Spectral distribution of the two-photon decay of He-like krypton

The 2 1S0 state in helium-like ions is forbidden to decay to the ground state by the emission of a single photon so the dominant decay mode is emission of two E1 photons. The energies of the individual photons have a continuous distribution with a broad peak at half the transition energy and the sum of the energies of the two photons is equal to the transition energy. The shape of the continuum single-photon spectrum provides a sensitive probe of the calculation of the transition probability for this decay and we have started a program to make a precision measurement of the spectral shape of the decay of the 2 1S0 level in He-like krypton in order to test the calculations