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Title: Broadband DC SQUID NMR spectrometry on metals.
Author: Digby, Megan Elizabeth.
ISNI:       0000 0001 3424 3279
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1999
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This Thesis describes the development of a broadband pulsed NMR spectrometer, based on a sensitive DC SQUID amplifier with wideband electronics, to observe directly the free precession of nuclear spins in bulk metallic samples (with broad NMR linewidths) at Larmor frequencies cß/2 ,r below 1 MHz. The sample is located inside a pickup coil, which forms a superconducting flux transformer with the input coil of the SQUID. The SQUID amplifier operates in a flux-locked-loop (FLL), hence it is sensitive to signals from DC up to the bandwidth of the FLL electronics. A modified commercial DC SQUID amplifier, with modulated feedback electronics, observed NMR signals from bulk platinum samples (T2 - 1.1 ms), at 1.5 K. The SQUID amplifier had a 50 kHz bandwidth, a dead-time - 50 μs, and a coupled energy sensitivity cc - 500h. The measurements showed that it is important to minimise the time-constant of eddy current decay in the sample, which scales with r2, as expected, where r is the sample dimension. A DC SQUID amplifier with additional positive feedback and wideband electronics configured using the direct offset integration technique, observed NMR signals from a bulk aluminium sample (T2 - 30 μs) at 20 mK. This SQUID amplifier had a 7.5 MHz bandwidth, the dead-time was 55 μs for small transmitter pulses and e,; - 600h. The use of a strongly coupled input coil with the SQUID necessitated damping across the coil to smooth out the SQUID flux-voltage characteristicThe NMR measurements showed that eddy current decay is less important if the NMR signal size is enhanced by cooling the sample. Measurements also confirmed that the NMR signal from bulk metal is proportional to 4c0, and that a reasonable estimate of the signal size is made by assuming the signal is due to spins within half the skin-depth of the surface
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available