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Register a new userFirst antineutrino energy spectrum from $^{235}$U fissions with the STEREO detector at ILL
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This article reports the measurement of the $^{235}$U-induced antineutrino spectrum shape by the STEREO experiment. 43000 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 $E_ u simeq 5.3$ MeV. A gaussian fit of this local excess leads to an amplitude of $A = 12.1 pm 3.4%$ (3.5$sigma$).
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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 $3.5times 10^6$ 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 $4.0sigma$ 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 $5.3sigma$. In the energy range of 4--6~MeV, a maximal local discrepancy of $6.3sigma$ is observed.
We report a measurement of the antineutrino rate from the fission of U-235 with the STEREO detector using 119 days of reactor turned on. In our analysis, we perform several detailed corrections and achieve the most precise single measurement at reactors with highly enriched U-235 fuel. We measure an IBD cross section per fission of $sigma_f$ = (6.34 $pm$ 0.06 [stat] $pm$ 0.15 [sys] $pm$ 0.15 [model]) $times$ 10${}^{-43}$ cm${}^{2}$/fission and observe a rate deficit of (5.2 $pm$ 0.8 [stat] $pm$ 2.3 [sys] $pm$ 2.3 [model])% compared to the model, consistent with the deficit of the world average. Testing U-235 as the sole source of the deficit, we find a tension between the results of lowly and highly enriched U-235 fuel of 2.1 standard deviations.
The PROSPECT and STEREO collaborations present a combined measurement of the pure $^{235}$U antineutrino spectrum, without site specific corrections or detector-dependent effects. The spectral measurements of the two highest precision experiments at research reactors are found to be compatible with $chi^2/mathrm{ndf} = 24.1/21$, allowing a joint unfolding of the prompt energy measurements into antineutrino energy. This $bar{ u}_e$ energy spectrum is provided to the community, and an excess of events relative to the Huber model is found in the 5-6 MeV region. When a Gaussian bump is fitted to the excess, the data-model $chi^2$ value is improved, corresponding to a $2.4sigma$ significance.
This Letter reports the first measurement of the $^{235}$U $overline{ u_{e}}$ energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9m from the 85MW$_{mathrm{th}}$ highly-enriched uranium (HEU) High Flux Isotope Reactor. With a surface-based, segmented detector, PROSPECT has observed 31678$pm$304 (stat.) $overline{ u_{e}}$-induced inverse beta decays (IBD), the largest sample from HEU fission to date, 99% of which are attributed to $^{235}$U. Despite broad agreement, comparison of the Huber $^{235}$U model to the measured spectrum produces a $chi^2/ndf = 51.4/31$, driven primarily by deviations in two localized energy regions. The measured $^{235}$U spectrum shape is consistent with a deviation relative to prediction equal in size to that observed at low-enriched uranium power reactors in the $overline{ u_{e}}$ energy region of 5-7MeV.
The normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio has been measured using the NIFFTE fission Time Projection Chamber from the reaction threshold to $30$~MeV. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time-of-flight. The $^{238}$U(n,f)/$^{235}$U(n,f) ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}$U(n,f) cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$beta$5 at 14.5 MeV.
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