A Pick-up coil assembly for multi-channel squid magnetometer

Abstract: A pick-up coil assembly, for a multi-channel SQUID magnetometer, includes: a base plate (62); a plurality of support bars (61) fixedly mounted on the base plate so as to form a matrix; and a plurality of pick-up coil units fixedly mounted on the support bars, each pick-up coil unit (50) having respective first and second pick-up coil blocks. Each of the blocks has a support body (50a, 50b), a pair of pick-up coils (52a,...52d), connection pads (54a,...54f) and connection lines (53a, 53b), the pair of pick-up coils, the connection pads and the connection lines being formed on the support body using lithography techniques. Such a pick-up coil assembly can be made to a desirably high precision, whereby the detection characteristics of the different coils can exhibit improved uniformity.

Description: The present invention relates to a pick-up coil assembly for a magnetometer using a plurality of super-conducting quantum interference devices (SQUID's). A SQUID is widely used for detecting a weak magnetic field with very high sensitivity. That is, a SQUID can respond to a change in a weak magnetic field based on a quantum interference effect of the magnetic field. Accordingly, a SQUID is mainly utilized for, particularly, a high sensitivity magnetometer having a pick-up coil assembly as a magnetic sensor. A SQUID magnetometer is utilized, for example, in the medical equipment field to measure the weak magnetic field generated by the heart of a human body. A detecting stage of a digital SQUID magnetometer (see Fig. 11) may be constituted by a superconductive pick-up coil for detecting a weak magnetic field from an object (human body), a super-conductive input coil provided in the SQUID and magnetically coupled to the pick-up coil, and a superconductive feedback coil magnetically coupled to these coils. In general, the pick-up coil assembly is constituted by a plurality of coils (multi-channel), each coil corresponding to one channel, since it is necessary to simultaneously detect the magnetic field over a wide area on the object. For example, about 36 individual pick-up coils are necessary for diagnosing the heart. In this case, since the pick-up coil assembly must detect a very weak magnetic field without dispersion of characteristics among pick-up coils, it is necessary to assemble the pick-up coil assembly with very high precision, particularly structural precision, so as to have high uniformity among coils. EP-A-0 313 132 discloses a multi-channel SQUID magnetometer having a pick-up coil assembly with the features of the preamble of accompanying claim 1. In this magnetometer, each channel is equipped with an assembly in which a SQUID is integrated with first and second pick-up coils on the same substrate. EP-A-0 184 670 discloses a pick-up coil assembly for a SQUID magnetometer in which multiple pick-up coils are formed on the same cylindrical support body, thereby allowing a second-order gradiometer to be formed using a single substrate. According to the present invention, there is provided a pick-up coil assembly for a multi-channel SQUID gradiometer, comprising a base plate, and a plurality of pick-up coil units mounted on the base plate, each of the units having a support body, a plurality of pick-up coils and connection lines, the pick-up coils and the connection lines being formed by a lithography technique on the support body; characterised in that: a plurality of support bars are fixedly mounted on the base plate so as to form an array configuration; each of the pick-up coil units is made up of a first and a second pick-up coil block having respective support bodies which are fixedly mounted on one of the support bars; each of the pick-up coil blocks has a pair of said pick-up coils, and connection pads which are also formed by said lithography technique on the support body; and in that the pick-up coil assembly constitutes a detecting stage of the gradiometer, said detecting stage being provided separately from a SQUID and being coupled thereto via an input coil. An embodiment of the present invention can provide such a pick-up coil assembly for a multi-channel SQUID gradiometer, having high precision and high uniformity of characteristics detected among coils. Preferably, the pick-up coil assembly includes insulating spacers, one of which is provided between the first and second pick-up coil block of each pick-up coil unit to insulate between the pick-up coils. In one embodiment of the present invention, the pair of pick-up coils of each block forms a first order gradiometer. In another embodiment of the present invention, a pick-up coil of the first block is cross-connected to a pick-up coil of the second block by superconductive wires so as to form a second order gradiometer. Preferably, in a pick-up coil assembly embodying the invention, the support body includes a penetrating hole along with a longitudinal axis for inserting the support bar. Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is one example of a pick-up coil assembly for a multi-channel SQUID magnetometer; Fig. 2 is another example of a pick-up coil assembly for a multi-channel SQUID magnetometer; Fig. 3 is a basic form of a first order gradiometer; Fig. 4 is a basic form of a second order gradiometer; Fig. 5 is a schematic view of a pick-up coil unit for a pick-up coil assembly embodying the invention; Fig. 6 is a view for explaining the assembly steps of a pick-up coil assembly shown in Fig. 5; Fig.7 is a schematic view of a pick-up coil assembly embodying the present invention; Figs. 8A to 8G are views for explaining the manufacturing process of the pick-up coil using the lithography technique; Fig. 9 is a view for explaining a wire pattern on a base plate; Figs 10A to 10C show respective schematic plan views of different forms of a part of the coil unit of Fig 5; and Fig. 11 is a schematic block diagram of a digital SQUID control system for use in a digital SQUID magnetometer. Figure 1 shows one type of pick-up coil assembly. This type of pick-up coil assembly 1 is constituted by a plurality of pick-up coils 11. Each of pick-up coils 11 has a coil bobbin 111 and a superconductive wire 112. The coil bobbin 111 is made of a non-magnetized material. In this case, the superconductive wire 112 is manually wound on the coil bobbin 111 along with a groove (not shown) to form the first or second order gradiometer shown in Fig. 3 or Fig. 4. In a manufacturing process of the above pick-up coil assembly, however, it is very difficult to obtain high uniformity among coils so that this winding work wastes much time. Further, since the magnetic field from the human body is very weak, it is necessary to manufacture the pickup coil with very high structural precision for sufficiently eliminating environmental magnetic noises. Since the intensity of the magnetic field from the heart is on the order of about 10⁻¹⁰ Tesla (10⁻⁶ Gauss), it is necessary to ensure the structural precision in line with the above intensity. However, limitation of structural precision of this type is about 10⁻³ so that it is necessary to measure weak magnetic field in a magnetically shielded room which can completely eliminate environmental magnetic noises. Figure 2 is another example of a pick-up coil assembly for a multi-channel SQUID magnetometer. This type of the pick-up coil assembly is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-133474 (corresponding U.S. Serial No. 4,749,946). This type of the pick-up coil assembly 1 is constituted by a support body 20 and a plurality of pick-up coils 21a and 21b provided on the support body 20. In this case, the pick-up coils 21b are provided on opposite surfaces of the pick-up coil 21a, and connected thereto by a connection line 22. In this case, each of pick-up coils 21a and 21b are formed by a lithography technique which is widely utilized in the field of integrated circuit (IC) manufacturing. There are, however, some problems in the manufacturing process using the lithography technique as explained in detail below. First, in general, in the case of the diagnosis of the heart, it is necessary to provide 36 individual pick-up coils to detect the weak magnetic field from the heart. In this case, the 36 coils are arranged in a matrix of 6 columns x 6 rows. Accordingly, since the diameter of one pick-up coil is about 2 to 3 cm, the size of one surface of the support body 20 becomes 15 cm x 15 cm to provide the pick-up coil assembly. However, it is difficult to provide a large lithography device which can print such large numbers of lithography patterns (i.e., a plurality of pick-up coils 21a and 21b) on both surfaces of the support body 20. Second, it is difficult to provide the lithography patterns connecting the pick-up coils 21a and 21b (i.e., connection lines 22) because very fine adjustment of the position is necessary for printing the above patterns in an additional manufacturing process. Third, as explained above, at least the surface area of 15 cm x 15 cm is necessary for diagnosing the heart. However, it is very difficult to provide uniformity among all of the pick-up coils, particularly, if the pick-up coils are printed on four corners. A pick-up coil assembly embodying the present invention is explained in detail hereinafter with reference to the accompanying drawings. Figure 5 is a schematic view of a pick-up coil unit of an assembly embodying the invention. In Fig. 5, the pick-up coil unit is constituted by a first and second pick-up coil blocks. In the first and second block, 50a and 50b denote support bodies made of, for example, glass block or bulk of silicon. Each of the support bodies 50a and 50b has a square configuration in a sectional view, and comprises penetrating holes 51a and 51b along a longitudinal axis thereof. The support body 50a comprises only one pair of the pick-up coils 52a and 52b on both plane surfaces around the penetrating hole 51a. That is, the first order gradiometer is formed by a pair of pick-up coils 52a and 52b. The support body 50b including a pair of pick-up coils 52c and 52d has the same structure as the support body 50a. In this case, these pick-up coils 52a, 52b, and 52c, 52d are made by the lithography technique through the manufacturing process as shown in detail in Fig. 8. The upper pick-up coil 52a is connected to the lower pick-up coil 52b through the connection line 53a printed by the lithography technique. Similarly, the upper pick-up coil 52c is connected to the lower pick-up coil 52d through the connection line 53b printed by the lithography technique. In this case, the lithography technique of the connection lines 53a and 53b is not so difficult to manufacture because the support bodies 50a and 50b are very small. Further, it is not difficult to obtain a flatness between the upper coil 52a and the lower coil 52b in the body 50a, and between the upper coil 52c and the lower coil 52d in the body 50b because the support bodies 50a and 50b are very small. Further, it is not difficult to obtain uniformity among the coils because the same mask pattern used for the first pick-up coil is used for manufacturing a plurality of pick-up coils. Further, the pick-up coil 52b of the upper block 50a is cross-connected to the pick-up coil 52c of the lower block 50b through the superconductive line 53c so as to form the second order gradiometer as shown in Fig. 4. In Fig. 5, 54a to 54f denote connection pads. The pad 54a is connected to an input stage (see Fig. 11) of the magnetometer, the pad 54b is connected to the pad 54c, the pad 54c is connected to the pad 54f, and the pad 54d is connected to the pad 54e. In this case, the connection pad 54 is also formed by the lithography technique, and the connection between the pads is effected by a superconductive soldering or screw for a superconductive line. In the drawing, arrows denote the direction of the current flowing in the pick-up coil. A pick-up coil assembly embodying the present invention is constituted by a plurality of pick-up coil units 50 shown in Fig. 5, by arranging the pick-up coil units in the matrix configuration as explained in detail hereinafter. Figure 6 is a view for explaining assembly steps of the pick-up coil assembly. In Fig. 6, 61 denotes a bar. 63 denotes a spacer for insulating between pick-up coils. A plurality of bars 61 are fixedly arranged in a base plate 62 in a matrix configuration. In the assembling steps, first, the support body 50b is inserted onto the bar 61 through the penetrating hole 51b; second, the spacer 63 is inserted onto the bar 61; third, the support body 50a is inserted onto the bar 61 through the penetrating hole 51a; finally, the pick-up coil 52b is connected to the pick-up coil 52c as explained above, then, the support bodies 50a and 50b are integrated as the pick-up unit 50. Figure 7 is a schematic view of the pick-up coil assembly. As shown in the drawing, a plurality of the pick-up coil units 50 are arranged in the matrix configuration so as to form the pick-up coil assembly having, for example, 36 (6 x 6) pick-up coil units. Figures 8A to 8G are views for explaining a manufacturing process of the pick-up coil using the lithography technique. In Figs. 8A to 8G, 80 denotes a niobium (Nb) thin layer, 81 a resist layer, and 82 a mask film. In Fig. 8A, the niobium thin layer 80 is formed on the glass (or silicon: Si) body 50a by using a sputtering method. In Fig. 8B, the resist material is spread on the niobium thin layer 80, the spread resist material is baked to be harden by using a pre-bake method, and the resist layer 81 is formed on the glass body 50a. In Fig. 8C, the mask film 82 having the pattern of the pick-up coil is put on the resist layer 81. In Fig. 8D, after exposing, developing and heating are performed by using a post-bake method so that the resist layer 81 remains along with the pattern of the pick-up coil 52a. In Fig. 8E, the niobium thin layer 80 which is not covered by the resist layer 81 is eliminated by using a dry-etching method. In Fig. 8F, the resist layer 81 is eliminated by a solvent, for example, an acetone solvent, and only the niobium thin layer 80 is formed on the glass body 50a. In Fig. 8G, finally, the pick-up coil 52a having the desired configuration is formed on the glass body 50a. The same manufacturing process as above is repeatedly performed for all surfaces of the glass bodies 50a and 50b. Figure 9 is a view for explaining a wire pattern on the base plate. The wire pattern 64 is formed by a printed circuit technique on the base plate 62. Figures 10A to 10C illustrate alternative forms of the support body shown in Fig. 5. As shown in Figs. 10A to 10C, the support body 50a″ is formed by an octagonal or hexagonal configuration. Figure 11 is a schematic block diagram of a digital SQUID control system used for the digital SQUID magnetometer. In Fig. 11, L₄ denotes a pick-up coil. The SQUID is constituted by two Josephson devices and a superconductive coil L₂. Further, L₁ denotes a superconductive input coil, and L₃ denotes a superconductive feedback coil. Reference number 191 denotes an amplitude modulation wave generator, 192 an up-down counter, 193 a digital-to-analog converter, 194 a low-pass filter and 195 a current converter. A pick-up coil assembly embodying the present invention can be applied to the pick-up coil L₄. Briefly, the pick-up coil L₄ detects weak magnetic field Φ c irradiated from the object to be measured. Accordingly, the current flows input coil L₁ in response to change of the magnetic field Φ c . The coil L₂ of the SQUID is arranged in the vicinity of the input coil L₁ so as to obtain magnetic coupling with each other. Further, an alternating bias current I b generated from the generator 191 flows in the SQUID through an injection terminal B. The bias current I b is superimposed on the current flowing in the SQUID. Reference I f denotes a feedback current flowing from the converter 195 to the coil L₃. A patent application for the digital SQUID control system discussed above has already been filed in the United States on March 27, 1990, in the names of the present inventors. As explained above, a pick-up coil assembly embodying the present invention is constituted by a plurality of pick-up coils arranged in an array configuration (for example, matrix) for enabling multi-channel measurement of magnetic field. Such a coil assembly may be advantageously utilized in a digital SQUID magnetometer.

Claims: A pick-up coil assembly for a multi-channel SQUID gradiometer, comprising a base plate (62), and a plurality of pick-up coil units mounted on the base plate (62), each of the units having a support body (50a, 50b), a plurality of pick-up coils (52a,52b;52c,52d) and connection lines, the pick-up coils and the connection lines (53a, 53b) being formed by a lithography technique on the support body (50a,50b); characterised in that: a plurality of support bars (61) are fixedly mounted on the base plate (62) so as to form an array configuration; each of the pick-up coil units (50) is made up of a first and a second pick-up coil block having respective support bodies (50a, 50b) which are fixedly mounted on one of the support bars (61); each of the pick-up coil blocks has a pair of said pick-up coils (52a,52b;52c,52d), and connection pads (54a...54f) which are also formed by said lithography technique on the support body (50a, 50b); and in that the pick-up coil assembly constitutes a detecting stage (L4) of the gradiometer, said detecting stage being provided separately from a SQUID (L2,J1,J2) and being coupled thereto via an input coil (L1). A pick-up coil assembly as claimed in claim 1, further comprising insulating spacers (63), one of which is provided between the first and the second pick-up coil block of each pick-up coil unit (50) for insulating between the pick-up coils (52b,52c). A pick-up coil assembly as claimed in claim 1, wherein the pair of pick-up coils (52a,52b;52c,52d) of each block forms a first order gradiometer. A pick-up coil assembly as claimed in claim 1, wherein a pick-up coil (52b) of the first block is cross-connected to a pick-up coil (52c) of the second block by superconductive wires (53c) so as to form a second order gradiometer. A pick-up coil assembly as claimed in claim 1, wherein the support body (50a,50b) comprises a penetrating hole (51a,51b) along the longitudinal axis thereof for inserting the support bar (61).

Inventors: GOTO TAKAO, HAYASHI TAKEHIKO, IGARASHI YUTAKA, GOTO, TAKAO, HAYASHI, TAKEHIKO, IGARASHI, YUTAKA
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G01R33
IPC codes: G01R33