We have developed an apparatus which allows measuring the sticking cross section of a neutral atom onto a mass selected charged cluster of known temperature. The main point is to reduce the kinetic energy dispersion in the mass selected cluster beam in order to work with ions of very low (near thermal) kinetic energy. A novel device is presented which focuses in energy with only a small loss in the beam intensity. An application is shown to the sticking of sodium atoms onto sodium clusters at an energy of a few tenths of an eV in the center-of-mass frame.