Interleaved NQR detection using atomic magnetometers

Interleaved Nuclear Quadrupole Resonance (NQR) detection was conducted on ammonium nitrate and potassium chlorate using two ⁸⁷Rb magnetometers, where potassium chlorate is measured during the T₁ limited recovery time of ammonium nitrate. The multi-pass magnetometers are rapidly matched to the NQR frequencies, 531 kHz and 423 kHz, with the use of a single tuning field. For ease of implementation, a double resonant tank circuit was used for excitation, but could be replaced by a broad-band transmitter. All work was done in an unshielded environment and compared to conventional coil detection. The two magnetometers were sensitive, base noise as low as 2 fT/Hz, and were shown to reduce ambient noise through signal subtraction. When an excitation pulse was introduced, however, residual ringing increased the noise floor; mitigation techniques are discussed. The two detection techniques resulted in comparable Signal-to-Noise Ratio (SNR). Interleaved detection using the atomic magnetometers took half the time of conventional detection and provided localization of the explosives.

Long single pulse NQR for broad resonance lines

This paper describes the experimental application of long single pulses to strongly inhomogeneously broadened NQR spectral lines, where the pulse length significantly exceeds the transverse relaxation time. A⁶³Cu NQR resonance in the mineral covellite (CuS) was used as an exemplar for study in this specific regime, which was motivated by the requirement to obtain useful signals in very large volume applications having radiofrequency power limitations. In this study, signal transients that followed the application of the long single pulses were measured over a large range of radiofrequency field strength and pulse width. The results indicate effective generation of signal amplitudes. This is in contrast to previously reported studies involving long pulses applied to relatively narrow resonances. The results are found to be well described by simulations of the modified Bloch equations.

Pulsed spin-locking of spin-3/2 nuclei: 39K-NQR of potassium chlorate

A model was developed for predicting a locked signal under a series of refocusing pulses for Nuclear Quadrupole Resonance (NQR) of spin I=32 and tested with a powder of KClO₃. This work represents the first direct NQR detection of the ³⁹K line of potassium chlorate. The characteristic time constants, T₁,T_2e and T₂^∗, were measured to determine the detectability of potassium chlorate via ³⁹K-NQR. The echo train T_2e was found to be strongly dependent on the refocusing pulse-spacing and weakly dependent on the refocusing pulse strength. The optimal angles of the excitation and echo pulse for a pulse train were also determined, as well as, the resonance-frequency dependence on sample temperature.

Design of a radio-frequency transceiver coil for landmine detection in Colombia by nuclear quadrupole resonance

This paper shows the design of a radio-frequency transceiver coil for landmine detection in Colombia by nuclear quadrupole resonance (NQR). The radio-frequency transceiver coil is of great importance as it is responsible for exciting the target explosive and for picking up the weak NQR signal; however, little detail is found on the literature about its design. The strategy followed on this work consisted on constructing and experimentally comparing five different radio-frequency transceiver coils, whose dimensions were selected according to four design parameters: noise rejection, magnetic flux density, coil sensitivity, and quality factor; taking into account the characteristics of landmines in Colombia, the second country most affected by anti-personnel mines in the world. The constructed coils were experimentally compared using a portable system and with three of them, the system was capable of detecting 200 g ammonium nitrate (the main substance used in Colombian landmines) up to 3 cm from the coil within 12 s, with a steady-state free precession pulse sequence. Conclusions from this work could help to guide RF coil design in other works that apply NQR for remote detection of substances in non-shielded environments and to direct future research about landmine detection in Colombia.

A rapid determination of nuclear quadrupole resonance frequencies using field-cycling magnetic resonance and frequency modulated RF excitations

A modification of Slusher-Hahn's double resonance technique is described and experimentally tested. It is based on application of multiple frequency sweeps and can be used for a rapid location of nuclear quadrupole resonance (NQR) frequencies. The resolution of the present technique is relatively low but, when the NQR frequencies are located, it is easy to use either the Slusher-Hahn's technique or pulse NQR to determine the NQR frequencies with a higher precision.

Measuring of short spin-spin relaxation times distributions using NQR nutation experiments

Experiments using nutation interferograms and nutation spectra in nuclear quadrupole resonance (NQR) are used for the first time to find the distribution of short spin-spin relaxation times in powder samples. Instead of the traditional method of the inversion of the Laplace transform, the inversion of the integral transformations of nutation interferograms and nutation spectra are used in this work. The use of the distributions of NQR relaxation times can provide new information on dynamic processes in complex molecular systems in the solid state. To illustrate the capabilities of the method, experimental results for ³⁵Cl, ⁷⁵As NQR and for ¹H NMR are provided.

Spectral descriptors and supervised classifier for ammonium nitrate detection in landmines by nuclear quadrupole resonance

The high specificity of Nuclear Quadrupole Resonance (NQR) makes it very suited for the detection of antipersonnel mines, where the intensity of the signal spectrum around the resonance frequency of the target substance is the standard decision parameter; however, radiofrequency interference, soil effects on the search coil, landmine size, burial depth, and target temperature affect signal intensity. To overcome this, the use of spectral descriptors and a supervised classifier are proposed in this work, where an assembly of decision trees was trained with NQR data collected on places where a target filled with ammonium nitrate was present and where it was not. A statistical test, comparing the proposed classifier and the solution based solely on the intensity of the signal spectrum, showed with significant evidence that the proposed classifier outperforms the traditional solution. A final blind experiment was conducted in a rural region of Colombia, where five landmines of different size filled with ammonium nitrate were shallowly buried in an area of 1.9 × 1.52 m, and the system with the proposed classifier detected four of them with three false alarms. This work is also novel in detecting ammonium nitrate in antipersonnel mines, which are typical in Colombia, the second most mined country in the world.

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

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.

An electronically tuned wideband probehead for NQR spectroscopy in the VHF range

Nuclear quadrupole resonance spectroscopy is an analytical method which allows to characterize materials which contain quadrupolar nuclei, i.e. nuclei with spin ⩾1. The measurement technology is similar to that of NMR except that no static magnetic field is necessary. In contrast to NMR, however, it is frequently necessary to scan spectra with a very large bandwidth with a span of several tens of % of the central frequency so as to localize unknown peaks. Standard NMR probeheads which are typically constructed as resonators must be tuned and matched to comparatively narrow bands and must thus be re-tuned and re-matched very frequently when scanning over a whole NQR spectrum. At low frequencies up to few MHz dedicated circuits without the need for tuning and matching have been developed, but many quadrupole nuclei have transitions in the VHF range between several tens of MHz up to several hundreds of MHz. Currently available commercial NQR probeheads employ stepper motors for setting mechanically tuneable capacitors in standard NMR resonators. These yield high quality factors (Q) and thus high SNR but are relatively large and clumsy and do not allow for fast frequency sweeps. This article presents a new concept for a NQR probehead which combines a previously published no-tune no-match wideband concept for the transmit (TX) pulse with an electronically tuneable receive (RX) part employing varactor diodes. The prototype coil provides a TX frequency range of 57MHz with a center frequency of 97.5MHz with a return loss of ⩽-15dB. During RX the resonator is tuned and matched automatically to the right frequency via control voltages which are read out from a previously generated lookup table, thus providing high SNR. The control voltages which bias the varactors settle very fast and allow for hopping to the next frequency point in the spectrum within less than 100μs. Experiments with a test sample of ZnBr2 proved the feasibility of the method.

(75)As NQR studies on FeAs2

(75)As NQR spectra and relaxation times of synthetic and natural FeAs2 samples have been studied at variable static magnetic field and temperature. FeAs2 is a well understood diamagnetic semiconductor and occurs as the natural mineral lollingite in selected ore deposits. We observed a spin-spin relaxation time enhancement of up to five in synthetic powders in the presence of a weak external static magnetic field. The effect is of interest with regard to signal-to-noise ratio improvement for materials characterization applications where broad NQR absorption lines are excited with wideband pulse sequences.

A miniaturized NQR spectrometer for a multi-channel NQR-based detection device

A low frequency (0.5-5 MHz) battery operated sensitive pulsed NQR spectrometer with a transmitter power up to 5 W and a total mass of about 3 kg aimed at detecting (14)N NQR signals, predominantly of illicit materials, was designed and assembled. This spectrometer uses a standard software defined radio (SDR) platform for the data acquisition unit. Signal processing is done with the LabView Virtual instrument on a personal computer. We successfully tested the spectrometer by measuring (14)N NQR signals from aminotetrazole monohydrate (ATMH), potassium nitrate (PN), paracetamol (PCM) and trinitrotoluene (TNT). Such a spectrometer is a feasible component of a portable single or multichannel (14)N NQR based detection device.

Heteronuclear dipolar coupling in spin-1 NQR pulsed spin locking

We investigate theoretically and experimentally the role of broadening due to heteronuclear dipolar coupling in spin-1 nuclear quadrupole resonance pulsed spin locking. We find the experimental conditions where heteronuclear dipolar coupling is refocused by a standard multipulse sequence. This experimental condition allows us to extend our previously reported ability to measure the homonuclear dipolar coupling of powder samples to include substances that have heteronuclear coupling. These results are useful for designing substance detection algorithms, and for performing sample characterization.

WURST-QCPMG sequence and "spin-lock" in ¹⁴N nuclear quadrupole resonance

¹⁴N nuclear quadrupole resonance (NQR) is a promising method for the analysis of pharmaceuticals or for the detection of nitrogen based illicit compounds, but so far, the technique is still not widely used, mostly due to the very low sensitivity. This problem is already acute in the preliminary NQR stage, when a compound is being examined for the first time and the NQR frequencies are being searched for, by scanning a wide frequency range step-by-step. In the present work, we experimentally show how to increase the efficiency of this initial stage by using a combination of a wideband excitation achieved with frequency swept pulses (WURST) and a "spin-lock" state obtained with a quadrupolar-CPMG (QCPMG) sequence. In the first part we show that WURST pulses provide a much larger excitation bandwidth compared to common rectangular pulses. This increased bandwidth allows to increase the frequency step and reduces the total number of steps in a scanning stage. In the second part we show that the "spin-lock" decay time T2eff obtained with the WURST-QCPMG combination is practically identical with the T2eff obtained with the most common "spin-lock" sequence, the SLSE, despite a very different nature and length of excitation pulses. This allows for a substantial S/N increase through echo averaging in every individual step and really allows to exploit all the advantages of the wider excitation in the NQR frequency scanning stage. Our experimental results were obtained on a sample of trinitrotoluene, but identical behavior is expected for all compounds where a "spin-lock" state can be created.