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Michael Cribier: Reactor Antineutrino Anomalies in Europe

March 13, 2013

The speaker started his speech by asking the question: “Do we need a fourth neutrino to explain these results ?” His answer is yes. This is streneghtened by a reevaluation of antineutrino spectrum emitted by reactors, adding the newest knowledge on nuclear databases, including the neutron lifetime which has decreased as determinations have become more accurate.

A reanalysis of the 19 previous experiments at reactors with very short baseline and a treatment of correlations between the various measurement explicitates a overall deficit, with a ratio observed/theoretically predicted flux of R=0.927+-0.023.

Moreover, there is the lingering gallium anomaly: experiments with solar neutrinos, calibrated using radioactive neutrino emitters of low energy, evidence a deficit R=0.75-0.86. Combining reactors and gallium results one gets best values of delta_m^2 in the 1.5-3 eV^2 range.

So there is a lot of expectation for what Planck may unveil next week.

Then Cribier mentioned the fact that a few days ago a preprint appeared in the arxiv (1303.0900v1) where P.Vogel et al. reanalyze the 19 SBL reactor antineutrinos adding a value of sin^2(2 theta_13)=0.089+-0.011, and they get a much reduced anomaly, with a ratio R=0.959+-0.009 +-0.027, which is just a 1.4-sigma effect. The Saclay group quickly tried to ascertain the result and has many questions. They reanalyzed the same way the data, but they still find a three-sigma deficit of neutrinos. So this disagreement remains in these latest results.

An intriguing result at the ILL reactor in Grenoble, taking data at 9 meters distance. A reevaluation in 1990 by 9.5% of the reactor power shows that the deficit in the 1980 data is evident and one can also fit the energy spectrum, with a dm^2 value of 2.3 eV^2, very close to what we agree to now.

The speaker said that we need new experimental input at very short distance. There are many experimental projects ongoing in this landscape. With sources, reactors, and beams. SoX in Borexino uses a 51Cr source.

Nucifer is a small detector with a baseline of 7 meters, at Saclay (70MW power, with 20% 235U and a compact core). The design of the detector was optimized for core monitoring. There were several experimental problems, including a bad liquid scintillator and large gamma backgrounds. Nonetheless, as a function of energy the flux might show a marked oscillation with six months of good data.

The Stereo experiment at Grenoble also is very close to the core of the 50MW reactor: 7 to 9 meters. Katrin is an experiment that may provide very interesting answers about the existence of a fourth neutrino, which would modify the end point of the energy spectrum from tritium.
The 51Cr source has been used in Gallex and Sage to calibrate solar neutrino experiments. It is produced by irradiation in intense thermal neutron fluxes. Its life time is of 40 days. Can be done in Russia. In a liquid scintillator detector the detection is by elastic scattering. Baksan proposed (arxiv:1006.2103) a new Gallium experiment, with well-known technology. Its design is of a central zone including the radioactive source, an outer zone of 42 tons. With a 3 Mci source (obtained by 50 days of irradiation).

CeLand is a proposed search for a fourth neutrino, exploiting the idea of oscillometry: put a radioactive source in the center of one of the large-volume liquid scintillators, and study the oscillation. When you look at the radius distribution of your interaction points you cancel the radius dependence of the oscillation term, and can extract the parameters cleanly. The advantage of antineutrinos are the detection by inverse beta decay, with high cross section. But you must have energies above 1.8 MeV. The other advantage is the delayed coincidence with neutron capture; also, the isotopes are easier to produce. There are several candidate radionuclides.

144Ce can be an option. Has a lifetime of 411 days. The power of a 75 kCi source (9 kg of CeO2) is of 600W. The spectrum however is not very well known. Half of antineutrinos are emitted above the energy threshold for inverse beta decay.
The production of the source involves many steps, which the speaker illustrated in detail. One has to work with cerium oxyde, which is a powder of 4-5 grams per cubic centimeter. The radioactive source of 75 kCi ends up weighing 9 kg and requires to be encapsulated in a proper shielding. One of the very difficult issues is the transportation of this object: one needs a 23 ton container. There are severe constraints based on regulations issued by IAEA. It is a long and costly process. The limit for transportation by air is of 16.2 kCi for cerium, so one would have to split the source in five. The container is not approved in Japan. One is looking at several months to complete the transportation.

There are several candidates to host such a source at their center. KamLand is ideal. One would first put the source close to the outer veto, close to the water in the exterior of the tank, with a very clear signal. 20,000 events could be collected with small background. Then you could transfer the source inside the center of KamLand, but this is not easy to do.

If we look at the performances of this endeavour, we see that the discovery potential is quite interesting.

In conclusion, the experimental landscape is unclear and we need to adddress this important issue with new ideas. There are many possibilities, and a clear answer could be obtained in few years.

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