Precision measurement of the H-like X-ray spectrum of Cl and the 1s Lamb shift

Abstract
The H-like spectrum of chlorine emitted from the Alcator C tokamak has been measured using a Bragg crystal X-ray spectrometer and in situ wavelength calibrations. The 1s-2p transition energies were determined to an accuracy of 34 p.p.m. allowing for a test of the theoretically calculated Lamb shift of the 1s level in Cl16+ at the 12% level. This new technique of spectroscopy of highly ionised atoms could be further refined to approach the 5 p.p.m. accuracy level in the absolute transition energy determination.