The cloud chamber: see the invisible

By Musée Curie

The cloud chamber is a scientific instrument invented between 1897 and 1899 by the Scottish physicist C.T.R Wilson (it is often called the “Wilson chamber”). It allows us to visualise the trajectories of ionizing particles including those from radioactive elements. Its use in particle physics was crucial from the `20s until the `50s. In particular, it allowed the discovery of the positron (or positive electron) in 1932, and of the muon, another elementary constituent of matter, in 1936.

Cloud chamber with variable pressure designed by Frédéric Joliot (Photographie 2012) by Frédéric Joliot. Photographie : Alexandre Lescure / Musée CurieMusée Curie

The scientific instrument

The cloud chamber or Wilson chamber is a device which allows, by suddenly increasing the volume of the chamber through the fall of a piston, to lower the temperature and thus artificially create a mist of steam or of a mixture water/alcohol. Droplets form by condensation along the path of charged particles. A radioactive element introduced into the chamber produces radiation which leads to the formation of ions during its passage: its trajectory becomes visible through the transparent window at the top of the device.

Cloud chamber with variable pressure designed by Frédéric Joliot, top view, Frédéric Joliot-Curie. Photographie : Alexandre Lescure / Musée Curie, 2012, From the collection of: Musée Curie
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Of the black paint that lined the bottom of the chamber originally, there are only traces. It allowed to better visualize the paths left by the radiation in the mist.

Picture taken with a Wilson chamber, taken by Irène Joliot-Curie in 1937, Irène Joliot-Curie. Source : Musée Curie (coll. ACJC), 1937, From the collection of: Musée Curie
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Un élément radioactif introduit dans la chambre produit un rayonnement qui conduit à la formation de charges électriques lors de son passage. La trajectoire du rayonnement devient donc visible à travers la fenêtre transparente au sommet de l’appareil.

Stereoscopic camera used with the cloud chamber (Photographie 2012) by Alexandre Lescure / Musée CurieMusée Curie

Images to see the invisible

The cloud chamber may be used together with electromagnetic coils that can deflect charged particles. But also with a light bulb to illuminate the droplets formed and make them visible to the naked eye. A stereoscopic camera allows us to obtain relief images of the path of these particles and thus to calculate by trigonometry their length: their physical characteristics are revealed.

Electromagnet coil used with the cloud chamber, Alexandre Lescure / Musée Curie, Photographie 2012, From the collection of: Musée Curie
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Electromagnet coil used with the cloud chamber, Alexandre Lescure / Musée Curie, Photographie 2012, From the collection of: Musée Curie
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Light bulb for side lighting of the cloud chamber, Alexandre Lescure / Musée Curie, Photographie 2012, From the collection of: Musée Curie
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Light bulb for side lighting of the cloud chamber, detail, Sacha Lenormand / Musée Curie, Photographie 2012, From the collection of: Musée Curie
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Traces of electrons in a picture taken using the Wilson chamber, Source : Musée Curie (coll. ACJC), 1939, From the collection of: Musée Curie
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Frédéric Joliot in front of the Wilson chamber in the Collège de France in 1938 (1938) by Source : Musée Curie (coll. ACJC)Musée Curie

Its use by the Joliot-Curie

The cloud chamber had a major impact on research into elementary particles of matter. It served to demonstrate the existence of new particles and nuclear fission. Frédéric and Irène Joliot-Curie used it often, whether at the Radium Institute or at the College de France when Frédéric became a professor there in 1937. Moreover, he invented a model of cloud chamber with variable pressure allowing him to obtain more precise images, like the one presented in the Musée Curie.

Irène Joliot-Curie in her laboratory at the Radium Institute in 1936, Source : Musée Curie (coll. ACJC), 1936, From the collection of: Musée Curie
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Frédéric Joliot-Curie holding elements of a Wilson chamber, Source : Musée Curie (coll. ACJC), 1938, From the collection of: Musée Curie
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Irène Joliot-Curie at the Radium institute, next to the Wilson chamber in 1946, Source : Musée Curie (coll. ACJC), 1946, From the collection of: Musée Curie
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Wilson chamber with variable pressure of Frédéric Joliot-Curie (1936) by Source : Musée Curie (coll. ACJC)Musée Curie

An easily reproducible instrument

It is an instrument which can be easily reproduced and it is not unusual to find cloud chambers in schools, universities or science centers. This equipment has an important educational interest because it allows relatively simply to “see” or at least to detect the passage of charged particles, coming from either radioactivity or cosmic radiation. Numerous videos, available on the Internet, offer the possibility to understand a phenomenon that surrounds us but remains invisible to the naked eye.

Wilson chamber in the laboratory of Irène Joliot-Curie, Photo SAFARA. Source : Musée Curie (coll. ACJC), 1935, From the collection of: Musée Curie
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Working of a cloud chamber during 50 min. [720p], Cloudylabs, 2014-01-02, From the collection of: Musée Curie
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Credits: Story

Conception : Musée Curie
Photographie 2012 : Alexandre Lescure, Institut Curie et Sacha Lenormand
Photographies anciennes : collection ACJC, Musée Curie
Vidéo : Cloudylabs, Youtube (https://www.youtube.com/watch?v=ZiscokCGOhs)

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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