CHINESE JOURNAL OF PHYSICS

VOL. 42, NO. 4-II

AUGUST 2004

High-Tc SQUID Detection System for Contaminants in Food and Drug
S. Tanaka,1 N. Hotta,1 S. Kudo,1 S. Tsuboi,1 Y. Hatsukade,1T. Nagaishi,2 K. Nishi,2 H. Ota,2 and S. Suzuki3
1Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
2Sumitomo Electric Hightechs Co., Ltd., 1-1,
Koyakita 1-chome Itami, Hyogo 664-0016, Japan
3Advance Food Technology Co., Ltd., 1-12-5 Kita Iwata, Toyohashi, Aichi 440-0031, Japan
There is a strong demand for detection of small contaminants in food and pharmaceuticalmedicines. High-Tc SQUID detection systems for a metallic contaminant in foodstuffs anddrugs are developed for safety. We developed two systems; one large system is for meat
blocks and the other small system is for powdered drugs or packaged foodstuffs. Bothsystems consist of SQUID magnetometers, a permanent magnet for magnetization and abelt conveyer. All the samples were magnetized before measurements and measured by highTc SQUIDs. As a result, we successfully measured small syringe needles with length of 2mm in a meat block and a stainless steel ball as small as 0.3 mm in diameter.
PACS numbers: 85.25, 81.70, 07.55

I. INTRODUCTION

Recently, opportunities to eat processed foodstuffs are increasing in our daily life.
Therefore there is a chance to eat unfavorable contaminants, which are accidentally mixed
with food. For example, they are small chips of processing machines and also broken syringe
needles used for immunization shot or hormone injections. According to the increase ofinternational concern regarding food safety, we should develop a high sensitive detector toensure the safety. Although an iron particle detection system has been already developed,
there is a few reports for food contaminants and no system for factory use [1–3]. Our
target is fabrication of detection system for factory use. Since the electric conductivityof the austenitic stainless steel is low, it is difficult to detect it using a conventional eddy
current method. An austenitic stainless steel material is originally non-magnetic. However
it shows properties like a ferromagnetic material after martensitic transformation during
its manufacturing process. Therefore it is possible to detect it by SQUID magnetometer[4–8].
In this paper, we describe the system for small stainless steel contaminants in foodstuffs or pharmaceutical drugs by using high-Tc SQUID magnetometer.

http://PSROC.phys.ntu.edu.tw/cjp

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c 2004 THE PHYSICAL SOCIETY
OF THE REPUBLIC OF CHINA

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FIG. 1: Block diagram of food contamination detection system. It consists of a permanent magnet,a conveyer, a magnetically shielded box and SQUIDs.
II. PRINCIPLES

The block diagram of the detection system is shown in Fig. 1. It consists of a permanent magnet, a conveyer, a magnetically shielded box and SQUIDs. All of the sample
moves from left to right and passes under the magnet tunnel before the detection. An
austenitic stainless steel material is originally non-magnetic. However it shows properties
like a ferromagnetic material after martensitic transformation by work hardening during
its manufacturing process [9]. Therefore the magnetization prior to the detection is effective also for austenitic stainless steel contaminants. The magnetic field from a metallic
contaminant in food is detected by the SQUID magnetometers when it passes under themagnetometer.

III. CONTAMINANT DETECTION SYSTEM FOR MEAT BLOCK

III-1. System design
The target detected by the system is piece of stainless steal syringe needle 0.9 mm×2mm L. The size of the meat block which is currently processed at a factory in Japan is
640L×400W×200H. Thus we decided the size of the conveyer to meet its size. It is fairly
large and 500 mm in width. The conveyer speed is also determined by the real speed in afactory and is 15 m per minute.
Firstly, we calculated the distribution of the sensitivity of a SQUID sensor to know
the best configuration of the sensors to cover the width of the conveyer. As shown in Fig. 2,
we supposed a magnetic dipole and simulated the magnetic field of z-axis Hz at the positionof the sensor. The simulated results are shown in Fig. 3. This shows that if three SQUIDs
are positioned with separation of 170 mm, you can keep the sensitivity at least more than
60% of maximum value in any place on the conveyer with width of 500 mm. Therefore,
we determined that three SQUIDs are needed for this system. The SQUID and its driving
electronics we employed here are Sumitomo Electric Hightechs made. The size of the pickup
loop is 10 mm×10 mm square and high Tc direct coupled type. The sensitivity of the SQUID

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FIG. 2: Calculation of magnetic field from a magnetic dipole. Magnetic field of z-axis Hz at theposition of the sensor was calculated.

FIG. 3: Distribution of SQUID sensitivity. Sensitivity more than 60% of maximum value can beobtained in any place on the conveyer with 500 mm width if three SQUIDs are employed.

is nominally 300 fT/Hz1/2 at 10 Hz. The SQUID driving electronics is non-modulation typeand its bandwidth is 300 kHz. The detail of the specification is shown in the web [10].
Fig. 4 shows the design drawing of the whole system. The size is 3305L×1290W×
1610H. The magnet is made of Nd base alloy and its magnetic field is 0.2 T. The LN2cryostat to keep the temperature of the SQUIDs at 77 K is made of G-10 glass epoxy resin
and its size is 620W×200D×312H. The volume of the cryostat is 7 litters and the liquid
nitrogen can be kept for 7 hours without filling. The magnetic shield covering the cryostat
consists of two layers of permalloy with thickness of 1 mm. Most of the frame was madeof stainless steal sus304. Although the rollers were originally made of steal, they werereplaced with the aluminum alloy to prevent from magnetic noise. The system was totallycontrolled by a PC and you can operate the system by touching the display panel in frontof the system. All the electronics and the vacuum pump for evacuation of the cryostat wereVOL. 42S. Tanakaa, N. HOTTA et al.

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FIG. 4: Design drawing of the large system for meat block. The size is 3305L×1290W×1610H.

FIG. 5: Noise spectra of the system. Several peaks around 3 to 5 Hz are due to the environmental
noise at the laboratory. The peak at 60 Hz is from the appliance frequency. The noise at 10 Hz is
300–400 fT/Hz1/2 .

installed underneath the magnetic shield. The signal was passed through a low-pass filter(LPF) at a frequency of 5 Hz or 10 Hz.
III-2. Performances
We have measured the magnetic field noise of the system without filter. The signalof the SQUID was measured by dynamic signal analyzer Agilent Technologies 35670A.
The noise spectra are shown in Fig. 5. Several peaks around 1 to 5 Hz are due to theenvironmental noise at the laboratory. It may come from a voltage stabilizer in the power

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FIG. 6: Time trace of the detected signal. A cut stainless steel syringe needle with dimension of 0.9
×2 mm was measured. The distance between the test piece and the SQUID sensor was set at 200mm.

system. The peak at 60 Hz is from the appliance frequency. The magnetic field noise levelat 10 Hz is 300-400 fT/Hz1/2.
For the performance test, a cut stainless steal syringe needle with dimension of 0.9 x2mm as a test piece was put vertically on the conveyer. The distance between the test pieceand the SQUID sensor was set at 200 mm. The sample was moved by the conveyer withspeed of 15 m/min. It was magnetized by the magnet when it passed thorough the magnettunnel. And then it was measured when it passed under the SQUID. The time trace ofthe signal is shown in Fig. 6. One peak as large as 150 pT can be seen in the middle ofthe trace. This peak is corresponding to the test piece. We could successfully measure thepiece of needle as small as 2mm with distance of 200mm.

IV. CONTAMINANT DETECTION SYSTEM FOR DRUG

IV-1. System design
We also designed a smaller system, which detects contaminants in a drug or a small
packaged food. The target to be detected is small stainless steal ball as small as 0.3 mm indiameter. Since the expected dimension of the drug or the packaged food is 150W×80H inmaximum, the width of the belt conveyer was determined as 200 mm and the height fromthe conveyer to the bottom of the cryostat was 80 mm. The conveyer speed is the same as
the large system and is 15 m/minute. Following the same manner as our large system, wesimulated the distribution of the sensing area of the SQUID. As a result, it was found that
not one but three SQUIDs are needed with separation of 35 mm because the sensing area

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FIG. 7: Simulation model of magnetic shield. Magnetic field of 50 mT was applied from the Ydirection for each model and the field distribution inside was calculated by FEM.

IV-2. Magnetic Shield Design
FIG. 8: Design drawing of the small system. The size is 2500L×235W×800H.

becomes smaller as decrease of the distance.
The magnetic shield is expensive and one of important components in the detection
system because high sensitive magnetometers are used in a factory where environmental
noise is considerably higher. Thus, before the design we performed the 2D simulationof magnetic field using four models as shown in Fig. 7. Simulation software Maxwell ofAnsoft Corporation was used. Magnetic field of 50 µT was applied from the Y-direction foreach model and the field distribution inside was calculated by FEM. The thickness of thepermalloy layer was 1 mm. The summary of the simulation results is shown in Table 1. The

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VOL. 42TABLE I: Applied field: 50 µT
Number of layer (Model No.) Length (mm) Flux density at center (µT) Attenuation (dB)
2 (a)
600
3.20-23.9
2 (b)
1200
0.21-47.5
3 (c)
1200
0.19-48.63 (d)1200 with flange
0.10-53.8

FIG. 9: Magnetic signal vs. distance. The signal intensity is inversely proportional to the cubic ofdistance.

most attenuated result could be obtained at the model (d). Thus we employed three layerswith flange as a magnetic shield. Fig. 8 depicts the design drawing of the small system weconstructed. The size is 2500L×235W×800H. The magnet is made of Nd base alloy and its
magnetic field is 0.1 T, which is half an intensity of the large system. The LN2 cryostat tokeep the temperature of the SQUIDs at 77 K is made of G-10 glass epoxy. The system wastotally controlled by a PC. The electronic system is almost the same as the large system
for meat as we described in the section III.
IV-3. Performances
We prepared a sus304 stainless steal ball of 0.3 mm. It was magnetized by thepermanent magnet and detected by the SQUIDs. The distance dependence of the signal
is shown in Fig. 9. It is demonstrated that the signal intensity is inversely proportional
to the cubic of distance. The laboratory noise level is indicated at the same time for thecomparison. Although the signal at the distance of 50 mm is six times larger than the noiselevel, the signal at 80 mm is just 1.5 times larger than the noise. Therefore this systemshould be used within the distance of 50mm to obtain a better signal noise ratio.
Detection using a stainless steal ball in an aluminum bag was also performed. The
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the SQUID.

V. CONCLUSION

We have constructed and demonstrated two types of detection systems for metallic
contaminants in a food or drug. One is for large meat block and the other is for a pharmaceutical drug or a small packaged food. Former system could successfully detect a stainlesssteel needle of 0.9×2 mm and latter system could detect a stainless steal ball as small as
0.3 mm in diameter. These detectable levels are above the requirement at a factory.

Acknowledgments
This work was supported in part by Ministry of Education, Culture, Sports, Scienceand Technology. We thank Japan Meat Technolgy Institute (JAMTI) for their financial
support to develop a detection system for a meat block.

References[1] S. Tanaka, M. Natsume, T. Matsuda, Z. Aspanut, and M. Uchida, Extended abstract of 9thInternational Superconductive Electronics Conference (ISEC03), PTh47 (2003).
[2] M. Bick, P. Sulllivan, D.L. Tilbrook, J. Du, B. Thorn, R. Binks, C. Sharman, K.E.Leslie, A.
Hinsch, K.Macrae, and C.P. Foley, Extended abstract of 9th International SuperconductiveElectronics Conference (ISEC03), PTh06 (2003).
[3] G. B. Donaldson, A. Cochran, and D. McKirdy, Fundamentals and Applications, ed. H. Weinstock (Dordrecht, Kluwer Academic Publishers, 1996), p. 599.
[4] S. Tanaka, O. Yamazaki, R. Shimizu, and Y. Saito, Jpn. J. Appl. Phys. 38, L505 (1999).
[5] S. Tanaka, R. Shimizu, Y. Saito, and K. Shin, IEICE Transactions E-83-C, 44 (2000).
[6] S. Tanaka, A. Hirata, Y. Saito. T. Mizoguchi, Y. Tamaki, I. Sakita, and M. Monden, IEEE
Transactions on Applied Superconductivity 11, 665 (2001).
[7] S. Tanaka, T. Mizoguchi, H. Ota, and Y.Kondo, IEICE Transactions Electron. E85-C No.3,687 (2002).
[8] S.Tanaka, K. Matsuda, O. Yamazaki, M. Natsume, and H. Ota, Superconductor Science andTechnology 15, 146 (2002).
[9] H. Huang , J. Ding, and P. G. Mccormick, Materials Science & Engineering A-216 (1-2), 178(1996).
[10] http://www.shs.co.jp/squid/