Saturday, April 18, 2020

How Diffusion Cloud Chamber Works?

Theory of operation of the Cloud Chamber is fairly simple yet brilliant.
The Diffusion Cloud Chamber is basically a sealed volume, filled with Alcohol Vapors.
Conditions are created for a steep vertical temperature gradient inside the volume.
Alcohol is evaporated from a reservoir near the top of the volume where the temperature is higher and vapors fall down as they cool, towards a very cold plate at the bottom.
Just above the cold plate, the alcohol vapors reach a supersaturated state due to the deep cooling (colder than -26℃). 
In this state, the air above the cold plate is so saturated with alcohol vapors and they are cooled beyond the threshold of condensation (dew point) so the vapors only need a trigger - a "seed" around which to start the process of condensing into small liquid droplets, just like a normal fog.
The ionization trail of a charged particle provides these condensation "seeds".

(c) 2020 Andrey Stoev

(please note that the graphical representations of the elements above are not to scale with each-other in order to improve the clarity of the diagram)

When a charged particle enters the volume (or it is created inside) and passes thru the Cloud Chamber's Active Zone, it leaves an ionization trail. 
Gas atoms of oxygen and nitrogen from the air become Ions in the path of the energetic particle as the particle collides with the gas atoms and deposits some of its energy while knocking off electrons.
Once the atoms are stripped from even a single electron, the charge balance is no longer present - the atom becomes positively charged due to the positive charge of its nucleus.
Since the Alcohol and Water molecules are slightly polar, the molecules are then attracted to these ions. The ions serve as "seeds" / condensation centers and droplets of fine mist form around these condensation centers, tracing the path of the particle.

The condensation trails appear only in the active zone where conditions are right (alcohol vapors are supersaturated) and are white in color just like the normal water-based fog.
Black background and intense tangential lighting helps to improve the contrast, bringing them out and making them more visible to the observer.

Improved observation conditions are achieved by deeper cooling which extends the thickness of the Active Zone layer by creating a steeper temperature gradient.

Another feature (optional) which greatly improves the performance of the Cloud Chamber is the Ion Scrubber. "Ion Scrubber" is the scientific term for High-Voltage Electric Field applied to the chamber's volume. Typically, between the cold plate (-) and an electrode (+) suspended above the cold plate. 
CERN recommends about 100V/cm field potential.
The electrostatic field improves the performance of the Cloud Chamber in the following ways:
- clears the volume from airborne dust particles and contaminants at startup - this removes unwanted condensation centers
- deflects the con-trails in the Active Zone for a better observation
- "resets" the chamber by quickly removing (scrubbing) the ions so condensation trails don't linger for too long and new ones can be observed
- prevents ions from creating an alcohol "rain" over the active zone.
- improves sensitivity of the Cloud Chamber and increases the definition of the con-trails, making them crisp and well defined by quickly stripping off the "fuzzy" outer part of the con-trail.

It is very important that the air inside of the chamber is perfectly still as the fine vapor trails are very "fragile" and they will quickly disperse in the presence of air movement / turbulence. Any air leaks from outside will also bring in warmer air which will interfere with the operation of the Active Zone - a good, air-tight seal of the volume should always be maintained during operation!

Large Cloud Chambers usually have a special port to bring in radioactive samples or inject radioactive gas (Radon) without much air disturbance. Such specimens can be introduced also by gently opening the volume (lifting the cover) but it will take a few seconds for the conditions inside to normalize and the con-trails to reappear.

Trivia (credit: Wikipedia)
The invention of the cloud was Charles Thomson Rees Wilson. This was his signature accomplishment, earning him the Nobel Prize for Physics in 1927.  The Cavendish laboratory praised him for the creation of "a novel and striking method of investigating the properties of ionized gases". The cloud chamber allowed huge experimental leaps forward in the study of subatomic particles and the field of particle physics, generally. Some have credited Wilson with making the study of particles possible at all.
Cloud chambers played a prominent role in experimental particle physics from the 1920s to the 1950s, until the advent of the bubble chamber. In particular, the discoveries of the positron in 1932 and the muon in 1936, both by Carl Anderson (awarded a Nobel Prize in Physics in 1936), used cloud chambers. Discovery of the kaon by George Rochester and Clifford Charles Butler in 1947, also was made using a cloud chamber as the detector. In each case, cosmic rays were the source of ionizing radiation.
The diffusion cloud chamber was developed in 1936 by Alexander Langsdorf.

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