On the Masses of28Si and the Proton Determined in a Penning Trap

The masses of the three silicon isotopes are important parameters for a re-definition of the kilogram based on atomic properties. Using the Penning trap mass spectrometer SMILETRAP at the Manne Siegbahn Laboratory the mass of 28 Si was determined from the cyclotron frequencies of 28 S 14+ , 28 S 13+ and 28 S 12+ using 12 C 6+ , 12 C 5+ and 12 C 4+ as well as H 2 + as mass references. Since H 2 + ions were used as intermediate mass reference the mass of the proton could be determined using in addition 20 Ne 10,9+ and 40 Ar 16+,14+ ions as mass references. By comparing the value of the proton mass determined this way with the accurate accepted value we could set an experimental limit of 0.35 ppb for the total uncertainties in our mass determinations. A value of 27.976 926 533.9 (80) u was obtained for the mass of 28 Si confirming the more accurate value obtained by the MIT-group using an entirely different method. Thus the mass of 28 Si has been determined by using two very different methods agreeing within an uncertainty <0.35 ppb.