A validity test of E = m.c2

The comparison of mass and energy variation in a nuclear reaction allows an experimental verification of Einstein's energy - mass equivalence principle. Mass measurements are performed in a high precision Penning trap and yield values in unified atomic mass units. The energies of emitted gamma radiation are determined via Laue-diffraction with perfect crystals. The according values of the gamma ray wave lengths are expressed in units of the crystal lattice constant. The comparison of masses and wave lengths requires a conversion factor, which represents the unified atomic mass unit within the SI unit system. The latter is given by the molar Planck constant N (A) h, which itself is known via its relation to the fine structure constant. In the present paper we report on measurements carried out until 2003 with an uncertainty level of 4 a <... 10(-7). We discuss the main limitations of these experiments and outline the possibilities for future measurements at the 10(-8) level. Such measurements would allow a direct representation of the unified atomic mass unit in terms of a Compton frequency and are of utmost importance for a future re-definition of the kilogram mass unit.