magnetic field dosimeter

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RAD 2012
The F irst International Conference on Radiation
And Dosimetry in Various Fields of Research
MAGNETIC FIELD DOSIMETER
Marjan Blagojević - IRC Sentronis AD, Niš, Serbia
Ljubomir Vračar - Faculty of Electronic Engineering University of Niš, Serbia
MAGNETIC FIELD DOSIMETER
A magnetic field always exists when there is an electric current flowing. A static
magnetic field is formed in the case of direct current, and a time-varying magnetic
field is produced by alternating current sources.
The fundamental vector quantities describing a magnetic field are field strength, H
(unit: A/m) and magnetic flux density, B (unit: T, tesla). These quantities are
related through B = µH, where µ is the magnetic permeability of the medium.
The term "dosimetry" is used to quantify exposure. Present understanding of
interaction mechanisms is insufficient to develop anything but preliminary
dosimetric concepts for static or ELF magnetic fields.
MAGNETIC FIELD DOSIMETER
• Sources of exposure
-Earth’s magnetic field
(from 30μT to 70μT)
- Transmission power lines
of direct current (≈20μT)
- Magnetic levitation trains
(from 100μT to several mT)
- Small permanent magnet
(local field higher than 0.5 mT )
-MRI diagnostic
(from 0.15T to 3T)
MAGNETIC FIELD DOSIMETER
• Sources of exposure
- Thermonuclear reactors
(9 T to 12 T internal field
and 50 mT external field)
- Magnetic-hydrodynamic
systems (5 T to 6 T
internal, 10 mT external
at a distance of 50 m and
0.1 mT at a distance
greater than 250 m)
- Superconducting
generators (6 T to 7 T
internal and external field
within personnel area less
than 0.1 mT)
MAGNETIC FIELD DOSIMETER
• Sources of exposure
- Thermonuclear reactors
(0.35 mT)
- Electrolytic process of
manufacturing aluminum
(several tens of mT)
- Permanent magnet
production (the magnetic
field on the workers
hands is typically 2 to
5 mT, on the chest and
hands 0.3 mT to 0.5 mT )
- Laboratory
electromagnet (2T)
MAGNETIC FIELD DOSIMETER
• Interaction mechanisms
Magnetic fields interact with the live matter
over three physical mechanisms:
-Magnetic induction (electro-dynamical
interactions with moveable electrolytes
and inductive electric fields and
currents);
-Magneto-mechanical effect
(magnetic orientation and magnetomechanical translation)
-Electronic interaction
MAGNETIC FIELD DOSIMETER
-Electro-dynamical interactions with moveable electrolytes
Schematic representation of the magnetohydrodynamic (MHD) force F. This MHD force
works across the channel in human blood vessels.
The Hall force, FH, acts against the direction of flow
and results from the interaction of current flow with
the magnetic field.
Magnetic induction is in the base of electro-dynamic interaction with moveable
electrolytes. Static magnetic field acts with a Lorentz force on moving ions, and
that way it inducts electric fields and currents:
F  qv  B 
where F is the force on the electric charge q with velocity v, while B is the
magnetic flux density.
MAGNETIC FIELD DOSIMETER
-Electro-dynamical interactions with moveable electrolytes
A direct consequence of the Lorentz force exerted on moving ionic currents, so
that blood flowing through a cylindrical vessels of diameter, d, will develop an
electrical potential, ψ given by the equation:
  Ei  d  v  B  d  sin 
where θ is the angle between B and the velocity vector.
MAGNETIC FIELD DOSIMETER
-Electro-dynamical interactions with moveable electrolytes
An example of an inducted electric potential caused by blood streaming through
the aorta in a persistent magnetic field, is demonstrated using an
electrocardiogram – ECG. The primary change of the ECG in the field is the change
of the signal’s amplitude in the place of T waves. Repolarisation of the ventricular
heart muscle, which increases T waves, occurs in a normal ECG, almost
simultaneously with the heart pumping the blood through the aorta. Therefore, it’s
expected that the potential inducted by the magnetic field, is superimposed to the
T wave.
Normal superimposed ECG and ECG measured in a
presence of a high magnetic field
MAGNETIC FIELD DOSIMETER
- Inductive electric fields and currents
An example of an interaction by magnetic induction is a case of time-dependent
magnetic field that inducts electric currents in living tissues, according to the
Faraday’s law of induction.
  r dB
J  E 

2 dt
J is the current density, E is the induced potential, r is the radius of the induced
loop, σ is the conductivity of the tissue, and (dB/dt) is the time rate of change of
the magnetic flux.
Induction of eddy currents in the human
body perpendicular to (a) a vertical
magnetic field and (b) horizontal
magnetic field.
MAGNETIC FIELD DOSIMETER
- Magneto-mechanical effect (magnetic orientation and magnetomechanical translation)
Magneto-Orientation
A magnetic dipole with moment m in a external magnetic field B experiences a
torque N=m x B
• Paramagnetic molecules experience a torque that minimizes their free energy
in the static magnetic field
• Forces are generally considered too small to affect biological tissues in vivo
Magneto-Translation
A magnetic dipole m in a static gradient magnetic field experiences a force of
F=(m·s)B
• Occurs in the presence of a magnetic field gradient
• Force in the direction of (diamagnetic) or against (paramagnetic) gradient
• e.g. 8 T magnet with 50 T/m falloff can decrease the depth of water in a
trough passing through the field
• Corresponds to <40mm H2O pressure, not enough to affect blood flow in a
human
MAGNETIC FIELD DOSIMETER
- Electronic interaction
Many studies use the possibility of a direct interaction between the magnetic field
and the DNA as an explanation for the changes in the biosynthesis. Spiral force
acts on electrons moving in the DNA, repulses them from each other, or even
breaks the chain. This increases the number of DNA multiplications.
Bending of the DNA chain.
MAGNETIC FIELD DOSIMETER
• Measurement of Magnetic Fields
The two most popular types of magnetic field probes are a search coil and a Hallprobe. Most of the commercially available magnetic field meters use one of them.
MAGNETIC FIELD DOSIMETER
• Integrated Hall sensor
The Hall voltage is given by:
1
VH 
IB
qnt
where q represents the charge of electrons, n is the average density of electrons
inside the plate, t is the thickness of the plate, I is the bias current, and B┴ is the
component of the magnetic field orthogonal to the plate’s surface.
In general, sensitivity parallel to the plate’s surface was achieved by using the
conventional Hall plate in a position orthogonal to the plate’s surface. The problem
is, that a structure like that isn’t compatible with conventional IC technology. The
solution for the vertical Hall element was discovered in 1984 by Popović.
MAGNETIC FIELD DOSIMETER
• Integrated Hall sensor
The principle of the vertical Hall element is illustrated on figure
Imaginary conformal transformation of a conventional Hall plate HP embedded vertically into
a chip CH (left), via an elastic stage (middle), into an integrated vertical Hall de vice VH
(right). The vertical Hall device has all terminals (C1, C2’, C2’’, S1 and S2) accessible at the
chip surface. In the right-hand structure, the solid arrows represent the current lines, while
the dashed line between the terminals S1 and S2 is the integration path for determining the
S2
Hall voltage by equation
VH   E H dS
S1
MAGNETIC FIELD DOSIMETER
• Integrated Hall sensor
Photograph of a fully integrated 3-axis Hall magnetic sensor. This is a CMOS int
grated circuit which consists of: the magnetic sensing part, the signal processing
part, and the connecting part. The magnetic sensing part, is shown on the righthand side. A single horizontal Hall plate (HH) measures the magnetic field
component perpendicular to the die plane and two pairs of vertical Hall de vices
(VH) measure each of the two in-plane components of a magnetic field. All of
these Hall elements are integrated on an area of about 150 μm x 150 μm and
have a depth of less than 10 μm. The die dimensions are 4300 μm x 640 μm x
550 μm (thickness)
MAGNETIC FIELD DOSIMETER
• Integrated Hall sensor
•
•
•
•
•
•
•
On-chip suppression of offset and 1/f noise
Negligible planar Hall effect
No parasitic inductive and capacitive coupling
High DC field resolution: up to 5μT at 20mT
High-frequency response: DC to 25 kHz
High spatial resolution: 0.05mm
Available ranges: 20mT, 200mT, 2T and 20T
MAGNETIC FIELD DOSIMETER
• A portable dosimeter
The design is based on a popular standard microcontroller PIC18F4520. It was
primary chosen because it has enough RAM memory to create a FAT file system,
that stores the data on the SD card. Also, the microcontroller controls the sensor’s
power supply, performs measurements with a built-in ADC convertor, and records
the information on the SD card.
Simplified schematic of the system
MAGNETIC FIELD DOSIMETER
• A portable dosimeter
At the start-up of the program, all crucial parameters
for device operation are configured, like microcontroller
ports and variables. The next procedure is switching
the SD card into MMC mode and creating a FAT file
system. After creating the file, the microcontroller
activates sensor’s power supply, and performs
measurements using a built-in 10-bit ADC. Conversion
of the received data to a desired format and storage to
the SD card follows. Next, the sensor’s power supply is
disconnected, and the microcontroller goes into SLEEP
mode. An internal Watchdog timer “wakes-up” the
microcontroller after a certain time, and the whole
cycle repeats itself. The microcontroller has a very low
consumption in SLEEP mode, only a few μA, this way
the whole system has a small average consumption.
This is very important considering that the system has
a portable battery supply.
MAGNETIC FIELD DOSIMETER
• A portable dosimeter
Photo of prototype dosimeter
The dimensions of the final prototype are only 28x48 mm, there is definitely a
possibility of further decrease, and developing a particular microsystem that would
significantly improve the ease of use.
MAGNETIC FIELD DOSIMETER
• Conclusion
- The dosimeter can be easily applied in many situations where there is strong
human exposure to the magnetic field.
- The transfer of measurement data, from the dosimeter to a computer, can be
performed using a SD memory card.
- The measurement data are processed using standard programs.
RAD 2012
The F irst International Conference on Radiation
And Dosimetry in Various Fields of Research
Thanks for your attention
THE END
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