In order to ensure that the interaction is electromagnetic in nature — and not nuclear — the distance of closest approach of the colliding nuclei has to be sufficiently large.
In particular, in low-energy Coulomb excitation (taking place at beam energies of a few megaelectronvolts per nucleon) the commonly adopted empirical criterion is that if the surfaces of the colliding nuclei are separated by at least 5 femtometers, the contribution of the short-range nuclear interaction to the excitation process can be neglected.
[1] From the measured excitation cross sections, electromagnetic transition probabilities between the nuclear energy levels can be extracted.
Moreover, it is the only experimental method in nuclear physics that is sensitive to electric quadrupole moments of excited nuclear states with lifetimes shorter than nanoseconds.
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