The main goal of the SoLid project is to search for a new particle in the form of a light sterile neutrino with a mass around 1 eV, a possible explanation for the reactor anomaly. The project will also measure, at the same time, the antineutrino energy spectrum from pure Uranium-235 fission, with percent level precision. The use of solid plastic scintillator technology will provide a unique dataset which could shed the light on the origin of the so-called bump seen in several measurements at power reactors.
The discovery of a new kind of matter in the form of a sterile neutrino would be a profound challenge for the theory to explain. The existence of a type of neutrino state with no standard model coupling may provide an important window into understanding the dark universe and opening a new the direction for dark matter search. The discovery of this new state through neutrino oscillation will also affect the understanding of the other oscillation parameters and their current precision.
The shape distortion of the antineutrino energy spectrum called the “5 MeV bump” is another conundrum that needs solving. It appears in all recent liquid scintillator detector data taken at power reactors which uses Low Enriched Uranium fuel. The size of the distortion increases with reactor power and therefore cannot be explained by an oscillation phenomenon. As of today, the origin of this distortion is not yet understood. The 5 MeV distortion is clearly pointing towards some issue with the prediction and more specifically with the reference method called the conversion method based on electron data. Despite a very large program to re-measure a number of isotopes, recent results from beta branches summation are in good agreement with conversion methods on the shape of the spectrum. Nuclear physicists working on antineutrino flux prediction are still looking for a credible explanation for this effect. Experimentally, there are very few constraints that can be used to help with this problem and one of them is to make a measurement of a pure Uranium-235 antineutrino spectrum. A precision measurement of a Uranium-235 core like BR2 with a non-liquid scintillator detector would at least reject some of the possible explanations like non-linear effects.
Resolving the confusing picture that has emerged at the short L/E oscillation region is therefore of utmost importance for the neutrino community and the SOLID project is well placed to gather a unique dataset to answer those questions. Through those measurements, SOLID also aims to demonstrate that the detector technology provides a fully integrated system suitable for monitoring nuclear reactors or waste repositories, a step closer to practical application in nuclear safeguards.
