Cloud chamber

Cloud chamber

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The cloud chamber, also known as the Wilson chamber, is a particle detector
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 high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...

 used for detecting ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

. In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

. When a charged particle (for example, an alpha
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...

 or beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...

) interacts with the mixture, it ionizes it. The resulting ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s act as condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and shows more evidence of deflection by collisions, while an electron's is thinner and straight). When any uniform magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law with two particles of opposite charge.

Cloud chambers played a prominent role in the experimental particle physics from 1920s till 1950s, until the advent of the bubble chamber
Bubble chamber
A bubble chamber is a vessel filled with a superheated transparent liquid used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded the 1960 Nobel Prize in Physics...

. In particular, the discoveries of the positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

 in 1932 (acknowledged by a Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

 in 1936) and the kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...

 in 1953 were made using cloud chambers as detectors.

Invention



Charles Thomson Rees Wilson
Charles Thomson Rees Wilson
Charles Thomson Rees Wilson, CH, FRS was a Scottish physicist and meteorologist who received the Nobel Prize in physics for his invention of the cloud chamber.- Biography:...

 (1869–1959), a Scottish physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...

, is credited with inventing the cloud chamber. Inspired by sightings of the Brocken spectre
Brocken spectre
A Brocken spectre , also called Brocken bow, mountain spectre or glockenspectre is the apparently enormous and magnified shadow of an observer, cast upon the upper surfaces of clouds opposite the sun...

 while working on the summit of Ben Nevis
Ben Nevis
Ben Nevis is the highest mountain in the British Isles. It is located at the western end of the Grampian Mountains in the Lochaber area of the Scottish Highlands, close to the town of Fort William....

 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 is used to expand the air inside the chamber (adiabatic expansion). This cools the air and water vapor starts to condense. 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. Wilson, along with Arthur Compton
Arthur Compton
Arthur Holly Compton was an American physicist and Nobel laureate in physics for his discovery of the Compton effect. He served as Chancellor of Washington University in St. Louis from 1945 to 1953.-Early years:...

, received the Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

 in 1927 for his work on the cloud chamber. This kind of chamber is also called a Pulsed Chamber, because the conditions for operation are not continuously maintained.

Further developments were made by Patrick Blackett who utilised a stiff spring to expand and compress the chamber very rapidly, making the chamber sensitive to particles several times a second. A cine film
Cine film
Ciné is usually used to refer to one or more of the home movie formats including 8 mm, 9.5 mm, 16 mm film, and Super 8. It is not generally used to refer to video formats or professional formats ....

 was used to record the images. The cloud chamber was the first detector of radioactivity and nuclear transmutation.

Structure and operation



A simple cloud chamber consists of the sealed environment, radioactive source (optionally), dry ice or a cold plate and some kind of alcohol source (it has to allow easy evaporation).

Lightweight methyl alcohol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

 vapour 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. The alcohol vapour condenses around ion trails left behind by the travelling ionizing particles. The result is cloud formation, seen in the cloud chamber by the presence of droplets falling down to the condenser. As particles pass through the chamber they leave ionization trails and because the alcohol vapour is supersaturated it condenses onto these trails. Since the tracks are emitted radially out from the source, their point of origin can easily be determined.

Just above the cold condenser plate there is an area of the chamber which is sensitive to radioactive tracks. At this height, most of the alcohol has not condensed. This means that the ion trail left by the radioactive particles provides an optimal trigger for condensation and cloud formation. This sensitive area is increased in height by employing a steep temperature gradient, little convection, and very 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. While tracks from sources can still be seen without a voltage supply, background tracks are very difficult to observe. In addition, the voltage can also serve to prevent large amounts of "rain" from obscuring the sensitive region of the chamber,caused by condensation forming above the sensitive area of the chamber. This means that ion trails left by radioactive particles are obscured by constant precipitation. The black background makes it easier to observe cloud tracks.

Before tracks can be visible, a tangential light source is needed. This illuminates the white droplets against the black background. Drops should be viewed from a horizontal position. If the chamber is working correctly, tiny droplets should be seen condensing. Often this condensation is not apparent until a shallow pool of alcohol is formed at the condenser plate. The tracks become much more obvious once temperatures and conditions have stabilized in the chamber. This requires the elimination of any significant drift currents (poor chamber sealing).

Other particle-detection chambers


The diffusion cloud chamber was developed in 1939 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 as cold as or colder than dry ice
Dry ice
Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

. Alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 vapor is also often used due to its different phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

 temperatures. Dry-ice-cooled cloud chambers are a common demonstration and hobbyist device; the most common fluid used in them is isopropyl alcohol
Isopropyl alcohol
Isopropyl alcohol is a common name for a chemical compound with the molecular formula C3H8O. It is a colorless, flammable chemical compound with a strong odor...

, though methyl alcohol can be encountered as well. There are also water-cooled diffusion cloud chambers, using ethylene glycol
Ethylene glycol
Ethylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid...

.

The bubble chamber
Bubble chamber
A bubble chamber is a vessel filled with a superheated transparent liquid used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded the 1960 Nobel Prize in Physics...

was invented by Donald A. Glaser
Donald A. Glaser
Donald Arthur Glaser , is an American physicist, neurobiologist, and Nobel Prize in Physics laureate for his invention of the Bubble chamber used in subatomic particle physics....

 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, but as trails of bubbles in a superheated liquid, usually liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

. Bubble chambers can be made physically larger than cloud chambers, and since they are filled with much-denser liquid material, they 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.

The newer spark chamber is an electrical device that uses a grid of uninsulated electric wires in a chamber, with voltages applied between the wires. Microscopic charged particles cause some ionization of the air along the path of the particle, and this ionization causes sparks to fly between the associated wires. 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.

Similar condensation effects can be observed as Wilson clouds, also called condensation clouds, at large explosions in humid air and other Prandtl–Glauert singularity effects.

See also

  • Bubble chamber
    Bubble chamber
    A bubble chamber is a vessel filled with a superheated transparent liquid used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded the 1960 Nobel Prize in Physics...

  • Nuclear emulsion
    Nuclear emulsion
    In a Particle and Nuclear physics, a nuclear emulsion plate is a photographic plate with a particularly thick emulsion layer and with a very uniform grain size. Like bubble chambers, cloud chambers, and wire chambers nuclear emulsion plates record the tracks of charged particles passing through...

    – also used to record and investigate fast charged particles

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