The Bubble Chamber

The bubble chamber was invented in 1952 by Donald Glaser, he eventually won a noble prize for it in 1960. A bubble chamber is a vessel filled with a superheated liquid, most commonly liquid hydrogen, and is used to detect charged particles moving through this liquid. It was cited that the idea came from the bubbles in a pint of beer, however this story was refuted with him saying that although beer was not the inspiration he had attempted to use it in earlier designs of the chamber!

Gargamelle, the heavy-liquid bubble chamber at CERN in which neutral currents were discovered

The CERN 2m hydrogen bubble chamber

An artistically enhanced picture of particle tracks in the BEBC

Gargamelle, the heavy-liquid bubble chamber at CERN in which neutral currents were discovered.

The CERN 2m hydrogen bubble chamberAn artistically enhanced picture of particle tracks in the BEBC

What is a Bubble Chamber?

A bubble chamber is a device developed in the 1950s which allows physicists to observe the interactions and decays of charged particles. It consists of a vessel filled with a superheated liquid, most probably hydrogen, that forms bubbles when an ionizing particle moves through the liquid- this process is explained later on. The chamber allows photos of the trails to be taken and then this can be analysed and from it the momentum and charge of the particles can be determined. It is like the trail that you see when an aeroplane circles above your head, as we all know the vapour trail is good evidence that the plane has been there recently so if two planes were to cross paths we would be able to identify this and more from the trail they would both leave behind. This is the same as in a bubble chamber, we get a photo of these trails and from that we can piece back together what has happened. In most detectors the beam of particles is fired into a target substance, containing the target nuclei and then what comes out is measured in a detector, the bubble chamber however is special in that it is both the target and detector all in one.

How Does it Work?

As charged particles travel they lose energy by radiating photons, these photons are then able to ionize atoms, in this case those of the liquid hydrogen. The liquid in the chamber is very sensitive to charged particles moving through it and will boil as a result of the charged particle ionizing the atoms in the liquid and deposit energy. The process is as so, very briefly, the liquid is prepared and held under a pressure of 5 atmosphere (1 atm=105Pa). Just before the beam of particles reaches the chamber from the accelerator the pressure is dropped to around 2 atmospheres and this causes super-heating of the liquid. As the charged beam particles move through the liquid they cause ionization and so the liquid boils along the particles path and it is this that gives the bubbles. Some of the beam particles collide with the nuclei of the liquid and interact to give new particles that, if charged, can be determined from their paths as they will also cause ionization along their path. This is not to say we can't detect neutral particles in the chamber, however we infer their existence by the production of particles from seemingly out of nowhere, as a neutral particle would produce no trail and from this we can work out its trajectory and see where it initially originated from. We allow the bubbles in the chamber to expand for a millisecond or so, this allows the bubbles to swell to about 1mm in diameter, and then take photos from various angles to enable a 3D view of the interactions to be formed. The pressure is then once more increased to the starting level to clear the bubbles and await the arrival of the next beam.

Some interesting diagrams can illustrate this. Click Here to view them.

In a bubble chamber a magnetic field will curve the trajectory of charged particles, with negativly charged particles curving in one direction and positivly charged particles curving in the opposite direction. Neutral particles, however, do not leave a trail of bubbles and so can not be seen in a bubble chamber photograph. Having said this, if a neutral particle is unstable it may decay into a pair of lighter particles (although many such particles leave the chamber before they decay). These lighter particles (one negative and one positive) are easily recognisable by the V shape which they leave, something which appears to have come out of nowhere (as you cannot see the neutral particle from which it originates).

Bubble chamber photographs

Below are two examples of photographs taken using a bubble chamber and what they show us. For more information on how to read bubble chamber photographs click here.

a bubble chamber picture

e=mc2

This shows an electron knocked out of a hydrogen atom. The electron spirals because it loses energy (momentum) at a considerable rate as it moves through the liquid in the bubble chamber, and the radius of curvature of charged particle moving in a magnetic field is proportional to its momentum. This is an example of Einstein's equation E=mc2

What are the problems?

  • The bubble chamber gives us a 2D image of what is a 3D process which makes it less convenient, in order to convert the 2D image into a 3D one requires us to take multiple photographs from various angles and piecing it together which can be time consuming and is far harder than letting a machine do it for you
  • The superheated phase must be ready at the exact moment of collision otherwise data is lost, as no trail forms and so this method finds it difficult to identify short-lived particles
  • Bubble chambers on the whole are relatively small and so in high energy collisions where particles may radiate out from a point in a large path and so may not be fully contained with in the detector this leads to a loss of data and so accurate calculations may not be possible.
  • Since it gives us a photographic readout instead of an electronic one it can be inconvenient to use especially in experiments where repeats are needed and we have to analyse the data from multiple readouts looking for similarities, as this has to be done by hand rather than allowing a computer to do it for us.

The development of the Bubble Chamber

The bubble chamber has a long and interesting history in which it has lead to many key discoveries.

Year Development
1952 Glaser invents the bubble chamber for detecting subatomic particles, he would recieve a nobel prize in 1960 for his discovery
1959 Serde and Chamberlain recieve nobel prize for demonstrating the existence of the anti-proton, using a bubble chamber
1961 Gell-Mann and Ne'eman both independently devise the " eightfold way", a table like format to order subatomic particles in a way much like the periodic table does with elements
1963

Gell-Mann and Zweig both postulate the existence of quarks, a fundamental particle with a fractional charge

1964

Omega-minus particle discovered from a bubble chamber picture completes the eightfold way and brings it acceptance, Gell-Mann would receive a Nobel prize in 1969 for this and his other contributions to the study of subatomic particles

1968

Alvarez receives Nobel prize for his discovery of many resonance states ( very short lived subatomic particles which only occur in high-energy collisions), This was made possible by the bubble chamber

1969 Murray Gell-Mann (US) recieved a Nobel prize for study of subatomic particles
1973 Neutral currents were discovered at CERN in the 2m x 4.8m freon filled Gargamelle bubble chamber
1974 7-foot Brookhaven bubble chamber started to operate. This was the first particle detector of its type in which the chamber through which the particles passed was surrounded by a superconducting magnet.

Credits

Images from http://teachers.web.cern.ch/teachers/archiv/HST2005/bubble_chambers/BCwebsite/index.htm and courtesy of CERN. This was reviewed by Ben Maybee and Megan Hopton in July 2012 as part of work experience placements.

Further Reading

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pdfpdf bubblechamber.pdf manage 347.0 K 23 Sep 2013 - 15:16 NathanaelASFarley Leaflet based on this page
Topic revision: r14 - 23 Sep 2013 - 15:17:18 - NathanaelASFarley
 
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