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

Abstract

In the search for a novel MRI contrast agent which relies on T 1 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 T 1 times. Moreover many experiments yield better results at low temperatures such as 77K (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 -116 MHz, both at room temperature and in LN. Especially for the first transition which exhibits extremely long T 1 times (64ms) the and low signal the new approach allows an acceleration factor by more than 100 when compared to classical methods.Keywords: nuclear quadrupole resonance, bismuth compounds, wideband cryo-probehead, varactor diodes, tuning and matching, interleaved subspectrum sampling Highlights  Fast electronically tuneable wideband cryo-coils for NMR and NQR spectroscopy  electronic tuning/matching allows for very short repetition times  fast interleaved subspectrum sampling for very short repetition times even at long T 1  strong acceleration of explorative wideband NQR scans by factors of 100 and more

Model – free approach to quadrupole spin relaxation in solid 209Bi-aryl compounds

“…For the measurements at RT and 310 K temperature stability was maintained by a flow of dry air controlled by a feedback unit using a type K thermocouple placed in the sample vial. Some of the coils were self-built 45 featuring a special low-temperature front-end 46 for diving them directly into liquid nitrogen. Temperature accuracy was 0.5 K for measurements at 300 K and 310 K respectively.…”

Aspects of structural order in 209Bi-containing particles for potential MRI contrast agents based on quadrupole enhanced relaxation

“…NQRS was carried out with a custom-built zero-field spectrometer as described in [27]. In order to achieve fast scanning of a wide frequency band, typical resonating NMR probe heads are not applicable because they have to be re-tuned and re-matched mechanically when changing the frequency more than the bandwidth of the resonator.…”

“…The coil sits at the end of a hollow nozzle made from teflon which can be immersed into liquid nitrogen so as to cool down the sample to 77 K which is beneficial for increasing the SNR in case of BiPh 3 due to an increase of the population difference between the energy levels of the QN. The used cryo-probe head operates in a frequency range of 79-116 MHz (type A4 from [27]). For cryostatisation a commercial thermos flask from stainless steel was adapted with a special snap-lock which establishes a tight and reliable electric connection between the RF ground plane of the probe head case and the thermos flask, thus not only providing thermal insulation but also forming an excellent shield against external RF perturbations.…”

“…All scans were carried out with a standard spin echo sequence. In case that the location of spectral peaks was not known beforehand, wideband scans were carried out with densely spaced frequency points ( < 5 kHz) and using the fast interleaved sub-spectrum sampling method presented in [27]. The latter was required to keep the scanning times within reasonable limits even in the presence of the significantly prolonged T 1 time constants which typically occur at low temperatures (LN).…”

“…As described in [1] this is also a method in the process of frequency tuning (condition 1) because it allows for the determination of Q cc and the so-called asymmetry parameter η. An important difference to standard NMR, however, is the need for scanning the RF frequency over very large bandwidths, usually many tens of MHz, which requires special, fast tuneable probe heads [27].…”