Institute of research into the fundamental laws of the Universe

Two methods for predicting a spectrum

Reactor antineutrinos are mostly emitted from the β^- decay of the fission products of uranium 235 and 238 and plutonium 239 and 241 in nuclear fuel (see figure 1). Since their first detection in 1956, they are widely used for probing neutrino fundamental properties, and more recently for searching for signs of new physics beyond the Standard Model. Determining the spectra of antineutrinos emitted by a reactor, which are essential for interpreting measurements, remains an arduous task, which still motivates huge experimental and theoretical efforts. Some first attempts were conducted to quantify the neutrino interaction cross-section from measurements and date back to the late 1950s. At this time, computational resources were much more limited than they are today, and the required input nuclear data were unreliable and very incomplete. Predictions then relied on a clever procedure using virtual beta branches to convert an electron spectrum, measured following irradiation of a uranium-235 foil by a thermal neutron flux, into an antineutrino spectrum [4]. This method, known as the conversion method, has since been extensively tested and refined, notably thanks to a remarkable series of high-precision measurements of electron spectra obtained in the 1980s at the high-flux reactor of the Institut Laue Langevin (ILL, Grenoble).

The more natural and direct summation method models and sums the beta spectra of all known fission products to predict the expected total spectrum (see figure 2). The first realistic calculations were published in the early 1970s, especially with the advent of the first evaluated nuclear databases [5]. However, the reliability of these calculations is severly questioned by a number of challenges, the main one being that the nuclear data are incomplete and often poorly known, since they are based on measurements that are often inconsistent. A blatant example of this major flaw, now well known to the nuclear data evaluation community, is the “pandemonium effect”, which biases the determination of decay patterns of excited nuclei by measuring their gamma cascades [6]. For these reasons, antineutrino spectra have long been predicted using the conversion method. Until very recently, the Huber-Mueller (HM) conversion model, based on electron measurements at ILL, was the reference in the community.

Results and outlook

All these developments have improved the predictive power of the summation model, while protecting it from possible calculation biases. Uncertainties of the order of one percent around 2 MeV to ten percent above 8 MeV have been obtained on the calculation of an antineutrino spectrum, making this new model competitive with conversion predictions. This new version of the summation model was then compared with the latest experimental measurements and state-of-the-art theoretical predictions. Figure 3 compares IBD measurements and predictions of antineutrino fluxes emitted by fission of uranium 235 and plutonium 239, expressed relatively to the reference HM prediction (in blue). Remarkably, the new summation model (in green) agrees very well with the latest result from STEREO (orange band), which has provided the most accurate measurement of antineutrino flux from uranium-235 fission to date. Interestingly, the new summation model shifts the IBD flux from uranium-235 fission by -(7.5 ± 3.9)% compared with the HM model. This shift would significantly reduce the statistical significance of the RAA. It also supports other suspicions that poor normalization of the uranium-235 electron data measured at ILL is at the root of the prediction bias in the HM reference model. Other comparative studies have been carried out, showing good agreement in rate and shape with the most accurate IBD measurements conducted at reactors (Daya Bay, STEREO, PROSPECT, etc.). These studies demonstrate the added value of this new model for analyzing and interpreting all available experimental data in a coherent manner, in order to shed light on the origin of reactor antineutrino anomalies. This last point is essential, particularly in view of future experimental programs involving measurements on reactors, whether using inverse beta decay or coherent elastic neutrino scattering on nuclei.