Reactor Neutrino Spectrum Research Articles

Potential impact of sub-structure on the determination of neutrino mass hierarchy at medium-baseline reactor neutrino oscillation experiments

In the past decade, the precise measurement of the lastly known neutrino mixing angle theta _{13} has enabled the resolution of neutrino mass hierarch (MH) at medium-baseline reactor neutrino oscillation (MBRO) experiments. Recent calculations of the reactor neutrino flux predict percent-level sub-structures in the bar{nu }_e spectrum due to Coulomb effects in beta decay. Such fine structure in the reactor spectrum has been an issue of concern for efforts to determine the neutrino MH for the MBRO approach, the concern being that the sub-dominant oscillation pattern used to discriminate different hierarchies will be obscured by fine structure. The energy resolutions of current reactor experiments are not sufficient to measure such fine structure, and therefore the size and location in energy of these predicted discontinuities has not been confirmed experimentally. There has been speculation that a near detector is required with sufficient energy resolution to resolve the fine structure. This article studies the impact of fine structure on the resolution of MH, based on predicted reactor neutrino spectra, using the measured spectrum from Daya Bay as a reference. We also investigate how a near detector could improve the sensitivity of neutrino MH resolution based on various assumptions of near detector energy resolution.

Mapping reactor neutrino spectra from TAO to JUNO

The Jiangmen Underground Neutrino Observatory (JUNO) project aims at probing, at the same time, the two main frequencies of three-flavor neutrino oscillations, as well as their interference related to the mass ordering (normal or inverted), at a distance of ~53 km from two powerful reactor complexes in China, at Yangjiang and Taishan. In the latter complex, the unoscillated spectrum from one reactor core is planned to be closely monitored by the Taishan Antineutrino Observatory (TAO), expected to have better resolution (x 1/2) and higher statistics (x 30) than JUNO. In the context of neutrino energy spectra endowed with fine-structure features from summation calculations, we analyze in detail the effects of energy resolution and nucleon recoil on observable event spectra. We show that a model spectrum in TAO can be mapped into a corresponding spectrum in JUNO through appropriate convolutions. The mapping is exact in the hypothetical case without oscillations, and holds to a very good accuracy in the real case with oscillations. We then analyze the sensitivity to mass ordering of JUNO (and its precision oscillometry capabilities) assuming a single reference spectrum, as well as bundles of variant spectra, as obtained by changing nuclear input uncertainties in summation calculations from a publicly available toolkit. We show through a chi-squared analysis that variant spectra induce little reduction of the sensitivity in JUNO, especially when TAO constraints are included. Subtle aspects of the statistical analysis of variant spectra are also discussed.

Why matter effects matter for JUNO

In this paper we focus on the Earth matter effects for the solar parameter determination by a medium baseline reactor experiment such as JUNO. We derive perturbative expansions for the mixing angles θ12 and θ13 as well as the Δm212 and Δm312 in terms of the matter potential relevant for JUNO. These expansions, up to second order in the matter potential, while simple, allow one to calculate the electron antineutrino survival probability to a precision much better than needed for the JUNO experiment. We use these perturbative expansions to semi-analytically explain and confirm the shift caused by the matter effects on the solar neutrino mixing parameters θ12 and Δm212 which were previously obtained by a purely numerical χ2 analysis. Since these shifts do not satisfy the naive expectations and are significant given the precision that can be achieved by the JUNO experiment, a totally independent cross check using a completely different method is of particular importance. We find that these matter effect shifts do not depend on any of the details of the detector characteristics apart from the baseline and earth mass density between reactor(s) and detector, but do depend on the normalized product of reactor neutrino spectrum times the inverse-beta decay cross-section. The results of this manuscript suggests an alternative analysis method for measuring sin2⁡θ12 and Δm212 in JUNO which would be a useful cross check of the standard analysis and for the understanding of the Wolfenstein matter effect. The explanation of these shifts together with a quantitative understanding, using a semi-analytical method, is the principal purpose of this paper.

Impact of theoretical reactor flux uncertainties and of the near detector on the JUNO measurements

JUNO is medium-baseline reactor neutrino experiment that will probe the neutrino spectrum mass ordering (MO) and will measure the mass-mixing oscillation parameters with unprecedented sub-percent precision. It will be complemented with a ton-level, high energy resolution liquid scintillator reference detector, TAO, that will measure the reactor neutrino spectrum with unprecedented accuracy. In this work we study if the limited knowledge of the reactor antineutrino spectrum and of its fine structure can have a significant effect on the mass ordering determination and on the precision measurements of the oscillation parameters, and try to assess the advantages of having the TAO reference detector.

Development the nuclear decay data sublibrary for fission product

Accurate and reliable nuclear decay data libraries are essential for calculation and design of the advanced nuclear systems. To meet the requirements of decay heat calculation, burn-up calculation, analysis neutrino spectra anomaly and the other related studies, the evaluated nuclear decay data sublibrary for fission product has been developed and constituted for the first time in China based on the several main national evaluated data libraries in the world. About 1415 nuclides were included in present database. And two major data format, that's ENSDF and ENDF were provided. Some decay heat calculation, reactor neutrino spectra analysis and decay chain design needed in fission yield evaluation were performed to test the accuracy and reliability of the nuclear decay data sublibrary.

Uncertainty in the reactor neutrino spectrum and mass hierarchy determination

One of the challenges that must be overcome in order to determine the neutrino mass hierarchy using reactor neutrinos is the theoretical uncertainty in the unoscillated reactor neutrino spectrum: this is one of the reasons why, recently, it was proposed to add a near detector to the JUNO experiment. A model-independent treatment of the spectrum uncertainty will be discussed, as well as the effect that it will have on the final result. Moreover, since the neutrino spectrum depends on the chemical composition of the fuel, the spectra at the near and far detectors will be different, because they will receive neutrinos from different cores. Taking into account the time evolution of the chemical composition of the fuel in the reactor core, it is possible to reconstruct the far detector spectrum from the near detector data. We will show that the method used to reconstruct the spectrum can affect sensitivity to the mass hierarchy, however if the near detector is large enough the difference will be negligible.

Precise measurement of reactor antineutrino spectrum flux and spectrum at RENO

The RENO(Reactor Experiment for Neutrino Oscillation) experiment is to measure the smallest neutrino mixing angle theta13 using anti-neutrinos emitted from the Hanbit nuclear power plant in Korea. It is essential to compare the observed and expected fluxes of reactor antineutrinos for determining the neutrino disappearance probability. The expected reactor neutrino flux is calculated from the reactor thermal power and the fission rate of individual fuel isotope. Time-dependent fuel composition changes not only neutrino fluxes but also it distort the antineutrino spectrum. In this presentation, we describe how to derive the expected reactor neutrino fluxes and spectrum at both near and far detectors and report an excess in the observed spectrum near 5 MeV at RENO.

Analysis of reactor-neutrino spectra fully based on gross theory of beta-decay emphasizing the special role of odd-odd FP nuclides

We applied the gross theory of β-decay to calculate the reactor antineutrino (νe ) spectra emitted from 235,238 U and 239,241 Pu samples under neutron irradiation by summing up all the contributions from a large number of decaying fission-products (FPs). Considering the special role of the odd(Z )-odd(N ) FPs in spectrum-shaping, we utilized the experimentally-known spin-parity of each odd-odd FP through the treatment proposed by Nakata, Tachibana and Yamada. Owing to this treatment, the consistency between calculated and experimental spectra was remarkably improved in a way expected from the nature of the gross theory of β-decay.

Investigating the spectral anomaly with different reactor antineutrino experiments

The spectral shape of reactor antineutrinos measured in recent experiments shows anomalies in comparison to neutrino reference spectra. New precision measurements of the reactor neutrino spectra as well as more complete input in nuclear data bases are needed to resolve the observed discrepancies between models and experimental results. This article proposes the combination of experiments at reactors which are highly enriched in ${}^{235}$U with commercial reactors with typically lower enrichment to gain new insights into the origin of the anomalous neutrino spectrum. The presented method clarifies, if the spectral anomaly is either solely or not at all related to the predicted ${}^{235}$U spectrum. Considering the current improvements of the energy scale uncertainty of present-day experiments, a significance of three sigma and above can be reached. As an example, we discuss the option of a direct comparison of the measured shape in the currently running Double Chooz near detector and the upcoming Stereo experiment. A quantitative feasibility study emphasizes that a precise understanding of the energy scale systematics is a crucial prerequisite in recent and next generation experiments investigating the spectral anomaly.

Measuring θ12 despite an uncertain reactor neutrino spectrum

The recently discovered 5–7 MeV excess in the reactor neutrino spectral structure, corresponding to a prompt energy of 4–6 MeV, highlights that the uncertainty in the reactor neutrino spectrum is far greater than some theoretical estimates. Medium baseline (about 50 km) reactor neutrino experiments will deliver by far the most precise ever measurements of θ12. However, the theoretical reactor neutrino spectra, as they were recalculated in 2011, do not reproduce this excess. As a result, if a medium baseline experiment attempted to determine sin2⁡(2θ12) using the theoretical spectrum, the result would have a systematic upward bias of 1%, much larger than the expected uncertainty. We show that by using recent measurements of the reactor neutrino spectrum the precision of a measurement of θ12 at a medium baseline reactor neutrino experiment can be improved appreciably. We estimate this precision as a function of the 9Li spallation background veto efficiency and dead time.

Vetoing cosmogenic muons in a large liquid scintillator

At upcoming medium baseline reactor neutrino experiments the spallation Li-9 background will be somewhat larger than the inverse beta decay reactor neutrino signal. We use new FLUKA simulations of spallation backgrounds to optimize a class of veto strategies and find that surprisingly the optimal veto for the mass hierarchy determination has a rejection efficiency below 90%. The unrejected background has only a modest effect on the physics goals. For example Delta chi(2) for the hierarchy determination falls by 1.4 to 3 points depending on the muon tracking ability. The optimal veto strategy is essentially insensitive to the tracking ability, consisting of 2 meter radius, 1.1 second cylindrical vetoes of well tracked muons with showering energies above 3 to 4 GeV and 0.7 second full detector vetoes for poorly tracked muons above 15 to 18 GeV. On the other hand, as the uncertainty in 012 will be dominated by the uncertainty in the reactor neutrino spectrum and not statistical fluctuations, the optimal rejection efficiency for the measurement of theta(12) is 93% in the case of perfect tracking.

Possible origins and implications of the shoulder in reactor neutrino spectra

We analyze within a nuclear database framework the shoulder observed in the antineutrino spectra in current reactor experiments. We find that the ENDF/B-VII.1 database predicts that the antineutrino shoulder arises from an analogous shoulder in the aggregate fission beta spectra. In contrast, the JEFF-3.1.1 database does not predict a shoulder for two out of three of the modern reactor neutrino experiments, and the shoulder that is predicted by JEFF-3.1.1 arises from $^{238}$U. We consider several possible origins of the shoulder, and find possible explanations. For example, there could be a problem with the measured aggregate beta spectra, or the harder neutron spectrum at a light-water power reactor could affect the distribution of beta-decaying isotopes. In addition to the fissile actinides, we find that $^{238}$U could also play a significant role in distorting the total antineutrino spectrum. Distinguishing these and quantifying whether there is an anomaly associated with measured reactor neutrino signals will require new short-baseline experiments, both at thermal reactors and at reactors with a sizable epithermal neutron component.

New results from RENO and prospects with RENO-50

RENO (Reactor Experiment for Neutrino Oscillation) has made a definitive measurement of the smallest mixing angle θ13 in 2012, based on the disappearance of electron antineutrinos. More precise measurements have been obtained and presented on the mixing angle and the reactor neutrino spectrum, using ∼800 days of data. We have observed an excess of inverse-beta-decay (IBD) prompt spectrum near 5 MeV with respect to the most commonly used models. The excess is found to be consistent with coming from reactors. We have also successfully measured the reactor neutrino flux with a delayed signal of neutron capture on hydrogen. A future reactor neutrino experiment of RENO-50 is proposed to determine the neutrino mass hierarchy and to make highly precise measurements of θ12, Δm212, and Δm312. Physics goals and sensitivities of RENO-50 are presented with a strategy of obtaining an extremely good energy resolution toward the neutrino mass hierarchy.

Measurement of the liquid scintillator nonlinear energy response to electron

Nonlinearity of the liquid scintillator energy response is a key to measuring the neutrino energy spectrum in reactor neutrino experiments such as Daya Bay and JUNO. We measured the nonlinearity of the linear alkyl benzene based liquid scintillator in the laboratory, which is used in Daya Bay and will be used in JUNO, via the Compton scattering process. By tagging the scattered gamma from the liquid scintillator sample simultaneously at seven angles, the instability of the system was largely cancelled. The accurately measured nonlinearity will improve the precision of the θ13, Δm2, and reactor neutrino spectrum measurements at Daya Bay.

Method of fission product beta spectra measurements for predicting reactor anti-neutrino emission

The nuclear fission process that occurs in the core of nuclear reactors results in unstable, neutron-rich fission products that subsequently beta decay and emit electron antineutrinos. These reactor neutrinos have served neutrino physics research from the initial discovery of the neutrino to today׳s precision measurements of neutrino mixing angles. The prediction of the absolute flux and energy spectrum of the emitted reactor neutrinos hinges upon a series of seminal papers based on measurements performed in the 1970s and 1980s. The steadily improving reactor neutrino measurement techniques and recent reconsiderations of the agreement between the predicted and observed reactor neutrino flux motivates revisiting the underlying beta spectra measurements. A method is proposed to use an accelerator proton beam delivered to an engineered target to yield a neutron field tailored to reproduce the neutron energy spectrum present in the core of an operating nuclear reactor. Foils of the primary reactor fissionable isotopes placed in this tailored neutron flux will ultimately emit beta particles from the resultant fission products. Measurement of these beta particles in a time projection chamber with a perpendicular magnetic field provides a distinctive set of systematic considerations for comparison to the original seminal beta spectra measurements. Ancillary measurements such as gamma-ray emission and post-irradiation radiochemical analysis will further constrain the absolute normalization of beta emissions per fission. The requirements for unfolding the beta spectra measured with this method into a predicted reactor neutrino spectrum are explored.

The high energy neutrino nuisance at a medium baseline reactor experiment

10 years from now medium baseline reactor experiments will attempt to determine the neutrino mass hierarchy from quantities associated to the Fourier transformed neutrino spectra. Recently Qian et al. have claimed that this goal may be impeded by the strong dependence of these quantities on the reactor neutrino flux and on slight variations of |ΔM322|. We demonstrate that this effect results from a spurious dependence of the quantities on the very high energy (8+ MeV) tail of the reactor neutrino spectrum. This dependence is spurious because the high energy tail depends upon decays of exotic isotopes and is insensitive to the mass hierarchy. An energy-dependent weight in the Fourier transform eliminates this spurious dependence without decreasing the chance of correctly determining the hierarchy.

Reactor Neutrinos

We review the status and the results of reactor neutrino experiments. Short-baseline experiments have provided the measurement of the reactor neutrino spectrum, and their interest has been recently revived by the discovery of the reactor antineutrino anomaly, a discrepancy between the reactor neutrino flux state of the art prediction and the measurements at baselines shorter than one kilometer. Middle and long-baseline oscillation experiments at Daya Bay, Double Chooz, and RENO provided very recently the most precise determination of the neutrino mixing angleθ13. This paper provides an overview of the upcoming experiments and of the projects under development, including the determination of the neutrino mass hierarchy and the possible use of neutrinos for society, for nonproliferation of nuclear materials, and geophysics.

Determination of the neutrino mass hierarchy at an intermediate baseline

It is generally believed that neutrino mass hierarchy can be determined at a long baseline experiment, often using accelerator neutrino beams. Reactor neutrino experiments at an intermediate baseline have the capability to distinguish normal or inverted hierarchy. Recently, it has been demonstrated that the mass hierarchy could possibly be identified using Fourier transform to the $L/E$ spectrum if the mixing angle ${sin}^{2}(2{\ensuremath{\theta}}_{13})>0.02$. In this study, a more sensitive Fourier analysis is introduced. We found that an ideal detector at an intermediate baseline ($\ensuremath{\sim}60\text{ }\text{ }\mathrm{km}$) could identify the mass hierarchy for a mixing angle ${sin}^{2}(2{\ensuremath{\theta}}_{13})>0.005$, without requirements on accurate information of reactor neutrino spectra and the value of $\ensuremath{\Delta}{m}_{32}^{2}$.

Reactor neutrinos

We review the status and the results of reactor neutrino experiments, that toe the cutting edge of neutrino research. Short baseline experiments have provided the measurement of the reactor neutrino spectrum, and are still searching for important phenomena such as the neutrino magnetic moment. They could open the door to the measurement of coherent neutrino scattering in the near future. Middle and long baseline oscillation experiments at Chooz and KamLAND have played a relevant role in neutrino oscillation physics in the last years. It is now widely accepted that a new middle baseline disappearance reactor neutrino experiment with multiple detectors could provide a clean measurement of the last undetermined neutrino mixing angle θ 13 . We conclude by opening on possible use of neutrinos for Society: the non-proliferation of nuclear materials and geophysics. To cite this article: Th. Lasserre, H.W. Sobel, C. R. Physique 6 (2005).

Search for θ13 with Reactor Experiments

The actual best limits on the missing mixing angle θ 13 are coming from the reactor experiment at Chooz, France. In this paper the prospectives of future reactor experiments in this field are shown. The main idea is to use two detectors at the same time, with a close one which is monitoring the reactor neutrino flux and spectrum. Hence, most of the systematic errors cancel and the sensitivity on θ 13 may be enhanced significantly.