Direct measurement of the 3 He + magnetic moments.

Helium-3 has nowadays become one of the most important candidates for studies in fundamental physics ^1-3 , nuclear and atomic structure ^4,5 , magnetometry and metrology ⁶ , as well as chemistry and medicine ^7,8 . In particular, ³ He nuclear magnetic resonance (NMR) probes have been proposed as a new standard for absolute magnetometry ^6,9 . This requires a high-accuracy value for the ³ He nuclear magnetic moment, which, however, has so far been determined only indirectly and with a relative precision of 12 parts per billon ^10,11 . Here we investigate the ³ He ⁺ ground-state hyperfine structure in a Penning trap to directly measure the nuclear g-factor of ³ He ⁺ [Formula: see text], the zero-field hyperfine splitting [Formula: see text] Hz and the bound electron g-factor [Formula: see text]. The latter is consistent with our theoretical value [Formula: see text] based on parameters and fundamental constants from ref. ¹² . Our measured value for the ³ He ⁺ nuclear g-factor enables determination of the g-factor of the bare nucleus [Formula: see text] via our accurate calculation of the diamagnetic shielding constant ¹³ [Formula: see text]. This constitutes a direct calibration for ³ He NMR probes and an improvement of the precision by one order of magnitude compared to previous indirect results. The measured zero-field hyperfine splitting improves the precision by two orders of magnitude compared to the previous most precise value ¹⁴ and enables us to determine the Zemach radius ¹⁵ to [Formula: see text] fm.
(© 2022. The Author(s).)