High field Enhancement of nucLear Polarisation In Noble Gases – HELPING

Experimental, computational, and theoretical investigations will be combined to advance current knowledge about hyperpolarisation at high field in alkali vapours (SEOP) and in gas discharges (MEOP, PAMP). This requires the development of suitable measurement tools, prototype devices, predictive models, and numerical simulations.
Experiments will be performed in a super-wide bore 7 T NMR spectrometer/imager, recently purchased by both partners. The instrument benefits from an advanced architecture and a broadband rf equipment suited for multi-channel spectroscopy and 3D imaging. The bore size allows for the design of a variety of prototype devices as well as for the implementation of several elements for complementary physical diagnoses.
For quantitative analysis, various experimental parameters must be specified. NMR measurements will be combined with optical diagnosis and monitoring of orientation. NMR yields straightforward access to the dynamics of nuclear polarisation build-up and decay. Laser probes can be used to measure atomic number densities of relevant optically active species or to measure the electronic orientation, for instance.
PAMP only requires excitation of the 3He gas by a strong discharge. MEOP typically involves a weak rf excitation that promotes some He atoms to a long-lived (metastable) excited energy level, where these minority atoms can absorb resonant infrared light from the optical pumping laser beam at 1083 nm. A 1083 nm probe beam can be used for measurements of the 3He gas polarisation thanks to a strong coupling between metastable and ground state atom angular momenta, enforced by frequent metastability exchange collisions. The use of sealed glass cells with various noble gas contents is planned.
Valved glass containers connected to a gas handling system are preferred for SEOP studies. The method requires heating and temperature control of the alkali number density. Operating conditions depend on several parameters such as the noble gas pressure, the type and number density of alkali atoms present in the vapour, or the presence/absence of buffer gas. NMR tools will be used to monitor the noble gas polarisation, optical tools to measure alkali number densities and polarisation.
The development of predictive models and of optimal experimental protocols will be based on the obtained body of data and knowledge.

The main results are announced on the project web site (short link: www.lkb.science/helping/)
They are reported in scientific publications and communications. Contents is shared through deposits in the open archive HAL.

Hyperpolarisation techniques are relevant for all areas where an enormous boost in nuclear polarisation is desirable. This primarily includes basic and applied physics but, also, all fields in research (and beyond) where NMR imaging or analytical tools are used. Hyperpolarisation is actually a major route to higher NMR sensitivity that advantageously complements, and sometimes substitutes for, the use of high magnetic field strength and sophisticated detection systems.
The super-wide bore 7 T NMR system installed at NIMBE has a rather unusual combination of attractive features, on which the project capitalises: an extra-large bore size, equipment for multi-channel operation, and a dual capability for spectroscopy and for imaging that meets the standards of both types of instruments. It is devoted to exploration of new concepts and innovative applications.
The findings will hopefully provide educated guidance for further upgrade of high-field HP devices. The prototype devices built during the project should already facilitate high-field NMR work. They might open new perspectives for cutting-edge NMR research, as described in the project web site.

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Nuclear Magnetic Resonance (NMR) is a sophisticated and powerful analytical method with numerous applications in research, health care, and industry. However, its use is limited by the low intrinsic sensitivity of inductive detection (owing to a low energy difference between nuclear spin states, even above 1 Tesla) and by the low polarisation that is established at thermodynamic equilibrium (i.e., the relative difference in populations for states with opposite spin orientations). Hyperpolarisation is relevant when a strong boost in sensitivity is needed. Polarisation enhancement factors can reach several orders of magnitude. This has paved the way to advances in science and applications.
Laser-polarised noble gases have found applications in functional lung imaging and biomedicine, as well as in basic research. Optical pumping techniques have been extensively studied at low magnetic field (1 - 10 mT). But detection of the NMR signal at high field is usually needed, for increased sensitivity (thanks to operation at high frequency) and spectral resolution (large chemical shifts). It allows a more detailed analysis of the system, at the expense of polarisation losses due to gas handling and transfer into the NMR apparatus.
Hyperpolarisation of noble gases at high magnetic field lies at the heart of the HELPING project. Scarcely explored up to now, it is of fundamental interest (it can shed light on polarisation build-up and decay processes) and it can help reduce, or even eliminate, the dead time for gas transportation in practice. New features are expected, owing to field-dependent atomic level structures, collisional interactions, and nuclear relaxation processes. Overall optimisation of polarised gas delivery also requires meeting inherent technical constraints (bore size, magnetic susceptibility of materials).
The HELPING project mainly deals with hyperpolarisation of spin-1/2 atoms, 129Xe et 3He, and includes tests with quadrupolar isotopes, 21Ne (spin 3/2) et 83Kr (spin 9/2). Xenon strongly interacts with its surrounding and 129Xe exhibits a wide range of frequency chemical shifts that are particularly relevant for NMR spectroscopy. Helium, smaller in size, interacts weakly with nearby atoms hence 3He has low nuclear relaxation rates; it is also an excellent gas probe for systems with nanoscale void spaces. 21Ne et 83Kr can both provide additional information in NMR studies in various materials or media, through data that are more easily interpreted (21Ne) or more sensitive to surface properties (83Kr).
The HELPING project relies on in-depth studies, with combined optical and NMR measurements, that are made possible by the recent implementation, at CEA Saclay, of a purchased 7 T NMR spectrometer/imager. It capitalises on the almost unique features of the instrument (a super-wide bore magnet, ratings for high quality MRS and 3-axes MRI, multi-channel detection). Work will focus, first, on hardware developments for noble gas hyperpolarisation in, or very close to, the measurement area. It will, then, consist in experimental investigations aiming at:
1- In-depth study of spin exchange optical pumping (SEOP) of 129Xe in alkali vapours and SEOP tests with 83Kr,
2- Assessment of the limits of metastability exchange optical pumping (MEOP) in pure 3He, extension to 3He-4He gas mixtures, and MEOP tests at cryogenic temperatures,
3- In-depth study of a new (non-optical) hyperpolarisation scheme, recently discovered in 3He gas discharges, called PAMP (Polarisation of Atoms in a Magnetised Plasma).
Computational models and simulations will be jointly developed to rationalise experimental results and propose novel sets of experimental parameters that may allow gas delivery with optimal nuclear polarisation or magnetisation (the product of polarisation and atom number density) for the selected isotopes. Finally, work will be applied to NMR characterisation of porous materials and magnetometry.