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 TLLS associated with the two nonequivalent Hα glycine protons, sustained by suitable radio-frequency irradiation, was found to be seven times longer than their spin-lattice relaxation time constant (TLLS/T₁ = 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.
The use of polarized protons as neutron spin filter is an attractive alternative to the well established neutron polarization techniques, as the large, spin-dependent neutron scattering cross-section ...for protons is useful up to the sub-MeV region. Employing optically excited triplet states for the dynamic nuclear polarization (DNP) of the protons relieves the stringent requirements of classical DNP schemes, i.e low temperatures and strong magnetic fields, making technically simpler systems with open geometries possible.
Using triplet DNP a record polarization of 71% has been achieved in a pentacene doped naphthalene single crystal at a field of 0.36T using a simple helium flow cryostat for cooling. Furthermore, by placing the polarized crystal in a neutron optics focus and de-focus scheme, the actual sample cross-section could be increased by a factor 35 corresponding to an effective spin filter cross-section of 18×18mm2.
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.
A novel neutron spin filter whose principle is based on the strong spin dependence of the neutron scattering on protons has been developed. The dimensions of this filter are small, and it works very ...efficiently and is stable even in inhomogeneous fields. The protons in the naphthalene spin filter crystal are polarized by a recently developed method of dynamic nuclear polarization using photoexcited triplet states. This technique allows the design of a very compact apparatus that can be placed at a close distance to the sample under investigation. The application of this filter as a polarization analyzer is demonstrated in a magnetic small‐angle neutron scattering experiment with the measurement of the spin‐dependent scattering signals of a CuFeNi alloy. This sample has a pronounced textured structure factor of ferromagnetic precipitates in a paramagnetic matrix. The performance of the spin filter as an analyzer is illustrated by the excellent agreement of the experimental data with simulations based on a model of homogeneously magnetized spherical particles which are ordered in a simple cubic paracrystalline lattice.
In dynamic nuclear polarisation (DNP), also called hyperpolarisation, a small amount of unpaired electron spins is added to the sample containing the nuclear spins, and the polarisation of these ...unpaired electron spins is transferred to the nuclear spins by means of a microwave field. Traditional DNP polarises the electron spin of stable paramagnetic centres by cooling down to low temperature and applying a strong magnetic field. Then weak continuous wave microwave fields are used to induce the polarisation transfer. Complicated cryogenic equipment and strong magnets can be avoided using short-lived photo-excited triplet states that are strongly aligned in the optical excitation process. However, a much faster transfer of the electron spin polarisation is needed and pulsed DNP methods like nuclear orientation via electron spin locking (NOVEL) and the integrated solid effect (ISE) are used.
To describe the polarisation transfer with the strong microwave fields in NOVEL and ISE, the usual perturbation methods cannot be used anymore. In the previous paper, we presented a theoretical approach to calculate the polarisation transfer in ISE. In the present paper, the theory is applied to the system naphthalene-h
8
doped with pentacene-d
14
yielding the photo-excited triplet states and compared with experimental results.
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.
A polarized neutron Laue diffraction experiment on a single crystal of neodymium‐doped lanthanum magnesium nitrate hydrate containing polarized proton spins is reported. By using dynamic nuclear ...polarization to polarize the proton spins, it is demonstrated that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal‐to‐noise ratio in neutron diffraction patterns of hydrogenous matter.
The PSI ultra-cold neutron source Anghel, A.; Atchison, F.; Blau, B. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2009, Letnik:
611, Številka:
2
Journal Article
Recenzirano
A new type of ultra-cold neutron (UCN) source based on the spallation process is under construction at PSI. The essential elements are a pulsed proton beam with highest intensity (
I
p
≥
2
mA
) and a ...low duty cycle (1%), a lead spallation target, a large
D
2
O
moderator and a solid deuterium (
sD
2
) converter system. Spallation neutrons are thermalized in the
D
2
O
, further cooled and partially downscattered into the ultra-cold neutron range in the
sD
2
. The expected UCN density is higher than
1000
UCN
/
cm
3
in typical experiments, an increase of almost two orders of magnitude over the best source currently available (at ILL). The new UCN source is expected to come into operation in 2009.
In a test of principle a neutron spin filter has been built, which is based on dynamic nuclear polarization (DNP) using photo-excited triplet states. This DNP method has advantages over classical ...concepts as the requirements for cryogenic equipment and magnets are much relaxed: the spin filter is operated in a field of 0.3T at a temperature of about 100K and has performed reliably over periods of several weeks.
The neutron beam was also used to analyze the polarization of the target employed as a spin filter. We obtained an independent measurement of the proton spin polarization of ∼0.13 in good agreement with the value determined with NMR. Moreover, the neutron beam was used to measure the proton spin polarization as a function of position in the naphthalene sample. The polarization was found to be homogeneous, even at low laser power, in contradiction to existing models describing the photo-excitation process.
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