and the outer circumferential surface of the superconducting block 5. It only exists in a small gap. When the shield body 4 is moved in this state,
Shield body 4 due to the perfect diamagnetic effect based on the superconducting state
External magnetic flux may penetrate into the inside of the shield body 4.
Inside, the magnetic flux remaining in the small gap mentioned above just spreads over the whole, and the magnetic flux density inside becomes extremely small.

When the shield body 4 is moved to a position covering the SQUID element 1 in this state, the SQUID element 1 is placed in an extremely small magnetic field environment and can be used almost without magnetic noise.

Note that the placement position of the superconductor block 5 is not limited to the position as in the above embodiment, but the superconductor block 5 may be provided closer to the tip of the probe than the SQUID element 1 as in this embodiment. This increases the effect of reducing the magnetic flux remaining within the superconducting shield body 4. That is, according to this arrangement, the shield body 4 and the block 5 are
It is possible to make the superconducting state before the D element 1, but in that state, the SQUID element 1 before becoming superconducting is covered and the magnetic field applied to the element 1 is kept low.
After completing the procedure of putting ID element 1 into a superconducting state, S
The amount of magnetic flux trapped in the QUID element 1 itself is reduced, and further reduction in element noise can be expected.

Here, the superconducting shield body 4 and the superconducting block 5
As long as a material whose critical temperature is sufficiently higher than that of the SQUID element 1 is selected, the above effect can be expected regardless of the positional relationship with respect to the SQUID element 1. That is, SQ
As the material of the UID element 1, for example, Nb with Tc=9.2
T and L are used as the shield body 4 and block 5.
=, 90 K of YB az Cu! If OK materials are used, the element 1 and the shield body 4. Even if the block 5 is cooled at the same time, the shield body 4 and the block 5 become superconducting much earlier, and the same effect as described above can be obtained.

<Effects of the Invention> As described above, according to the present invention, the superconducting shield for magnetically shielding the SQUID element is slidably displaceable with respect to the support of the probe, and the shield body is used to shield the SQUID element and the SQUID element. Since it is possible to selectively cover any of the separately provided superconductor blocks, SQU
It becomes possible to cover the ID element, and the magnetic field applied to the SQUID element can be suppressed to an extremely low level compared to conventional measures. Furthermore, the structure of the present invention is simpler than that shown in FIG. 3, and there is no fear of metal fatigue, resulting in significantly superior durability.

[Brief explanation of drawings]

Figure 1 is a front view showing the structure of an embodiment of the present invention, Figure 2 is an explanatory diagram of a conventional SQUID probe, and Figure 3 is an explanatory diagram of a conventional shield structure designed to reduce magnetic flux density within the shield. . 1... SQUID element 2... Pick-up coil 3... Support body 4... Superconducting shield body 5... Superconducting block 6... Support for shield movement

Claims (1)

[Claims] A probe comprising a pick-up coil for picking up the magnetic flux to be measured and a SQUID element magnetically coupled to the pick-up coil, each mounted on a support body,
It includes a superconducting magnetic shield for covering the SQUID element, and a superconducting block having a shape that roughly follows the inner peripheral surface of the superconducting shield, and the superconducting shield is slidable along the support. , a SQUID configured such that a state in which the SQUID element is covered and a state in which the superconductor block is covered can be selected.
probe.