Ultra-low Field Nuclear Magnetic Resonance Measurements with SQUIDs Michelle Espy SQUID Team Biological and Quantum Physics, P-21

A SQUID is a magnetic flux to voltage converter

Magnetometers and Gradiometers

T < 0.5 K can be a problem for SQUIDs The low temperature is required to keep the 3He diffusion coefficient sufficiently high. The white component of SQUID noise energy is proportional to temperature. But the same is not necessarily true for the 1/f component. Most SQUIDs are optimized for operation at 4 K.

Use the SQUIDs to measure 3 and get rid of your dependence on B Any difference in for Eparallel or Eantiparallel is your signal for an EDM Use the SQUIDs to measure 3 and get rid of your dependence on B

Introduction to Ultra-Low Field NMR By ULF we mean measurement fields from 1 – 100 mT EDM ~0.3 mT Separate polarizing and measurement fields (B0 vs Bm ) Dramatically smaller and lower-cost systems Narrow line widths (wFWHM a B0 or Bm) Reduced susceptibility ‘artifact’ (a B0 or Bm) Compatible with MEG (simultaneous MEG/MRI) Potential to directly measure neuronal currents Micro MRI possible

Ultra Low Field NMR This decaying NMR signal is called the Free Induction Decay, or FID.  The actual spectrum is recovered from the FID via Fourier transformation, which transforms the time interferogram into a frequency spectrum. The signal frequency, w0 = gB0 (Larmor frequency) scales with B0.

Line width for frog (not anesthetized) ~ 2 Hz First ultra-low-field NMR for living animal Line width for frog (not anesthetized) ~ 2 Hz B0 = 60 G, Bm = 47 µT

G 1-D gradient image of a water phantom at 50 mT 1-volume and 3-section water phantoms: G Feasibility of imaging demonstrated by these data!

2-D gradient image of a water phantom at 50 mT

First-ever simultaneous NMR-MEG recording Simultaneous MEG ultra-low-field NMR measurements: B0 = 60 G, Bm ~ 5.5 mT

Jena Gradiometer Planar gradiometer with 2 cm x 2 cm coils connected in series Baseline is 4 cm. Effective area of 7.1 mm2 (one loop). The measured white noise level 5 – 10 mF/sqrt(Hz) Magnetic field gradient resolution 36 – 72 fT m-1 Hz-1/2 Corresponds to a field resolution in one gradiometer loop from 1.45 – 2.9 fT Hz-1/2. Measurements in a uniform field showed a balance from 104 m-1 to 103 m-1.

How does our water signal compare to what we expect? At 1 mT and 298K

We have developed electronics such that the SQUID survives the pulsed fields Dead time after SQUID reset: SQUID/electronics to recover = 20 ms Bp to SQUID = 5-10 ms (best 1.5 ms)

UIUC apparatus

SQUID gradiometer

Simultaneous dressed spin – will SQUID survive? Proposed dressed spin field ~ 1 mT at 1 kHz SQUID sensitivity is 1 nT/F - for 1000 F/s That is a high slew rate for SQUID (though not impossible) Use of a gradiometer would solve the problem Balance level 103 for Jena gradiometer Reduce signal to 1 F/s Expected signal at ~ 3 Hz – so other filtering methods can also be used