Average CsI Neutron Density Distribution from COHERENT Data

Using the coherent elastic neutrino-nucleus scattering data of the COHERENT experiment, we determine for the first time the average neutron rms radius of Cs-133 and I-127. We obtain the practically model-independent value R-n = 5.5(-1.1)(+0.9) fm using the symmetrized Fermi and Helm form factors. We also point out that the COHERENT data show a 2.3 sigma evidence of the nuclear structure suppression of the full coherence.