RESUMEN
In standard Dynamic Nuclear Polarization (DNP) electron spins are polarized at low temperatures in a strong magnetic field and this polarization is transferred to the nuclear spins by means of a microwave field. To obtain high nuclear polarizations cryogenic equipment reaching temperatures of 1 K or below and superconducting magnets delivering several Tesla are required. This equipment strongly limits applications in nuclear and particle physics where beams of particles interact with the polarized nuclei, as well as in neutron scattering science. The problem can be solved using short-lived optically excited triplet states delivering the electron spin. The spin is polarized in the optical excitation process and both the cryogenic equipment and magnet can be simplified significantly. A versatile apparatus is described that allows to perform pulsed dynamic nuclear polarization experiments at X-band using short-lived optically excited triplet sates. The efficient (4)He flow cryostat that cools the sample to temperatures between 4 K and 300 K has an optical access with a coupling stage for a fiber transporting the light from a dedicated laser system. It is further designed to be operated on a neutron beam. A combined pulse ESR/DNP spectrometer has been developed to observe and characterize the triplet states and to perform pulse DNP experiments. The ESR probe is based on a dielectric ring resonator of 7 mm inner diameter that can accommodate cubic samples of 5mm length needed for neutron experiments. NMR measurements can be performed during DNP with a coil integrated in the cavity. With the presented apparatus a proton polarization of 0.5 has been achieved at 0.3 T.
RESUMEN
A high throughput method was designed to produce hyperpolarized gases by combining low-temperature dynamic nuclear polarization with a sublimation procedure. It is illustrated by applications to 129Xe nuclear magnetic resonance in xenon gas, leading to a signal enhancement of 3 to 4 orders of magnitude compared to the room-temperature thermal equilibrium signal at 7.05 T.
Asunto(s)
Espectroscopía de Resonancia Magnética/métodos , Transición de Fase , Xenón/química , TemperaturaRESUMEN
Major breakthroughs have recently been reported that can help overcome two inherent drawbacks of NMR: the lack of sensitivity and the limited memory of longitudinal magnetization. Dynamic nuclear polarization (DNP) couples nuclear spins to the large reservoir of electrons, thus making it possible to detect dilute endogenous substances in magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI). We have designed a method to preserve enhanced ("hyperpolarized") magnetization by conversion into long-lived states (LLS). It is shown that these enhanced long-lived states can be generated for proton spins, which afford sensitive detection. Even in complex molecules such as peptides, long-lived proton states can be sustained effectively over time intervals on the order of tens of seconds, thus allowing hyperpolarized substrates to reach target areas and affording access to slow metabolic pathways. The natural abundance carbon-13 polarization has been enhanced ex situ by almost four orders of magnitude in the dipeptide Ala-Gly. The sample was transferred by the dissolution process to a high-resolution magnet where the carbon-13 polarization was converted into a long-lived state associated with a pair of protons. In Ala-Gly, the lifetime T(LLS) associated with the two nonequivalent H(alpha) glycine protons, sustained by suitable radio-frequency irradiation, was found to be seven times longer than their spin-lattice relaxation time constant (T(LLS)/T(1) = 7). At desired intervals, small fractions of the populations of long-lived states were converted into observable magnetization. This opens the way to observing slow chemical reactions and slow transport phenomena such as diffusion by enhanced magnetic resonance.
Asunto(s)
Magnetismo , Dipéptidos/química , Imagen por Resonancia Magnética , Espectroscopía de Resonancia Magnética , Protones , Factores de TiempoRESUMEN
We report on a novel neutron radiography technique that uses the Ramsey principle, a method similar to neutron spin echo. For the first time quantitative imaging measurements of magnetic objects and fields could be performed. The strength of the spin-dependent magnetic interaction is detected by a change in the Larmor precession frequency of the neutron spins. Hence, one obtains in addition to the normal attenuation radiography image a so-called neutron spin phase image, which provides a two-dimensional projection of the magnetic field integrated over the neutron flight path.
RESUMEN
New low-temperature inserts compatible with an existing hyperpolarizer were developed to dynamically polarize nuclei in large samples. The performance of the system was tested on 8 ml glassy frozen solutions containing 13C-labeled molecules and doped with nitroxyl free radicals. The obtained 13C low-temperature polarization was comparable to the one measured on 20 times smaller sample volume with only 3-4 times higher microwave power. By using a dissolution insert that fits to the new design, it was possible to obtain about 120 ml of room-temperature hyperpolarized solution. The polarization as well as the molecule concentration was comparable to the values obtained in standard size hyperpolarized samples. Such large samples are interesting for future studies on larger animals and possibly for potential clinical applications.
Asunto(s)
Isótopos de Carbono/química , Isótopos de Carbono/aislamiento & purificación , Fraccionamiento Químico/métodos , Marcaje Isotópico/métodos , Espectroscopía de Resonancia Magnética/métodos , Soluciones/química , Soluciones/aislamiento & purificación , Electricidad EstáticaRESUMEN
Apart from their very classical use to build polarized targets for particle physics, the methods of dynamic nuclear polarization (DNP) have more recently found application for sensitivity enhancement in high-resolution NMR, both in the solid and in the liquid state. It is often thought that the possible signal enhancement in such applications deteriorates when the DNP is performed at higher fields. We show that for a dissolution-DNP method that uses conventional (2,2,6,6-tetramethylpiperidine 1-oxyl) radicals as the paramagnetic agent, this is not the case for fields up to 5 T.
Asunto(s)
Espectroscopía de Resonancia Magnética/métodos , Óxidos N-Cíclicos , Radicales Libres , Espectroscopía de Resonancia Magnética/instrumentación , MagnetismoRESUMEN
A measurement of the production of ultracold neutrons from velocity-selected cold neutrons on gaseous and solid deuterium targets is reported. The expected energy dependence for two-particle collisions with well defined neutron and Maxwell-Boltzmann distributed molecular velocities is found for the gas target. The solid target data agree in shape with the phonon density-of-states curve and provide strong evidence for the phonon model including multiphonon excitations.
RESUMEN
The total scattering cross sections for slow neutrons with energies in the range 100 neV to 3 meV for solid ortho-2H2 at 18 and 5 K, frozen from the liquid, have been measured. The 18 K cross sections are found to be in excellent agreement with theoretical expectations and for ultracold neutrons dominated by thermal up scattering. At 5 K the total scattering cross sections are found to be dominated by the crystal defects originating in temperature induced stress but not deteriorated by temperature cycles between 5 and 10 K.
RESUMEN
The total scattering cross sections for slow neutrons with energies E in the range 300 neV to 3 meV for gaseous and liquid ortho-2H2 have been measured. The cross sections for 2H2 gas are found to be in excellent agreement with both the Hamermesh and Schwinger and the Young and Koppel models. For liquid 2H(2), we confirm the existing experimental data in the cold neutron range and the discrepancy with the gas models. We find a clear 1 / square root[E'] dependence at low energies for both states. A simple explanation for the liquid 2H2 cross section is offered.
RESUMEN
Solid deuterium (sD2) will be used for the production of ultra-cold neutrons (UCN) in a new generation of UCN sources. Scattering cross sections of UCN in sD2 determine the source yield but until now have not been investigated. We report first results from transmission and scattering experiments with cold, very cold and ultra-cold neutrons on sD2 along with light transmission and Raman scattering studies showing the influence of the sD2 crystal properties.