Dielectronic recombination of Pb53+ has been studied and a resonance is det
ected only similar to 10(-4) eV above the ground state. The possibility to
determine the Pb53+ (4p(1/2)) - Ph53+ (4s(1/2)) energy splitting with a sim
ilar accuracy from the determination of the resonance position is discussed
. Such a precision can only be achieved by calculations which treat quantum
electrodynamics (QED) in a many-body environment at a level which can stil
l not be reached. A fully relativistic many-body calculation of the splitti
ng is described and the uncertainties are discussed.