SMILETRAP—A Penning trap facility for precision mass measurements using highly charged ions

The precision of mass measurements in a Penning trap increases linearly with the charge of the ion. Therefore we have attached a Penning trap, named SMILETRAP, to the electron beam ion source CRYSIS at MSL. CRYSIS is via an isotope separator connected to an ion source that can deliver singly charged ions of practically any element. In CRYSIS charge state breeding occurs by intense electron bombardment. We have shown that it is possible to produce, catch and measure the cyclotron frequencies of ions in the charge region 1+ to 52+. The relevant observable in mass measurements using a Penning trap is the ratio of the cyclotron frequencies of the ion of interest and ion used as a mass reference. High precision requires that the two frequencies are measured after one another in the shortest possible time. For reasons of convenience the precision trap operates at room temperature. So far it has been believed that warm traps working at 4 K are required for high mass precision with exactly one ion in the trap at a time. In this paper we demonstrate that mass precision of a few parts in 1010 also can be obtained in a warm trap at a pressure of about 5×10−12 mbar by stabilizing the pressure in the He-dewar, the trap temperature and the frequency synthesizer. In order to reduce the influence of changes of the magnetic field to a level below 10−10, the scanning of the frequencies close to the resonances of both the ion of interest and the reference ion is done in a total time <2 min. Trapping of ions is a statistical procedure, allowing more than one ion to be trapped in each measurement cycle. However, after completing the measurements it is possible to reject all information except for events based on 1 and 2 trapped ions. The procedures of producing, transporting, catching, exciting and measuring the cyclotron resonance frequencies of highly charged ions and the mass reference ions with the time-of-flight method are described. In routine measurements with 1 s excitation time lasting for about 24 h, atomic masses can be determined at an uncertainty of about 1 pbb. In the case of q/A doublet measurements a mass uncertainty close to 0.1 ppb can be obtained as illustrated by a mass measurement of 4He2+. The mass measurements so far performed are either related to fundamental constants or to masses the accuracy of which is needed for some current questions in physics.