An RFQ Cooler – Mass Filter – Buncher System for Experiments with Fusion-Evaporation Products∗

At the Penning trap facility SHIPTRAP mass measurements of transfermium elements as well as nuclides in the vicinity of the proton drip-line have been performed [1, 2]. Going towards even more exotic proton-rich elements, the SHIPTRAP experiment is hampered by the abundant unwanted reaction products. In order to quickly identify the ions produced, to select the nuclei of interest out of all fusion products and to increase the sensitivity of the experiment, an RFQ mass filter has been developed in a matched combination with an RFQ cooler and RFQ buncher. Instrumental developments. The RFQ cooler, mass filter and buncher have been designed [3, 4] and built, see figure 1. The mass filter is constructed out of round rods with a diameter of 18 mm and uses Brubaker lenses [5] at its entrance and exit. Special care has been taken to assure that the cooler and mass filter have the best possible alignment, in order to avoid a degradation in resolution and transmission. A control system for operation of the mass filter at a fixed working point (mass selection), in 1D scanning mode (acquisition of mass spectrum) or 2D scanning mode (mapping of stability diagram) has been developed. The RF voltages are generated by high-quality resonance circuits. Since the RF amplitude is easily affected by e.g. temperature changes and the stability of RF amplitude has a direct impact on the achievable resolution and transmission, an effort has been made to stabilize the RF voltages to the 0.1 % level. Electronic circuits for precise measurement of the RF amplitude has been developed, and a PID controller has been implemented to the control program. The RFQ buncher consists of a 40 cm cooling section, in which the axial field is created by additional DC rods [6], and a trapping section consisting of 5 segments. These allow the user to increase the efficiency by implementing a pretrap.