---
title: "Reactor Antineutrino Coincidence Interactive Lab | Sandbox Physics"
description: "Can you widen the timing window to catch more reactor antineutrinos without letting in too many false pairs? Up to a point."
canonical_url: "https://sandboxphysics.com/labs/reactor-coincidence/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/reactor-coincidence/"
last_modified: "2026-10-27"
---

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 simulation: open this page in a web browser to change the parameters and run the model.

This is a teaching simulation, not the Cowan–Reines data. Every pulse is sorted correctly as prompt or delayed. Neutron capture times follow an exponential with a 30 microsecond lifetime, and unrelated single pulses arrive independently at a steady rate. Every pair with the delayed pulse after the prompt one is counted. Energy cuts, dead time, correlated neutron background and the full detector response are left out. The apparatus drawing is schematic; the counts and plots come from the same generated record.

Recorded data from a teaching simulation

## Inspect the measurement

Preparing a reproducible teaching record…

Read a sample of the record

[Scientific reference ↗](https://doi.org/10.1103/PhysRev.117.159) [Read the worldwide history →](https://sandboxphysics.com/stories/neutrinos/) [2026 Nobel Prize in Physics →](https://sandboxphysics.com/nobel/2026/physics/)

Physics tutorial

## Choosing the time window is part of the experiment

**Background** Inverse 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 matters** Does 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

$$
\epsilon(T)=\int_0^T\frac{e^{-t/\tau}}{\tau}\,dt=1-e^{-T/\tau}
$$

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

$$
N_{\mathrm{acc}}\simeq R_pR_dT\,t_{\mathrm{exp}}
$$

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 misconception** Any prompt pulse followed by a delayed one must come from a neutrino.

**Better mental model** Two 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.

Physical conclusion

Compare different windows on the same recorded data. Take new data only when you change the source or the background.

## 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 here **Reactor Antineutrino Coincidence** Select prompt and delayed signals against accidental pairs.](https://sandboxphysics.com/labs/reactor-coincidence/)
2. [Lab **Cherenkov Light & Detector Rings** Turn charged-particle geometry into a recorded light pattern.](https://sandboxphysics.com/labs/cherenkov-rings/)
3. [Lab **Liquid Argon Time Projection Chamber** Turn electron arrival times and wire projections into calibrated slice centroids.](https://sandboxphysics.com/labs/liquid-argon-tpc/)
4. [Lab **IceCube: From Sensor Hits to a Track** Infer a track direction from the recorded sensor times.](https://sandboxphysics.com/labs/icecube-reconstruction/)
5. [Lab **Supernova Neutrino Burst Observatory** Scan a frozen burst record, then inspect one interaction at the optical timescale.](https://sandboxphysics.com/labs/supernova-neutrino-burst/)
6. [Lab **Neutrino Sky Search Observatory** Search frozen neutrino directions and compare a local hotspot with paired finite-grid background trials.](https://sandboxphysics.com/labs/neutrino-sky-search/)

Neutrinos · Reading path

### Read reactors and the Earth

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

1. [Article **Where does a reactor’s antineutrino spectrum come from?**](https://sandboxphysics.com/stories/neutrinos/reactor-antineutrino-spectrum-origins/)
2. [Article **KamLAND: testing a solar mystery with reactors on Earth**](https://sandboxphysics.com/stories/neutrinos/kamland-reactor-oscillations/)
3. [Article **Daya Bay: why compare a near detector with a far one?**](https://sandboxphysics.com/stories/neutrinos/daya-bay-near-far/)
4. [Lab **Reactor Near–Far Spectra**](https://sandboxphysics.com/labs/reactor-near-far-spectra/)
5. [Article **RENO and Double Chooz: why repeat the mixing-angle measurement?**](https://sandboxphysics.com/stories/neutrinos/reno-double-chooz-independent-mixing-tests/)
6. [Lab **Neutrino Flavor Propagation**](https://sandboxphysics.com/labs/neutrino-flavor-propagation/)
7. [Article **JUNO: why must a huge detector resolve tiny spectral features?**](https://sandboxphysics.com/stories/neutrinos/juno-fine-spectrum-and-mass-ordering/)
8. [Article **The reactor anomaly and the spectrum bump ask different questions**](https://sandboxphysics.com/stories/neutrinos/reactor-anomaly-and-spectrum-bump/)
9. [Article **Short reactor baselines: look for a change with distance**](https://sandboxphysics.com/stories/neutrinos/short-baseline-reactor-sterile-tests/)
10. [Article **Geoneutrinos: what can particles tell us about the Earth’s heat?**](https://sandboxphysics.com/stories/neutrinos/geoneutrinos-earth-radiogenic-heat/)

Neutrinos · Reading path

### Start with the prize

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

1. [Article **How does a faint signal become a neutrino discovery?**](https://sandboxphysics.com/stories/neutrinos/neutrino-discovery-evidence-chain/)
2. [Lab **IceCube: From Sensor Hits to a Track**](https://sandboxphysics.com/labs/icecube-reconstruction/)
3. [Article **Why do neutrinos keep appearing in the Nobel Prize?**](https://sandboxphysics.com/stories/neutrinos/neutrinos-and-nobel-prizes/)
4. [Lab **Cherenkov Light & Detector Rings**](https://sandboxphysics.com/labs/cherenkov-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. [Article **Why put a neutrino experiment under rock, sea or ice?**](https://sandboxphysics.com/stories/neutrinos/underground-underwater-neutrino-sites/)
2. [Lab **IceCube: From Sensor Hits to a Track**](https://sandboxphysics.com/labs/icecube-reconstruction/)
3. [Article **Water, scintillator and argon: what does each detector see?**](https://sandboxphysics.com/stories/neutrinos/neutrino-detector-measurement-chains/)
4. [Lab **Liquid Argon Time Projection Chamber**](https://sandboxphysics.com/labs/liquid-argon-tpc/)
5. [Lab **Cherenkov Light & Detector Rings**](https://sandboxphysics.com/labs/cherenkov-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. [Article **Cowan and Reines: finding the neutrino in two flashes**](https://sandboxphysics.com/stories/neutrinos/reactor-first-detection/)

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. [Article **SN 1987A: a supernova seen from underground**](https://sandboxphysics.com/stories/neutrinos/sn1987a-burst/)
2. [Lab **Supernova Neutrino Burst Observatory**](https://sandboxphysics.com/labs/supernova-neutrino-burst/)
3. [Lab **Cherenkov Light & Detector Rings**](https://sandboxphysics.com/labs/cherenkov-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. [Article **Bothe and Geiger ask whether two signals belong together**](https://sandboxphysics.com/stories/particle-evidence/bothe-coincidence-method/)

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)