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.
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.
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.
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.
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.
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.
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.
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)
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