A major background for CENNS measurements*

Many collaborations are closing in on a measurement of Coherent Elastic Neutrino-Nucleus Scattering (CENNS) using reactor sources. These measurements will open a new window onto neutrino parameters and other physics like sterile neutrino searches. We investigated a potentially major background source due to nuclear recoils created by capturing thermal neutrons. The level of background from the sea-level thermal neutrons is already far too high for silicon and germanium detectors. On the other hand a sea-level thermal neutron flux is an ambitious background goal so near to a fission reactor core. Mitigation techniques are available and will be discussed.