A cloud chamber, also known as a Wilson chamber, is a

particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing elementary particle, particles, such as t ...

used for visualizing the passage of

ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...

. A cloud chamber consists of a sealed environment containing a

supersaturated In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a ...

vapor In physics, a vapor (American English) or vapour (Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R ...

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

. An energetic charged particle (for example, an

alpha Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...

beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and � ...

) interacts with the gaseous mixture by knocking

electrons The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

off gas molecules via

electrostatic Electrostatics is a branch of physics that studies slow-moving or stationary electric charges. Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word (), mean ...

forces during collisions, resulting in a trail of ionized gas particles. The resulting

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s act as condensation centers around which a mist-like trail of small droplets form if the gas mixture is at the point of condensation. These droplets are visible as a "cloud" track that persists for several seconds while the droplets fall through the vapor. These tracks have characteristic shapes. For example, an alpha particle track is thick and straight, while a beta particle track is wispy and shows more evidence of deflections by collisions. Cloud chambers were invented in the early 1900s by the Scottish physicist

Charles Thomson Rees Wilson Charles Thomson Rees Wilson (14 February 1869 – 15 November 1959) was a Scottish meteorologist and particle physicist who shared the 1927 Nobel Prize in Physics with Arthur Compton for his invention of the cloud chamber. Education and earl ...

. They played a prominent role in experimental particle physics from the 1920s to the 1950s, until the advent of the

bubble chamber A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded th ...

. In particular, the discoveries of the

positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...

in 1932 (see Fig. 1) and the

muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...

in 1936, both by Carl Anderson (awarded a

Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...

in 1936), used cloud chambers. The discovery of the

kaon In particle physics, a kaon, also called a K meson and denoted , is any of a group of four mesons distinguished by a quantum number called strangeness. In the quark model they are understood to be bound states of a strange quark (or antiquark ...

George Rochester George Dixon Rochester, Fellow of the Royal Society, FRS (4 February 1908 – 26 December 2001) was a England, British physicist known for having co-discovered, with Sir Clifford Charles Butler, a subatomic particle called the kaon. Biography ...

and

Clifford Charles Butler Sir Clifford Charles Butler FRS (20 May 1922 – 30 June 1999) was an English physicist, best known for the discovery of the hyperon and meson types of particles. In later life, Butler was involved in educational policy, serving as director of ...

in 1947 was made using a cloud chamber as the detector. In each of these cases,

cosmic rays Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar ...

were the source of ionizing radiation. Yet they were also used with artificial sources of particles, for example in

radiography Radiography is an imaging technology, imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical ("diagnostic" radiog ...

applications as part of the

Manhattan Project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...

Invention

(1869–1959), a

Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European language family native to Scotland *Scottish English *Scottish national identity, the Scottish ide ...

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

, is credited with inventing the cloud chamber. Inspired by sightings of the

Brocken spectre A Brocken spectre (British English; American spelling: Brocken specter; ), also called Brocken bow, mountain spectre, or spectre of the Brocken is the magnified (and apparently enormous) shadow of an observer cast in mid air upon any type of cl ...

while working on the summit of

Ben Nevis Ben Nevis ( ; , ) is the highest mountain in Scotland, the United Kingdom, and the British Isles. Ben Nevis stands at the western end of the Grampian Mountains in the Highland region of Lochaber, close to the town of Fort William. The mount ...

in 1894, he began to develop expansion chambers for studying cloud formation and optical phenomena in moist air. Very rapidly he discovered that ions could act as centers for water droplet formation in such chambers. He pursued the application of this discovery and perfected the first cloud chamber in 1911. In Wilson's original chamber, the air inside the sealed device was saturated with water vapor, then a diaphragm was used to expand the air inside the chamber ( adiabatic expansion), cooling the air and starting to condense water vapor. Hence the name expansion cloud chamber is used. When an ionizing particle passes through the chamber, water vapor condenses on the resulting ions and the trail of the particle is visible in the vapor cloud. A cine film was used to record the images. Further developments were made by

Patrick Blackett Patrick Maynard Stuart Blackett, Baron Blackett (18 November 1897 – 13 July 1974) was an English physicist who received the 1948 Nobel Prize in Physics. In 1925, he was the first person to prove that radioactivity could cause the nuclear tr ...

who utilised a stiff spring to expand and compress the chamber very rapidly, making the chamber sensitive to particles several times a second. This kind of chamber is also called a pulsed chamber because the conditions for operation are not continuously maintained. Wilson received half the

in 1927 for his work on the cloud chamber (the same year as Arthur Compton received half the prize for the

Compton Effect Compton scattering (or the Compton effect) is the quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron. Specifically, when the photon hits electrons, it releases loosely bound e ...

). The diffusion cloud chamber was developed in 1936 by Alexander Langsdorf. This chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation, and in that the bottom must be cooled to a rather low temperature, generally colder than . Instead of water vapor,

is used because of its lower

freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

. Cloud chambers cooled by

dry ice Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and Sublimation (phase transition), sublimes directly from the solid state to the gas ...

or Peltier effect

thermoelectric cooling Thermoelectric cooling uses the Peltier effect to create a heat flux at the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a Solid-state physics, solid-state active heat pump which transfers ...

are common demonstration and hobbyist devices; the alcohol used in them is commonly

isopropyl alcohol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable, organic compound with a pungent alcoholic odor. Isopropyl alcohol, an organic polar molecule, is miscible in water, ethanol, an ...

methylated spirit Denatured alcohol, also known as methylated spirits, metho, or meths in Australia, Ireland, New Zealand, South Africa, and the United Kingdom, and as denatured rectified spirit, is ethanol that has additives to make it poisonous, bad-tasting, f ...

Structure and operation

Diagram of a continuous operation cloud chamber

Diffusion-type cloud chambers will be discussed here. A simple cloud chamber consists of the sealed environment, a warm top plate and a cold bottom plate (See Fig. 3). It requires a source of liquid alcohol at the warm side of the chamber where the liquid evaporates, forming a vapor that cools as it falls through the gas and condenses on the cold bottom plate. Some sort of ionizing radiation is needed.

Isopropanol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable, organic compound with a pungent alcoholic odor. Isopropyl alcohol, an organic polar molecule, is miscible in water, ethanol, an ...

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

, or other alcohol vapor saturates the chamber. The alcohol falls as it cools down and the cold condenser provides a steep temperature gradient. The result is a supersaturated environment. As energetic charged particles pass through the gas they leave ionization trails. The alcohol vapor condenses around gaseous ion trails left behind by the ionizing particles. This occurs because alcohol and water molecules are polar, resulting in a net attractive force toward a nearby free charge (See Fig. 4). The result is a misty cloud-like formation, seen by the presence of droplets falling down to the condenser. When the tracks are emitted from a source, their point of origin can easily be determined.Zani, G. Dept. of Physics, Brown University, RI USA
"Wilson Cloud Chamber"
. Updated 05/13/2016. Fig. 5 shows an example of an alpha particle from a Pb-210 pin-type source undergoing

Rutherford scattering The Rutherford scattering experiments were a landmark series of experiments by which scientists learned that every atom has a Atomic nucleus, nucleus where all of its positive charge and most of its mass is concentrated. They deduced this after ...

. Just above the cold condenser plate there is a volume of the chamber which is sensitive to ionization tracks. The ion trail left by the radioactive particles provides an optimal trigger for condensation and cloud formation. This sensitive volume is increased in height by employing a steep temperature gradient, and stable conditions. A strong electric field is often used to draw cloud tracks down to the sensitive region of the chamber and increase the sensitivity of the chamber. The electric field can also serve to prevent large amounts of background "rain" from obscuring the sensitive region of the chamber, caused by condensation forming above the sensitive volume of the chamber, thereby obscuring tracks by constant precipitation. A black background makes it easier to observe cloud tracks, and typically a tangential light source is needed to illuminate the white droplets against the black background. Often the tracks are not apparent until a shallow pool of alcohol is formed at the condenser plate. If a

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the

Lorentz force law Lorentz is a name derived from the Roman surname, Laurentius, which means "from Laurentum Laurentum was an ancient Rome, ancient Roman city of Latium situated between Ostia Antica, Ostia and Lavinium, on the west coast of the Italian Peninsul ...

; strong-enough fields are difficult to achieve with small hobbyist setups. This method was also used to prove the existence of the

in 1932, in accordance with

Paul Dirac Paul Adrien Maurice Dirac ( ; 8 August 1902 – 20 October 1984) was an English mathematician and Theoretical physics, theoretical physicist who is considered to be one of the founders of quantum mechanics. Dirac laid the foundations for bot ...

's theoretical proof, published in 1928.

Other particle detectors

The

was invented by

Donald A. Glaser Donald Arthur Glaser (September 21, 1926 – February 28, 2013) was an American physicist who received the Nobel Prize in Physics in 1960 for his invention of the bubble chamber. Education Born in Cleveland, Ohio, Glaser completed his Bachelor ...

of the United States in 1952, and for this, he was awarded the Nobel Prize in Physics in 1960. The bubble chamber similarly reveals the tracks of subatomic particles, and inverts the principle of the cloud chamber to detect them as trails of bubbles in a superheated liquid, usually

liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...

, rather than as trails of drops in a supersaturated vapor. Bubble chambers can be made physically larger than cloud chambers, and since they are filled with much-denser liquid material, they can reveal the tracks of much more energetic particles. These factors rapidly made the bubble chamber the predominant particle detector for a number of decades, so that cloud chambers were effectively superseded in fundamental research by the start of the 1960s. A

spark chamber A spark chamber is a particle detector: a device used in particle physics for detecting electrically charged Subatomic particle, particles. They were most widely used as research tools from the 1930s to the 1960s and have since been superseded by ...

is an electrical device that uses a grid of uninsulated electric wires in a gas-filled chamber, with high voltages applied between the wires. Energetic charged particles cause ionization of the gas along the path of the particle in the same way as in the Wilson cloud chamber; in this case the ambient electric fields are high enough to precipitate full-scale gas breakdown in the form of sparks at the position of the initial ionization. The presence and location of these sparks is then registered electrically, and the information is stored for later analysis, such as by a

digital computer A computer is a machine that can be programmed to automatically carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic sets of operations known as ''programs'', wh ...

. Similar condensation effects can be observed as

Wilson cloud A transient condensation cloud, also called a Wilson cloud, is observable surrounding large explosions in humid air. When a nuclear weapon or high explosive is detonated in sufficiently humid air, the "negative phase" of the shock wave causes ...

s, also called condensation clouds, at large explosions in humid air and other

Prandtl–Glauert singularity The Prandtl–Glauert singularity is a theoretical construct in flow physics, often incorrectly used to explain vapor cones in transonic flows. It is the prediction by the Prandtl–Glauert transformation that infinite pressures would be expe ...

effects.

Gallery

File:Home Made Cloud Chamber.webm, A home-made cloud chamber.thin: β-particles). See also animated version File:Cloudylabs cloud chamber.jpg, Example of watercooled thermoelectric cloud chamber File:Radioactivity of a Thorite mineral seen in a cloud chamber.jpg, Radioactivity of a thorite mineral seen in a cloud chamber File:Cloud Chamber with Ion Scrubber.jpg, Diffusion cloud chamber with 4.6kV ion scrubber File:Homebrewed Diffusion Cloud Chamber.jpg, Homebrewed TEC / water-cooled (5-stage) diffusion cloud chamber File:Wilson chamber.webm, Video of a Wilson chamber File:FZU_outreach_11.jpg, Cloud chamber during the European Researchers' Night at FZU. File:Cloud chamber bionerd.jpg, Cloud chamber with visible tracks from

(short, thick: α-particles; long, File:Particle Tracks in AWAN Expansion Cloud Chamber.jpg, Particle tracks from radioisotopes in an expansion cloud chamber. (Left) Alpha tracks from Am-241 source, with one beta track possibly from its daughter radionuclide, Pa-233. (Right) Beta tracks from Sr-90/Y-90 source. File:Nuclear particle in a diffusion cloud chamber.png, Image taken in the Pic du Midi at 2877 m in a Phywe PJ45 cloud chamber (size of surface is 45 × 45 cm). This rare picture shows in a single shot the 4 particles that are detectable in a cloud chamber : proton, electron, muon (probably) and alpha File:Cloud chambers played an important role of particle detectors.jpg, Another cloud chamber in a display case

Notes

References

External links

* ,

Peter Wothers Peter David Wothers, , is a British chemist and author of several popular textbooks aimed at university students. He is a teaching fellow in the Department of Chemistry at the University of Cambridge and is a fellow of St Catharine's College, C ...

Royal Institution The Royal Institution of Great Britain (often the Royal Institution, Ri or RI) is an organisation for scientific education and research, based in the City of Westminster. It was founded in 1799 by the leading British scientists of the age, inc ...

, December 2012] {{DEFAULTSORT:Cloud Chamber Particle detectors Radioactivity Scottish inventions Ionising radiation detectors Articles containing video clips