Reactor-based Neutrino Oscillation Experiments

TL;DR

This paper surveys reactor-based neutrino oscillation experiments, detailing how low-energy $\bar{\nu}_e$ from nuclear reactors enable disappearance measurements with minimal matter effects and complement accelerator-based studies. It explains the end-to-end flux and spectrum predictions, including fission-rate modeling, beta-decay spectral conversions, and the inverse-beta decay detection channel, along with calibration and systematic controls that yield ~2% flux-spectral accuracy. The Chooz and Palo Verde experiments demonstrate the feasibility of controlled reactor-based searches and constrain $\sin^2 2\theta_{13}$, while KamLAND is shown to have the potential to test the solar MSW Large Mixing Angle solution with baselines exceeding 100 km and a large, low-background detector. The work highlights the unique capabilities of reactor experiments to probe very small $\Delta m^2$ regions and discusses future prospects for extending sensitivity to smaller mixing angles and exploring geoneutrinos and other astrophysical phenomena.

Abstract

The status of neutrino oscillation searches employing nuclear reactors as sources is reviewed. This technique, a direct continuation of the experiments that proved the existence of neutrinos, is today an essential tool in investigating the indications of oscillations found in studying neutrinos produced in the sun and in the earth's atmosphere. The low-energy of the reactor \nuebar makes them an ideal tool to explore oscillations with small mass differences and relatively large mixing angles. In the last several years the determination of the reactor anti-neutrino flux and spectrum has reached a high degree of accuracy. Hence measurements of these quantities at a given distance L can be readily compared with the expectation at L = 0, thus testing \nuebar disappearance. While two experiments, Chooz and Palo Verde, with baselines of about 1 km and thus sensitive to the neutrino mass differences associated with the atmospheric neutrino anomaly, have collected data and published results recently, an ambitious project with a baseline of more than 100 km, Kamland, is preparing to take data. This ultimate reactor experiment will have a sensitivity sufficient to explore part of the oscillation phase space relevant to solar neutrino scenarios. It is the only envisioned experiment with a terrestrial source of neutrinos capable of addressing the solar neutrino puzzle.

Figures (39)

• Figure 1: Phase-space for neutrino oscillations. The existing limits on $\rm \nu_e - \nu_{\mu}$ are compared with current and future experiments and the regions obtained by interpreting the solar, atmospheric and LSND neutrino anomalies as due to oscillations (some of these effects are not necessarily $\rm \nu_e - \nu_{\mu}$ oscillations.) The MSW mechanism is used in plotting some of the solar neutrino regions. The sensitivity of reactor experiments is the same for $\nu_{\rm e} - \nu_{\tau}$ oscillations. Limits are at 90% CL.
• Figure 2: Reactor $\bar{\nu}_e\;$ flux, inverse beta decay cross section, and $\bar{\nu}_e\;$ interaction spectrum at a detector based on such reaction.
• Figure 3: Neutrino $\Delta m^2$ sensitivity as a function of total reactor power and detector fiducial mass for detection based on the inverse-$\beta$ reaction discussed in the text. The baseline scales with the $\Delta m^2$ sensitivity sought according to Eq. (\ref{['eq:oscleq']}). The fiducial-mass$\times$power necessary for the experiment grows with the square of the baseline. The past experiments are labelled by the name of the reactor complex used. The approximate year of the experiment is also indicated to show that the increased baseline and $\Delta m^2$ sensivity followed more or less the chronological order.
• Figure 4: Expected positron energy spectra for no oscillations (full line) and oscillations with parameters $\Delta m^2 = 7.2\times 10^{-3}$ eV$^2$ and $\sin^2 2\theta = 1$ at the Chooz ($L\simeq 1$ km) (dashed line) and Palo Verde ($L\simeq 0.8$ km) (dotted line) experiments. Adapted from Harrison, Perkins and Scott (1996).
• Figure 5: Yields (in %) for $^{235}$U thermal neutron fission (normalized to 2 for the two fragments)