Key points are not available for this paper at this time.
A 303-MHz hyperfine transition in the ground state of 9Be+ was used as a basis of a frequency standard. The ions were stored in a Penning ion trap. Linewidths as narrow as 900μHz were obtained. The frequency stability was measured to be better than 3 x 10 -12 τ -1/2 . The inaccuracy in the second-order .Doppler shift was reduced to 5 parts in 10 15 by laser cooling. An apparent pressure shift with an unexpectedly large value was discovered which limits the accuracy of the current experiment to approximately 1 part in 10 13 .
Bollinger et al. (Mon,) studied this question.