Bubble Chamber

garytse86

Why don't neutral particles leave tracks in the bubble chamber?

How is the bubble formed, is it condensation around ionised hydrogen, or around the particles?

Mk

A bubble chamber is made by filling a large cylinder with a liquid just below its boiling point, while the whole chamber is under a constant magnetic field. As particles enter the chamber, a piston suddenly decreases the pressure in the chamber. This brings the liquid to a superheated state, in which a tiny effect, such as the passing of a charged particle near an atom, is sufficient to make a bubble of gas. A camera records what's happening. The magnetic field causes charged particles to travel in helical paths whose radius is determined by the ratio of charge to mass of the particle. In this way charged particles can be observed and their mass measured, however, there is no way to know their velocity. Because the detection of the particle relies on the magnetic field-particle interaction, neutral particles don't work.

garytse86

so the bubble forms around the atom, not the charged particle?

Why dont neutral particles work?

Mk

If protons are put in, bubbles form around the protons, if atoms go in, bubbles form around those, of course the particles have to be charged.

I said why neutral particles don't work in the last sentance.

Or do you want me to explain it diffrently?

garytse86

Neutrons can ionise hydrogen atoms and so forming charged particles, so why can't neutrons be directly detected?

kevinalm

I see what you mean. Iirc, most (perhaps all) neutral particles don't leave an ionization trail strong enough to trigger a bubble trail. They don't interact with the liquid strongly enough.

garytse86

surely a proton and a neutron have similar ionising powers?

Caper

The particles aren't observed by what they ionize, they are observed by the bubble trail they leave behind as they pass through. The reason they need to have a charge is so the magnetic field affects them and they take on a spiral path, or else they go in a straight line and you get no useful data.

Editted spelling

kevinalm

Depends on what you mean by ionizing powers. If they have equal energy then yes they will create roughly the same total number of ions as each slows to a stop. But a neutron takes longer to slow than a proton, no charge to strip electrons from atoms near its path. A neutron does have a magnetic field, but this isn't nearly as effective for causing an ionization event. The net effect is that the density of ions along the track is lower. To low for a bubble trace to form.

garytse86

Alright, I understand now.

Thanks for the help. :D

Gary

kevinalm

Glad to help. By the way, did you ever hear the story of how the bubble chamber was invented. Particle physists had run into a problem in that cloud chambers weren't accumlating enough collision events (a particle actually striking a nucleus) due to the high energies of the particles and the low density of the gas filled cloud chamber. One evening the inventor (whose name I don't recall atm) was having a beer with friends at the local watering hole and noticed the bubbles forming in the glass. He started wondering what caused bubbles to form and... True story supposedly.

jtbell

The guy was Donald Glaser and it happened at the University of Michigan. When I was a grad student there in the late 1970s / early 1980s, I heard that story a lot, because I did my Ph.D. with the bubble chamber group there. Glaser had been gone from Michigan for years by then. I used to know which watering hole was supposedly the site, but it seems to have slipped out of my memory.