CosI: Coherent Neutrino Scattering with Cesium Iodide
University of Chicago
Coherent neutrino scattering is a process that the standard model of particle physics predicts but has never been observed. Neutrinos of the right energy, O(10 MeV), should interact coherently with the entire nucleus, enhancing the scattering cross section by a factor of the number of neutrons squared. Observing this process would be an additional test of the standard model, and once observed it can be applied to diverse scientific areas from nuclear reactor monitoring to the direct detection of dark matter.
The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory can provide an intense source of medium-energy neutrinos with a well-defined time structure suited to seeking coherent neutrino scattering. The pulsed beam structure allows for a background rejection factor of 6x10^-4 for backgrounds that are not beam-associated. The high flux of the SNS will result in about 1,300 neutrino events per year in a 15 kg crystal of CsI(Na) at a distance of only 20 m from the source.
CsI(Na) is an inorganic scintillator that is ideal for seeking coherent neutrino scattering. It has a high light yield of 39,000 photons/MeV, which is well matched to the wavelength sensitivity of a standard bialkali PMT. It is reasonably affordable at approximately $1,000 per kg. Most importantly, both cesium and iodine are good targets for coherent neutrino scattering, as each has a large (78 and 74, respectively) number of neutrons. By making a low-threshold, low-background CsI(Na) detector, observing coherent neutrino scattering should be possible at the SNS.