This article reviews the physics and technology of producing large quantities of highly spin-polarized 3He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping. Both technical developments and deeper understanding of the physical processes involved have led to substantial improvements in the capabilities of both methods. For SEOP, the use of spectrally narrowed lasers and K-Rb mixtures has substantially increased the achievable polarization and polarizing rate. For MEOP nearly lossless compression allows for rapid production of polarized 3He and operation in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and revealed new phenomena. Both methods have benefitted from development of storage methods that allow for spin-relaxation times of hundreds of hours, and specialized precision methods for polarimetry. SEOP and MEOP are now widely applied for spin-polarized targets, neutron spin filters, magnetic resonance imaging, and precision measurements.

Optically polarized 3 He

We review experimental techniques in our laboratory for nuclear magnetic resonance (NMR) in zero and ultralow magnetic field (below 0.1 μT) where detection is based on a low-cost, non-cryogenic, spin-exchange relaxation free 87Rb atomic magnetometer. The typical sensitivity is 20-30 fT/Hz1/2 for signal frequencies below 1 kHz and NMR linewidths range from Hz all the way down to tens of mHz. These features enable precision measurements of chemically informative nuclear spin-spin couplings as well as nuclear spin precession in ultralow magnetic fields.

Invited Review Article: Instrumentation for nuclear magnetic resonance in zero and ultralow magnetic field

https://cds.cern.ch/record/2310268/files/1803.07547.pdf

High magnetic fields for fundamental physics

The Baryon Antibaryon Symmetry Experiment (BASE) aims at performing a stringent test of the combined charge parity and time reversal (CPT) symmetry by comparing the magnetic moments of the proton and the antiproton with high precision. Using single particles in a Penning trap, the proton/antiproton g-factors, i.e. the magnetic moment in units of the nuclear magneton, are determined by measuring the respective ratio of the spin-precession frequency to the cyclotron frequency. The spin precession frequency is measured by non-destructive detection of spin quantum transitions using the continuous Stern-Gerlach effect, and the cyclotron frequency is determined from the particle*s motional eigenfrequencies in the Penning trap using the invariance theorem. By application of the double Penning-trap method we expect that in our measurements a fractional precision of δg/g 10−9 can be achieved. The successful application of this method to the antiproton will consist a factor 1000 improvement in the fractional precision of its magnetic moment. The BASE collaboration has constructed and commissioned a new experiment at the Antiproton Decelerator (AD) of CERN. This article describes and summarizes the physical and technical aspects of this new experiment.

/pdf/base-the-baryon-antibaryon-symmetry-experiment-t0e683ivtb.pdf

BASE - The Baryon Antibaryon Symmetry Experiment

We present a laser-based atomic magnetometer that allows inferring the modulus of a magnetic field from the free Larmor precession of spin-oriented Cs vapour atoms. The detection of free spin precession (FSP) is not subject to systematic readout errors that occur in phase feedback-controlled magnetometers in which the spin precession is actively driven by an oscillating field or a modulation of light parameters, such as frequency, amplitude, or polarization. We demonstrate that an FSP-magnetometer can achieve a ∼200 fT/√Hz sensitivity (<100 fT/√Hz in the shotnoise limit) and an absolute accuracy at the same level.

/pdf/a-sensitive-and-accurate-atomic-magnetometer-based-on-free-gqbkf07ltv.pdf

A sensitive and accurate atomic magnetometer based on free spin precession

We describe a 3He
magnetometer capable to measure high magnetic fields (B> 0.1 T) with a
relative accuracy of better than 10-12. Our approach is based on the measurement of the free
induction decay of gaseous, nuclear spin polarized 3He following a resonant radio
frequency pulse excitation. The measurement sensitivity can be attributed to the long
coherent spin precession time T2
 ∗ being of order minutes which is achieved for spherical
sample cells in the regime of “motional narrowing” where the disturbing influence of field
inhomogeneities is strongly suppressed. The 3He gas is spin polarized in situ using a new,
non-standard variant of the metastability exchange optical pumping. We show that
miniaturization helps to increase T2
 ∗ further and that the measurement sensitivity is not
significantly affected by temporal field fluctuations of order 10-4.

Ultrasensitive 3He magnetometer for measurements of high magnetic fields

High magnetic fields (> 1 T) are measured by NMR magnetometers with un-rivaled precision if the precessing spin sample provides long coherence times. The longest coherence times are found in diluted ${}^{3}$He samples, which can be hyperpolarized for sufficient signal strength. In order to have minimal influence on the homogeneity and value of the measured magnetic field the optimal container for the ${}^{3}$He should be a perfect sphere. A fused silica sphere with an inner diameter of 8 mm and an outer diameter of 12 mm was made from two hemispheres by diffusion bonding leaving only a small hole for cleaning and evacuation. This hole was closed in vacuum by a CO${}_{2}$ laser and the inner volume was filled with a few mbars of ${}^3$He via wall permeation. NMR-measurements on such a sample had coherence times of 5 min. While the hemispheres were produced with < 1 $\mu$m deviation from sphericity, the bonding left a step of ca. 50 $\mu$m at maximum. The influence of such a mismatch, its orientation and materials in the direct vicinity of the sample are analyzed by FEM-simulations and discussed in view of coherence times and absolute fields.

Spherical Fused Silica Cells Filled with Pure Helium for NMR-Magnetometry

High magnetic fields (>1 T) are measured by NMR magnetometers with unrivaled precision if the precessing spin sample provides long coherence times. The longest coherence times are found in diluted (3)He samples, which can be hyperpolarized for sufficient signal strength. In order to have minimal influence on the homogeneity and value of the measured magnetic field, the optimal container for the (3)He should be a perfect sphere. A fused silica sphere with an inner diameter of 8 mm and an outer diameter of 12 mm was made from two hemispheres by diffusion bonding leaving only a small hole for cleaning and evacuation. This hole was closed in vacuum by a CO2 laser and the inner volume was filled with a few mbars of (3)He via wall permeation. NMR-measurements on such a sample had coherence times of 5 min. While the hemispheres were produced with <1 μm deviation from sphericity, the bonding left a step of ca. 50 μm at maximum. The influence of such a mismatch, its orientation, and the immediate environment of the sample is analyzed by FEM-simulations and discussed in view of coherence times and absolute field measurements.

Anna Nikiel

Papers

Ultrasensitive 3He magnetometer for measurements of high magnetic fields

Spherical Fused Silica Cells Filled with Pure Helium for NMR-Magnetometry

Spherical fused silica cells filled with pure helium for nuclear magnetic resonance-magnetometry