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 ¹³³Cs and ¹²⁷I. We obtain the practically model-independent value Rₙ=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 2.3σ evidence of the nuclear structure suppression of the full coherence.