Michel Cribier, Maximilien Fechner, Thierry Lasserre, Alain Letourneau, David Lhuillier, Guillaume Mention, Davide Franco, Vasily Kornoukhov, Stefan Schoenert
Corresponding author : Thierry Lasserre (firstname.lastname@example.org)
Several observed anomalies in neutrino oscillation data can be explained by a hypothetical fourth neutrino separated from the three standard neutrinos by a squared mass difference of a few eV2. This is becoming one of the most important topics to be addressed in neutrino physics. In particular the reactor neutrino and the gallium anomalies can be tested with a PBq (ten kilocurie scale) 144Ce antineutrino beta-source deployed at the center of a large low background liquid scintillator detector, such like Borexino, KamLAND, and SNO+. The compact size of such a source could yield an energy-dependent oscillating pattern in event spatial distribution that would unambiguously determine neutrino mass differences and mixing angles. The proposed program aims to perform the necessary research and developments to produce and deploy an intense antineutrino source in a large liquid scintillator detector.
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A sterile neutrino, by definition, is not able to induce an interaction allowing its direct detection. Nevertheless theory predicts oscillation between the three ordinary neutrinos and the sterile one. Thus the experimental signature of a sterile neutrino consists in the observation of interactions of the ordinary neutrinos with a modulation in energy and/or distance controlled by the mixing and masses of the fourth neutrino. The mass (eVscale) and coupling of the sterile neutrino able to explain the reactor antineutrino anomaly is such than interactions of neutrino/antineutrino of typical energy of 1-2 MeV would induce a spatial modulation of several per cent over few meters. Hence if an intense source of neutrino is placed at the center of a spherical liquid scintillator detector (see Figure 1), the radial distribution of the interaction vertex will deviate from a flat distribution with a sinusoidal modulation (see Figure 2 left). The spatial period is inversely proportional to the mass of the sterile neutrino whereas the amplitude is a function of the coupling between the fourth and the usual electron neutrino.
Starting from the characteristics of the potential existing liquid scintillator detectors and using the interaction cross-section, it is possible to estimate the required activity of the antineutrino source: a 1.85 PBq (50 kCi) is needed to produce 40 000 interactions in a year, leading to a negligible statistical error. The suitable antineutrino source must have energy above 1.8 MeV and a lifetime long enough to allow the production, transport and measurement. For individual nuclei, these requirements are contradictory so we searched for candidate sources involving a long-lived nucleus that decays to a short-lived but energetic nucleus. Among the 3-4 potential candidates that we identified, the pair 144Ce-144Pr seems the most promising thanks to its abundant presence in the fission products of 235U (5%) and 239Pu (3%) and to the relatively easy technique to isolate it. In particular these techniques of hot chemistry are understood and available in Russia, France, and Germany. 144Ce decays in 144Pr with a suitable long half-life of 285 days, and 144Pr emits quickly (17 min.) an energetic antineutrino whose spectrum extend up to 3 MeV. To reach the required activity only 14 g of 144Ce are needed but it seems unavoidable to have it mixed with other Cerium isotopes (≈1.5 kg). For radioprotection and to reduce unwanted backgrounds this Cerium is encapsulated in a ≈30 cm radius shield composed mainly of tungsten.
An international collaboration must be organized in order prepare the deployment the intense antineutrino source at the center of an existing detector like, Borexino, KamLAND, SNO+, or LENA.