--- title: "Wilson turns ionisation into a visible track" description: "Why does a passing particle leave a thin trail of cloud?" canonical_url: "https://sandboxphysics.com/stories/particle-evidence/wilson-cloud-chamber/" language: "en" page_type: "story" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/stories/particle-evidence/wilson-cloud-chamber/" last_modified: "2026-10-24" --- [Detecting a passing particle](https://sandboxphysics.com/stories/particle-evidence/#detectors) Particles · Experiments · Discovery # Wilson turns ionisation into a visible track Why does a passing particle leave a thin trail of cloud? Sandbox Physics · 2026-10-24 Nobel contribution: 1927-2 1. Prepare moist clean gas 2. Expand and cool it 3. Grow droplets on the ion trail 4. Illuminate and photograph Droplets enlarge the ionisation left behind by a charged particle. ## The particle leaves the seeds of a cloud In the spring of 1911, C. T. R. Wilson exposed an expansion chamber to X-rays and saw fine threads of cloud appear inside. The X-rays had knocked electrons out of atoms. Those moving electrons ionised more gas along their paths, leaving charged atoms and free electrons behind them. Water condensed around the resulting ions. A fleeting journey became a line of droplets large enough to illuminate and photograph. Wilson had come to the problem through clouds and condensation. Moist air can hold invisible water vapour. Cool it quickly and it can contain more vapour than would be stable at the new temperature. That condition is called supersaturation. A small seed helps droplets begin growing, rather as a tiny scratch can help a crystal start in a solution. Dust can supply such seeds, but a chamber full of dust makes a general fog that hides the delicate particle trails. The experiment needed clean gas and a carefully chosen expansion. Too little cooling left the ions invisible. Too much produced unwanted condensation throughout the chamber. Between those conditions, the charged remnants of a particle’s passage supplied especially useful places for water to gather. The visible track was made of ordinary droplets; its extraordinary feature was the line along which they appeared. Sources: [1] [2] ## A camera sees the aftermath Expansion did two jobs at once: it cooled the gas and opened a short interval during which fresh ionisation could grow into visible droplets. A flash illuminated the chamber, and a camera recorded the pattern before the droplets dispersed or fell. The timing had to suit both the gas and the camera. An excellent optical system was little use if the chamber had already lost its sensitivity when a particle arrived. A track should therefore be read as a record of what the particle did to its surroundings. It is not a photograph of the particle itself, enlarged to droplet size. The particle has passed on by the time the droplets are photographed. Its deposited charge has been converted into another, much slower physical process. The same basic strategy appears throughout particle detection: an invisible event creates a signal that an instrument can amplify and keep. Track shapes also carry information. A heavy alpha particle often leaves a dense, comparatively straight trail. An electron is lighter and can be turned sharply by encounters in the gas, producing a more irregular path. A neutral X-ray does not draw a continuous ion trail of its own; an electron released in one of its interactions can do so. That distinction lets a photograph reveal how one form of radiation produces another. Sources: [1] [2] ## A fresh track gradually loses its sharpness The ions do not stay fixed after a particle passes. Gas molecules move and collide, and the charge distribution gradually spreads. Positive and negative charges can also recombine. A delay therefore makes the original path harder to recover, even before any droplets have grown. The chamber has to connect the short-lived ion pattern to a slower visible cloud while enough of the pattern survives. This explains both the power and the timing limitation of the method. One particle leaves many local marks along a path, giving an extended image with bends and stopping points. Yet the chamber cannot remain equally sensitive through every moment of an expansion-and-recovery cycle. A track seen clearly and a particle missed entirely can belong to different parts of that cycle. Later counters helped choose the right moment to expand. Sources: [1] [2] ## One view can hide a three-dimensional crossing The chamber contains a volume, whereas a photograph flattens that volume onto a sheet. Two unrelated tracks at different depths may look as though they meet. Photographs from different directions can recover the geometry and test whether a supposed collision really has a common point. That is why the illumination and the camera arrangement form part of the measurement, alongside the gas itself. The records invited new experiments. Put a chamber in a magnetic field and a charged particle curves, with the sense of the bend depending on its charge and direction of travel. Add material and study what happens at the boundary. Later experimenters used such comparisons to identify the positron and to examine showers of particles. Wilson supplied the means to see trajectories; extracting a particle’s identity still required those further physical tests. Wilson shared the 1927 physics prize with Arthur Compton, whose recognised work concerned scattering. The two contributions meet in the cloud chamber: a scattered electron can make the transfer of energy visible as a track. In Wilson’s method, a cloud ceased to be an obstacle to seeing and became the thing that allowed a microscopic event to be seen. Sources: [1] Continue with the physics ## Try the related models ### [Bend a charged-particle beam](https://sandboxphysics.com/labs/charged-particle/) Change the electric and magnetic fields and follow the trajectory. The model follows a nonrelativistic particle through fixed, uniform fields. ## Sources and reading notes 1. [C. T. R. Wilson, On the cloud method of making visible ions and the tracks of ionising particles (1927)](https://www.nobelprize.org/uploads/2018/06/wilson-lecture.pdf) Wilson recounts condensation studies, his 1911 first tracks and the illumination and expansion apparatus. 2. [C. T. R. Wilson, On an expansion apparatus for making visible the tracks of ionising particles in gases (1912)](https://doi.org/10.1098/rspa.1912.0081) The original apparatus paper connects expansion conditions and photographed ionisation tracks. ## Follow the next question [Blackett and Occhialini let a particle take its own photograph](https://sandboxphysics.com/stories/particle-evidence/blackett-counter-controlled-cloud-chamber/) [The lead plate that revealed a positron](https://sandboxphysics.com/stories/particle-evidence/anderson-positron-lead-plate/) [Particles, detectors and the rules of matter](https://sandboxphysics.com/stories/particle-evidence/) [Lenard lets cathode rays out of the tube](https://sandboxphysics.com/stories/particle-evidence/lenard-cathode-ray-window/) [Thomson finds a common particle in cathode rays](https://sandboxphysics.com/stories/particle-evidence/thomson-electron-charge-mass/) [Bothe and Geiger ask whether two signals belong together](https://sandboxphysics.com/stories/particle-evidence/bothe-coincidence-method/) [Blackett and Occhialini let a particle take its own photograph](https://sandboxphysics.com/stories/particle-evidence/blackett-counter-controlled-cloud-chamber/) [The pion leaves a second track in photographic emulsion](https://sandboxphysics.com/stories/particle-evidence/powell-pion-emulsion/) [Glaser makes a particle trail start the boiling](https://sandboxphysics.com/stories/particle-evidence/glaser-bubble-chamber/) [Alvarez’s group finds short-lived particles in combinations of tracks](https://sandboxphysics.com/stories/particle-evidence/alvarez-resonance-events/) [Cherenkov follows the blue light to a moving charge](https://sandboxphysics.com/stories/particle-evidence/cherenkov-light-cone/) [Charpak turns a particle’s position into an electrical signal](https://sandboxphysics.com/stories/particle-evidence/charpak-multiwire-chamber/)