11,940 research outputs found
Study of the properties of hyperpolarized xenon-129 for magnetic resonance imaging
Produkce hyperpolarizovaných plynů, především helia (3He) nebo xenonu (129Xe), nachází stále rostoucí rozsah aplikací v zobrazování magnetickou rezonancí (MRI). Helium ani xenon nejsou obyčejně obsaženy v těle a experimenty tedy nejsou ovlivněny nechtěným signálem z okolních tkání. Ukázalo se, že několika hyperpolarizačními technikami může být magnetická polarizace (magnetizace) jader vzácných plynů zvýšena na hladinu, se kterou jsou praktické aplikace proveditelné. Hyperpolarizované plyny mohou tedy být užitečným nástrojem pro neinvazivní zkoumání lidského dýchání, dovolující statické zobrazování během zadržení dechu nebo zkoumání dynamiky výdechu nebo nádechu, nebo funkčního zobrazování. V neživé přírodě, mohou být hyperpolarizovaný plyny využity jako kontrastní látka při studiu mikroporézních materiálů, jako jsou zeolity, stavební látky a hmoty, atd. V této doktorské práci je popsán vývoj a konstrukce aparatury pro hyperpolarizaci xenonu (izotopu 129Xe). Nákup hyperpolarizovaného xenonu od jiných výzkumných center v zahraničí a jeho dovážení by ovšem nebylo efektivní a to zejména z důvodu náročnosti zajištění potřebných fyzikálních podmínek pro přepravu hyperpolarizovaného plynu. Toto bylo hlavní motivací k vývoji vlastní technologie pro přípravu hyperpolarizovaného xenonu. Se zvládnutou technologií by bylo možné navázat spolupráci s medicínskými zařízeními, nebo s týmy zabývající se živou nebo neživou přírodou (např. při studiu mikroporézních materiálů, gelů, v zemědělských aplikacích nebo při výzkumu využívajících zvířat, atd.). Cílem této práce je studium teorie hyperpolarizovaných vzácných plynů se zaměřením na 129Xe a experimentální ověření a změření relaxačních časů pomocí jaderné magnetické rezonance. Vzhledem k tomu, že je možné hyperpolarizované vzácné plyny skladovat pro pozdější využití, se tato práce také zabývá možnostmi zásobníku hyperpolarizovaného vzácného plynu a jeho teoretickým a experimentálním řešením. V této práci jsou popsány především dva základní typy experimentů přípravy hyperpolarizovaného xenonu. V obou jsou využity zatavené válcové skleněné vzorky naplněné xenonem a doplňujícím plynem – dusíkem, heliem. První z experimentů se zabývá měřením vlastností termálně polarizovaného xenonu a druhý měřením vlastností hyperpolarizovaného xenonu. Pro hyperpolarizaci 129Xe bylo použito výkonového laseru a experimentálně byla zkoumána jednak míra polarizace na základě změny spektrální hustoty čerpacího laserového svazku a dále pak optimální doba optického čerpání 129Xe a relaxační časy xenonu.The production of hyperpolarized gases (HpG), predominantly helium (3He) or xenon (129Xe), have found a steadily increasing range of applications in magnetic resonance imaging (MRI). Neither helium nor xenon are normally present in the body, thus the magnetic resonance experiments do not suffer from unwanted background signals. It has been demonstrated by several techniques of hyperpolarization that the magnetic polarization (magnetization) of the noble gas nuclei can be increased to levels that make practical application feasible. Hence, hyperpolarized gases may become a useful tool for non-invasive investigation of human lung ventilation, permitting static imaging during breathhold or probing the dynamics of inhalation/exhalation, or functional imaging. In inanimate nature, hyperpolarized gas can be used as a contrast medium for microporous materials, such zeolites, constructive materials in civil engineering, etc. This thesis describes the development and construction of a xenon (129Xe) hyperpolarization (Hp) device. Buying hyperpolarized xenon from other research centres abroad is inefficient mainly because of a need of a fast transport of HpXe under specific conditions. That was the main motivation for developing of our own technology for production of HpXe. Well-handled technology could allow a medical cooperation or cooperation with teams dealing with in/animate nature (microporous material, gels, agriculture, animals, etc.). The aim of this work is to study the hyperpolarized noble gases theory with concern to 129Xe and to experimentally prove and measure xenon relaxation times by the NMR. Since it is possible to store hyperpolarized noble gases for later use, this doctoral thesis also explores the potentials of hyperpolarized noble gas storage system and its theoretical and experimental solution. Mainly two types of experiments are described in the thesis. In both experiments, sealed cylindrical Simax sample filled with xenon and supplement gas – nitrogen, helium were used. The first type of experiment is based on thermally polarized xenon and the second on hyperpolarized xenon. For hyperpolarization of 129Xe a high-power laser was used. In this experiment, the relation between power spectral density of optical pumping beam and efficiency of HpXe production process was investigated. The optimal duration of optical pumping and relaxation times of HpXe were investigated too.
Molecular effects in the ionization of N, O and F by intense laser fields
In this paper we study the response in time of N, O and F to
laser pulses having a wavelength of 390nm. We find single ionization
suppression in O and its absence in F, in accordance with experimental
results at nm. Within our framework of time-dependent density
functional theory we are able to explain deviations from the predictions of
Intense-Field Many-Body -Matrix Theory (IMST). We confirm the connection of
ionization suppression with destructive interference of outgoing electron waves
from the ionized electron orbital. However, the prediction of ionization
suppression, justified within the IMST approach through the symmetry of the
highest occupied molecular orbital (HOMO), is not reliable since it turns out
that, e.g. in the case of F, the electronic response to the laser pulse is
rather complicated and does not lead to dominant depletion of the HOMO.
Therefore, the symmetry of the HOMO is not sufficient to predict ionization
suppression. However, at least for F, the symmetry of the dominantly
ionized orbital is consistent with the non-suppression of ionization.Comment: 19 pages, 5 figure
Experimental Heat-Bath Cooling of Spins
Algorithmic cooling (AC) is a method to purify quantum systems, such as
ensembles of nuclear spins, or cold atoms in an optical lattice. When applied
to spins, AC produces ensembles of highly polarized spins, which enhance the
signal strength in nuclear magnetic resonance (NMR). According to this cooling
approach, spin-half nuclei in a constant magnetic field are considered as bits,
or more precisely, quantum bits, in a known probability distribution.
Algorithmic steps on these bits are then translated into specially designed NMR
pulse sequences using common NMR quantum computation tools. The
cooling of spins is achieved by alternately combining reversible,
entropy-preserving manipulations (borrowed from data compression algorithms)
with , the transfer of entropy from selected spins to the
environment. In theory, applying algorithmic cooling to sufficiently large spin
systems may produce polarizations far beyond the limits due to conservation of
Shannon entropy.
Here, only selective reset steps are performed, hence we prefer to call this
process "heat-bath" cooling, rather than algorithmic cooling. We experimentally
implement here two consecutive steps of selective reset that transfer entropy
from two selected spins to the environment. We performed such cooling
experiments with commercially-available labeled molecules, on standard
liquid-state NMR spectrometers. Our experiments yielded polarizations that
- , so that the entire
spin-system was cooled. This paper was initially submitted in 2005, first to
Science and then to PNAS, and includes additional results from subsequent years
(e.g. for resubmission in 2007). The Postscriptum includes more details.Comment: 20 pages, 8 figures, replaces quant-ph/051115
Non-thermal nuclear magnetic resonance quantum computing using hyperpolarized Xenon
Current experiments in liquid-state nuclear magnetic resonance quantum
computing are limited by low initial polarization. To address this problem, we
have investigated the use of optical pumping techniques to enhance the
polarization of a 2-qubit NMR quantum computer (13C and 1H in 13CHCl3). To
efficiently use the increased polarization, we have generalized the procedure
for effective pure state preparation. With this new, more flexible scheme, an
effective pure state was prepared with polarization-enhancement of a factor of
10 compared to the thermal state. An implementation of Grover's quantum search
algorithm was demonstrated using this new technique.Comment: 4 pages, 3 figures. Submitted for publicatio
Theoretical studies of solar-pumped lasers
Possible types of lasers were surveyed for solar power conversion. The types considered were (1) liquid dye lasers, (2) vapor dye lasers, and (3) nondissociative molecular lasers. These are discussed
Transport Properties of operational gas mixtures used at LHC
This report summarizes some useful data on the transport characteristics of
gas mixtures which are required for detection of charged particles in gas
detectors. We try to replace Freon used for RPC detector in the CMS experiment
with another gas while maintaining the good properties of the Freon gas mixture
unchanged. We try to switch to freonless gas mixture because Freon is not a
green gas, it is very expensive and its availability is decreasing. Noble gases
like Ar, He, Ne and Xe (with some quenchers like carbon dioxide, methane,
ethane and isobutene) are investigated. Transport parameters like drift
velocity, diffusion, Townsend coefficient, attachment coefficient and Lorentz
angle are computed using Garfield software for different gas mixtures and
compared with experimental data.Comment: 35 page
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