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Bubble chamber

A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) 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.

Anecdotally, Glaser was inspired by the bubbles in a glass of beer; however, in a 2006 talk, he refuted this story,[1] saying that although beer was not the inspiration for the bubble chamber, he did experiment with using beer to fill early prototypes.

Function and use

The bubble chamber is similar to a cloud chamber in application and basic principle. It is normally made by filling a large cylinder with a liquid heated to just below its boiling point. As particles enter the chamber, a piston suddenly decreases its pressure, and the liquid enters into a superheated, metastable phase. Charged particles create an ionization track, around which the liquid vaporizes, forming microscopic bubbles. Bubble density around a track is proportional to a particle's energy loss.

Bubbles grow in size as the chamber expands, until they are large enough to be seen or photographed. Several cameras are mounted around it, allowing a tridimensional image of an event to be captured. Bubble chambers with resolutions down to a few μm have been operated.

The whole chamber is subject to a constant magnetic field, which causes charged particles to travel in helical paths whose radius is determined by their charge-to-mass ratios. Given that for all known charged long-lived subatomic particles, the magnitude of their charge is that of an electron, their radius of curvature is thus proportional to their momentum.

Recently, bubble chambers have been used in research on dark matter (WIMPs). [2]

A bubble chamber

First tracks observed in a liquid hydrogen bubble chamber.

Bubble chamber tracks of the decay of a charmed baryon, first published in 1975. At the time, this identification was tentative. Later work has confirmed that the baryon involved was indeed charmed, now known as the Σc++. From Brookhaven National Laboratory

Drawbacks

Although bubble chambers were very successful in the past, they are of only limited use in current very-high-energy experiments, for a variety of reasons:

* The need for a photographic readout rather than tri-dimensional electronic data makes it less convenient, especially in experiments which must be reset, repeated and analysed many times.

* The superheated phase must be ready at the precise moment of collision, which complicates the detection of short-lived particles.

* Bubble chambers are neither large nor massive enough to analyze high-energy collisions, where all products should be contained inside the detector.

* The high-energy particles' path radii may be too large to allow the precise estimation of momentum in a relatively small chamber.

Due to these issues, bubble chambers have largely been replaced by wire chambers, which allow particle energies to be measured at the same time. Another alternative technique is the spark chamber.

Recent uses of bubble chambers: searches for dark matter (WIMPs): COUPP web site

Notes

1. ^ http://www.lbl.gov/Publications/Currents/Archive/Jul-21-2006.html

2. ^ COUPP web site

Articles and references

* General Information

* Donald A. Glaser (1952). "Some Effects of Ionizing Radiation on the Formation of Bubbles in Liquids". Phys. Rev. 87 (4): 665 - 665. doi:10.1103/PhysRev.87.665.

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