In 2011, a recalculation of the antineutrino flux expected from nuclear reactors found that the measured rate at short-baseline experiments was about 6% lower than predicted. The discrepancy became known as the reactor antineutrino anomaly, and the most exciting interpretation was that some reactor antineutrinos were oscillating into a sterile neutrino with mass around 1 eV — a particle not predicted by the Standard Model.
Testing this interpretation directly required putting a detector very close to a reactor core. At distances of about 10 meters from the reactor, conventional three-flavor oscillations would not yet have developed, but sterile-neutrino oscillation (driven by a mass-squared difference around 1 eV², far larger than the active-neutrino mass differences) would already be in full effect. A measurement of the antineutrino energy spectrum and arrival rates as a function of distance would either confirm or rule out the sterile hypothesis.
Two experiments were built to perform exactly this measurement. PROSPECT at the High Flux Isotope Reactor in Oak Ridge, Tennessee, and STEREO at the Institut Laue-Langevin reactor in Grenoble, France, both reported their results across the late 2010s and early 2020s. Neither found the sterile neutrino. Both placed strong constraints on the parameter space that the anomaly had originally pointed toward.
Why very short baselines
The standard formula for two-flavor neutrino oscillation predicts that the survival probability oscillates as a function of L/E — the distance divided by neutrino energy. For three known active flavors, the oscillation length corresponding to atmospheric-scale mass-squared differences is around 1 kilometer at GeV energies, and around 100 kilometers at reactor energies (few MeV). For solar-scale mass-squared differences, the oscillation length is much longer.
A sterile-neutrino mass-squared difference of order 1 eV² has a much shorter oscillation length — roughly 2 meters at MeV energies. This means that at a baseline of 10 meters, the survival probability has gone through multiple oscillation cycles for the sterile mode, while the active modes have not yet begun to oscillate.
The experimental signature would be:
- An overall deficit in the measured antineutrino rate (the average suppression from the sterile mode).
- A spectral distortion that depends on the baseline (because different energies oscillate at different L/E values).
- A position-dependent rate inside the detector (segments closer to the reactor see less oscillation than segments further away).
The third feature is particularly powerful. By using a segmented detector, an experiment can measure the same neutrino flux at multiple effective baselines simultaneously, eliminating most systematic uncertainties about the absolute reactor flux. Only the rate ratio between segments matters for sterile-neutrino constraints.
The PROSPECT design
PROSPECT was deployed at the HFIR research reactor at Oak Ridge National Laboratory. HFIR is a compact 85-megawatt research reactor that operates on highly-enriched uranium-235 — making it a relatively pure source of antineutrinos from a single fissioning isotope, which simplifies the spectral analysis.
The PROSPECT detector consisted of 4 tons of lithium-loaded liquid scintillator segmented into 154 rectangular cells, each about 14 cm × 14 cm × 117 cm, arranged in an 11 × 14 array. The lithium-6 loading provides clean neutron-capture signals (lithium-6 + neutron → tritium + alpha) that flag inverse-beta-decay events through their characteristic delayed-coincidence signature.
The detector sat in a service room about 7-9 meters from the HFIR reactor core. The depth into the detector array provided multiple effective baselines, ranging from about 6.5 meters at the near face to 9.2 meters at the far face. The segmented design allowed PROSPECT to measure the spectrum and rate as a function of baseline simultaneously.
PROSPECT operated from 2018 to 2019 before various technical issues led to its decommissioning. Final results based on its accumulated data were published in 2024.
The STEREO design
STEREO was a similar concept deployed at the Institut Laue-Langevin in Grenoble, a research facility with a 58-megawatt reactor used primarily for neutron-beam experiments. Like HFIR, the ILL reactor uses highly-enriched uranium-235, providing a clean spectrum.
The STEREO detector held about 1.7 tons of gadolinium-loaded liquid scintillator divided into six cells of roughly 250 liters each, arranged linearly. The detector was placed 9-11 meters from the reactor core. Each of the six cells provided an independent baseline measurement.
STEREO operated from 2017 to 2020, accumulating substantially more statistics than PROSPECT before completing its physics run. Final results were published in 2023.
What both experiments found
Despite their different host reactors, different detector technologies, and different teams, PROSPECT and STEREO reached consistent conclusions.
No evidence for sterile-neutrino oscillation. The measured antineutrino spectra and rates as a function of baseline were consistent with no oscillation beyond what three-flavor active neutrinos predict at the relevant L/E values. The expected sterile-neutrino signature was not seen.
Strong exclusion of the original parameter space. The combined limits from PROSPECT and STEREO exclude most of the parameter region originally suggested by the reactor antineutrino anomaly. A sterile neutrino at the parameters that would explain the 6% rate deficit is now ruled out at high confidence.
Continued spectral distortion at 5 MeV. Both experiments confirmed the existence of the 5 MeV bump — the unexpected excess of events around 5 MeV that other reactor experiments had also seen. The bump appears at the same location and with the same magnitude in PROSPECT and STEREO data, strengthening the case that it is a real feature of reactor antineutrino emission rather than an experimental artifact. The leading current interpretation involves miscalculated forbidden beta-decay transitions in fission fragments.
How the picture has evolved
The reactor antineutrino anomaly was, in 2011, a serious candidate for new-physics evidence. The combination of theoretical flux recalculation and consistent experimental deficits across multiple reactors produced a coherent picture pointing to a sterile-neutrino interpretation.
A decade and a half of follow-up work has narrowed the picture substantially.
The flux calculations have been improved. Updated summation-method calculations with better nuclear data reproduce more of the measured spectrum than the original 2011 calculations did. The size of the remaining “anomalous deficit” has shrunk.
The fuel composition dependence is now understood. Some of the original anomaly was driven by uranium-235-specific issues that affect different reactors differently. Modern measurements broken down by fuel composition show patterns consistent with flux-calculation errors rather than sterile-neutrino oscillation.
Direct very-short-baseline experiments don’t see the signal. PROSPECT, STEREO, and other very-short-baseline experiments (DANSS, NEOS, Neutrino-4) have looked for the oscillation pattern and not found the simplest sterile-neutrino interpretation. The Neutrino-4 collaboration has reported a positive signal at parameters somewhat off from where the original anomaly pointed, but this claim is widely disputed in the broader community.
Gallium anomaly stands somewhat apart. The separate gallium anomaly — a roughly 20% deficit in calibration runs of GALLEX and SAGE, recently strengthened by the BEST experiment — does still favor a sterile-neutrino interpretation at parameters not too far from where the reactor anomaly originally pointed. The tension between gallium and reactor results is a current research focus.
What PROSPECT and STEREO contributed beyond the headline result
Even setting aside the sterile-neutrino question, the two experiments produced useful science.
Precision spectrum measurements. Both experiments measured the antineutrino spectrum from uranium-235 fission with high precision, providing calibration for the broader reactor neutrino physics program.
The 5 MeV bump confirmed. The independent observation of the 5 MeV spectral feature at HFIR and ILL strengthens the interpretation as a real reactor-emission phenomenon rather than detector-specific artifact.
Compact detector techniques. The technology developed for PROSPECT and STEREO — segmented liquid scintillator with neutron-capture flagging at very low overburden — has informed subsequent detector designs for similar applications.
Reactor monitoring potential. Both detectors demonstrate, at the technology-demonstration level, that compact antineutrino detectors near reactors can characterize the reactor’s fuel composition and power output non-invasively. This has potential applications in nuclear non-proliferation monitoring.
After PROSPECT and STEREO
Both experiments have completed their primary physics runs. Future very-short-baseline reactor work will be carried by other programs. CONUS+ in Germany is using cryogenic detectors to look for CEvNS at reactors, with secondary sensitivity to short-baseline oscillations. CHOOZ-style reactor experiments at larger baselines continue to refine the longer-distance picture.
The sterile-neutrino question itself has shifted to other channels. The LSND/MiniBooNE anomaly tested by MicroBooNE, the gallium anomaly tested by BEST, and various cosmological constraints together form the current frontier of sterile-neutrino searches.
A measured exclusion
PROSPECT and STEREO are examples of how science can carefully and decisively rule out a specific new-physics scenario without producing a dramatic discovery. The reactor antineutrino anomaly was an interesting puzzle for a decade. The most natural interpretation pointed to a sterile neutrino. Direct experimental tests at the relevant parameters did not find it. The simplest version of the hypothesis is now excluded.
The work is not exciting in the way a discovery would have been. But the constraints it provided are now part of the foundation that future neutrino-physics work builds on. When some future experiment claims a sterile-neutrino-related signal at parameters relevant to reactor experiments, the question “but didn’t PROSPECT and STEREO rule that out?” will be a real and important check.
That kind of careful, narrowing work is, in many ways, what most of modern particle physics actually looks like.
For the underlying anomaly, see The reactor antineutrino anomaly. For the parallel sterile-neutrino searches at accelerators, see MicroBooNE. For the gallium-related counterpart, see GALLEX and SAGE. For the broader sterile picture, see Sterile neutrinos: a stubborn maybe.
Frequently asked
What is a very-short-baseline reactor experiment?
A neutrino oscillation experiment in which the detector is placed within about 10-20 meters of the reactor core, much closer than conventional reactor neutrino experiments (which typically operate at distances of hundreds of meters to kilometers). At such short baselines, ordinary three-flavor neutrino oscillations have not yet had time to develop, so any observed flavor-changing behavior would indicate the existence of a sterile-neutrino state with a much larger mass-squared difference.
What is PROSPECT?
PROSPECT (Precision Reactor Oscillation and SPECTrum experiment) was a compact antineutrino detector deployed at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory in Tennessee. It used 4 tons of segmented liquid scintillator placed about 7-9 meters from the reactor core, providing simultaneous measurements at multiple baselines through its segmented design.
What is STEREO?
STEREO was a similar experiment installed at the Institut Laue-Langevin (ILL) research reactor in Grenoble, France. It used about 1.7 tons of gadolinium-loaded liquid scintillator divided into six cells, positioned 9-11 meters from the reactor core. Like PROSPECT, STEREO was specifically designed to test the sterile-neutrino interpretation of the reactor antineutrino anomaly.
What did they find?
Neither experiment found evidence for a sterile neutrino at the parameters predicted to explain the reactor antineutrino anomaly. PROSPECT's final results (published in 2024) excluded most of the relevant parameter space at high confidence. STEREO's final results (2023) reached similar conclusions. Combined with other very-short-baseline experiments, the simplest sterile-neutrino-at-1-eV interpretation of the reactor anomaly is now strongly disfavored.
Are sterile neutrinos completely ruled out then?
No. The PROSPECT and STEREO results rule out a specific simple sterile-neutrino scenario with parameters tuned to explain the reactor anomaly. More elaborate sterile-neutrino models — with multiple states, decay channels, or non-standard interactions — remain viable in some parameter regions. But the simplest 'one sterile state at 1 eV mixing with electrons' picture is largely excluded.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, November 8). PROSPECT and STEREO: hunting sterile neutrinos meters from reactor cores. Neutrino Times. https://neutrino-times.com/articles/prospect-stereo-very-short-baseline-reactor/
Chicago
Neutrino Times Editorial Team. "PROSPECT and STEREO: hunting sterile neutrinos meters from reactor cores." Neutrino Times, November 8, 2025. https://neutrino-times.com/articles/prospect-stereo-very-short-baseline-reactor/.
MLA
Neutrino Times Editorial Team. "PROSPECT and STEREO: hunting sterile neutrinos meters from reactor cores." Neutrino Times, 8 Nov. 2025, https://neutrino-times.com/articles/prospect-stereo-very-short-baseline-reactor/.
BibTeX
@misc{neutrino-times-prospect-stereo-very-short-baseline-reactor,
author = {Neutrino Times Editorial Team},
title = {PROSPECT and STEREO: hunting sterile neutrinos meters from reactor cores},
howpublished = {Neutrino Times},
year = {2025},
month = {nov},
url = {https://neutrino-times.com/articles/prospect-stereo-very-short-baseline-reactor/},
note = {Accessed: 2025-11-08}
} RIS
TY - GEN TI - PROSPECT and STEREO: hunting sterile neutrinos meters from reactor cores AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-11-08 PB - Neutrino Times UR - https://neutrino-times.com/articles/prospect-stereo-very-short-baseline-reactor/ ER -