---
title: "Liquid Argon Time Projection Chamber Interactive Lab | Sandbox Physics"
description: "How does a liquid argon detector use timing to get a track’s third dimension? By timing the drift."
canonical_url: "https://sandboxphysics.com/labs/liquid-argon-tpc/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/liquid-argon-tpc/"
last_modified: "2026-10-27"
---

Neutrinos · D10 · From ionization to an image

# Liquid Argon Time Projection Chamber

**How does a liquid argon detector use timing to get a track’s third dimension?** By timing the drift. Ionization electrons drift toward the wire planes at a known speed, so the time from the start of the event to their arrival tells you how far they traveled: the depth. If the start time or the drift speed is wrong, every depth comes out wrong, and adding more wire views cannot fix that.

Charged particles leave a trail of ionization in a liquid-argon chamber. Drift the freed electrons to wire planes with three orientations, freeze the readout, and then recover the center of charge in each time slice from those same projections.

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

This is a schematic single-phase LArTPC, not a replica of a real detector or a recorded neutrino event. The field is uniform and the argon is at 87 K. The energy deposits are prescribed, with constant stopping power and fixed recombination. The wire images show ideal charge projections after deconvolution, not raw bipolar induction waveforms. The wires are coarse, and the model includes Gaussian diffusion and electronic noise; the diffusion coefficients are fixed illustrative values. With the trigger and calibration known, reconstruction gives the center of charge in each time slice. It does not recover individual tracks, identify particles or measure neutrino energy. The full assumptions and primary sources are listed below.

One charge deposit in three projections

## The chamber records charge against arrival time

Focus on the recorded signal · uncheck for the full window

### U $60^\circ$

Use in recovery

### V $-60^\circ$

Use in recovery

### W $0^\circ$

Use in recovery

Simulated charge after ideal deconvolution; U and V are not raw induction currents. Colors use a common logarithmic display scale. Linear signed pixel charges remain in the exported record; three maps do not mean three times the collected charge.

Inspect arrival slice $\text{—}$

### Recover the charge centroids

Show prescribed deposits

A point is one time-slice charge centroid, not an identified particle. Coincident branches merge; a centroid may lie between tracks. Dots outside the chamber expose a wrong assumed clock or speed and are not clipped into the physical volume.

Turn arrival times back into positions

### What must the clock know?

Known event start Assumed trigger offset $0\,\mathrm{\mu s}$ Drift-speed calibration $1.00$

Projection consistency RMS $\text{—}$

Two independent orientations determine a centroid; the third checks its projection. The residual is not an uncertainty or a significance. Start-time ambiguity remains even with three planes.

Exact transport, slice values & model assumptions

Initial free charge $\text{—}$

Expected surviving charge $\text{—}$

Maximum transverse spread $\text{—}$

The active region is 2.4 m deep, 1.8 m high and 2.8 m long. Wire spacing is 25 mm and time bins are 16.875 microseconds. The readout starts 100 microseconds before the event. Three idealized orientations share the same effective anode and arrival time; actual separated-plane transit is omitted. Display markers are enlarged. Drift at 87 K follows the ICARUS low-field parameterization used in LArSoft, restricted to 300–600 V/cm.

The charge maps include Gaussian diffusion and an 8 microsecond electronics response, plus independent zero-mean Gaussian noise in each pixel. The model leaves out ionization fluctuations at the source, field-dependent recombination, space charge, scintillation transport, wire weighting fields, transit between planes, induction deconvolution, dead channels and detailed showers. The three track shapes are set geometrically; they do not simulate neutrino interactions or particle types.

Recovery keeps pixels above three noise standard deviations or 20 electron equivalents and requires at least 1000 electron equivalents per plane per slice (or 20 noise scales). Thresholds, finite bins and overlapping deposits bias centroids; the algorithm does not infer individual correspondence or a confidence region. CSV includes every observed and expected pixel, source deposit and the current recovered centroids.

[MicroBooNE · detector ↗](https://microboone.fnal.gov/detector/) [ProtoDUNE-SP · 2020 ↗](https://arxiv.org/abs/2007.06722) [MicroBooNE · diffusion ↗](https://arxiv.org/abs/2104.06551) [LArSoft · transport model ↗](https://code-doc.larsoft.org/docs/latest/html/DetectorPropertiesStandard_8cxx_source.html)

[DUNE: why liquid argon? →](https://sandboxphysics.com/stories/neutrinos/dune-liquid-argon-and-long-baseline/) [Water, scintillator and argon →](https://sandboxphysics.com/stories/neutrinos/neutrino-detector-measurement-chains/)

Physics tutorial

## Time supplies the missing coordinate.

**Background** ICARUS developed large liquid-argon time projection chambers, and MicroBooNE and ProtoDUNE extended how they are measured and calibrated. The incoming neutrino leaves no glowing track of its own. The charged particles it produces deposit energy in the argon, which creates ionization and scintillation light.

**Why it matters** The three wire orientations give projections across the chamber. The arrival time gives the distance from the anode, but only if you know when the event started and how fast the electrons drift. This Lab looks at that chain of measurements on its own.

Start with the essentials

- **Focus question**: Can the same wire images imply different depths when the clock or speed calibration changes?
- **One-sentence intuition**: Changing the assumed clock moves every recovered depth while the recorded images stay the same, and more projections cannot make up for an unknown event start time.

### Core mathematical model

#### Coordinates and field direction

$$
\mathbf E=E\hat{\mathbf x},\quad\mathbf v_e=-v_d\hat{\mathbf x},\quad t-t_0=\frac{x}{v_d}
$$

The anode sits at zero and the cathode at 2400 mm. The field points toward the cathode, so electrons drift the other way. All three wire orientations share one effective arrival time at the anode.

#### Charge produced and lost to attachment

$$
Q_0=\frac{0.65\,\Delta E}{23.6\,\mathrm{eV}},\qquad Q(t)=Q_0e^{-t/\tau}
$$

The stopping power is a constant 2.1 MeV/cm, and a fixed fraction of 0.65 of the charge survives recombination. These are prescribed teaching deposits, not a field-dependent calorimetric calibration. The lifetime describes charge lost to attachment during the drift; it does not slow down the electrons that survive.

#### Diffusion and electronics

$$
\sigma_T^2=2D_Tt,\qquad\sigma_t^2=\frac{2D_Lt}{v_d^2}+(8\,\mathrm{\mu s})^2
$$

With the multiplier at one, the transverse and longitudinal diffusion coefficients are 12 and 4 square centimeters per second. These are illustrative constants, not field-dependent measurements. The electronics add a fixed Gaussian spread in time, and charge is summed over finite bins.

#### Three charge projections

$$
w_k=y\sin\theta_k+z\cos\theta_k,\qquad\theta_k\in\{-60^\circ,0,60^\circ\}
$$

Each wire coordinate is measured perpendicular to the wires. The 25 mm pitch is deliberately coarse. The positive signal maps approximate the charge after ideal deconvolution. Real induction signals are bipolar and carry a field response, which is left out here.

#### Recover one point per time slice, not every particle

$$
\hat x=v_{\mathrm{cal}}(t-t_{0,\mathrm{cal}}),\qquad (\hat y,\hat z)=\arg\min_{y,z}\sum_k(\bar w_k-y\sin\theta_k-z\cos\theta_k)^2
$$

Charge above threshold gives one centroid per plane in each time slice. You need at least two distinct orientations: two projections fit exactly, and a third checks whether they agree. Overlapping branches merge into one centroid and may fail that check. The residual is not a confidence interval.

### Common difficulties

#### A readout projection is not a photograph

**Typical misconception** Each positive wire image is the raw current on an induction plane.

**Better mental model** U and V show idealized charge projections after deconvolution. The model leaves out weighting-field induction, wire transparency and the raw bipolar pulse shapes.

#### The true deposit and the recovered centroids are separate

**Typical misconception** The three-dimensional deposit in the chamber view is reconstructed from the measured pixels.

**Better mental model** The chamber view shows the prescribed source, labeled as such, and how its charge drifts. The separate recovery plot uses only the observed image bins and the assumed calibration. Its optional dashed truth line is there for comparison and is not fed into the reconstruction.

### Run the experiment

1. 01

   #### Watch the electrons arrive

   Acquire a straight deposit, select Side, then move the playback time from zero until all the charge has arrived.

   **What to observe:** The electrons drift toward the anode, and the original charged-particle deposit stays where it was. Gold points mark charge that has already been collected. The markers are drawn larger than life, and playback is slowed down.

2. 02

   #### Read three views of one record

   Look at the U, V and W images. Pick a time slice by clicking or with the arrow keys.

   **What to observe:** All three images are projections of the same drifting charge. Their bright features look different because the wires point in different directions, not because they show three separate events.

3. 03

   #### Remove the clock

   Switch off Known event start, then switch it back on and change the assumed trigger offset.

   **What to observe:** Without the event start, the absolute depth cannot be determined. Changing the assumed start moves the reconstructed depth, but every measured pixel stays the same.

4. 04

   #### Change purity and geometry

   Try Impure argon and Branched deposit, then remove one or two projections.

   **What to observe:** Attachment leaves less charge to collect. When several tracks share a time slice, the recovered center of charge can fall between them, and it should not be called an individually recovered track.

Physical conclusion

A liquid-argon chamber turns charge, wire orientation and a calibrated clock into positions in space. The argon purity, the detector response and knowing the start time are all part of that result.

## 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. [Lab **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. [You are here **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

### Make a source and test its journey

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

1. [Article **DUNE: what detailed argon images and a long baseline can answer**](https://sandboxphysics.com/stories/neutrinos/dune-liquid-argon-and-long-baseline/)
2. [Lab **Long-Baseline CP Observatory**](https://sandboxphysics.com/labs/long-baseline-cp/)
3. [Lab **Neutrino Flavor Propagation**](https://sandboxphysics.com/labs/neutrino-flavor-propagation/)

Neutrinos · Reading path

### Follow the measurement chain

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

1. [Article **Water, scintillator and argon: what does each detector see?**](https://sandboxphysics.com/stories/neutrinos/neutrino-detector-measurement-chains/)
2. [Lab **Cherenkov Light & Detector Rings**](https://sandboxphysics.com/labs/cherenkov-rings/)
3. [Lab **Reactor Antineutrino Coincidence**](https://sandboxphysics.com/labs/reactor-coincidence/)

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)