First antineutrino energy spectrum from $^{235}$U fissions with the STEREO detector at ILL

The collaboration
J.Phys.G 48 (2021) 075107, 2021.

Abstract (data abstract)
These data support the measurement of the $^{235}$U-induced antineutrino spectrum shape by the STEREO experiment. 43 000 antineutrinos have been detected at about 10 m from the highly enriched core of the ILL reactor during 118 full days equivalent at nominal power. The measured inverse beta decay spectrum is unfolded to provide a pure $^{235}$U spectrum in antineutrino energy. A careful study of the unfolding procedure, including a cross-validation by an independent framework, has shown that no major biases are introduced by the method. A significant local distortion is found with respect to predictions around $E_\nu = 5.3$ MeV. A gaussian fit of this local excess leads to an amplitude of 12.1 $\pm$ 3.4% (3.5$\sigma$).

• #### Measured spectrum and normalized prediction for Phase-II

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Data from Figure 13 – Measured IBD yield spectrum and area-normalized HM-based prediction. Here, error bars inlude only uncorrelated uncertainties,...

• #### Experimental covariance matrix

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Total covariance matrix of the measured spectrum, including statistics and all systematic uncertainties. It is denoted $V_\text{pr}$ in eqn.(18).

• #### Response matrix

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STEREO Detector Response Matrix, sampled using STEREO's simulation using neutrinos with energy distributed according to HFR's IBD yield prediction. The...

• #### Selection efficiency

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Data from Figure 6 – Selection efficiency as a function of $E_\nu$.

• #### Spectrum prediction in neutrino energy

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Spectrum prediction for ILL's High Flux Reactor, given in 50keV-wide $E_\nu$ bins (centers ranging from 1.8 to 10 MeV). Huber's...

• #### Unfolded $^{235}$U spectrum

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Data from Figure 14 - STEREO pure-$^{235}$U unfolded spectrum. Error bars are square-roots of diagonal coefficients of the covariance matrix,...

• #### Covariance matrix of the unfolded spectrum

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Covariance matrix of the unfolded $^{235}$U spectrum, including statistical and systematic uncertainties. It is denoted $V_\Phi$ in Section 8.