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Particle Detectors

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A particle detector is a device used to detect, track, and/or identify ionizing particles.

These particles may have been produced by nuclear decay, cosmic radiation, or reactions in a particle accelerator.

Particle detectors can measure the particle’s energy, momentum, spin, charge, particle type, in addition to merely registering the presence of the particle.

Cloud Chamber

(Adapted from Wikipedia)

cloud chamber, also known as a Wilson cloud chamber, is a particle detector used for visualizing the passage of ionizing radiation.

A cloud chamber consists of a sealed environment containing a supersaturated vapor of water or alcohol.

An energetic charged particle (for example, an alpha or beta particle) interacts with the gaseous mixture:

it knocks electrons off gas molecules via electrostatic forces during collisions

This results in a trail of ionized gas particles. They act as condensation centers : 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 persist 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 an electron track is wispy and shows more evidence of deflections by collisions.

Cloud chambers played a prominent role in the experimental particle physics from the 1920s to the 1950s, until the advent of the bubble chamber.

This is a Diffusion Cloud Chamber used for public demonstrations at the Museum of Technology in Berlin. The first part shows the alpha and beta radiation occurring around us all the time, thanks to normal activity in the atmosphere. Then a sample of Radon 220 (half-life 55 sec) is inserted into the chamber and all hell breaks loose as an alpha-decay party ensues!

Source: Derek McKenzie, Physics Footnotes, http://physicsfootnotes.com/radon-cloud-chamber/


Here is an example of two particles colliding within an accelerator, and decaying into a variety of other products.



particles colliding LHC

Let’s look at some detailed examples. We’ll see photographs of the particle detector, then we’ll see cutaway diagrams showing us what is inside the detector.

While each detector is different – designed for a different task – they all have some basic elements in common. Each has a set of wires that make a signal if a particle flies through them. These wires are arrayed around the target area – the place where the particles are forced to collide.

When a collision occurs, some particles are broken free and fly outwards.

More remarkably, when a collision occurs, some particles are actually created – we generate particles that weren’t even there before. How is that possible? Short version, Einstein’s theory of mass-energy equivalence means that matter can be converted into energy, or vice-versa. The massive energy in these collisions creates many new sub-atomic particles. Some of these may be permanent, others might exist for only short periods of time.

ALICE, A Large Ion Collider Experiment in the LHC at CERN

This animation shows what happens when electrons and positrons collide in the ILD detector, one of the planned detectors for the future ILC. Many collisions will happen at the same time around the clock, producing a vast array of possible events. This shows one possible collision event involving the Higgs boson.



“With the uncertainty principle and the observer effects in mind, how do these devices measure both the position and momentum of sub-atomic particles with the kind of accuracy that they seem to get, with the beautiful color pictures?”

How do these devices measure both the position and momentum of particles without violating the Heisenberg Uncertainty principle?




how Particle Accelerators Work



The Particle Adventure app lets us discover: The Standard Model, Accelerators and Particle Detectors, Higgs Boson Discovered, Unsolved Mysteries, Particle Decays and Annihilations.

   Android – The Particle Adveture .   iOS (Apple) The Particle Adventure

Interactive website sims

The Particle Adventure

CPEP Contemporary Physics Education Project


Further reading

Symmetry Magazine (for high school students)


Learning Standards

SAT Subject Test: Physics

Quantum phenomena, such as photons and photoelectric effect. Atomic, such as the Rutherford and Bohr models, atomic energy levels, and atomic spectra. Nuclear and particle physics, such as radioactivity, nuclear reactions, and fundamental particles.
Relativity, such as time dilation, length contraction, and mass-energy equivalence

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

Electromagnetic radiation can be modeled as a wave of changing electric and magnetic fields or as particles called photons. The wave model is useful for explaining many features of electromagnetic radiation, and the particle model explains other features. Quantum theory relates the two models…. Knowledge of quantum physics enabled the development of semiconductors, computer chips, and lasers, all of which are now essential components of modern imaging, communications, and information technologies.


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