ultra sensitive magnetic field detectors · ultra sensitive magnetic field detectors gary green...

Ultra SensitiveMagnetic

FieldDetectors

Gary Green

Outline

MagneticFields

a simplemagnetometer

Squids

Atomic Clocks

Arrays of mag-netometers

The future

Summary

Questions

Ultra Sensitive Magnetic Field Detectors

Gary Green

York NeuroImaging Centre, UK

January 8, 2014


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

1 Magnetic Fields

2 a simple magnetometer

3 Squids

4 Atomic Clocks

5 Arrays of magnetometers

6 The future

7 Summary

8 Questions


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Magnetic Fields - a bit of history

• Oersted (1819) demonstrated an electric current generatesa magnetic field

• Ampere (1820) demonstrated parallel wires attract ifcurrent flows in the same direction in each wire

• Biot and Savart (1820) predicted the magnetic fieldaround the current

• Faraday (1831) demonstrated induction where a changingmagnetic field is associated with an encircling electric field

• Maxwell (1865) published unifying theory of theelectromagnetic field

• Einstein (1905) electric and magnetic fields are the same -different reference frames


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Magnetic fields - some simpleobervations

• Due to a moving electrical charge -right hand rule

• The magnetic moment of an elementary particle due tospin

• Magnetic flux• often denoted as Φ or ΦB

• - units are in Webers (Wb)

• Magnetic flux density - the flux per unit area• often denoted as B• - units are in Tesla

• B in Tesla = Φ/m2

• 1 Tesla = 1 Weber/m2

• 1 Tesla = 10000 Gauss


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

What are we measuring?


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

What are we measuring - moredetail?


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

A simple magnetometer

A coil of wire

• If we have a changing magnetic flux and a loop of wire

• the EMF (voltage) in the wire is related to the change offlux with time

• ε = −N dΦBdt

• where N is the number of turns in the coil


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

How sensitive are simple coils asmagnetometers?

• what do we mean by sensitive?

• how small a flux density can we detect?

• what is the noise level? and where does the noise comefrom?

• cf Induction Coil Sensors a Review, by Tumanski

• Measurement Science and Technology, Vol. 18, No. 3.(19 January 2007), pp. R31-R46,doi:10.1088/0957-0233/18/3/r01


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

an example

• if the coil is made from Litz wire and coupled to highlymatched electronics

• stranded, only effective at high frequencies• noise level can be 10fT/(root Hz)at 1kHz but 2.5pT at

10Hz• so have been used for MRI and for MCG

• K.P. Estola, J. Malmivuo, Air-core induction coilmagnetometer design, J. Phys. E., vol. 15, 1982, pp.110-1113

• air coil sensor with 10 000 turns and diameter 1 m -picoTesla for MCG


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

So - a much more sensitive magnetometer is required

As brain signals are from subfemtotesla to nanoTesla

This has, historically been fulfilled by using SQUIDs

Superconducting Quantum Interference Devices


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Squids

• ring of superconducting wire which has Josephsonjunction(s) in ring

• can introduce a current though the ring - the bias current

• magnetic flux through the ring core affects the currentaround the ring

• current will flow to null the magnetic flux

• a junction allows quantum tunnelling when the biascurrent is low and a superconducting current flows

• when the bias current goes above a critical level, thejunction becomes resistive


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Squids

Current Voltage curve - no magnetic field

upto critical bias current, no voltage across ring appears

pairs of electrons, Cooper pairs, tunnel through the junction


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

The effect of the magnetic field

Current flows in the ring, in one direction around the ring

This takes the current above the critical current on one side ofthe ring and below the critical current on the other side.

But, if we continue to increase the magnetic flux, beyond aspecific value, half the magnetic flux quantum, then aninteresting effect occurs, the current switches direction andincreases the flux until it reaches the magnetic flux quantumvalue.

As the current goes above the critical bias current, the voltageacross the ring is a period function of magnetic flux


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

voltage as a function of fluxA magnetic fux quantum is h/2e and is 2.0678x10−15Wb

So, in a practical application, you can set the bias current to bejust above the critical current, then measure the voltage andhave a feedback circuit to cancel the current in the ring. Thisfeedback current is a measure of the magnetic flux and,crucially, allows you to measure flux smaller than the fluxquantum.


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Problem?

noise increases dramatically below about 4Hz

the best MEG sensors in whole head systems have a sensitivityof about 1fT Is that enough?

Requires a cryogen

There is a minimum distance from the head because ofinsulation. Most systems use pickup, superconducting, coils

Squids only work well in low fields

but do have a very wide bandwidth - many KHz is easy,upto/into the MHz is possible


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Atomic Clock Magnetometers


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Atomic clock magnetometers

• Subfemtotesla (0.5fT√Hz achieved using 0.6cm3 (Sheng

et al. 2013)

• operate in very low fields (SERF domain)

• low density means that oven needed

• but - capable of working with ferrite shields and evenunshielded

• very wide dynamic range but only 100Hz bandwidth so far


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Room temperature atomic clockmagnetometer

• Chalupczak et al. 2012 - National Physics lab, UK

• Uses Caesium - no requirement for oven

• single laser

• femtoTesla sensitivity

• can be used in wide range of DC magnetic fields


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Arrays of Atomic ClocksJohnson et al. (2013)

Shah & Wakai (2013)


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

The Future?

Volegov et al (2004)

Simultaneous NMR and MEG


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Summary

• Modern squids capable of about 1 fT sensitivity

• Atomic clocks offer higher sensitivity without cryogens

• Other magnetometers in development - Nitrogen vacanciesin diamond/ silicon

• Molecular imaging & neural dynamics simultaneously is onthe horizon


FieldDetectors

Gary Green

Outline

MagneticFields


Squids

Atomic Clocks


The future

Summary

Questions

Questions

• Thank you for your attention

• Thank you to my colleagues in York

• and to Witold Chalupczak, NPL

• Any Questions?

........

ultra sensitive magnetic field detectors · ultra sensitive magnetic field detectors gary green...

Documents