42 research outputs found
Development of Triplet Dynamic Nuclear Polarization for Polarization Analysis in Small-Angle Neutron Scattering
Get PDFWe present the status of the development of a novel neutron spin filter based on the dynamic nuclear polarization (DNP) of protons in a naphthalene single crystal that uses highly polarized optically excited triplet states of pentacene as the polarizing agent (PA). The filter is applied as spin analyzer in small-angle neutron scattering (SANS) experiments to study magnetism.
In order to improve the spin filter performance, a better understanding of the electron polarization creation was essential. For this purpose, careful light absorption measurements have been performed an a theory developed to describe the light propagation and absorption in the biaxial anisotropic absorptive pentacene:naphthalene single crystal and the subsequent triplet production of pentacene molecules. The DNP build-up in a crystal of given size can now be simulated and optimized by the proper choice of the experimental parameters, e.g. the type of excitation light source or the pentacene dopant concentration. As a result 80% proton polarization can now be routinely achieved, close to the theoretical maximum, with extremely long relaxation times. This significantly improved the figure of merit of the spin filter and furthermore allowed to implement a new scheme of filter operation that greatly facilitates its operation in the environment of a large-scale neutron scattering facility. We have made the device transportable, i.e. the filter is conveniently polarized under optimum conditions in the laboratory and then transferred to the neutron beam line where it can be operated during several days with practically frozen polarization while requiring only a minimum of equipment. This saves cost on instrumentation, beam time and work.
These improvement allowed to apply the spin filter as neutron polarization analyzer in a series of polarized SANS experiments to study magnetism on the nano-scale. These studies focused on an exotic and elusive physical phenomena – the defect-induced Dzyaloshinskii- Moriya interaction (DMI) in a nanocrystalline two-phase alloy Fe73Si16B7Nb3Cu1. An asymmetric signal is observed in the difference between the two spin-flip cross sections, which is a key signature directly related to the DMI. The result supports the generic relevance of the DMI for the magnetic structure of defect-rich ferromagnets.
Two additional studies are presented that do not directly relate to the spin filter subject but further exploit the unique properties of highly polarized proton spin systems with extremely long relaxation times that now can be prepared as a result of all the optimized processes. The first addresses a fundamental issue of DNP that storage and transport of hyperpolarized samples is severely restricted. A procedure and equipment is presented to transport polarized samples over long distance and provide hyperpolarized nuclear spins to users that are not in the possession of DNP equipment. The second studies the long-range nuclear magnetic ordering (ferromagnetic or antiferromagnetic) that is created by adiabatic demagnetization in the rotating frame. We focus on the antiferromagnetic nuclear spin configuration of hydrogen nuclei in a naphthalene single crystal, similar to the antiferromagnetic structure of electron spins ordered by the Heisenberg exchange interaction
A Ramsey apparatus for proton spins in flowing water
Get PDFWe present an apparatus that applies Ramsey's method of separated oscillatory
fields to proton spins in water molecules. The setup consists of a water
circuit, a spin polarizer, a magnetically shielded interaction region with
various radio frequency elements, and a nuclear magnetic resonance system to
measure the spin polarization. We show that this apparatus can be used for Rabi
resonance measurements and to investigate magnetic and pseudomagnetic field
effects in Ramsey-type precision measurements with a sensitivity below 100 pT
Defect-induced Dzyaloshinskii–Moriya interaction in a nanocrystalline two-phase alloy
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A novel broad-band neutron spin filter based on dynamically polarized protons using photo-excited triplet states
Get PDFThe use of polarized protons as a broad-band neutron spin filter is an attractive alternative to the well-established neutron polarization techniques, namely polarized 3He gas and super mirrors, since the spin-dependent neutron proton scattering cross-section is large in a broad wavelength range. We have developed a novel neutron spin filter where we create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) that uses photo-excited triplet states. This requires only moderate experimental means and allows a compact design. In order to quantify the efficiency of the spin filter, we have measured the relevant spin-dependent and spin-independent terms of the neutron scattering cross-section of a naphthalene single crystal. The data allows to estimate the triplet spin filter performance over a broad wavelength range. With the recently achieved proton polarization of 80% the triplet filter compares well with a state of the art 3He filter
A novel broad-band neutron spin filter based on dynamically polarized protons using photo-excited triplet states
No full textThe use of polarized protons as a broad-band neutron spin filter is an attractive alternative to the well-established neutron polarization techniques, namely polarized 3He gas and super mirrors, since the spin-dependent neutron proton scattering cross-section is large in a broad wavelength range. We have developed a novel neutron spin filter where we create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) that uses photo-excited triplet states. This requires only moderate experimental means and allows a compact design. In order to quantify the efficiency of the spin filter, we have measured the relevant spin-dependent and spin-independent terms of the neutron scattering cross-section of a naphthalene single crystal. The data allows to estimate the triplet spin filter performance over a broad wavelength range. With the recently achieved proton polarization of 80% the triplet filter compares well with a state of the art 3He filter
Impact of the neutron-depolarization effect on polarized neutron scattering in ferromagnets
No full textIt has been known for decades that a ferromagnetic sample can depolarize a transmitted neutron beam. This effect was used and developed into the neutron-depolarization technique to investigate the magnetic structure of ferromagnetic materials. Since the polarization evolves continuously as the neutrons move through the sample, the initial spin states on scattering will be different at different depths within the sample. This leads to a contamination of the measured spin-dependent neutron-scattering intensities by the other spin-dependent cross sections. The effect has rarely been considered in polarized neutron-scattering experiments even though it has a crucial impact on the observable signal. A model is proposed to describe the depolarization of a neutron beam traversing a ferromagnetic sample, provide the procedure for data correction and give guidelines to choose the optimum sample thickness. It is experimentally verified for a small-angle neutron-scattering geometry with samples of the nanocristalline soft-magnet Vitroperm (Fe73Si16B7Nb3Cu1). The model is general enough to be adapted to other types of neutron-diffraction experiments and sample geometries
Automated transfer and injection of hyperpolarized molecules with polarization measurement prior to in vivo NMR
No full textInternational audienceyperpolarized magnetic resonance via dissolution dynamic nuclear polarization necessitates the transfer of thehyperpolarized molecules from the polarizer to the imager prior to in vivo measurements. This process leads to unavoid-able losses in nuclear polarization, which are difficult to evaluate once the solution has been injected into an animal. Wepropose a method to measure the polarization of the hyperpolarized molecules inside the imager bore, 3 s following dis-solution, at the time of the injection, using a precise quantification of the infusate concentration. This in situ quantifica-tion allows for distinguishing between signal modulations related to variations in the nuclear polarization at the time ofthe injection and signal modulations related to physiological processes such as tissue perfusion. In addition, our methodincludes a radical scavenging process that leads to a minor reduction in sample concentration and takes place within acouple of seconds following the dissolution in order to minimize the losses due to the presence of paramagnetic polar-izing agent in the infusate. We showed that proton exchange between vitamin C, the scavenging molecule and the deu-terated solvent shortens the long carboxyl13C longitudinal relaxation time in [1-13C]acetate. This additional source ofdipolar relaxation can be avoided by using deuterated ascorbate. Overall, the method allows for a substantial gain inpolarization and also leads to an extension of the time window available for in vivo measurements
Ultra High-Resolution NMR: Sustained Induction Decays of Long-Lived Coherences
No full textLong-lived coherences (LLCs) in homonuclear pairs of chemically inequivalent spins can be excited and sustained during protracted radio frequency irradiation periods that alternate with brief windows for signal observation. Fourier transformation of the sustained induction decays recorded in a single scan yields NMR spectra with line-widths in the range 10 < Delta v < 100 mHz, even in moderately inhomogeneous magnetic fields. The resulting doublets, which are reminiscent of J-spectra, allow one to determine the sum of scalar and residual dipolar interactions in partly oriented media. The signal intensity can be boosted by several orders of magnitude by "dissolution" dynamic nuclear polarization (DNP).LRM
Dynamic Nuclear Polarization using short-lived photo-excited triplet states: experiments and applications
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