Speaker
Description
The current method for disseminating the unit Tesla relies on NMR measurements on field polarized proton spins, using water [1]. However, the relative measurement uncertainties are restricted to values between $10^{-4}$ and $10^{-6}$ by limited SNR at fields below 2 mT and a rather insensitive absorption technique at fields above 10 mT. To overcome these limitations, we explore the use of hyperpolarized $^3$He gas, which offers field independent enhanced SNR in combination with longer precession times $T_2^*$ [2], thereby promising higher precision over a very wide magnetic field range. Using metastability exchange optical pumping (MEOP) to achieve hyperpolarization [3] promises fast buildup times and avoids interaction of the polarized alkali atoms on $^3$He precession and the application of complicated pulse sequences for sufficient decoupling thereof[4].
Our experimental setup $^3$He MEOP magnetometry includes a commercial table-top four-layer magnetic shield with integrated coils, enabling the generation of a constant B$_0$ field in the µT-range alongside a perpendicular, resonant B$_1$ field to initiate spin precession. Utilizing a commercial Rubidium (Rb) optical pumped magnetic gradiometer (OMG) with a 2.3 cm baseline, we have measured the $^3He$ magnetization precession, allowing us to deduce the Larmor frequency at varying B$_0$ magnetic field strengths. With the setup we determine polarization buildup times and T$_2^*$ relaxation times. By varying the flip angle, we show systematic effects of residual longitudinal magnetization in the $^3$He on the detected Larmor frequency. This allows conclusions to be drawn about the imperfect sphericity of the cell geometry.
To validate the precision of our setup, experiments are planned within a large magnetically shielded room, which offers a more stable magnetic field and enables the measurement with superconducting quantum interference devices (SQUIDs) for the $^3$He magnetization precession. Using SQUIDS facilitates higher SNR and measurements at lower B$_0$ fields compared to the Rb gradiometer. These investigations will compare the detectable B$_0$ field by $^3$He nuclei using different sensor types—OMG versus SQUID—enhancing our understanding of the influence of sensors on the precision of magnetic field metrology.
[1] K. Weyand, IEEE Trans. Inst. Meas. (2001) 50, 470 .
[2] C. Gemmel et al., Eur. Phys. J. D (2010) 57, 303.
[3] T. R. Gentile et al., Rev. Mod. Phys. (2017) 89, 045004.
[4] M. E. Limes et al., Phys. Rev. Lett. (2018) 120, 033401.
