Nuclear Quadrupole Resonance Signals Research Articles

Integrating superregenerative principles in a compact, power-efficient NMR/NQR spectrometer: A novel approach with pulsed excitation

We present a new approach to Nuclear Quadrupole Resonance (NQR)/Nuclear Magnetic Resonance (NMR) spectroscopy, the Damp-Enhanced Superregenerative Nuclear Spin Analyser (DESSA). This system integrates Superregenerative principles with pulsed sample excitation and detection, offering significant advancements over traditional Super-Regenerative Receivers (SRRs). Our approach overcomes certain limitations associated with traditional Super-Regenerative Receivers (SRRs) by integrating direct digital processing of the oscillator response delay time (Td) and an electronic damp unit to regulate the excitation pulse decay time (Te). The essence is combining pulsed excitation with a reception inspired by, but distinct from, conventional SRRs. The damp unit allows a rapid termination of the oscillation pulse and the initiation of detection within microseconds, and direct digital processing avoids the need for a second lower frequency which is used for quenching in a traditional SRRs, thereby avoiding the formation of sidebands. We demonstrate the effectiveness of DESSA on a NaClO3 sample containing the isotope Chlorine-35 where it accurately detects the NQR signal with sub-kHz resolution.

Improving Detection of a Portable NQR System for Humanitarian Demining Using Machine Learning

This article presents an approach to enhance the detection of buried landmines by applying machine learning (ML) to signals from a nuclear quadrupole resonance (NQR) system. This custom-made, low-cost, and portable NQR system has been developed for deployment in humanitarian demining, where strong radio frequency (RF) interference and a low signal-to-noise ratio (SNR) are important challenges for accurate detection. To tackle these problems, we have applied and tested various ML techniques to NQR signals acquired by our system in laboratory experiments with the explosive Research Department X (RDX). Results suggest that ML methods can indeed improve the detection accuracy of the NQR device, and this is confirmed further using data from field trials with our device. Importantly, the trained classifiers can be implemented with our device’s field-programmable gate array (FPGA) architecture and can run with little time penalty compared with simpler but less-efficient fast Fourier transform (FFT)-based energy detection methods.

Improvement of simulated nuclear quadrupole resonance signals from explosive detection via a Red-Pitaya board

In Nuclear quadrupole resonance (NQR), the interaction of the nuclear magnetic moments of quadrupolar nuclei (spin greater than 1/2) with the electric field gradient of the surrounding molecular orbitals produces an energy splitting. Because the resonant frequency is very specific to the molecular structure, the NQR can be used to detect explosive materials very accurately and it is extremely useful for detecting modern bombs whose containers made from plastics and wood instead of metals. However, NQR signals are generally very weak so they are difficult to be detected. Recently, Red-Pitaya boards, a Field Programmable Gate Array (FPGA) on Single Board Computers, have been being utilized in many electronic applications due to their small size and low cost. Since the boards can generate and acquire radio frequency signals, they can be taken as the console of portable bomb detectors. In this work, we study an improvement of the NQR signals of an explosive, ammonium nitrate with a resonant frequency of 423.6 kHz, acquired by using a Red-Pitaya board (STEMlab 125-14). To construct the NQR signals, we simulate free induction decay (FID) signals (exponential decay of sinusoidal functions) and add real measured noises from an input port of the Red-Pitaya board. To mimic real situations, the FID amplitude is varied, frequency fluctuations and phase shifts are added. The results show that averaging of signals from repeat measurements can improve the signals in all cases. To distinguish the signals from the noises, a minimal number of measurements is required. This necessary number of repeat measurements increases with frequency fluctuations and phase shifts but decreases when the FID amplitude grows.

Investigation of magnetic noise from conductive shields in the 10–300 kHz frequency range

We present experimental and theoretical investigations of magnetic noise originating from radio-frequency (RF) conductive shields of flat geometry in the inductance-dominated impedance regime below 300 kHz. The measurement is based on a Q-factor determination of a coil that provides a sufficient sensitivity, placed at the position where the shield magnetic noise is measured. The theory is based on calculations of magnetic field and inductance of one or a few flat rings that emulate a conductive shield. The theoretical model is found to be in close agreement with experimental data. It can be used to predict the magnetic noise of a conductive shield with different thicknesses, conductivities, and temperatures at different distances for a wide range of frequencies. Although the model can be generalized for a more arbitrary shield geometry, in its presented form, it can be applied to the magnetic noise predictions when the shield surface curvature is not large. One important conclusion is that the RF conductive shield can generate the magnetic noise much lower than femtotesla, and, thus, it can be used in many precision experiments targeting minute high-frequency magnetic signals, such as detection of magnetic resonance imaging and nuclear quadrupole resonance signals and search for axion dark matter.

Nuclear Quadrupole Resonance (NQR)—A Useful Spectroscopic Tool in Pharmacy for the Study of Polymorphism

Nuclear Quadrupole Resonance (NQR) spectroscopy has been known for 70 years. It is suitable for the study of measured (poly)crystalline chemical compounds containing quadrupole nuclei (nuclei with spin I ≥ 1) where the characteristic NQR frequencies represent the fingerprints of these compounds. In several cases, 14N NQR can distinguish between the polymorphic crystalline phases of active pharmaceutical ingredients (APIs). In order to further stimulate 14N NQR studies, we review here several results of API polymorphism studies obtained in Ljubljana laboratories: (a) In sulfanilamide, a clear distinction between three known polymorphs (α, β, γ) was demonstrated. (b) In famotidine, the full spectra of all seven different nitrogen positions were measured; two polymorphs were distinguished. (c) In piroxicam, the 14N NQR data helped in confirming the new polymorphic form V. (d) The compaction pressure in the tablet production of paracetamol, which is connected with linewidth change, can be used to distinguish between producers of paracetamol. We established that paracetamol in the tablets of six different manufacturers can be identified by 14N NQR linewidth. (e) Finally, in order to get an extremely sensitive 14N NQR spectrometer, the optical detection of the 14N NQR signal is mentioned.

Rapid survey of nuclear quadrupole resonance by broadband excitation with comb modulation and dual-mode acquisition.

Nuclear Quadrupole Resonance (NQR) provides spectra carrying information as to the electric-field gradient around nuclei with a spin quantum number I > 1/2 and offers helpful clues toward characterizing the electronic structure of materials of chemical interest. A major challenge in NQR is finding hitherto unknown resonance frequencies, which can scatter over a wide range, requiring time consuming repetitive measurements with stepwise frequency increments. Here, we report on an efficient, two-step NQR protocol by bringing rapid-scan and frequency-comb together. In the first step, wideband excitation and simultaneous signal acquisition, both realized by a non-adiabatic, frequency-swept hyperbolic secant (HS) pulse with comb modulation, offers a clue for the existence/absence of the resonance within the frequency region under investigation. When and only when the sign of the resonance has been detected, the second step is implemented to compensate the limited detection bandwidth of the first and to unambiguously determine the NQR frequency. We also study the spin dynamics under the comb-modulated HS pulse by numerical simulations, and experimentally demonstrate the feasibility of the proposed scheme, which is referred to as RApid-Scan with GApped excitation with Dual-mode Operation (RASGADO) NQR.

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.

Simulation modeling of the digital quadrature receiver of the nuclear quadrupole resonance signals

Simulation modeling of the digital quadrature receiver of the nuclear quadrupole resonance signals

Robust Authentication of Consumables With Extrinsic Tags and Chemical Fingerprinting

Consumables-from food to pharmaceuticals and supplements-are becoming increasingly vulnerable to various modes of counterfeiting due to the growing complexity of their supply chain. Mislabeling, re-branding, and false advertising are prevalent in this sector. Existing physical authentication techniques fail to adequately verify integrity of these products and protect the end-users. In this paper, we aim at addressing this critical problem through the development of a novel authentication solution. It builds on the chemical analysis properties of a powerful spectroscopy technique, namely, nuclear quadrupole resonance (NQR) that is quantitative, non-invasive, low-cost, and amenable for miniaturization (to hand-held form factors). The method is sensitive to small variations in the solid-state chemical structure of a sample, which change the NQR signal properties. These attributes can be unique for various manufacturers, enabling their use as manufacturer-specific watermarks. However, NQR spectroscopy only works reliably (i.e., provides good sensitivity) on compounds that contain certain nuclear isotopes. We take advantage of the intrinsic properties of NQR-sensitive isotopes to use them as extrinsic tags in NQR-insensitive products. The NQR spectra of these extrinsic tags act as unique watermarks that can be analyzed using machine learning methods to authenticate any consumable with high confidence. In particular, we use support vector machines to classify the measured spectra and confirm the identity of items under test. We have assessed this approach on a variety of consumables utilizing semi-custom equipment and verified that it results in high (>95%) classification accuracy. In order to prove the unclonability of such extrinsic tags, we have also performed a mathematical analysis that proves the randomness of the extrinsic tag and confirms its robustness to brute-force attacks.

A novel wavelets method for cancelling time-varying interference in NQR signal detection

Abstract Interference cancelation is a very important aspect of Nuclear Quadrupole Resonance (NQR) signal detection, and can become really difficult when the interference is considerably time-varying. We propose a novel wavelets method to effectively remove (or reduce) time-varying interference in the data and facilitate a valid detection of the NQR signal. The proposed algorithm uses an extended Gabor-Morlet wavelets basis to approximate interference with complicated time-varying properties. The proposed algorithm utilizes our well designed cost function to extract the interference components out from NQR data strategically. Mathematical derivations and numerical results from both simulated and measured data demonstrate that the proposed algorithm can precisely cancel strong time-varying interference without distorting signal of interest improving NQR detection, even when interference and signal of interest are severely overlapped. The proposed algorithm is beyond normal wavelets methods such as standard wavelets denoising methods, and exhibits better performance than normal Fourier analysis and related frequency selective methods, and general adaptive filtering methods.

An automated instrument for polarization-enhanced broadband nuclear quadrupole resonance (NQR) spectroscopy.

An automated instrument for improving the sensitivity of nuclear quadrupole resonance (NQR) spectroscopy is presented. The device is capable of pre-polarizing samples within a custom Halbach-type permanent magnet and then moving them into an external probe for zero-field NQR detection. Polarization transfer between protons and nitrogen (14N) nuclei in the sample occurs during demagnetization, thus increasing the amplitude of the detected NQR signals. The sample motion profile is completely programmable, thus providing a high level of control over the sample position and velocity for optimizing the polarization transfer process for various samples. Moreover, the magnet and motion controller are combined with a shielded sample probe and ultra-broadband front-end electronics (both designed in-house) to realize a complete scientific instrument for 14N NQR experiments. Compared with previous work in the field, the system is designed to be programmable, robust, and easy to use. Experimental results from several samples are also presented.

An advanced beamforming approach based on two-channel echo-train system to cancel interference within an NQR signal resonance spectrum

Abstract Interference can be a huge challenge for Nuclear Quadrupole Resonance (NQR) signal detection in real life settings. The problem is particularly challenging when interference is strong around the resonant frequency band of the targeted NQR signal (to which we refer as the NQR band). This paper first proves the beamforming characteristics of a designed two-channel echo-train (TE) data acquisition system, and then presents a novel beamforming approach which is based on the TE system and is able to cancel interference effectively within the NQR band. After interference cancellation, NQR signal detection can be done successfully by applying an ”echo train” approximate maximum likelihood (ETAML) algorithm to the residual data. The proposed algorithm is shown to be superior to previously proposed algorithms, including the classical beamforming algorithms/detectors constructed on the TE system, for detecting NQR signal polluted by interference.

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

ABSTRACTQuadrupole relaxation enhancement (QRE) has been suggested as the key mechanism for a novel class of field-selective, potentially responsive magnetic resonance imaging contrast agents. In previous publications, QRE has been confirmed for solid compounds containing 209Bi as the quadrupolar nucleus (QN). For QRE to be effective in aqueous dispersions, several conditions must be met, i.e. high transition probability of the QN at the 1H Larmor frequency, water exchange with the bulk and comparatively slow motion of the Bi-carrying particles. In this paper, the potential influence of structural order within the compounds (‘crystallinity’) on QRE was studied by nuclear quadrupole resonance (NQR) spectroscopy in one crystalline and two amorphous preparations of Triphenylbismuth (BiPh3). The amorphous preparations comprised (1) a shock-frozen melt and (2) a granulate of polystyrene which contained homogeneously distributed BiPh3 after common dissolution in THF and subsequent evaporation of the solvent. In contrast to the crystalline powder which exhibits strong, narrow NQR peaks the amorphous preparations did not reveal any NQR signals above the noise floor. From these findings, we conclude that the amorphous state leads to a significant spectral peak broadening and that for efficient QRE in potential contrast agents structures with a high degree of order (near crystalline) are required.

Authentication of dietary supplements through Nuclear Quadrupole Resonance (NQR) spectroscopy

SummaryAs the industry grows, adulteration of many products by mislabelling, re‐branding and false advertising is becoming prevalent practice. Existing solutions for analysis often require extensive sample preparation or are limited in terms of detecting different types of integrity issues. We describe a novel authentication method based on Nuclear Quadrupole Resonance (NQR) spectroscopy which is quantitative, non‐invasive and non‐destructive. It is sensitive to small deviation in the solid‐state chemical structure of a product, which changes the NQR signal properties. These characteristics are unique for different manufacturers, resulting in manufacturer‐specific watermarks. We show that nominally identical dietary supplements from different manufacturers can be accurately classified based on features from NQR spectra. Specifically, we use a machine learning‐based classification called support vector machines (SVMs) to verify the authenticity of products under test. This approach has been evaluated on three products using semi‐custom hardware and shows promising results, with typical classification accuracy of over 95%.

Detection of extremely weak NQR signals using stochastic resonance and neural network theories

Nuclear Quadrupole Resonance (NQR) signal detection is widely used for searching related substances of interest, such as explosives, petroleum, drugs, etc. NQR responses from these substances are usually very weak compared to background noise. Moreover, in some applications such as landmine detection, NQR responses decay with time quickly, and the required scanning times are usually prohibitively long. This paper presents a novel approach which can detect NQR signals of very low SNRs in such scenarios, by combining a stochastic resonance framework and neural network theory. Firstly, the approach relies on the design of a stochastic resonance (SR) system which can transform the original data into a nonlinear waveform with special SR features. Secondly, a (feedforward) robust neural network is trained to discern this nonlinear waveform accurately, in order to identify the NQR signal. Our results demonstrate that the neural network approach outer-performs traditional signal processing detection and estimation methods. Moreover, this stochastic resonance neural network approach (SRNN) can be designed to detect a variety of NQR signals which have similar NQR parameters. The SRNN approach can also be effective in cases where both noise and radio frequency interference are strong relative to the NQR response.

Emergent Weyl Fermion Excitations in TaP Explored byTa181Quadrupole Resonance

The ^{181}Ta quadrupole resonance [nuclear quadrupole resonance (NQR)] technique is utilized to investigate the microscopic magnetic properties of the Weyl semimetal TaP. We find three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with I=7/2 nuclear spin. A quadrupole coupling constant, ν_{Q}=19.250 MHz, and an asymmetric parameter of the electric field gradient, η=0.423, are extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin-lattice relaxation rate (1/T_{1}T) is measured for the f_{2} line (±5/2↔±3/2 transition). We find that there exist two regimes with quite different relaxation processes. Above T^{*}≈30 K, a pronounced (1/T_{1}T)∝T^{2} behavior is found, which is attributed to the magnetic excitations at the Weyl nodes with temperature-dependent orbital hyperfine coupling. Below T^{*}, the relaxation is mainly governed by a Korringa process with 1/T_{1}T=const, accompanied by an additional T^{-1/2}-type dependence to fit our experimental data. We show that Ta NQR is a novel probe for the bulk Weyl fermions and their excitations.

Detecting NQR signals severely polluted by interference

Nuclear Quadrupole Resonance (NQR) signal detection can be severely obstructed by interference in real life settings, especially when the interference is strong, nonstationary, and its frequencies are close to that of the NQR signal. A novel algorithm is proposed to effectively remove (or reduce) interference components in the data and facilitate a valid detection of the NQR signal. The proposed method exhibits better performance compared to the previously proposed ETAML and FETAML algorithms, when applied to both simulated and measured data. Importantly, the present algorithm directly operates on the original primary data, without requiring any secondary data (NQR signal-free data) for acquiring prior knowledge of the interference.

Broadband quantitative NQR for authentication of vitamins and dietary supplements

We describe hardware, pulse sequences, and algorithms for nuclear quadrupole resonance (NQR) spectroscopy of medicines and dietary supplements. Medicine and food safety is a pressing problem that has drawn more and more attention. NQR is an ideal technique for authenticating these substances because it is a non-invasive method for chemical identification. We have recently developed a broadband NQR front-end that can excite and detect 14N NQR signals over a wide frequency range; its operating frequency can be rapidly set by software, while sensitivity is comparable to conventional narrowband front-ends over the entire range. This front-end improves the accuracy of authentication by enabling multiple-frequency experiments. We have also developed calibration and signal processing techniques to convert measured NQR signal amplitudes into nuclear spin densities, thus enabling its use as a quantitative technique. Experimental results from several samples are used to illustrate the proposed methods.

Optical detection of low frequency NQR signals: a step forward from conventional NQR

In searching for the more sensitive 14N nuclear quadrupole resonance (NQR) detecting system for illicit substances, a promising combination of a classic RF pulse NQR spectrometer and a K optically pumped magnetometer was tested. The initial results are encouraging. The principles of such a combination are described, and the detection limits in the low frequency RF region, where the 14N pulse NQR frequencies are usually positioned, are presented. Several illicit substances which are difficult to detect with a classic pulse NQR spectrometer were detected with both types of spectrometers. We noticed that with the proposed combination of classic RF excitation of 14N nuclei, using a pulse NQR spectrometer and subsequent optical detection of the sample’s response, a gain in S/N of up to a factor of 10 was possible.

On RF-Pulse-Phase Dependence of Nuclear Quadrupole Resonance Signal Under Short-Repetition-Time Pulse Sequences

In the most conventional pulsed nuclear quadrupole resonance (NQR) systems, radio frequency (RF) pulses for irradiation are produced by chopping a continuous RF wave with a DC pulse. Therefore, RF pulses have phase coherency but NQR signals after each RF pulse have different initial phase. In this case, a phase-sensitive detection (PSD) using the same continuous RF wave is necessary to accumulate NQR signal. Meanwhile, recent improvement of device technology enables a phase controlled RF pulse and a direct sampling of NQR signal in the laboratory frame. We developed a new pulse NQR system using phase controlled RF pulse, in which RF pulses with the same initial phase are produced and NQR signal after each pulse is directly acquired in the laboratory frame, and then the accumulated signal is processed by digital quadrature detection and fast Fourier transform. This paper provides and compares the feature of NQR signals from both types of pulse NQR system. Main findings are that NQR signal obtained by a train of phase coherent pulses and PSD shows the periodical variation of the intensity depending on the offset frequency of the irradiation RF pulse from the resonance frequency but the signal obtained by the phase controlled RF pulse train does not. This feature would be important for the application of NQR to detection system for such as explosives.