Gary Green. January 8, 2014

Transcription

1 s Arrays of s Ultra Sensitive York NeuroImaging Centre, UK January 8, 2014

2 s Arrays of s 1 s Arrays of s 6 7 8

3 s - a bit of history s Arrays of s Oersted (1819) demonstrated an electric current generates a magnetic field Ampere (1820) demonstrated parallel wires attract if current flows in the same direction in each wire Biot and Savart (1820) predicted the magnetic field around the current Faraday (1831) demonstrated induction where a changing magnetic field is associated with an encircling electric field Maxwell (1865) published unifying theory of the electromagnetic field Einstein (1905) electric and magnetic fields are the same - different reference frames

4 s Arrays of s fields - some simple obervations Due to a moving electrical charge -right hand rule The magnetic moment of an elementary particle due to spin flux often denoted as Φ or Φ B - units are in Webers (Wb) flux density - the flux per unit area often denoted as B - units are in Tesla B in Tesla = Φ/m 2 1 Tesla = 1 Weber/m 2 1 Tesla = Gauss

5 What are we measuring? s Arrays of s

6 What are we measuring - more detail? s Arrays of s

7 s Arrays of s A coil of wire A simple If we have a changing magnetic flux and a loop of wire the EMF (voltage) in the wire is related to the change of flux with time ε = N dφ B dt where N is the number of turns in the coil

8 How sensitive are simple coils as s? s Arrays of s what do we mean by sensitive? how small a flux density can we detect? what is the noise level? and where does the noise come from? cf Induction Coil Sensors a Review, by Tumanski Measurement Science and Technology, Vol. 18, No. 3. (19 January 2007), pp. R31-R46, doi: / /18/3/r01

9 an example s Arrays of s if the coil is made from Litz wire and coupled to highly matched 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 coil design, J. Phys. E., vol. 15, 1982, pp air coil sensor with turns and diameter 1 m - picotesla for MCG

10 s Arrays of s So - a much more sensitive is required As brain signals are from subfemtotesla to nanotesla This has, historically been fulfilled by using SQUIDs Superconducting Quantum Interference Devices

11 s Arrays of s ring of superconducting wire which has Josephson junction(s) in ring can introduce a current though the ring - the bias current magnetic flux through the ring core affects the current around the ring current will flow to null the magnetic flux a junction allows quantum tunnelling when the bias current is low and a superconducting current flows when the bias current goes above a critical level, the junction becomes resistive

12 s Current Voltage curve - no magnetic field Arrays of s upto critical bias current, no voltage across ring appears pairs of electrons, Cooper pairs, tunnel through the junction

13 The effect of the magnetic field s Arrays of s Current flows in the ring, in one direction around the ring This takes the current above the critical current on one side of the ring and below the critical current on the other side. But, if we continue to increase the magnetic flux, beyond a specific value, half the magnetic flux quantum, then an interesting effect occurs, the current switches direction and increases the flux until it reaches the magnetic flux quantum value. As the current goes above the critical bias current, the voltage across the ring is a period function of magnetic flux

14 voltage as a function of flux A magnetic fux quantum is h/2e and is x10 15 Wb s Arrays of s So, in a practical application, you can set the bias current to be just above the critical current, then measure the voltage and have a feedback circuit to cancel the current in the ring. This feedback current is a measure of the magnetic flux and, crucially, allows you to measure flux smaller than the flux quantum.

15 Problem? s Arrays of s noise increases dramatically below about 4Hz the best MEG sensors in whole head systems have a sensitivity of about 1fT Is that enough? Requires a cryogen There is a minimum distance from the head because of insulation. Most systems use pickup, superconducting, coils only work well in low fields but do have a very wide bandwidth - many KHz is easy, upto/into the MHz is possible

16 Atomic Clock Magnetometers s Arrays of s

17 Atomic Clock Magnetometers s Arrays of s

18 Atomic clock s s Arrays of s Subfemtotesla (0.5fT Hz achieved using 0.6cm 3 (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 even unshielded very wide dynamic range but only 100Hz bandwidth so far

19 Room temperature atomic clock s Arrays of s Chalupczak et al National Physics lab, UK Uses Caesium - no requirement for oven single laser femtotesla sensitivity can be used in wide range of DC magnetic fields

20 Johnson et al. (2013) Arrays of s Arrays of s Shah & Wakai (2013)

21 The Future? s Volegov et al (2004) Arrays of s Simultaneous NMR and MEG

22 s Arrays of s Modern squids capable of about 1 ft sensitivity Atomic clocks offer higher sensitivity without cryogens Other s in development - Nitrogen vacancies in diamond/ silicon Molecular imaging & neural dynamics simultaneously is on the horizon

23 s Arrays of s Thank you for your attention Thank you to my colleagues in York and to Witold Chalupczak, NPL Any?...