The 23P0,1,2 fine structure interval of 4He can be determined to 10-8 accuracy both theoretically and experimentally. It can be used either to determine the fine structure constant or to test the quantum electrodynamics theory. To reach this goal, it is necessary to measure the fine structure splitting to sub kHz accuracy by increasing the signal-to-noise ratio and eliminating the systematic deviations. In the experimental configuration of present study, transverse laser cooling is used to obtain an intense metastable helium atom beam. The triple state metastable atoms are also bent from the original atomic beam to reduce the background noise. The spectral scanning will be accomplished by tuning the sideband of a frequency-locked diode laser to maintain sufficient frequency stability during the scan. The experimental method has been tested on the setup recently built, and the analysis shows that a sub-kHz precision is feasible.
Systematic error caused by stray magnetic fields is often one of the major sources of the uncertainties in many precision measurements. It is necessary to accurately control the magnetic fields in these measurements. The spectrum of the fine structure of helium can be used to determine the fine structure constant, and also to test the quantum electrodynamic theory of multi-electron systems. In this work, we present a magnetic shield that can eliminate stray magnetic fields and a cosine coil that generates a uniform magnetic field. The central magnetic strength produced by the coil is measured through the spectrum of a 4He metastable atomic beam. The magnetic shield reduces the strength of the external magnetic field down to less than 0.8 mGs. The variation of the magnetic strength in the spectrum region is less than 10 mGs at a magnetic strength up to 20 Gs. In this case, the uncertainty introduced by the magnetic field will be less than in the spectrum of helium fine structure.
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