Département des accélérateurs, de cryogénie et de magnétisme

 

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.

 

Two main options are possible: an intense neutrino source, like those used in solar neutrino experiments (Gallex and Sage), or an antineutrino source. In the first case, interaction in a liquid scintillator detector is only by scattering on electron and thus very similar to any Compton scattering induced by gammas. This background prevents the efficient use of a neutrino source. On the other hand starting with antineutrinos the detection is via the inverse beta decay where an antineutrino interacts with a proton and produces a positron and a neutron. This selective reaction allows an efficient rejection of the background, but only antineutrinos above 1.8 MeV are able to induce this reaction.

 

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.