Asymptotic and intermediate long-time behavior of nuclear free induction decays in polycrystalline solids and powders

journal articleFree induction decay (FID) measured by nuclear magnetic resonance (NMR) in a polycrystalline solid is the isotropic average of the FIDs for individual single crystallites. It has been recently proposed theoretically and verified experimentally that the long-time behavior of single-crystal FIDs has the universal form of exponentially decaying sinusoidal oscillations. Polycrystalline averaging complicates the situation theoretically, while the available experimental evidence is also ambiguous. Exponentially decaying sinusoidal oscillations have been observed for 129Xe in polycrystalline solid xenon but not for 19F in the powder of CaF2. In this paper, we present the first principles FID calculations for the powders of both CaF2 and solid xenon. In both cases, the asymptotic long-time behavior has the expected form of exponentially decaying sinusoidal oscillations, which is determined by the single crystallite FID with the slowest exponential decay. However, this behavior appears only at rather small values of the signal that have not yet been measured in experiments. At intermediate times accessible experimentally, a polycrystalline FID depends on the distribution of the exponential decay constants and oscillation frequencies for single crystallite FIDs. In CaF2, these parameters are relatively broadly distributed, and as a result, the sinusoidal long-time oscillations become somewhat washed out. In contrast, the single crystallite parameters are more clustered in solid xenon, and, as a result, the experimentally observable range is characterized by well-defined oscillation frequency and exponential decay constant even though both of these parameters do not represent the true long-time behavior. The above difference of the intermediate FID behavior originates from the difference of the crystal structures of solid xenon and CaF2

Gas-phase spin relaxation of 129Xe

Journal ArticleWe have completed an extensive study of 129Xe longitudinal spin relaxation in the gas phase, involving both intrinsic and extrinsic mechanisms. The dominant intrinsic relaxation is mediated by the formation of persistent Xe2 van der Waals dimers. The dependence of this relaxation on applied magnetic field yields the relative contributions of the spin-rotation and chemical-shift-anisotropy interactions; the former dominates at magnetic fields below a few tesla. This relaxation also shows an inverse quadratic dependence on temperature T; the maximum low-field intrinsic relaxation for pure xenon at room temperature (measured here to be 4.6 h, in agreement with previous work) increases by ~60% for T=100 °C. The dominant extrinsic relaxation is mediated by collisions with the walls of the glass container. Wall relaxation was studied in silicone-coated alkali-metal-free cells, which showed long (many hours or more) and robust relaxation times, even at the low magnetic fields typical for spin-exchange optical pumping (~3 mT). The further suppression of wall relaxation for magnetic fields above a few tesla is consistent with the interaction of 129Xe with paramagnetic spins on or inside the surface coating. At 14.1 T and sufficiently low xenon density, we measured a relaxation time T1 =99 h, with an inferred wall-relaxation time of 174 h. A prototype large storage cell (12 cm diameter) was constructed to take advantage of the apparent increase in wall-relaxation time for cells with a smaller surfaceto- volume ratio. The measured relaxation time in this cell at 3 mT and 100 °C was 5.75 h. Such a cell (or one even larger) could be used to store many liters of hyperpolarized 129Xe produced by a flow-through polarizer and accumulator for up to three times longer than currently implemented schemes involving freezing xenon in liquid nitrogen

Nuclear spin relaxation of 129Xe due to persistent xenon dimers

Journal ArticleWe have measured longitudinal nuclear relaxation rates of 129Xe in Xe-N2 mixtures at densities below 0.5 amagats in a magnetic field of 8.0 T. We find that intrinsic spin relaxation in this regime is principally due to fluctuations in the intramolecular spin-rotation (SR) and chemical-shift-anisotropy (CSA) interactions, mediated by the formation of 129Xe-Xe persistent dimers. Our results are consistent with previous work done in one case at much lower applied fields where the CSA interaction is negligible and in another case at much higher gas densities where transient xenon dimers mediate the interactions. We have verified that a large applied field suppresses the persistent-dimer mechanism, consistent with standard relaxation theory, allowing us to measure room-temperature gas-phase relaxation times T1 for 129Xe greater than 25 h at 8.0 T. These data also yield a maximum possible low-field T1 for pure xenon gas at room temperature of 5.45 ± 0.2 h. The coupling strengths for the SR and CSA interactions that we extract are in fair agreement with estimates based both on previous experimental work and on ab initio calculations. Our results have potential implications for the production and storage of large quantities of hyperpolarized 129Xe for use in various applications

Phase relationship between the long-time beats of free induction decays and spin echoes in solids

Recent theoretical work on the role of microscopic chaos in the dynamics and relaxation of many-body quantum systems has made several experimentally confirmed predictions about the systems of interacting nuclear spins in solids, focusing, in particular, on the shapes of spin echo responses measured by nuclear magnetic resonance (NMR). These predictions were based on the idea that the transverse nuclear spin decays evolve in a manner governed at long times by the slowest decaying eigenmode of the quantum system, analogous to a chaotic resonance in a classical system. The present paper extends the above investigations both theoretically and experimentally. On the theoretical side, the notion of chaotic eigenmodes is used to make predictions about the relationships between the long-time oscillation phase of the nuclear free induction decay (FID) and the amplitudes and phases of spin echoes. On the experimental side, the above predictions are tested for the nuclear spin decays of F-19 in CaF2 crystals and Xe-129 in frozen xenon. Good agreement between the theory and the experiment is found.Comment: 20 pages, 9 figures, significant new experimental content in comparison with version

Analytical Investigation of Innovation Dynamics Considering Stochasticity in the Evaluation of Fitness

We investigate a selection-mutation model for the dynamics of technological innovation,a special case of reaction-diffusion equations. Although mutations are assumed to increase the variety of technologies, not their average success ("fitness"), they are an essential prerequisite for innovation. Together with a selection of above-average technologies due to imitation behavior, they are the "driving force" for the continuous increase in fitness. We will give analytical solutions for the probability distribution of technologies for special cases and in the limit of large times. The selection dynamics is modelled by a "proportional imitation" of better technologies. However, the assessment of a technology's fitness may be imperfect and, therefore, vary stochastically. We will derive conditions, under which wrong assessment of fitness can accelerate the innovation dynamics, as it has been found in some surprising numerical investigations.Comment: For related work see http://www.helbing.or

Motivasidanhasilstudisiswamiskinsmpnegeri3kecamatan Pujud

Motivation and study result of SMPN 3 PUJUD's poor students: Budi Pramaharja, 1205188179, 2014. Study program: guidance and counseling; science department, Education and science faculty: Riau University. Counselor I: Prof. Dr. Zulfan Saam, M. Si; Counselor II: Dra. Tri Umari, M. Si.This research aims to know the illustration of poor student' study motivation in SMPN 3 PUJUD that back grounded by some reason that seen in school such as : the problem about low motivation and the study result of poor student in SMPN 3 PUJUD.The method that be used in this research is Descriptive method in percentage, the location of research is in SMPN 3 PUJUD. This research subject is VII, VIII, IX students' class. Giving the study motivation is taken by sample technique or saturate sample. Because the sum of population in this research is small. Data and the instrument of data collection is questionnaire form about motivation and study of result poor student in SMPN 3 PUJUD that done by researcher and be guided at lattice work with the alternative of multiple choices: frequent, often, sometimes, seldom and never. Data analysis technique uses the percentage formula (Anas Sudijono, 2004:170)Based on the research and data processing that had been done, can be resumed : (1) based on the result of data processing can be know the general descriptive of study motivation: VII, VIII, IX students' class of SMPN 3 PUJUD, PUJUD district, school year 2013-2014, that is by paying attention the intensity of giving the poor students' study motivation, with the result that low level category. The poor students' study motivation is be categorized low level, be market by : a) have not the lesson book, b) less the parent's attention, c) seldom visit the library, d) less the media in giving the assist to the development of potential students in studying, so that become the adult human and responsible. (2) the description of poor students' study result in SMPN 3 PUJUD can be know that is the students of VII, VIII, IX class, school year 2013-2014, low level dominantly . the low subject is under KKM on Pendidikan Pancasila dan Kewarganegaraan (citizen) lesson by rating 47,5 before remedial, and on IPA (science) lesson by rating 45 before remedial. (3) constraints that be faced by the poor students like as helping parent, long distance from house to school, this case result the low study motivation and study result for students