Schematics of the DESIREE storage rings and the method to determine the final states in MN reactions. The left panel shows an overview of the two storage rings and the common merging section, where MN reactions may be studied with precise control of the center-of-mass collision energy. The imaging detector, marked as “ID”, provides information on the separation (r) of the two neutrals formed in MN reactions, as shown in the upper-right zoom-in. The r value depends on the kinetic energy release (E_KER) in the MN reaction, which is related to the excitation energy (E_exc) of the neutral products through the potential energy curves for the initial and final states shown schematically in the lower-right panel.