Nano-Science Center > Seminarer og events > The Ubiquitous SQUID -...

The Ubiquitous SQUID - From Axions to Magnetic Resonance Imaging

Colloquium by John Clarke, Department of Physics, University of California, Berkeley

Abstract
The SQUID (Superconducting QUantum Interference Device) combines the phenomena of Josephson tunneling and flux quantization to make an ultrasensitive detector of magnetic flux. Coupled to a superconducting flux transformer, the SQUID achieves a magnetic field noise below 10^-15 THz^-1/2 . I briefly review a variety of SQUID applications. An amplifier consisting of a SQUID with a resonant input circuit achieves a noise temperature within 20% of the quantum limit at 0.8 GHz. This amplifier is being installed in the axion detector at Lawrence Livermore National Laboratory, and is expected to increase the frequency scan rate by three orders of magnitude. We use a SQUID magnetometer, together with prepolarization of the nuclear spins, to obtain magnetic resonance images (MRI) in a magnetic field of 132 microtesla, corresponding to a proton Larmor frequency of 5.6 kHz-four orders of magnitude lower than in conventional MRI. The spatial resolution is typically 1 mm. The combination of prepolarization, spin relaxation in an adjustable field and detection in a microtesla field enables us to obtain relaxation time (T_1 )-weighted contrast images in fields ranging from 1 microtesla to 0.3 tesla. Values of T_1 measured in ex vivo specimens of surgically removed prostate tissue at 132 microtesla are typically 60% higher in normal tissue than in tumors. Preliminary ex vivo images suggest that microtesla MRI may well have clinical applications.