A cryostatic, fast scanning, wideband NQR spectrometer for the VHF range

Abstract

In the search for a novel MRI contrast agent which relies on T₁ shortening due to quadrupolar interaction between Bi nuclei and protons, a fast scanning wideband system for zero-field nuclear quadrupole resonance (NQR) spectroscopy is required. Established NQR probeheads with motor-driven tune/match stages are usually bulky and slow, which can be prohibitive if it comes to Bi compounds with low SNR (excessive averaging) and long quadrupolar T₁ times. Moreover many experiments yield better results at low temperatures such as 77 K (liquid nitrogen, LN) thus requiring easy to use cryo-probeheads. In this paper we present electronically tuned wideband probeheads for bands in the frequency range 20-120 MHz which can be immersed in LN and which enable very fast explorative scans over the whole range. To this end we apply an interleaved subspectrum sampling strategy (ISS) which relies on the electronic tuning capability. The superiority of the new concept is demonstrated with an experimental scan of triphenylbismuth from 24 to 116 MHz, both at room temperature and in LN. Especially for the first transition which exhibits extremely long T₁ times (64 ms) the and low signal the new approach allows an acceleration factor by more than 100 when compared to classical methods.

Keywords: Bismuth compounds; Interleaved subspectrum sampling; Nuclear quadrupole resonance; Tuning and matching; Varactor diodes; Wideband cryo-probehead.