The ERDRE groupe works in both neutrino fundamental and applied physics

Neutrino oscillation

During the last decade, our knowledge on neutrinos has impressively increased. Several experiments have shown that neutrinos are massive particles and as a consequence can oscillate from one leptonic flavor to another [1]. Despite most of the parameters involved in these flavor oscillations have been measured with good accurancy, it still remains great uncertainty on the value of the mixing angle θ₁₃ on which depends also the value of the phase δ characterizing the violation of CP symmetry in the leptonic sector. Antineutrinos emitted by nuclear reactors are used since the beginning of neutrino physics to study their fundamental properties. In particular, Reines and Cowan have demonstrated experimentally the existence of these particles by placing a detector near the Savannah River reactor in 1956 [2]. Since then, several pioneering experiments were performed by French teams, including the Chooz experiment that provided the best constraint on the value of θ₁₃ available until mid-2011[3]. In 2011, MINOS and T2K experiments, which are located respectively in the United States and Japan, have measured the appearence of electronic neutrinos in a beam of muon neutrinos [4] [5], indicating a non-zero value for the θ₁₃ mixing angle. This result has been reinforced by a combined fit of the results of solar neutrino experiments with KamLAND [6], Minos and T2K [7]. More precise measurements of the mixing angle have been obtained in 2012 by the disappearance experiments carried out near nuclear reactors: Double Chooz [8], Daya Bay [9] and Reno [10]. In its final result the Double Chooz experiment [11], in which the ERDRE group is heavily involved, measured 8249 electronic antineutrinos during 227.93 days of data taking (live time) in the far detector located at 1050m from the two reactors (type N4) of the Chooz power plant, exposed to 33.71 GW-ton-years (reactor power detector x mass x LiveTime). We would expect 8937 events in the assumption of a zero value for θ₁₃. This deficit has been interpreted as evidence of disappearance of electronic antineutrinos. Using an analysis in normalization and shape of measured antineutrino energy spectrum, we obtain the following value: sin² 2θ₁₃ = 0.109 ± 0.030 (stat) ± 0.025 (syst). The data exclude the hypothesis of zero-oscillation with 99.9% CL (3.1σ).

A priori there are three neutrino flavors. But a few years ago the LSND experiment has observed a signal which could be explained by the existence of sterile neutrinos, interacting only by the gravitational interaction. This signal was not confirmed by any experiment but could not be totally excluded by new measurements. In 2011, a new calculation of the energy spectra of antineutrinos led to new spectra for the main fissionable nuclei ²³⁵U, ²³⁹Pu, ²⁴¹Pu, and ²³⁸U [12], with a normalization about 3% higher than the previous reference spectra. This renormalization has given rise to a reanalysis of the results of reactor neutrino experiments of 80's and 90's. The summary of the results from experiments whose distance from reactor was less than 100m gives a relationship between measured and predicted fluxes of 0.943 ± 0.023, inducing a deviation from 1 to 98.6% CL [13]. This deficit is called "reactor anomaly". One of the possible hypotheses to explain this result is the existence of sterile neutrinos with Δm2 ≈ 1 eV2 [14], i.e. in a different zone respect to the one predicted by LSND experiment. The group is involved in the Nucifer experiment (collaboration with CEA). One of the motivations for this experiment is the measurement of oscillations at short distance by using a detector placed at 7m from the OSIRIS research reactor at Saclay (France). The group is also involved in the SoLiD experiment, mainly designed for sterile neutrino detection.

Another hypothesis to explain the "reactor anomaly" comes from the present evaluation method of calculated antineutrino spectra. Indeed, some of the terms used in the calculation still have a large uncertainty (see P. Huber [12]), in particular the weak magnetism term. Beta spectra measurements performed at the ILL high neutron flux reactor by the team of K. Schreckenbach in the 80's are still the most precise measurements up to now [15]. These measures are presently used to calculate the reference antineutrino spectra, thanks to a conversion method which has been revisited recently [12]. No further measures have confirmed the accuracy of these measurements of beta spectra. The only alternative method is the construction of spectra by summing each single spectrum associated to each beta decay branch of the fission products. The ERDRE group is involved in these calculations [16], but also in nuclear physics experiments aiming at improving the precision of the spectra obtained by the summation method, these experiments are performed by using the TAGS technique.

A new tool for nuclear reactor monitoring: the antineutrinos

During this quest, was born the idea that antineutrinos produced at reactors could carry a direct image of the core, outside the reactor, that could be exploited for a remote control of nuclear power plants [17]. Indeed, large quantities of antineutrinos are produced in the reactor due to β decays of the fission products and about 10²¹ /s are emitted in a 1 GW_e reactor core. The distribution of fission fragments depends on the fissile isotopes (²³⁵U, ²³⁸U, ²³⁹Pu and ²⁴¹Pu) and on the energy of neutron flux in the core. The released energy per fission, the average number of emitted antineutrinos and their mean energy depend also directly on the fissile isotope that undergoes fission, cf. Table 1. Consequently, all the differences in the fissioning tread lead to significant discrepancies in the associated antineutrino spectrum which will reflect the thermal power emitted by the core and its composition.

Table 1 Main characteristics of antineutrinos originating from ²³⁵U, ²³⁸U, ²³⁹Pu and ²⁴¹Pu fission, in standard PWR [18,19].

Two pioneering experiments performed at the Rovno power plant in the former USSR and at the Bugey power plant in France have demonstrated the proportionality between the antineutrino counting rate and the thermal power measured by the operators [20, 17]. Mikaelian et al. have demonstrated the direct relation between the antineutrino flux and energy spectrum with thermal power and fuel content of a reactor core [21]. At a fixed power, the neutrino flux as well as the shape of the energy spectrum are affected by any change of the fuel composition. As an example, a hypothetical reactor able to use only ²³⁵U would produce an antineutrino flux 40% higher than the same reactor burning only ²³⁹Pu. Linked with the energy dependency of the detection interaction cross-section, the detected neutrino flux would be 60% higher. Estimation of the thermal power hence requires the knowledge of the fuel history (initial composition) and the simulation of its evolution in time.

Antineutrinos not only provide informations on the reactor power but also on its isotopic content, opening several application possibilities such as burnup monitoring for fuel economy and safeguards aspects. The International Atomic Energy Agency (IAEA), the United Nations agency in charge of the development of peaceful use of atomic energy, is also the authority in charge of the application of the Treaty on the non-proliferation of Nuclear Weapons. Looking for innovative and complementary methods, the IAEA has asked member states a feasibility study to determine whether antineutrino detection methods could provide practical safeguards tools for selected applications [22].

During the last decades tremendous progresses in neutrino detection have been achieved, and the fundamental knowledge acquired on neutrinos gives opportunities for applied neutrino physics. A worldwide effort concentrated on applied antineutrino physics has started since the IAEA request a few years ago, in the US, France, Brazil, Japan and Russia.

The ERDRE group is involved in this reserch through its participation at the Double Chooz and Nucifer experiments and the effort in simulation of possible proliferation scenario.

 Experiments : Double Chooz, Nucifer, SoLiD, JUNO

Bibliography

[1] S. Fukuda et al., Phys. Lett. B, 359, 179-187 (2002). KamLAND: K. Eguchi et al., Phys. Rev. Lett., 90, 021802 (2003). SNO: Q. R. Ahmad et al. Phys. Rev. Lett. volume 89, No. 1, 011301 (2002). MINOS: D.G. Michael et al., Phys. Rev. Lett. 97, 191801 (2006)

[2] C.L. Cowan, Jr., F. Reines, F.B. Harrison, H.W. Kruse and A.D. McGuire, Science 124, 103 (1956).

[3] Apollonio, M. et al., 2003. Search for neutrino oscillations on a long baseline at the CHOOZ nuclear power station. Eur. Phys. J. C27 (2003) 331-374.

[4] P. Adamson et al., Phys. Rev. Lett. 107, 181802 (2011).

[5] K. Abe et al., Phys. Rev. Lett. 107, 041801 (2011).

[6] KamLAND Collab., Phys. Rev. D83, 052002 (2011).

[7] G.L. Fogli, E. Lisi, A. Marrone, A. Palazzo, A.M. Rotunno, Phys. Rev. D84, 053007 (2011). T. Schwetz et al., New J.
Phys. 13, 109401 (2011).

[8] Y. Abe et al., Phys. Rev. Lett. 108, 131801 (2012).

[9] F. P. An et al., Phys. Rev. Lett. 108, 171803 (2012).

[10] J. K. Ahn et al., Phys. Rev. Lett. 108, 191802 (2012).

[11] Y. Abe et al., Phys. Rev. D 86, 052008 (2012).

[12] Th. Mueller et al., Phys. Rev. C83, 054615 (2011). P. Huber, Phys. Rev. C84, 024617 (2011).

[13] G. Mention et al., Phys. Rev. D83, 073006 (2011).

[14] K. N. Abazajian et al., http://arxiv.org/abs/1204.5379.

[15] K. Schreckenbach et al., Phys. Lett. 99B, 251 (1981). K. Schreckenbach et al., Phys. Lett. 160B, 325 (1985). F. von Feilitzsch, A. A. Hahn and K. Schreckenbach, Phys. Lett. 118B, 162 (1982). A.A. Hahn et al., Phys. Lett. B218 (1989) 365.

[16] M. Fallot et al. http://arxiv.org/abs/1208.3877.

[17] Mikaelian, L.A., 1977, Proc. Int. Conf. Neutrino-77, v.2, p.383.

[18] Kopeikin V. I., Mikaelyan L. A., and Sinev V. V., 1997. Spectrum of electronic reactor anti-neutrinos. Phys. Atom. Nucl. 60 (1997) 172.

[19] Vogel P. et al., 1981. Reactor Anti-neutrino Spectra and Their Application to Anti-neutrino Induced Reactions. Phys. Rev. C24 (1981) 1543.

[20] Klimov, Yu.V., Kopeikin, V.I. . Rev, Mikaélyan, L.A., et. al., 1994. Neutrino method remote measurement of reactor power and power output. Atomic Energy, 76, No.2, 123-127.

[21] Korovkin, V.A. et. al., 1988. Measuring nuclear plant power output by neurtino detection. Atomic Energy, 65, No. 3, 712-718.

[22] IAEA report 17-18 December 2003 «  Meeting to evaluate Potential Applicability of Antineutrino Detection Technologies for Safeguards Purposes