Sealed-Off Hg^198 Atomic-Beam Light Source

Abstract

A sealed-off atomic-beam light source which utilizes the single isotope Hg198 is described. The emitted 2537 A line was investigated interferometrically with Fabry-Perot interferometers. Interferograms are shown for retardations of 0.4, 1.53, and 2.04 m with order numbers 1.6, 6.0, and 8.1 million, respectively. For each retardation, the theoretical contour of the observed fringes is shown. Theoretically predicted fringe contours are shown for retardations up to 6 m, the approximate limit of interference. It is concluded from the interferograms that the Hg198 2537 A line has a half-width of 0.0016 cm−1, as compared to 0.012 cm−1 for the Kr86 6056 A line proposed as the new primary standard of length. Owing to the small half-width and the extremely low level of pertubation in the atomic beam, this Hg198 line would be suitable for the primary standard of length.