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What is a Bubble Chamber?

Brendan McGuigan
Brendan McGuigan

A bubble chamber is a device used in physics to detect charged particles. It was invented by Donald Glaser in 1952, and he was subsequently awarded the Nobel Prize for its invention. Although once the prevalent way of detecting particles, the bubble chamber currently is not frequently used, in large part because of some drawbacks that become apparent when dealing with extremely high-energy particles.

The principle behind the bubble chamber, and indeed most particle detectors, is quite simple. It can be thought of as analogous to watching the sky for trails left behind by airplanes. Even if a jet streaks by so quickly you don’t notice it pass, you will see its trail for some time, allowing you to reconstruct the path it took. A bubble chamber works along a similar principle, with particles leaving a trail of bubbles that can be photographed.

Scientist with beakers
Scientist with beakers

The chamber itself is filled with some sort of transparent and unstable liquid, often superheated hydrogen. The liquid is made superheated by keeping it under pressure, and releasing it slightly at the moment the particles are introduced. As charged particles make their way through the chamber, they cause the liquid to boil as they pass, creating a trail of bubbles. The particles themselves take only a few nanoseconds to pass through the chamber, but the bubbles take millions of times longer to expand, generally taking around 10ms. In that time, photographs can be taken from various angles, creating a three-dimensional representation of the particle path.

The bubbles are then eliminated by pressurizing the chamber, and the procedure is repeated with the next batch of particles. Each set of photographs is taken in what we might consider a short period of time, requiring only a few seconds each, but this is actually quite long by scientific standards. Modern detectors are able to do the entire procedure in milliseconds, allowing for hundreds or thousands of bursts of particles to be documented in a few seconds. Modern detectors also capture images digitally, making them easier to analyze and store.

As a result, the bubble chamber is rarely used in modern particle detection. Another problem is that because bubble chambers are fairly small, they are also incapable of properly documenting collisions of high-energy particles, further reducing their usefulness in modern experiments. Finally, the point at which the liquid becomes superheated must coincide exactly with when the instant particles hit each other, which can be nearly impossible to coordinate with particles that have extremely-short lifespans.

In spite of their relative obsolescence, the images from bubble chambers are still quite useful for teaching purposes. Because they are photographs of physical trails, they are generally much easier for people to understand than more complex descriptions of interactions, or other abstracted data. Students can look at an image captured of a bubble trail and see precisely the interactions of various particles, and how the particles decay during their time in the chamber. For these reasons, although not widely used in cutting-edge research, bubble chambers continue to see some use university laboratories, and photographs taken historically are often seen in textbooks.

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      Scientist with beakers