Improved determination of the U235 and Pu239<

Abstract

We present the results of a combined fit of the reactor antineutrino rates and the Daya Bay measurement of σ f,235 and σ f,239 . The combined fit leads to a better determination of the two cross sections per fission: σ f,235 = 6.29 ± 0.08 and σ f,239 = 4.24 ± 0.21 in units of 10 −43 cm 2 /fission, with respective uncertainties of about 1.2% and 4.9%. Since the respective deviations from the theoretical cross sections per fission are 2.5σ and 0.7σ, we conclude that, if the reactor antineutrino anomaly is not due to active-sterile neutrino oscillations, it is likely that it can be solved with a revaluation of the 235 U reactor antineutrino flux. However, the 238 U, 239 Pu, and 241 Pu fluxes, which have larger uncertainties, could also be significantly different from the theoretical predictions.The flux of electron antineutrinos produced in nuclear reactors is generated by the β decays of the fission products of 235 U, 238 U, 239 Pu, and 241 Pu. The 2011 recalculation [1, 2] of the four fluxes led to the discovery of the reactor antineutrino anomaly [3], which is a deficit of the rate of electron antineutrinos measured in several reactor neutrino experiments. There are two known possible explanations of the reactor antineutrino anomaly: 1) a miscalculation of one or more of the four electron antineutrino fluxes [4,5] and 2) active-sterile neutrino oscillations (see Ref.[6] and references therein). In this paper we consider the first possibility and we present an improvement of the results presented in Refs. [4,5] The cross section per fission σ f,235 of the 235 U electron antineutrino flux was determined in Ref.[4] with a fit of the reactor rates by taking into account the different fuel compositions. Recently the Daya Bay Collaboration presented a determination of σ f,235 and σ f,239 obtained by measuring the correlations between the reactor core fuel evolution and the changes in the reactor antineutrino flux and energy spectrum [5]. In this paper we present a combined fit of the reactor rates and the Daya Bay measurement of σ f,235 and σ f,239 which leads to a better determination of both cross sections per fission.In the analysis of the reactor rates, we consider the theoretical ratios [4]where f a k is the antineutrino flux fraction from the fission of the isotope with atomic mass k and the coefficient r k is the corresponding correction of the theoretical cross section per fission σ SH f,k which is needed to fit the data (k = 235, 238, 239, 241, denotes, respectively We analyze the data of the reactor rates with the leastsquares statisticwhere R exp a are the measured reactor rates listed in Table 1 of Ref. [6] and V R is the covariance matrix constructed with the corresponding uncertainties. The second term in Eq. (2) serves to keep under control the variation of the rates of the minor fissionable isotopes 238 U and 241 Pu, which are not well determined by the fit [4]. We consider ∆r 238 = 15% and ∆r 241 = 10%, which are significantly larger than the nominal theoretical uncertainties (respectively, 8.15% and...

Improved measurement of the reactor antineutrino flux at Daya Bay

“…[15][16][17] that the allowed parameter space is compatible with earlier anomalies from LSND [18,19], MiniBooNE [20], GALLEX [21], and SAGE [22]. On the other hand, a number of authors [23][24][25][26] have argued that the RAA may be due to the theoretical uncertainties in the flux calculations. Recent antineutrino flux evolution results from Daya Bay are in tension with the sterile-neutrino-only explanation of RAA [27].…”

Prospects for improved understanding of isotopic reactor antineutrino fluxes

“…In this study, we refer to these isotopic IBD yields as σ 5 , σ 8 , σ 9 , and σ 1 for the isotopes Pu, respectively. To obtain isotopic IBD yields in this analysis, we use a least-squares test statistic similar to those utilized in previous studies [38,40,41]:…”

“…Measurement of variation in inverse beta decay (IBD) detection rates arising from evolution of reactor fuel content at the Daya Bay experiment has provided evidence that incorrect modeling ofν e production by individual fission isotopes may be at least partially responsible for the flux anomaly [38]. New global fits comparing measured IBD spectra [39] and integrated yields [40,41] between reactor experiments have suggested that modeling of…”

“…We begin by reproducing aspects of the global fit in Refs. [38,41,42] as a consistency check, while highlighting discontinuities present between measurements at Daya Bay and at HEU cores. We then show that a combined fit to all IBD yield data can also be utilized to produce a measurement ofν e production by 238 U and that the central value of this measurement differs from existing predictions.…”

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

“…One possibility is a partial mistake in the currently adopted reactor neutrino flux models, confirmed also by the presence of the "bump" (discussed above) in the region of the spectrum between 4 and 6 MeV and by the partial inconsistency with the fuel evolution results observed by Daya Bay and RENO. As underlined in [66][67][68], such a solution for the reactor antineutrino anomaly would require mainly a theoretical revaluation of the 235 U reactor antineutrino flux.…”