Search of Oscillations at very short distance with Lithium6 Detector at SCK-CEN BR2 – SoLid

The SoLid detector is a highly 3D segmented detector (8000 voxels/m3) based on a dual scintillation technology. Electron antineutrinos will interact primarily in the active detector volume via inverse beta decay (IBD) on hydrogen nuclei, producing a positron and a neutron in the final state: ¯?e + p ? e+ + n.
Experimental approaches use the coincidence technique, which consists of detecting both the positron and the neutron, within a short time window, typically up to hundreds of microseconds. The neutron generally thermalizes via elastic collisions in the detector, after which it can be captured by nuclei with a high neutron capture cross section. As such, it typically induces a scintillation signal that is delayed in time with respect to the scintillation light caused by the positron and its corresponding annihilation gamma-ray photons. The time delay between the two signals can be tuned by the choice of neutron capture elements and their concentration and distribution in the detector.
SoLid opted for a combination of two scintillators. One is polyvinyltoluene (PVT), a relatively cheap plastic scintillator that is generally easy to machine in any desired shape or geometry, and the other is ZnS(Ag) used together with 6LiF to capture thermal neutrons via the reaction: 6Li + n ? 3H + a (4.78 MeV), for which the decay products in turn induces scintillation in the ZnS(Ag) scintillator. The PVT-based scintillator is of the type EJ-200 produced by ELJEN Technology. It is a general purpose plastic scintillator that emits on average 10 000 photons per MeV of energy deposited by electrons in the blue-violet wavelength band with a peak emission wavelength of 425 nm. The choice of PVT is mainly motivated by its good light yield and its linear energy response over a wide range of energies ranging from 100 keV to several MeV. It combines a long optical attenuation length of about 380 cm, with a scintillation pulse decay time of 2.1 ns. The 6LiF:ZnS(Ag) scintillator for neutron detection is produced by SCINTACOR, in the form of thin screens. These so-called neutron detection screens, emit photons at a peak emission wavelength of 450 nm. The nature of the neutron capture reaction and the longer scintillation decay time of 10 µs for the 6LiF:ZnS(Ag) scintillator allows for a pulse shape discrimination between signals induced in the neutron detection screens via nuclear interaction and signals induced via electromagnetic processes in the PVT.

The IBD analysis, has been built over the years thanks to all the contributions of the French groups, in the context of this ANR. The analysis has demonstrated relatively good performances given the huge background level faced by SoLid. Another analysis using uniform BDT (uBDT) from our UK colleagues has shown similar results. This is illustrated on the figure (left) where the signal over background ratio (S/B) as a function of the IBD-candidates excess over the background per day, which depends on the BDT score cut, is represented for both analyses. The gBDT analysis is in good agreement with the prediction while the uBDT analysis is more fluctuating around the prediction. These results have been used for an oscillation search and preliminary sensitivity contours have been produced as shown on the figure (right). These results have been presented in 2021 at the summer conferences. In 2022, seen the last progress. We hope to exclude at better than 90% CL the RAA best fit.

In 2022, based on the accumulated work of the last years, the return on experience allowed to reach a very satisfactory sensitivity for the search of oscillation at short distance. The collaboration is finally finalizing its analyses which will allow to scan a part of the phase parameter complementary to the other experiments (Stereo, Prospect). In addition, the use of a different technology does reinforce this complementarity (different detector systematics errors). The first results will be released mid 2023.
Finally, in spite of the final objective that had to be reevaluated in terms of sensitivity (S/B lower than expected), the ANR funding allowed the production of a complete measurement for the search of short distance oscillation closing the chapter of the sterile neutrino at eV2 mass. It also allowed to train young PhD students and postdoctoral researchers in the neutrino experimental field. Finally, the SoLid technology has opened the way to breakthrough research in terms of detection of reactor antineutrino and more generally of charged particles. Part of the consortium is now involved in the LiquidO project based on Opaque liquid scintillator medium readout by wavelength shifting fibres and SiPMs. The segmentation researched by SoLid will this time be brought by opaticity of the medium and higher light yield is expected. Its R&D is ongoing and the ANR has funded one of its innovation parts (LPET-OTECH 2021) as well as Europe (AM-OTECH, EIC PATHFINDER 2021). The experience accumulated in SoLid is not at all foreign to these breakthrough developments. The expertise gained from the ANR SoLid project is thus fundamental for further innovation and for probing more precisely the standard model and physics beyond.

- Y. Abreu et al. “A novel segmented-scintillator antineutrino detector”. In: JINST 12.04 (2017), P04024. doi: 10.1088/1748-0221/12/04/P04024.
- Y. Abreu et al. “Performance of a full scale prototype detector at the BR2 reactor for the SoLid experiment”. In: JINST 13.05 (2018), P05005. doi: 10.1088/1748- 0221/13/05/P05005
- Y. Abreu et al. “Commissioning and Operation of the Readout System for the SoLid Neutrino Detector”. In: JINST 14.11 (2019), P11003. doi: 10 . 1088 / 1748 - 0221 / 14 / 11 / P11003
- Y. Abreu et al. “Development of a Quality Assurance Process for the SoLid Experiment”. In: JINST 14.02 (2019), P02014. doi: 10.1088/1748-0221/14/02/P02014.
- Y. Abreu et al. “SoLid: a short baseline reactor neutrino experiment”. In: JINST 16.02 (2021), P02025. doi: 10.1088/1748-0221/16/02/P02025

The neutrino is one of the most enigmatic ingredient of the standard model of particle physics. Because of its weak interaction with matter and despite enormous experimental progress, its nature and its fundamental properties remain unknown: Dirac / Majorana, CP violation, absolute mass scale, other flavors... Recent results from the t13 experiments Double Chooz, Daya Bay and RENO have uncovered an intriguing excess of events detected in the 4-6 MeV reconstructed energy range with respect to predictions. This spectral distortion may be a suggestion of discrepancies in models of antineutrino production in reactors. Moreover, three independent experimental anomalies (reactor anomaly, Gallium and LSND/ MiniBooNE) support the hypothesis of the existence of a new neutrino family, called sterile because not interacting through weak interaction. In this context, new data from a precise pure U235 Antineutrino spectrum are needed to resolve this open issue and to clarify the reactor anomaly.

The SoLid project is an unique opportunity for the community to obtain sufficiently large and accurate data of neutrino flux at very short distance from a nuclear reactor, and then provide a reference measurement of pure 235U, essential for neutrino flux predictions used in current and future neutrino measurements. It proposes to confirm or refute the anomaly reactor and test ultimately the fourth sterile flavor.

The strength of the SoLid proposal relies on both the antineutrino source and the technology of detection, which are unique. The experience takes place at BR2 research reactor of SCK-CEN (Mol, Belgium). It allows oscillation measurements at distance varying from 5.5 to 12 m from the core. In addition to this large range, the site is distinguished by its exceptionally low background environment and by having no-access to site constraint. The detector is based on an innovative technology of neutron detector, finely segmented. The use of 6LiF: ZnS layers allows a distinct discrimination of the neutron signal. In addition, the segmentation allows to locate the antineutrinos interactions and then effectively reject significant background sources. Combined with the favorable environment at BR2, our experiment provides an unprecedented sensitivity.

Early 2015 a large-scale module 288kg (SM1) has been built and took “reactor ON” data during several days. This systems clearly demonstrates the capabilities of the segmented design of the detector, when combined with sophisticated data analysis techniques, leads to gains of orders of magnitude in background rejection. The physics run, with the full detector, will begin in October 2016 for a duration of two years minimum.This project is led by an international collaboration composed of eleven laboratories involving fifty physicists. Since the beginning, the three partners, Subatech, LAL and LPC have key contributions to the project: mechanical design, BR2 modelization, antineutrino spectra, Geant4/MCNP simulations and data analysis. This strong involvement allowed the coordinator to take responsablity of the SoLid analysis.

Our proposal is to build 10 detection planes (320 kg) to increase the fiducial mass and the detector length, allowing us to probe the lower Dm2 phase space region. The French groups foresee to lead several specific studies into the oscillation framework to effectively probe the anomaly reactor, but also, comparing
the pure U235 spectrum measured at BR2 with the data coming from the Double Chooz near detector to get an first insight in the “ 5 MeV bump” understanding. This specific contribution will consolidate our leadership in a experiment that promises to settle the question about the existence of sterile neutrino. In the longer term and for the international neutrino community, more precise neutrino flux measurements will allow to push the precision for future experiments.