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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - SOLID (Search for a new form of matter: the sterile neutrino)

Teaser

Summary

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 â€œ5 MeV bumpâ€ 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.

Work performed

The first period of the project has focused on the successful realisation of the SOLID project through the construction and deployment of two detector modules at the BR2 research reactor. This period is therefore a critical step in the success of the whole project.

In the period from June 2016 to December 2016 completion of optimisation studied delivered the targeted performance of the detector system. During this period we also finalised the design of the detector system, coordinating and planning the construction effort across the various institutes. It was decided at this stage to embed the detector system in a ISO-freight container to ensure good stability over time of the detector response and electrical noise reduction.
The electronics read out electronics design was also finalised during that period and a first production of prototype boards was scheduled in December 2016.

During the period from December 2016 to September 2017, the production of the various detector components (scintillator cubes, read out boards) started. Construction and quality insurance of the detector modules was conducted at the Gent nuclear laboratory. The shipping container customisation was completed in June 2017 at the Rutherford Appleton Laboratory. It was shipped to Gent for assembly of the detector modules and testing of the water cooling system. The 5120 PVT cubes were machined from slabs and assembled into wrapped scintillating voxels. The planes were assembled one by one and tested using a XY radiation source scanner. It took 9 months to assemble all five detector modules including the ones for the ERC project.

From October 2017 to December 2017 the SOLID detector modules were instrumented with the new read out electronics. The detector planes were tested one by one and synchronised to trigger as one detector system. The commissioning of the system was performed with the detector in place in the container and the trigger was successfully commissioned, reading out data from all four first modules. The last modules was still going through the quality insurance tests when it was decided to start the installation on site at the BR2 reactor. The commissioning of the shielding enclosure was started in October and all four sides of the shielding were up by mid-December. On December the 5th we started taking data in physics mode with partial shielding. The system operated smoothly with very few issues. The DAQ and read out electronics operated well providing a first set of commissioning data that was used subsequently to tune our trigger firmware.

From December 2018 to April 2018, the detector was taking data covering a second reactor cycle in February. We developed at the time the in-situ calibration method and used this data to update and optimise the neutron trigger as to get maximum trigger efficiency.

Since April 2018, the detector is taking physics quality data.

The reactor data taken from April 2018 to April 2019 will be part of a first analysis of 6 reactor cycles (~140 days) that is currently in underway.

Final results

In the next period of the project we expect:
1. to make a first sterile neutrino search using 140 days of reactor ON
2. Measure for a similar duration and based on a good signal to background figure, the pure U-235 antineutrino spectrum with good precision.

We plan to make some improvements in our sensitivity by pushing the reconstruction threshold to a few photon Avalanche (PA) signal and develop a machine learning classification that takes into account all possible correlations between the physics quantities.

A second result is planned by the end of the project period using between 300 and 450 days of reactor ON to improve on the first result in an attempt to reach our ultimate goal to solve the reactor anomaly and the origin of the bump.