In order to study subatomic particles you need a method of detecting them. Over the years physicists have developed devices that can show the presence of particles and reveal their properties by the tracks that they leave. Two of the most important early detecting devices were cloud and bubble chambers. In modern high-energy research these devices are now obsolete. Spark and drift chambers are used as faster alternatives.
All of these devices work on a common principle: charged particles that pass through leave a path of detectable ionised particles. The technical details of each detector are slightly different but this principle is true nonetheless.
A simple cloud chamber is shown in the diagram below. As ionising radiation passes through the cold damp air, tiny water droplets form around the ions produced. An alpha particle passing through the cloud chamber would leave a thicker, shorted track of droplets than a beta particle. Why? Because the alpha particle is more ionising and larger, so it loses it energy more rapidly in the chamber.
Here are some original photos by C.T.R Wilson taken in 1912. He actually won the nobel prize for developing the cloud chamber.
The bubble chamber was the main tool for research in the 1950s and 60s. In consists of a tank of superheated liquid hydrogen kept just on the verge of boiling. When a particle moves through it can cause ions in the hydrogen around which tiny bubbles form. The tank is kept in a strong magnetic field so charged particles follow curved paths.
Here is a photo of bubble chamber tracks:
When the bubble pathways have formed, a camera can be used to take a picture and record the events. The tank is then reset, by increasing the pressure and collapsing the bubbles. This process takes around a second - a long time by modern standards!
This type of detector is much faster than the bubble chamber. It is formed from a set of thin metal plates, spaced closely together in an inert gas. When a charged particle passes through, it leaves a trail of ionised particles. A spark can cascade along this trail when a high potential difference is applied to the plates. As in a bubble chamber a photograph can be taken of the spark's path. The sparking clears the ions away quickly when the voltage is turned off so the chamber is ready for reuse in a fraction of a second.
Drift chambers are an improvement on spark chambers. They consist of many separate wires instead of the plates in the spark chamber. The wires are arranged in such a way, that a computer can detect the arrival of the ions created when they drift to a detection wire.
The pathways left in the detectors above are complex to analyse but much information can be gleaned by noticing a few simple points about particle paths within a magnetic field:
- The momentum of particle is proportional to the radius of its track.
- If the field is directed into the paper, negative particles curve clockwise; positive particles curve anticlockwise. (This follows from Fleming ' s left-hand rule.)
- Neutral particles do not leave tracks because they produce very little ionisation. They travel in straight lines and their paths can often be inferred from the presence of tell-tale gaps between visible tracks. It is possible to determine their momentum by applying the principle of conservation of mometum.
The two spiralling tracks in this bubble-chamber diagram were made by an electron and a positron. These particles were created by a high-energy gamma ray in a collision with the electron of a hydrogen atom in the bubble chamber. The long slightly curved downward track was made by the recoiling electron.
Beta Particle Diagram: