MAGNETIC FIELD DOSIMETER Marjan Blagojević - IRC Sentronis AD, Niš, Serbia Ljubomir Vračar -...

MAGNETIC FIELD DOSIMETER

Marjan Blagojević - IRC Sentronis AD, Niš, SerbiaLjubomir Vračar - Faculty of Electronic Engineering University of Niš, Serbia

RAD 2012The F irst International Conference on Radiation

And Dosimetry in Various Fields of Research


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.


• 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)



- 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)



- 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)


• 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 magneto-mechanical translation)

-Electronic interaction


Schematic representation of the magneto-hydrodynamic (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:

BvqF where F is the force on the electric charge q with velocity v, while B is the magnetic flux density.

-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:

sin dBvdE iwhere θ is the angle between B and the velocity vector.



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


- 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.

dt

dBrEJ

2

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.


- Magneto-mechanical effect (magnetic orientation and magneto-mechanical translation)

Magneto-OrientationA 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 vivoMagneto-TranslationA magnetic dipole m in a static gradient magnetic field experiences a force ofF=(m·)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


- 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.


• Measurement of Magnetic Fields The two most popular types of magnetic field probes are a search coil

and a Hall-probe. Most of the commercially available magnetic field meters use one of them.


• Integrated Hall sensor

The Hall voltage is given by:

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.

IBqnt

VH1

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.



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 Hall voltage by equation

2

1

S

SHH dSEV



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 right-hand 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)



• 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


• 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



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.



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.


• 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.

THE ENDTHE END

RAD 2012The F irst International Conference on Radiation

And Dosimetry in Various Fields of Research

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MAGNETIC FIELD DOSIMETER Marjan Blagojević - IRC Sentronis AD, Niš, Serbia Ljubomir Vračar -...

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