Reports: DNI548873-DNI5: Nuclear Spin Relaxation and Restricted Diffusion in Microtesla Magnetic Fields

Robert McDermott , University of Wisconsin (Madison)

We have continued our development of a low-field nuclear magnetic resonance (NMR) scanner based on a Superconducting QUantum Interference Device (SQUID) detector. We have operated the system with both tuned and untuned receiver circuits. In the case of the tuned receiver, the SQUID is operated with a resonant input circuit tuned to 4.8 kHz, corresponding to a proton Larmor frequency around 110 microtesla. In the case of the untuned circuit, we make a superconducting connection between the receiver coil, which is configured as a 1+2+1 turn second-order gradiometer, and an integrated input coil fabricated directly on the SQUID chip. The untuned detector provides for frequency-independent sensitivity to external magnetic fields and therefore allows complete freedom in the choice of proton Larmor frequency. In either case, the experimental protocol involves polarization of the protons in a relatively high field of order 100 mT; adiabatic removal of the polarizing field, which causes the spins in the sample to reorient toward the weak measurement field; application of audiofrequency excitation pulses to induce spin precession and form spin echos; and detection of the precessing magnetization from the sample using the SQUID, which is operated in a flux-locked loop as a null detector of magnetic flux.

To develop this low-field scanner, we have had to overcome a number of challenges to enable the detection of extremely weak NMR signals in the presence of significant noise and environmental interference in the detection band. First, we have fabricated a custom liquid helium cryostat to house the SQUID sensor. The cryostat contains a minimal amount of normal metal, to minimize magnetic noise from Nyquist currents in metallic components. Instead of using copper mesh for the thermal shield of the cryostat, we have implemented a thermal shield made from thin alumina rods epoxied to a thin-walled G10 fiberglass frame; the electrically insulating alumina allows for efficient heat transport, while introducing no magnetic noise. In addition, we have used a minimal amount of aluminized mylar in the tail section of the cryostat; moreover, the aluminized mylar was repeatedly crushed prior to assembly of the cryostat in order to disrupt electrical continuity of the aluminum thin film and prevent the flow of noise currents in large-area circuits. All of the shells, flanges, and end caps of the cryostat vessels were fabricated from nonconducting G10 fiberglass. These unique design features have enabled us to reduce the magnetic field noise from the cryostat to 3 fT/sqrt(Hz) in the measurement band, while maintaining an acceptable liquid helium boiloff rate of 5 liters per day.

In addition, we have had to suppress the environmental magnetic field noise at our chosen detection frequency, typically in the range from 2-5 kHz. To do this, we have employed a gradiometric configuration for the SQUID input circuit, consisting of sets of counterwound coils to eliminate sensitivity to distant sources of interference and broadband magnetic noise. The lower coil rests on the bottom of the cryostat inner vessel, and is tightly coupled to the sample placed just under the end cap of the outer vessel. In addition, we have surrounded our experiment with a homemade eddy current shield made from 1/8”-thick plates of 5052 aluminum. The skin depth in this alloy is around 1 mm at 5 kHz; the shield thus suppresses the environmental contribution to the magnetic noise within our detection band by more than an order of magnitude.

Initially we employed a feedback damping scheme with a high-Q tuned input circuit in order to realize a broadband, low-noise SQUID-based detector. More recently we have opted to operate with an all superconducting input circuit. This configuration yields low noise in the signal band, but it is also susceptible to out-of-band noise and interference. We have had to develop robust schemes for filtering power line harmonics and for suppressing external radiofrequency interference in order to take advantage of the full dynamic range of the A/D converter and to ensure stable operation of the SQUID receiver in the unshielded magnetic environment.

In order to improve the signal-to-noise ratio of our scanner, we have employed a prepolarization scheme to enhance the sample magnetization far above the thermal magnetization in our weak detection field. We have designed custom electronics to realize the necessary pulsed magnetic fields. Currents of order 100 A are used to prepolarize the spins in fields of order 100 mT. Switching of the field is accomplished by embedding the polarization coil in an LC resonant circuit, where the capacitance is provided by a bank of electrolytic capacitors. Upon opening an IGBT switch, the magnetic energy of the polarization coil is transferred in a quarter period of the LC circuit to the capacitor bank, which is heavily damped by power resistors that dissipate the stored energy prior to the next polarization cycle.

With this system we have undertaken NMR experiments to validate detector performance and to fine-tune the detector and optimize signal-to-noise. In addition, we have developed electronics for pulsed magnetic field gradient control to enable pulsed-gradient spin echo (PGSE) experiments to probe restricted diffusion. In the absence of spurious internal-field gradients, the application of controlled gradient pulses can enable determination of pore surface-area-to-volume ratio and tortuosity. When combined with relaxation data, this information can be used to extract key flow parameters such as permeability.

The project initiated under this PRF grant represented a departure from our core research direction, namely superconducting quantum information processing. The PRF funds enabled us to employ one Medical Physics graduate student, Matt Christensen, to develop the necessary hardware and to initiate the NMR and pulsed gradient experiments. We intend to pursue internal funding and to use undergraduate researchers to move this research effort forward, and thereby leverage the considerable investment we have made in constructing and commissioning the low-field SQUID NMR scanner.  

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