INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGYSupercond. Sci. Technol. 17 (2004) 620–623

PII: S0953-2048(04)66355-X

Measurement of metallic contaminants infood with a high-Tc SQUID
Saburo Tanaka, Miyuki Natsume, Masashi Uchida, Naoki Hotta,
Takemasa Matsuda, Zarina A Spanut and Yoshimi HatsukadeToyohashi University of Technology, 1-1 Hibarigaoka Tempaku-cho,
Toyohashi Aichi 441-8580, Japan

Received 17 July 2003
Published 19 February 2004
Online at stacks.iop.org/SUST/17/620 (DOI: 10.1088/0953-2048/17/4/009)
Abstract
We have proposed and demonstrated a high-Tc SQUID system for detecting
metallic contaminants in foodstuffs. There is a demand for the developmentof systems for detecting not only magnetic materials but also non-magnetic
materials such as Cu and aluminium in foodstuffs to ensure food safety. The
system consists of a SQUID magnetometer, an excitation coil and apermanent magnet. For a non-magnetic sample, an AC magnetic field isapplied during detection to induce an eddy current in the sample. For amagnetizable sample, a strong magnetic field is applied to the sample prior
to the detection attempt. We were able to detect a stainless steel ball with adiameter of 0.1 mm and a Cu ball less than 1 mm in diameter, for example.

1. Introduction
Recently, processed foodstuffs have become common.
Therefore there is a chance of individuals ingesting
contaminants which have been accidentally mixed with food.
For example, small chips from processing machines and also
broken syringe needles used for administering immunization
shots or hormone injections, which are mostly metallic
materials, have been found in food. In view of the increase
in international concern regarding food safety, we need todevelop a highly sensitive detector to ensure safety. Althoughan iron particle detection system has already been developed,
there is no system for detecting food contaminants [1–3]. It isdifficult to detect all possible contaminant materials with one
detection system. However, if targets are limited to metallic
contaminants, a high-Tc SQUID is one candidate sensitive
detector for finding contaminants in foodstuffs.
In this paper, we describe a system for detecting small
amounts of metallic substances by using a high-Tc SQUID
magnetometer.

2. Principles
Figure 1 shows the principles of our detection system. Two
methods are employed. One is for dealing with non-magnetic
samples such as Cu and silver. The other is for dealing withmagnetizable samples. For the non-magnetic sample case, an
AC magnetic field is applied during detection to induce an eddy
current in the sample. The SQUID signal is lock-in amplified
and recorded. For the magnetizable sample case, a strong
0953-2048/04/040620+04$30.00 © 2004 IOP Publishing Ltd

magnetic field is applied to the sample prior to the detection
attempt. The direct signal from the SQUID electronics isrecorded through a low pass filter, which eliminates the AC
field component.

3. Experimental details
3.1. SQUIDThe SQUID is made of Y1 Ba2 Cu3 O7−y thin film [4, 5]. The
junctions utilized in the SQUID are of the step-edge type.
The washer size of the SQUID is about 5.5 × 5.0 mm2 andthe effective area is about 0.1 mm2 . When the SQUID wasoperated in a flux-locked loop with a flux modulation frequencyof 256 kHz, the magnetic flux noise in the white noise region
was about 20 µφ0 Hz−1/2 , as shown in figure 2.
3.2. SystemThe detection system was located in a magnetically shieldedroom in our laboratory. Figure 3 shows a schematic drawing of
the detection system. A cryostat which was specially designed
for use with a SQUID microscope was employed. The crosssectional view of the cryostat is shown in figure 4. The
SQUID was located inside a vacuum, face up, and separated
by a 200 µm thick sapphire window. A more detailed
description can be found elsewhere [6]. A rectangular shaped
Helmholtz coil (1000 turns × 2; bore: 80 mm × 50 mm) was
located above the cryostat via an engineering plastic robust
table. A sample was slid onto the surface of the rectangularcoil tube at constant speed. In this condition the spacing

Printed in the UK620

Measurement of metallic contaminants in food with a high-Tc SQUID

Figure 1. Principles of the detection system. For the non-magnetic
sample case, an AC magnetic field is applied during the detection toinduce an eddy current in the sample. For the magnetizable sample
case, a strong magnetic field is applied to the sample prior to thedetection attempt.
(This figure is in colour only in the electronic version)

Figure 3. A schematic drawing of the detection system. A
rectangular shaped Helmholtz coil was located above the cryostat
via a table. A sample was slid onto the surface of the rectangularcoil tube with constant speed.

Flux noise[φ0 /Hz 1/2 ]

10 -3

10 -4

10 -5
1

10100

1k

Frequency[Hz]Figure 2. The spectral density of the flux noise of the SQUID. The
magnetic flux noise in the white noise region was about
20 µφ0 Hz−1/2 .

between the sample and the SQUID was from 10 to 50 mm. A
sinusoidal AC current with a frequency of 900 Hz–2 kHz wasdirected to the coils; the peak to peak amplitude of the magnetic
field generated from the coil was 2.5 × 10−4 T(peak−peak). The
modulated signal associated with the metallic sample was thendemodulated by the lock-in amplifier. The AC magnetic field
was applied to the samples regardless of magnetic properties,
such as whether they were non-magnetic or magnetizablesamples. For non-magnetic samples, the output signals fromthe lock-in amplifier were directed to the A/D converter ofa PC through a high pass filter with a frequency of 0.1 Hz,
which eliminates any dc offset. In this case the z componentsof the magnetic field at the centre of the coils cancel eachother in principle when there is no sample. Therefore theSQUID position was carefully adjusted before measurement,so that the SQUID output signal without a sample becamezero [7, 8]. For magnetizable samples, the direct signal fromthe SQUID electronics is recorded through the low pass filter,
which eliminates the AC field component. We note that thesamples were magnetized by a 0.5 T permanent magnet prior
to the measurements.

Figure 4. A cross-sectional view of the cryostat. This is especiallydesigned for use with a SQUID microscope. The minimum
separation between the SQUID and the sapphire window is 200 µm.

3.3. Samples
Austenitic stainless steel balls, stainless steel chips and small
copper balls with different sizes were prepared as samples.
The size range of the samples was from 0.1 to 1.5 mm indiameter. Since the electric conductivity of austenitic stainlesssteel is low, it is difficult to detect it using a conventional eddy
current method. The austenitic stainless steel material was
originally non-magnetic. However, it shows properties like aferromagnetic material after martensitic transformation during
its manufacturing process [9]. Therefore a stainless steel ballsample can be magnetized. So the samples were magnetizedfor 5 s with a strong permanent magnet before measurement.

4. Results and discussion
Firstly, a stainless steel chip, which was a cut sample of asyringe needle, was measured. It was 0.95 mm in diameterand 1 mm in length. Before the measurement the sample was621

S Tanaka et al

Magnetic signal [ nT ]2.0Sensor

φ0.95mm x 1mm
D=50mm1.51.0SQUID Output

H1 = q m / 4πµ0 r12H1H 2 = −q m / 4πµ r

θ120 10.5

Lock in AmpOutput

r1 = (r − (L / 2)) / cos θ1
r2 = (r + (L / 2)) / cos θ 20.0

θ 2 = Tan −1[X /(r + (L / 2))]
H Z = H1 cos θ1 + H 2 cos θ 20.01.0

2.0

3.0

4.0

5.0

6.0

rL/2

θ1 = Tan −1[X /(r − (L / 2))]-0.5
-1.0qm

HzL/2

θ2-qm

H2X

Time [sec]4080120Distance [mm]160200Figure 5. Time domain signal traces for a stainless steel chip. TheSQUID output signal shows a large peak: as high as 2.7 nT (peak topeak). The lock-in amplifier output shows a small bump at aroundthe same time.

magnetized with a permanent magnet and then measured. The
separation between the sample and the sensor was 50 mm.
Time domain signal traces are shown in figure 5, (the secondaxis at the bottom gives the position of the sample). The
SQUID output signal shows a large peak: as high as 2.7 nT(peak to peak). The lock-in amplifier output shows a small
bump at 100 mm (2.8 s). This means that the conductivityof the stainless steel is relatively small and as a result theinduced eddy current was not large. Meanwhile, it was shownthat the sample had a remanent magnetism. Therefore, forthe detection of the stainless steel, magnetization is essential
to obtain higher sensitivity. Here we calculate the remanent
magnetic signal distribution of each sample in order to compare
the measured signals. Figure 6 shows the model and theequations used for the calculation. The z component of themagnetic field intensity Hz is a function of r and L, where r is
the distance from the dipole to the sensor and L is the length ofthe dipole. The calculation was performed under the conditionsof r = 50 mm and L = 1 mm. The calculated magnetic field
distribution of the sample is shown in figure 7. The measured
signal is asymmetric in shape, unlike the calculated signal.
The cause of the asymmetry is that the sample was not verticalbut slightly tilted. The width of the foot of the measured
signal is almost 130 mm, the same as the calculated value.
The calculated distribution shows also that the SQUID sensor
can cover only a width of 50 mm while keeping a sensitivityof more than 60% of the peak value. This result suggests thatmultiple SQUID sensors are needed to cover a wide area. Thisnarrow area problem becomes more severe in the case where
the sample rotates or tilts upon magnetization.
Secondly, austenitic stainless balls with different sizes
were investigated. After magnetization, the sample was slid
above the SQUID at a distance of 10 mm. The time signal
traces are shown in figure 8. For the diameters 0.6 mm (a) and
0.3 mm (b) of samples, there exists a clear peak at 90 mm(2.6 s). The small dimple at 140 mm (4.2 s) in figure 8(b) is
noise coming from the vibration. For the sample of diameter
0.1 mm, a FFT filtering was applied after the data acquisition,
because it was hard to identify the peak. The result shownin figure 8(c) is that after filtering. One can identify a small
622Figure 6. The model and the equations used for the calculation. Thez component of the magnetic field intensity Hz is a function of r andL, where r is the distance from the dipole to the sensor and L is thelength of the dipole.

Normalized Signal Intensity

01.2
1.0
0.8
0.6
0.4
0.2
0-100-50

050

100Distance [mm]Figure 7. The calculated magnetic field distribution of the sample.
The measured signal is asymmetric in shape, unlike the calculated
signal, because of the tilt of the sample. The width of the foot of themeasured signal is almost 130 mm, the same as the calculated value.

step at around 90 mm (2.6 s) in the figure. The signal peakmust be proportional to the volume of the sample. Actually
the signal depends on the cube of the diameter. This meansthat the signals are properly measured. Then we tried to put asample at some distance from the sensor. Figure 9 shows oneof the results for a 0.3 mm stainless steel ball at 30 mm from thesensor. A peak of 0.8 nT was clearly observed at 90 mm (2.6 s).
One can compare the peak value with that of figure 8(b). The
diameter of the sample is the same. The peak value is inversely
proportional to the cube of the distance from the sensor to thesample. Even if the distance became wider than 40 mm, a
substantial signal peak could still be detected. The detectable
minimum size in the conventional eddy current method for the
stainless steel ball is several millimetres. The sensitivity of oursystem is almost an order of magnitude higher than that of theconventional method. The detectable minimum for our system
is adequate for finding contaminants in foodstuffs except for
the narrow area problem that we discussed above.
Lastly, we attempted to detect copper ball samples underthe modulation scheme. A sinusoidal AC magnetic field wasapplied along the horizontal direction so that the field did
not enter the SQUID. This field induces eddy currents on thesurface of the sample. The depth of the current path depends onthe frequency. From the calculation, we found that the current
flows in bulk for a Cu ball to be less than 1.5 mm in diameter.
The distance from the sensor to the sample was 10 mm. TheMeasurement of metallic contaminants in food with a high-Tc SQUID

2φ0.6mm

φ0.3mm120

Magnetic Signal [nT]

10080
60
40
200-200.0030

1.0

2.0
3.0
Time [sec]4080Distance [mm]4.0120Magnetic Signal [nT]

0-1

5.0
160

-20.0

1.0

2.0

3.0

4.0

5.0

Time [sec](b)0

φ0.3mm4080

120160Distance [mm]20Figure 9. The time signal trace of a 0.3 mm stainless steel ball at30 mm from the sensor. A peak of 0.9 nT can be clearly observed.
100.25
00.0

1.0

2.0
3.0
Time [sec]04080Distance [mm]1.2Magnetic Signal [nT]

D=30mm
14.0

120

5.0

160(c)

Magnetic Signal [nT]

Magnetic Signal [nT]

160 (a)140

D=10mm

0.20
0.15
0.10
0.05
0.00

1.0
φ0.1mm

0.8

1.0

1.1

1.2

1.3

1.4

1.5

Size [mm]Figure 10. The dependence of the signal intensity on the Cu ballsize. At the distance of 10 mm, the detectable minimum size for theCu balls was 1.0 mm. The signal was proportional to the second tothird power of the diameter.

0.60.40.2
0.00.001.02.03.0
Time [sec]4080Distance [mm]4.01205.0160Figure 8. The time signal traces for the diameters (a) 0.6 mm,(b) 0.3 mm and (c) 0.1 mm of samples. For the sample of diameter
0.1 mm, a FFT filtering was applied after the data acquisition.

dependence of the signal intensity on the Cu ball size is shownin figure 10. The detectable minimum size for the Cu balls
was 1.0 mm. The signals were proportional to the second to
third power of the diameter. This suggests that the current
follows not only the surface but also the bulk of the sample.
We think that the detectable minimum size could be reduced
by improving the system so that it can generate a larger AC
magnetic field.

5. Conclusion
We have constructed and demonstrated a high-Tc SQUID
system for detection of metallic contaminants in foodstuffs. It

can successfully detect 0.6–0.1 mm stainless steel balls. The
signal showed a good agreement with the calculated value.
As regards the detection of non-metallic substances, Cu balls
1.0 mm in size were detected. This detection level is better
than market demands. There now needs to be development ofa good magnetic shield appropriate to a factory site.

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