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

• Published in: arXiv.org 2015-09
• Main Authors: Maul, Andreas, Blümler, Peter, Heil, Werner, Nikiel, Anna, Otten, Ernst, Petrich, Andreas, Schmidt, Thomas
• Format: Article
• Language: English
• Subjects: Bonding; Computer simulation; Finite element method; Fused silica; Helium; Hemispheres; Magnetic fields; Magnetic measurement; Magnetometers; NMR; Nuclear magnetic resonance; Signal strength; Silicon dioxide
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1508.00838

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.