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Neutrinos · D02 · Time correlations

Reactor Antineutrino Coincidence

Can you widen the timing window to catch more reactor antineutrinos without letting in too many false pairs? Up to a point. A reactor antineutrino gives a prompt flash, then a delayed one when the freed neutron is captured. Widening the time window between them keeps more real captures but also lets in more unrelated flashes that happen to pair up. Compare selection choices on the same data, and measure the accidental rate directly with a time-shifted window or a run with the reactor off.

Record twenty seconds of detector pulses and look for a prompt flash followed by a delayed one. Widen the time window, compare with a time-shifted control sample, then turn the reactor off and see which pairs remain.

Interactive modelReactor Antineutrino Coincidence
Selected pairs—\text{—}
Time-shift control—\text{—}
Control-subtracted pairs—\text{—}
True pairs · simulation only—\text{—}
Accidental pairs · truth—\text{—}
Ideal time-window acceptance—\text{—}

Recorded data from a teaching simulation

Inspect the measurement

Preparing a reproducible teaching record…

Read a sample of the record
Scientific reference ↗Read the worldwide history →2026 Nobel Prize in Physics →

Physics tutorial

Choosing the time window is part of the experiment

BackgroundInverse beta decay produces a positron and a neutron, which give a prompt pulse and, once the neutron is captured, a delayed pulse. In this teaching model every pulse is labeled correctly as prompt or delayed. Real events and unrelated single pulses are both sampled in continuous time.

Why it mattersDoes widening the window improve the evidence, or just the count?

Start with the essentials

Focus question
Does widening the window improve the evidence, or just the count?
One-sentence intuition
Compare different windows on the same recorded data. Take new data only when you change the source or the background.

Core mathematical model

Fraction of neutron captures inside the window

ϵ(T)=∫0Te−t/ττ dt=1−e−T/τ\epsilon(T)=\int_0^T\frac{e^{-t/\tau}}{\tau}\,dt=1-e^{-T/\tau}

The model assumes a mean capture time of 30 microseconds, and the window opens at zero delay. This formula gives the expected fraction for comparison; the event counts come from the simulated record, not from this formula.

Accidental pairs from unrelated pulses

Nacc≃RpRdT texpN_{\mathrm{acc}}\simeq R_pR_dT\,t_{\mathrm{exp}}

The two rates are the total pulse rates in the prompt and delayed classes. Every pair with the delayed pulse after the prompt one is counted, so one pulse can belong to several pairs. Shifting the delayed pulses in time keeps both rates but breaks up the real short-delay pairs.

Common difficulties

Choosing the time window is part of the experiment

Typical misconceptionAny prompt pulse followed by a delayed one must come from a neutrino.

Better mental modelTwo unrelated pulses can pass the same timing rule by chance. You need a control measurement and further tests specific to the reaction.

Run the experiment

  1. 01

    Sweep the window on one record

    Acquire once, then sweep the window from 2 to 200 microseconds.

    What to observe: The number of true pairs levels off, while accidental pairs keep growing as the window widens.
  2. 02

    Check the control sample

    Choose Crowded record. Compare the time-shifted pairs with the true number of accidental pairs from the simulation.

    What to observe: The two numbers fluctuate independently, so subtracting the control gives only an estimate of the signal. The excess can even come out negative.
  3. 03

    Turn the source off

    Choose Reactor off, then acquire another record.

    What to observe: Accidental pairs remain. In this model turning the reactor off removes only the signal; a real experiment must also look for backgrounds that produce correlated pairs.

Related experiments and articles

Continue exploring

How detectors see a neutrino

A neutrino leaves only light, charge or a pair of signals close in time. How does each detector turn that into a direction, an energy or a source?

  1. You are hereReactor Antineutrino CoincidenceSelect prompt and delayed signals against accidental pairs.
  2. LabCherenkov Light & Detector RingsTurn charged-particle geometry into a recorded light pattern.
  3. LabLiquid Argon Time Projection ChamberTurn electron arrival times and wire projections into calibrated slice centroids.
  4. LabIceCube: From Sensor Hits to a TrackInfer a track direction from the recorded sensor times.
  5. LabSupernova Neutrino Burst ObservatoryScan a frozen burst record, then inspect one interaction at the optical timescale.
  6. LabNeutrino Sky Search ObservatorySearch frozen neutrino directions and compare a local hotspot with paired finite-grid background trials.

Neutrinos · Reading path

Read reactors and the Earth

Articles on this path that use this experiment, followed by the other experiments they suggest.

  1. ArticleWhere does a reactor’s antineutrino spectrum come from?
  2. ArticleKamLAND: testing a solar mystery with reactors on Earth
  3. ArticleDaya Bay: why compare a near detector with a far one?
  4. LabReactor Near–Far Spectra
  5. ArticleRENO and Double Chooz: why repeat the mixing-angle measurement?
  6. LabNeutrino Flavor Propagation
  7. ArticleJUNO: why must a huge detector resolve tiny spectral features?
  8. ArticleThe reactor anomaly and the spectrum bump ask different questions
  9. ArticleShort reactor baselines: look for a change with distance
  10. ArticleGeoneutrinos: what can particles tell us about the Earth’s heat?

Neutrinos · Reading path

Start with the prize

Articles on this path that use this experiment, followed by the other experiments they suggest.

  1. ArticleHow does a faint signal become a neutrino discovery?
  2. LabIceCube: From Sensor Hits to a Track
  3. ArticleWhy do neutrinos keep appearing in the Nobel Prize?
  4. LabCherenkov Light & Detector Rings
4 more groups

Neutrinos · Reading path

Follow the measurement chain

Articles on this path that use this experiment, followed by the other experiments they suggest.

  1. ArticleWhy put a neutrino experiment under rock, sea or ice?
  2. LabIceCube: From Sensor Hits to a Track
  3. ArticleWater, scintillator and argon: what does each detector see?
  4. LabLiquid Argon Time Projection Chamber
  5. LabCherenkov Light & Detector Rings

Neutrinos · Reading path

From missing energy to a new particle

Articles on this path that use this experiment, followed by the other experiments they suggest.

  1. ArticleCowan and Reines: finding the neutrino in two flashes

Neutrinos · Reading path

Build a new way to observe the sky

Articles on this path that use this experiment, followed by the other experiments they suggest.

  1. ArticleSN 1987A: a supernova seen from underground
  2. LabSupernova Neutrino Burst Observatory
  3. LabCherenkov Light & Detector Rings

Particles and detectors · Reading path

Detecting a passing particle

Articles on this path that use this experiment, followed by the other experiments they suggest.

  1. ArticleBothe and Geiger ask whether two signals belong together
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