Reactor Neutrinos

Nuclear reactors used for power generation produce enormous numbers of electron anti-neutrinos (typically > 1020 every second) from the beta decays of the fission products. The flux of reactor anti-neutrinos can be accurately calculated from the number of decays, which is related to the measured thermal power produced by each reactor. The precise shape of the anti-neutrino energy spectrum depends on the nuclear composition of the fuel used, which in general, is also well known for each reactor. Having a well known yield of anti-neutrinos with a well defined spectrum that is produced a known distance from the detector makes reactor anti-neutrinos very powerful tools to study neutrino oscillations.

In addition to the first detection of neutrinos very close to a reactor, and recent measurements of neutrino oscillation parameters a few km from reactors, the KamLAND collaboration has made measurements a few hundred km from reactors [1], producing the most precise measurement to date of Δm221 [2], which is one of the parameters used to describe neutrino oscillations.  Currently, this measurement is in mild tension with the value obtained using solar neutrinos.

 

In scintillator experiments, electron anti-neutrinos (ν̅e) are detected through the inverse beta decay reaction with protons (p):

                                                                            ν̅e + p → e⁺ + n.

The positron (e⁺) is produced with a kinetic energy of approximately 1.8 MeV less than the energy of the anti-neutrino, Eν, and annihilates with a nearby electron almost immediately, depositing a total of approximately (Eν - 0.8) MeV in the detector. The tight relationship between the kinetic energies of the detected positron and the anti-neutrino makes analysis of anti-neutrino energy spectra relatively straightforward. The neutron (n) typically captures on a hydrogen atom a few hundred microseconds later, producing a time coincident signal with the positron. This coincidence provides powerful background rejection.

 

Anti-neutrinos oscillate, which means that as they propagate, some fraction of the electron anti-neutrinos produced in the nuclear reactors will change into muon or tau anti-neutrinos, which are not detected. For a fixed distance, the probability that a neutrino oscillates depends on its energy; so the energy spectrum of electron anti-neutrinos observed at the detector differs significantly from the spectrum at production. The reactor anti-neutrino spectrum that we expect to arrive at SNO+ (compared with what it would look like if there were no oscillations) is shown in Figure 1, below. 

Our sensitivity to oscillation parameter Δm221 is illustrated by comparing the shape of the energy spectrum assuming the value determined by KamLAND (Fig. 1) with that using solar neutrinos (Fig. 2). 

ReactorNuEnergy Default

Figure 1. The expected reactor anti-neutrino spectrum arriving at SNO+ with Δm221 = 7.37e-5 eV2.  The unoscillated spectrum is also shown for comparison.

ReactorNuEnergy Solar

Figure 2. The expected reactor anti-neutrino spectrum arriving at SNO+ with Δm221 = 4.80e-5 eV2.

 

SNO+ will primarily measure anti-neutrinos from the Bruce, Pickering, and Darlington nuclear generating stations. Although it is not expected to detect as many events as KamLAND, the peaked structure in the summed oscillation spectrum (shown in Figure 1 above) is much sharper for SNO+ owing to the fortuitous positions of these three Canadian reactors.

References

[1] Araki, T. et al, Phys. Rev. Lett. 94 081801 (2005)

[2] Gando, A. et al., Phys. Rev. D 88 no.3, 033001 (2013)