Temperature dependence of nuclear magnetization and relaxation

The temperature dependences of nuclear magnetization and relaxation rates are reviewed theoretically and experimentally in order to quantify the effects of temperature on NMR signals acquired by common imaging techniques. Using common sequences, the temperature dependences of the equilibrium nuclear magnetization and relaxation times must each be considered to fully understand the effects of temperature on NMR images. The temperature dependence of the equilibrium nuclear magnetization is negative because of Boltzmann's distribution for all substances at all temperatures, but the combined temperature dependences of the equilibrium magnetization and relaxation can be negative, weak or positive depending on the temperature (T), echo time (T(E)), repetition time (T(R)), and the temperature dependences of the relaxation times T(1)(T) and T(2)(T) in a pulse sequence. As a result, the magnitude of the NMR signal from a given substance can decrease, increase or stay somewhat constant with increasing temperature. Nuclear thermal coefficients are defined and predictions for spin echo and other simple sequences are verified experimentally using a number of substances representing various thermal and NMR properties.     

Solvent Suppression in High-Resolution 1H NMR Spectroscopy Using Conventional and Phase Alternated Continuous Wave Free Precession

Solvent suppression is frequently mandatory in ¹H high-resolution nuclear magnetic resonance (NMR), especially for those experiments designed for non-deuterated solvent, normally used in protein and in vivo analysis, and also in liquid chromatography-NMR. Here, simple pulse sequences, which are based on continuous wave free precession (CWFP), consisting of a train of pulses separated by a time interval $ T_{\text{p}} \ll T_{2}^{*} $ , is applied to suppress one or more solvent signals in ¹H high-resolution NMR experiments, because of its dependency on the offset frequency. The conventional CWFP pulse sequence, that uses pulses with the same phase and duration, introduces some phase anomaly in the Fourier-transformed spectrum. This problem is minimized when the pulses are applied with phase alternation by π/2 in relation to the preceding pulse. Some problems with signal intensity can also be minimized using a shorter pulse width. Both CWFP and phase alternated CWFP can be easily used to suppress two solvent signals simultaneously, just using the correct T _p value, that must be equal to the inverse of frequency difference (?ν) between both signals to be suppressed. After modifications, we could introduce the CWFP train into 2D routine pulse sequences.     

Use of Carr-Purcell pulse sequence with low refocusing flip angle to measure T₁ and T₂ in a single experiment

The Carr-Purcell pulse sequence, with low refocusing flip angle, produces echoes midway between refocusing pulses that decay to a minimum value dependent on T(2). When the refocusing flip angle was π/2 (CP(90)) and τ>T(2), the signal after the minimum value, increased to reach a steady-state free precession regime (SSFP), composed of a free induction decay signal after each pulse and an echo, before the next pulse. When τ相似文献   

Nuclear spin relaxation induced by a mechanical resonator

We report on measurements of the spin lifetime of nuclear spins strongly coupled to a micromechanical cantilever as used in magnetic resonance force microscopy. We find that the rotating-frame correlation time of the statistical nuclear polarization is set by the magnetomechanical noise originating from the thermal motion of the cantilever. Evidence is based on the effect of three parameters: (1) the magnetic field gradient (the coupling strength), (2) the Rabi frequency of the spins (the transition energy), and (3) the temperature of the low-frequency mechanical modes. Experimental results are compared to relaxation rates calculated from the spectral density of the magnetomechanical noise.     

Measurement of thermal diffusivity by magnetic resonance imaging

Nuclear magnetic resonance (NMR) may be used for monitoring temperature changes within samples based on measurements of relaxation times, the diffusion coefficient of liquids, proton resonance frequency or phase shifts. Such methods may be extended to the explicit measurement of the thermal diffusivity of materials by NMR imaging. A method based on measuring nuclear spin phase shifts or changes in the equilibrium nuclear magnetization has been developed for measuring transient thermal diffusion effects and thermal diffusivity with potential applications in NMR thermotherapy and materials science. In this method, a thermal pulse is applied to a medium, and the resultant temporal variations of the nuclear spin phase or of the magnitude of the nuclear magnetization produced by the thermal pulse are monitored at a spatial distance. The results obtained on common fluids agree well with the data from other methods.     

Nuclear spin lattice relaxation of186ReFe and182TaFe: the role of spin wave mechanisms

Using the thermal cycling method, the nuclear spin lattice relaxation of¹⁸⁶ReFe and¹⁸²TaFe has been measured in polarizing fields of 0.1 to 1.2 T. The results for the relaxation constants in the high-field limit are 87(2) and 470(10) ms K, respectively. The relaxation rates are about a factor of three larger than expected from ab initio calculations. It is proposed to interprete this discrepancy as proof for a significant spin wave contribution to the relaxation.     

Nuclear spin-lattice relaxation in antiferromagnetic RbMnF3

The ⁸⁷Rb and ¹⁹F nuclear relaxation rates in antiferromagnetic RbMnF₃ are calculated as a function of temperature. The large difference in the relaxation rates of both nuclei as well as their temperature dependences are well accounted for over a considerable temperature range by a two-magnon process induced by the dipolar and “contact” parts of the hyperfine interaction.     

Nuclear magnetic relaxation in rare-earth compound crystals due to fluctuations of hyperfine magnetic fields

Some results on NMR and relaxation studies of the Van Vleck paramagnet TmES (thulium ethylsulphate) and the Ising ferromagnet DyES are summarized. Complicated but regular quasistatic internal magnetic fields are created by Tm and Dy ions in these compounds. These fields fluctuate due to the thermal excitation of tne ions and the energy transfer from one ion to another. Fluctuations give rise to NMR line shifts, broadening of the lines and spin-lattice relaxation, the shifts, linewidth and spin-lattice relaxation rate being proportional to exp(−Δ/kT) at low temperatures (kT≪Δ, Δ is an excitation energy). Pre-exponential factors depend on fluctuating fields in a definite but complicated manner, so estimates of the correlation time (electron spin-spin relaxation time) can be obtained from measurements of nuclear relaxation rates.     

Kernresonanz-und leitfähigkeitsmessungen zu diffusion und fehlordnung in LiBr

Electrical conductivity and nuclear magnetic relaxation rates were measured with pure and doped LiBr between 400 K and the melting point (824 K). Prevalent intrinsinc disorder was observed down to 470 K. The degree of thermal disorder is 5.10⁻⁷ at 470 K and 5.10⁻³ at the melting point. From the relaxation rates of ⁷Li, which are caused by Li-diffusion and nuclear dipole interaction, mean jump frequencies of the cations are derived. Conductivities calculated from these frequencies for a jump process via neighbouing cation vacancies are in perfect agreement with directly measured conductivities. From relaxation rates of ⁸¹Br with MgBr₂-doped crystals jump frequencies of vacancies were obtained which are again in good agreement with those derived from the conductivity data. The energies resulting from the measurements are (0.43 ± 0.03) eV for migration of cation vacancies and (1.46 ± 0.05) eV for thermal disorder. From motional narrowing of the ⁸¹Br absorption line the jump frequency of the anions is obtained, which is much smaller than for the cations. Since this motional narrowing is not influenced by any doping, it is concluded that anion transport mainly occurs via pairs of cation and anion vacancies.     

Rotational relaxation time in CO free jets

The electron beam fluorescence technique was applied to measure the rotational energy relaxation cross-sections in carbon monoxide free jets. The data were obtained in the low temperature region from 20 to 200 K and compared with the results of ultrasound, thermal conductivity, free jet and line broadening experiments. The overall scatter of the experimental data runs to one order of magnitude, and their temperature dependence differs. This fact calls for further theoretical and experimental investigations of rotational relaxation rates in carbon monoxide.     

Transport properties of the A phase of superfluid 3He at low temperatures

The results of a calculation of transport properties of the A phase of superfluid ³He are reported. The temperature dependence of the normal fluid density, viscosity, thermal conductivity, and nuclear spin relaxation time are given.     

Nuclear quadrupole resonance studies of transparent conducting oxides

We report ^63,65Cu spin–lattice relaxation rates measured by nuclear quadrupole resonance (NQR) in the delafossite compound CuYO₂ and CuYO₂:Ca over a temperature range from 200 to 450 K. CuYO₂:Ca is a prototype transparent oxide exhibiting p-type electrical conductivity. Relaxation rates in CuYO₂:Ca are enhanced by one to two orders of magnitude relative to undoped material, exhibit much stronger temperature dependence, and contain contributions from magnetic and quadrupolar relaxation mechanisms with roughly equal strengths. Relaxation in undoped CuYO₂ is of purely quadrupolar origin and is attributed to interactions with lattice phonons. The main focus of this paper is the magnetic contribution to the relaxation rate in CuYO₂:Ca which is attributed to the hyperfine fields of carriers. It is argued that the dynamics of the hyperfine field are dominated by the hopping rate for carrier transfers between neighboring atoms in the copper planes of the delafossite structure. Comparison of the magnetic relaxation rates with the DC conductivity permits an estimate of the carrier concentration and mobility.     

Nuclear magnetic resonance study of the relaxation mechanisms of the Be, Al, and Si nuclei in Cr-doped Be3Al2Si6O18 crystals

The variations with temperature of the relaxation mechanisms of the ⁹Be, ²⁷Al, and ²⁹Si nuclei in emerald (Be₃Al₂Si₆O₁₈:Cr³⁺) single crystals were investigated by using a pulse NMR spectrometer. The values of the spin-lattice relaxation rates for the three nuclei are different, and these differences are attributed to the differences between their Larmor frequencies and their local nuclear environments. The relaxation rates of the ⁹Be and ²⁷Al nuclei undergo no abrupt changes within the temperature range 180-400 K, which indicates that there are no phase transitions within this temperature range. The spin-lattice relaxation rates of ⁹Be and ²⁷Al were found to be proportional to temperature. Therefore, the nuclear spin-lattice relaxation processes of these two nuclei proceed via single-phonon processes.     

Deuterium NMR lineshape and relaxation study of a hydrated cross-linked polymer

A combined study of²H nuclear magnetic resonance lineshape and spin-lattice and spin-spin relaxation times as functions of temperature and the amount of hydration water in a cross-linked copolymer of sucrose and 1,4-butadienol diglycidyl ether in the hydrogel phase is reported. The results show strong evidence that the onset of the relaxation mechanisms is driven by anomalous water molecule diffusion depending on both temperature and the hydration degree of the hydrogel. In addition, these results are correlated with the transitions observed by differential thermal analysis.     

Modification of the spectra of correlated charm-anticharm quark pairs in the quark-gluon plasma

Heavy quarks play an important role in probing the properties of strongly interacting quark-gluon plasma(QGP)created in ultra-relativistic heavy-ion collisions.We study the interactions of single heavy(charm)quarks and correlated charm and anticharm(ccˉ)quark pairs with the medium constituents of QGP by performing fireball+Langevin simulations of the pertinent Brownian motion with elastic collisions.Besides studying the traditional observables,the nuclear modification factor and the elliptic flow of single heavy quarks in QGP for different thermal relaxation rates,we also study the broadening of the azimuthal correlations of charm and anticharm quark pairs in the QGP medium for different relaxation rates and transverse momenta classes.We quantified the smearing of ccˉpair azimuthal correlations with an increasing thermal relaxation rate:while the(nearly)back-to-back correlations among ccˉpairs are almost completely washed out at low transverse momentum(pT),these correlations at high pT largely survive the pair diffusion.This provides a novel observable for diagnosing the properties of QGP.     

Effects of temperature on the relaxation to equilibrium and stationary nonequilibrium states of some Langevin systems

We investigate the effects of temperature on the properties of the time relaxation to equilibrium and nonequilibrium steady states of correlation functions of some Langevin harmonic systems. We consider commonly used dissipative and conservative Langevin dynamics, and show that the time relaxation rate depends on the temperature in the case of thermal reservoirs at different temperatures connected to the system, but it does not happen in the case of relaxation to equilibrium, i.e., if all the heat bath are at the same temperature. Our formalism maps the initial stochastic problem on a noncanonical quantum field theory, and the calculations of the relaxation rates are based on a perturbative analysis. We argue to show the reliability of the perturbative computation.     

Longitudinal-phonon relaxation mechanisms in cubic crystals of germanium,silicon, and diamond

The relaxation rates of thermal and high-frequency longitudinal phonons are calculated using an anisotropic-continuum model. Three-phonon scattering mechanisms (L ? L + L, L ? T + L) for the phonon relaxation are considered. Anisotropic anharmonic phonon scattering in cubic crystals is described in terms of the second-and third-order elastic moduli. The parameters determining the longitudinal-phonon relaxation rates are found for germanium, silicon, and diamond crystals. The long-wavelength limit and the transition to the isotropic-medium model are considered, and the dependences of the relaxation rates of thermal and high-frequency phonons on temperature and phonon wave vector are analyzed for these crystals.     

<Superscript>29</Superscript>Si nuclear spin relaxation in microcrystals of plastically deformed Si: B samples

Single crystals and microcrystals Si: B enriched with ²⁹Si isotopes have been studied using nuclear magnetic resonance and electron paramagnetic resonance (EPR) in the temperature range from 300 to 800 K. It has been found that an increase in the temperature from 300 to 500 K leads to a change in the kinetics of the relaxation of the saturated nuclear spin system. At 300 K, the relaxation kinetics corresponds to direct electron–nuclear interaction with inhomogeneously distributed paramagnetic centers introduced by the plastic deformation of the crystals. At 500 K, the spin relaxation occurs through the nuclear spin diffusion and electron–nuclear interaction with an acceptor impurity. It has been revealed that the plastic deformation affects the EPR spectra at 9 K.     

Estimation of temperature elevation generated by ultrasonic irradiation in biological tissues using the thermal wave method

In most previous models,simulation of the temperature generation in tissue is based on the Pennes bio-heat transfer equation,which implies an instantaneous thermal energy deposition in the medium.Due to the long thermal relaxation time τ(20 s-30 s) in biological tissues,the actual temperature elevation during clinical treatments could be different from the value predicted by the Pennes bioheat equation.The thermal wave model of bio-heat transfer(TWMBT) defines a thermal relaxation time to describe the tissue heating from ultrasound exposure.In this paper,COMSOL Multiphysics 3.5a,a finite element method software package,is used to simulate the temperature response in tissues based on Pennes and TWMBT equations.We further discuss different factors in the bio-heat transfer model on the influence of the temperature rising and it is found that the temperature response in tissue under ultrasound exposure is a rising process with a declining rate.The thermal relaxation time inhibits the temperature elevation at the beginning of ultrasonic heating.Besides,thermal relaxation in TWMBT leads to lower temperature estimation than that based on Pennes equation during the same period of time.The blood flow carrying heat dominates most to the decline of temperature rising rate and the influence increases with temperature rising.On the contrary,heat diffusion,which can be described by thermal conductivity,has little effect on the temperature rising.