Extraction of the U235 and Pu239

Abstract

This Letter reports the first extraction of individual antineutrino spectra from 235 U and 239 Pu fission and an improved measurement of the prompt energy spectrum of reactor antineutrinos at Daya Bay. The analysis uses 3.5 × 10 6 inverse beta-decay candidates in four near antineutrino detectors in 1958 days. The individual antineutrino spectra of the two dominant isotopes, 235 U and 239 Pu, are extracted using the evolution of the prompt spectrum as a function of the isotope fission fractions. In the energy window of 4-6 MeV, a 7% (9%) excess of events is observed for the 235 U (239 Pu) spectrum compared with the normalized Huber-Mueller model prediction. The significance of discrepancy is 4.0σ for 235 U spectral shape compared with the Huber-Mueller model prediction. The shape of the measured inverse beta-decay prompt energy spectrum disagrees with the prediction of the Huber-Mueller model at 5.3σ. In the energy range of 4-6 MeV, a maximal local discrepancy of 6.3σ is observed.

Mapping reactor neutrino spectra from TAO to JUNO

“…The coordinates span the AE2σ ranges for the mass splitting and AE1σ ranges for the mixing angles in the units of Eqs. (26)- (29). The Δχ 2 value increases noticeably by increasing δm 2 or by decreasing Δm 2 ; in other words, the mass ordering test in JUNO improves when the ratio ρ ¼ δm 2 =Δm 2 ee increases, even if by small amounts (conversely, the mass ordering would become eventually untestable for vanishing ρ).…”

“…The central values (and to some extent the errors) of the oscillation parameters in Eqs. (26)- (29) have changed, in particular for the mass splittings (about þ1σ for Δm 2 ee and −1σ for δm 2 ee ). Concerning Φ, we use the reference neutrino flux from the Oklo toolkit, corresponding to the neutrino spectrum S ν ¼ Φσ ν in Fig.…”

“…where the first term includes statistical errors only; the second term includes penalties for variations of the oscillation parameters, governed by the priors in Eqs. (26)- (29); the third term contains normalization errors for the geo-ν Th and U fluxes, normalization and (polynomial) shape systematics for the reactor fluxes, and (polynomial) energyscale systematics. The second and third terms contain up to N sys ¼ 18 systematics, treated as nuisance parameters that are marginalized away in the χ 2 J minimization [78].…”

“…In the last decade, these approaches have been challenged by new data (or by reanalyses of old data) that do not compare well with computed spectra, even invoking nonstandard physics such as sterile neutrinos [19][20][21] (not considered herein). A primary example is the unexpected "bump" observed around 5 MeV in current SBL oscillation experiments [10][11][12]22] (and possibly in older data [23]), whose understanding is still entangled with many issues, including normalization anomalies in the total flux and its fuel components [24][25][26][27][28][29][30][31], incomplete information in nuclear databases [32][33][34][35], possible energy-scale systematics [36], suppression of β-decay spectra systematics [37] via total absorption [38][39][40][41][42][43] and other techniques [44], and, on the theory side, improved calculations of (allowed and forbidden) β decay spectra [45][46][47][48][49][50][51][52][53].…”

Present and Future Contributions of Reactor Experiments to Mass Ordering and Neutrino Oscillation Studies

“…This anomaly has been recently confirmed also by NEOS (Neutrino Experiment for Oscillation at Short baseline) [60], a short-baseline reactor experiment running since 2015 in Korea, with the main aim to verify the possible existence of a sterile neutrino at the eV mass scale. At present there is no unique final explanation of this "anomaly" and the problem is still under investigation, but the different analyses [61][62][63] seem to indicate a probable correlation with the reactor power and fuel composition evolution and with the knowledge of the flux from the different fissile products, mainly from the ones generated by 235 U and 239 Pu. Alternative solutions based on a particle origin of the anomaly and predicting non standard interactions are still viable, but less probable [64].…”