Absolute gravimeter helium immersion experiment

Operating an FG-5 absolute gravimeter in a helium atmosphere should reduce the noise that is caused by vibrations of the air-vacuum interface where the interferometer laser beam enters and leaves the vacuum dropping chamber. The helium atmosphere will decrease the change in refractivity across the interface by up to 88% over the value in air, depending on the purity of the helium. A marked reduction in the post-fit residuals is observed at frequencies of about 50 Hz and little or no effect at higher frequencies. A qualitative explanation of the frequency cutoff in terms of motion of air under compression is developed. Further experiments to measure and characterize the effects of the vibrations are planned.[PUBLICATION ABSTRACT]