Free Induction Decay Amplitude Research Articles

Multitype Noise Suppression in Magnetic Resonance Sounding Data Based on a Time–Frequency Fully Convolutional Neural Network

Magnetic resonance sounding (MRS) is one of the technical applications of nuclear magnetic resonance (NMR) used to directly detect and quantify groundwater content. MRS suffers from a low signal-to-noise ratio (SNR) due to the low amplitude of free induction decay (FID) signals and an inability to shield environmental noise. In this paper, a time-frequency fully convolutional neural network (TFCN) was proposed to suppress random, harmonic, and spike noise from MRS data. The TFCN parameters were trained with the time-frequency spectrum obtained by the short-time Fourier transform (STFT) of the MRS datasets as the input and the noise-free FID signals as the output. Based on the results of synthetic and field data experiments, the TFCN was compared with existing denoising methods. The results showed that the TFCN extracted the envelope of the FID signals from low-SNR random noise with higher accuracy than other methods. Moreover, the TFCN simultaneously suppressed multiple types of noise and exhibited high computational efficiency.

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.

A method for calibrating coil constants by using the free induction decay of noble gases

We propose a precise method to calibrate the coil constants of spin-precession gyroscopes and optical atomic magnetometers. This method is based on measuring the initial amplitude of Free Induction Decay (FID) of noble gases, from which the π/2 pulse duration can be calculated, since it is inversely proportional to the amplitude of the π/2 pulse. Therefore, the coil constants can be calibrated by measuring the π/2 pulse duration. Compared with the method based on the Larmor precession frequency of atoms, our method can avoid the effect of the pump and probe powers. We experimentally validated the method in a Nuclear Magnetic Resonance Gyroscope (NMRG), and the experimental results show that the coil constants are 436.63±0.04 nT/mA and 428.94±0.02 nT/mA in the x and y directions, respectively.

Reversible absorption of weak fields revealed in coherent transients

It is shown that the absorbtion of a weak field in a thick resonant absorber is a reversible process even in the case of the homogeneous broadening of the absorption line. As an example, the propagation of a long rectangular pulse with sharp edges in an optically dense resonant medium is studied theoretically in the linear response approximation. Transient nutation (TN), free induction decay (FID), and transients, induced by the phase switch of the incident pulse, are considered. It is shown that in exact resonance the amplitude of FID increases with the length of the medium. FID arises due to the scattered radiation field (dipoles ringing). In a thick medium the scattered field is almost of the same amplitude but opposite in phase with the incident radiation field. Both fields interfere destructively to produce what is seen as radiation damping at the output of the medium. The scattered field needs time to develop. Therefore, the leading edge of the pulse is not absorbed, demonstrating temporal transparency followed by TN. Phase shift (180${}^{\ensuremath{\circ}}$) of the pulse brings the incident pulse in phase with the scattered radiation. Constructive interference of the pulse with the scattered radiation field produces a short pulse with an amplitude that is two times larger than the amplitude of the incident pulse. If the input pulse is detuned from resonance, for a particular detuning and optical thickness of the medium the amplitude of the transient pulse, induced by the phase shift, is nearly three times larger than the amplitude of the incident pulse. This is explained by the interference of the scattered field, the phase-shifted input field, and the slowly propagating part of the pulse, developed before the phase shift.

Optical Free Induction Decay in Cold 85Rb Atoms

We have observed optical free induction decay (FID) in cold 85Rb atoms. The shortening of the FID lifetime with increasing optical depth of the trap has been found. This shortening is explained by an anomalous absorption for FID propagating in a homogeneous and resonant sample. Theoretical calculations are consistent with experimental results. The Rabi oscillations of FID amplitude have been observed by increasing the pulse area.

Rapid imaging of free radicals in vivo using field cycled PEDRI

Imaging of free radicals in vivo using an interleaved field-cycled proton-electron double-resonance imaging (FC-PEDRI) pulse sequence has recently been investigated. In this work, in order to reduce the EPR (electron paramagnetic resonance) irradiation power required and the imaging time, a centric reordered snapshot FC-PEDRI pulse sequence has been implemented. This is based on the FLASH pulse sequence with a very short repetition time and the use of centric reordering of the phase-encoding gradient, allowing the most significant free induction decay (FID) signals to be collected before the signal enhancement decays significantly. A new technique of signal phaseshift correction was required to eliminate ghost artefacts caused by the instability of the main magnetic field after field cycling. An FID amplitude correction scheme has also been implemented to reduce edge enhancement artefacts caused by the rapid change of magnetization population before reaching the steady state. Using the rapid pulse sequence, the time required for acquisition of a 64 × 64 pixel FC-PEDRI image was reduced to 6 s per image compared with about 2.5 min with the conventional pulse sequence. The EPR irradiation power applied to the sample was reduced by a factor of approximately 64. Although the resulting images obtained by the rapid pulse sequence have a lower signal to noise than those obtained by a normal interleaved FC-PEDRI pulse sequence, the results show that rapid imaging of free radicals in vivo using snapshot FC-PEDRI is possible.

Proton free induction decay evolution during hydration of white synthetic cement

Nuclear magnetic resonance (NMR) of protons in hydratlng cement paste(l) can be employed to differentiate between protons whlch are In llquld phases, such as bulk-llke water or very fluld gel, and those embedded In rlgld structures such as crystalline water or hydroxyl groups in portlandlte, CH. As the hydratlon proceeds (2), progresslvely more of the water Inltlally added to the cllnker becomes crystalllne water and hydroxyls of CH. The total amplltude of the proton Nt4R free induction decay (FID) followlng a 90 ° r.f. excltatlon, whlch Is a measure of the total number of protons In the paste, is expected to remaln Independent of the hydratlon time provided there is no loss of water by evaporatlon. Thus, during the hydration the total proton FID amplitude does not change, as Its solld-llke component grows at the expense of its llquld-llke component. The solld-llke FID can be very easily differentiated from the llquld-llke FID as their respectlve shapes differ drastically. Whlle the solld-llke FID is Gausslan, wlth a characterlstlc decay time T on the order of 20 ~s, the llquld-llke FID Is nearly 2

Colorado Green River Formation oil shale as viewed by pulsed n.m.r.

Wideband proton pulsed n.m.r. techniques have been applied to selected Colorado Green River Formation oil shales. Among the various parameters generated by this technique, one correlates with total organic matter contents and another correlates with Fischer assay oil yields. The free induction decay (FID) amplitude quantitatively measures total oil shale proton contents when 1. (1) the amplitude is extrapolated up to the midpoint of the radio frequency pulse, and 2. (2) an adequate trigger repetition rate ( t r ) is used. A 1 s value of t r , within experimental precision, masks the crystalline inorganic protons. The maximum FID amplitude with a 1 s t r correlates with the total organic matter content. T 1 decays for the organic matter have two relaxation components. The fast component has relaxation times ⪡ 5 ms. Maximum FID amplitudes using a 1 s t r , reduced by the fraction of protons with the fast T 1 decays, correlate directly with Fischer assay oil yields. This T 1 correction factor, thus, reflects the organic matter unrecoverable as oil by the Fischer assay. The reported procedure cannot alone resolve oil-shale free and clay interlayer water protons from the organic protons. This work, along with literature references, shows these protons to be present in minor quantities as compared to the organic proton content of commercial oil shales. If such protons should be present in significant quantities, their concentration can be greatly reduced by finely crushing the oil shale and drying at 110 °C overnight.

Pulsed n.m.r. in nickel

The dependence of the n.m.r. free induction decay amplitude on the strength of the r.f. field and on pulse length has been studied in nickel metal powder. The results can be explained by the model of Stearns and Overhauser which hypothesizes boundary pinning of domains. We estimate a maximum enchancement factor in nickel of [Formula: see text].

Spin-Echo and Free-Induction-Decay Measurements in Pure Fe and Fe-Rich Ferromagnetic Alloys: Domain-Wall Dynamics

Measurements were made of the free-induction-decay (FID) and spin-echo amplitudes of ${\mathrm{Fe}}^{57}$ in pure Fe and some of its alloys. Complex spectra are observed consisting of many oscillations often covering a range of many Mc/sec. The amplitudes are very dependent on the pulse length $\ensuremath{\tau}$, the applied rf field ${B}_{1}$, and the exciting frequency $\ensuremath{\omega}$. Excellent agreement is found between the experimental and calculated amplitudes when a model for domain-wall motion is used which takes into consideration the following features: 1. There is an averaging over the orientation of ${\mathbf{B}}_{1}$ with respect to the local magnetization. 2. There is a variation of the enhancement factor across the domain wall due to the spin arrangement in the wall. 3. The domain wall is considered to be composed of area segments which are immobile along their perimeters and whose motion resembles that of a vibrating drumhead. 4. These domain-wall segments have a distribution in areas. The restricted motion of the domain walls imposed by the last two features is essential in order to obtain agreement with the experimental results. This is in contrast to the usual model, which represents the domain-wall motion as given by rigid simple-harmonic-like oscillations; such a model shows great discrepancy with the experimental results presented here. In some of the spectra, asymmetries are observed in the variation of the amplitudes as a function of frequency. No satisfactory explanation for these asymmetries is known although some possible sources are suggested. Such asymmetries make it appear very doubtful that the Fe-alloy spectra obtained with this technique can be reliably unfolded to give the variation of the internal field for second- or higher-neighbor impurity atoms. FID amplitude measurements were also made on Fe in an externally applied dc field. No variation in the value of the maximum enhancement factor was observed for dc fields varying from 0 to about 5 kG.