Session Q20: Neutrinos V - Reactor Neutrinos

Live

The Daya Bay Reactor Antineutrino Experiment consists of eight identically-designed antineutrino detectors used for measuring the $\theta_{13}$ neutrino mixing angle. With a data set of millions of inverse beta decay reactor antineutrino events, Daya Bay is able to probe the mechanisms of antineutrino production in nuclear reactors. The absolute reactor antineutrino flux is updated following improved precision of the neutron detection efficiency. Two new studies, of individual $\,^{235}$U and $\,^{239}$Pu antineutrino spectra, and of flux and spectral evolution, have also been reported. The latest flux measurement continues to confirm the discrepancy with the Huber-Mueller model prediction (known as the reactor antineutrino anomaly) resulting in a ratio of measured to predicted yield of $0.952 \pm 0.014 \text{ (exp.)} \pm 0.023 \text{ (model)}$). The individual $\,^{235}$U and $\,^{239}$Pu spectral measurements, extracted from a reactor antineutrino spectrum for the first time, show spectral shape disagreements with the Huber-Mueller prediction in both isotopes. The flux and spectral evolution (due to changing reactor fuel composition) suggest that $\,^{235}$U modeling may be the primary contributor to the reactor antineutrino anomaly.   

Live

Nuclear databases have played an important role in understanding the production of electron antineutrinos in nuclear reactors, which is currently of importance for refining our understanding of neutrino oscillations, reactor monitoring, and non-proliferation. In order to provide more reliable results, the ENDF/B decay data sub-library has been recently updated to include the latest available \(\beta\)-decay results from various neutron rich nuclides that utilized TAGS and high-resolution gamma spectroscopy techniques. Additionally, we have added half-lives and delayed neutron probabilities from the recent IAEA Coordinated Research Project on the subject. With this new version of the sub-library we have calculated Inverse Beta Decay antineutrino spectra as well as yields as function of fuel burn-up, which were compared to the recently published Daya Bay and RENO results.   

Live

Coherent elastic neutrino-nucleus scattering, CE$\nu$NS, was observed for the first time in 2017 after forty years of experimentation, with neutrino energies about 10 times larger than those of reactor neutrinos. Here we assume that neutrinos from reactors and other MeV-sources eventually will be detected using CE$\nu$NS. CE$\nu$NS is attractive for applications in nuclear security since it may allow for significantly reduced detector masses, as well as detection of neutrinos from breeding reactions and spent nuclear fuel. On the other hand, inverse beta decay (IBD) has been routinely used for decades for the detection of reactor neutrinos. Therefore, it appears timely to compare the potential of CE$\nu$NS and IBD. We will use neutrino fluxes measured from reactors and their cross sections to compute the energy spectra of neutrinos from $^{235}$U, $^{238}$U, $^{239}$Pu, and $^{241}$Pu, and determine and compare neutrino detection event counts using either IBD or CE$\nu$NS. This characterization will inform future detector choices and is directly applicable to various neutrino sources, including reactor neutrinos, spent fuel neutrinos, and geoneutrinos. The result is potentially useful in monitoring spent nuclear fuel and reactors in support of nuclear nonproliferation safeguards.   

Live

Independent fission yields (IFYs), \textit{i.e.}, the probability of a nuclide to be produced in a fission event represent, along with decay data, the key quantity needed to predict reactor antineutrino spectra. It has been recently speculated by A.C.~Hayes and collaborators that the excess of antineutrinos at 5~MeV - colloquially known as 'the bump' - could be due to deficient knowledge of the $^{238}$U antineutrino spectrum. The last evaluation of $^{238}$U dates back to the 1990's, thus revisiting it is timely. \\ We present an effort to produce new FY recommended experimental values for fast fission of $^{238}$U. We explored different methods to evaluate IFYs, from simply adopting the new experimental data published since the last evaluation, to employing fission models to correct and use results from innovative experimental techniques (such as inverse kinematics) that are not traditionally included in FY evaluations. The various sets of $^{238}$U recommended values, and their effects in reactor antineutrino calculations will be discussed and compared.   

Live

Fission process of heavy nuclei $^{\mathrm{235}}$U, $^{\mathrm{238}}$U, $^{\mathrm{239}}$Pu and $^{\mathrm{241}}$Pu generates energy in power reactors. A fraction of this energy and the anti-neutrino emission originate from the decay of fission products. Measurements performed with the Modular Total Absorption Spectrometer (MTAS) help to understand the energy release and emitted anti-neutrino properties. MTAS array of about 1 ton total weight constructed from 19 NaI(Tl) modules packed in a close geometry has been commissioned and used for the decay studies of nearly 80 fission products at ORNL and ANL Analysis of MTAS spectra resulted in the modification of beta-gamma decay schemes and following beta-strength distribution. It was found that the average beta and anti-neutrino energy per decay are substantially reduced and the average gamma energy is increased. The status of MTAS data and their impact on the reactor anti-neutrino spectra properties will be presented.   

Live

Reactor neutrino experiments are well-suited for probing the existence of a light sterile neutrino in the region of a sub-eV$^2$ mass splitting. Using eight functionally identical antineutrino detectors, the Daya Bay experiment measures the electron antineutrinos produced by six nuclear reactors located near Shenzhen, China. A 1230-day sample of antineutrinos was used to set the most stringent limits to date on the mixing of sterile neutrinos for $2\times 10^{-4}\, < \Delta m^2_{41} < 0.3$ eV$^2$, based on two independent statistical methods, Feldman-Cousins and CL$_{\mathrm{s}}$, which gave consistent results. Going further, sensitivity was extended to larger $\Delta m^2_{41}$ using data from the Bugey-3 short-baseline reactor experiment and MINOS/MINOS+ accelerator experiments. The joint analysis excludes the LSND and MiniBooNE allowed regions for $\Delta m^2_{41} < 5$ $(1.2)$ eV$^2$ at $90\%$ $(99\%)$ C.L., strongly increasing the tension between a four-flavor interpretation of their observations and the null results of other experiments.   

Live

Since discovering its non-zero value in 2012, the Daya Bay Reactor Neutrino Experiment continually makes evermore precise measurements of $\theta $13, the smallest mixing angle in the three-neutrino mixing framework. The experiment is composed of eight functionally identical detectors located underground at different baselines from three pairs of reactors. This arrangement provides Daya Bay with the ability to precisely look for the disappearance of reactor electron antineutrinos. In this talk, I will give a brief overview of the experiment and present the most recent neutrino oscillation results using inverse beta decay events tagged by neutron capture on gadolinium and on hydrogen.