The physics behind the isobar separation of 36Cl and 10Be at the French AMS national facility ASTER

The French AMS national facility ASTER, based on a 5 MV Tandetron accelerator, uses a degrader foil installed behind the focal plane of the 90°-high energy magnet for 10Be and 36Cl analyses. Ions passing the 1000 nm silicon nitride degrader foil are analyzed with a 35°-Electrostatic Analyser (ESA) and a 30°-magnet (vertical plane). The horizontal beam profiles were measured. For 30 MeV Cl ions, we observed a very narrow beam width, which is well described by a normal distribution. Corresponding scans of 9Be and 10B at 11.2 MeV, however, revealed much wider beam widths and showed tails on both sides that could not be described by a normal distribution. These non-normal distributions can be interpreted to result from the coulomb-explosion that occurs during dissociation of the BeO− or BO− molecules in the terminal. In an additional experiment, the energy loss in the silicon nitride foil and the associated energy straggling were measured and compared with models. Finally, charge state distributions were determined for 36Cl exiting the degrader foil at 23.9 and 19.8 MeV. These distributions are well described by Gaussian functions and the associated positions and widths agree quite well with published values for carbon foils.