Development and simulation of a pumping barrier for the Penning trap mass spectrometer SHIPTRAP

Penning traps are ideal tools to perform high-precision mass measurements. For this purpose the cyclotron frequency of the stored charged particles is measured. In case of short-lived radionuclides the ions get in general first prepared and cooled by buffer-gas collisions in a preparation trap to reduce their motional amplitude and are then transported to a precision trap for the cyclotron frequency determination. In the double Penning trap mass spectrometer SHIPTRAP at GSI the traps are placed in the homogeneous region of one superconducting magnet with a distance of only 20 cm to avoid transport losses. Because the gas pressure inside the measurement trap has to be very low in order to minimize the damping of the ion motion with the rest gas molecules while the frequency determination, a pumping barrier is installed between both traps. This barrier consists of a 47 mm long channel with an inner diameter of either 1.5, 2 or 3mm. To predict the pressure difference between the two traps in the region of molecular gas flow the movement of each particle can be simulated without consideration of the other particles. Thus it is possible to calculate the transit probability through a tube of a given geometry. The results are compared with the experimentally obtained pressure differences