# Why dont neutral particles in a bubble chamber leave tracks?

Even if they have kinetic energy, neutral particles dont leave tracks behind. Why is that so?

Also, can someone please explain how a cloud chamber works?

Thanks!!

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tiny-tim
Homework Helper
Welcome to PF!

Hi merry! Welcome to PF!
Even if they have kinetic energy, neutral particles dont leave tracks behind. Why is that so?

Also, can someone please explain how a cloud chamber works?
Because neutral particles won't ionise anything …

Charged particles create an ionisation track, around which the liquid vaporises, forming microscopic bubbles …
See http://en.wikipedia.org/wiki/Bubble_chamber and http://en.wikipedia.org/wiki/Cloud_chamber for details.

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I once saw a nice analogy about these chambers which goes as follows:
You are an observer in a helicopter flying above a cornfield when somebody fires a cannonball through the corn.The cannonball represents the ionising particle and the track of broken corn represent the track of ions produced.A flock of blackbirds sensing the broken corn descend down and begin to feast.The blackbirds represent the line of bubbles as in the bubble chamber or the line of liquid drops as in the cloud chamber.Having feasted the blackbirds fly away and the tracks can no longer be seen.

Charged particles ionize matter per the Bethe-Bloch equation. The charged particles generally lose energy at about 2 MeV per gram/cm^2. These ions seed bubbles in the bubble chamber when the piston lowers the pressure on the liquid hydrogen. Ions in cloud chambers also are the seed for water droplets in the saturated gas. See
http://en.wikipedia.org/wiki/Bethe_formula

Hi merry! Welcome to PF!

Because neutral particles won't ionise anything …

See http://en.wikipedia.org/wiki/Bubble_chamber and http://en.wikipedia.org/wiki/Cloud_chamber for details.
Thanks! =D

I thought that the moving particles had kinetic energy which they transferred to the liquid they came in contact with thus giving them enough energy to become a gas...
Apparently, my assumption was wrong X.X
I am confused with the ionisation stuff though...
Heres what wiki says
"Charged particles create an ionisation track, around which the liquid vaporises, forming microscopic bubbles. Bubble density around a track is proportional to a particle's energy loss."
http://en.wikipedia.org/wiki/Bubble_chamber

Is the liquid ionised? If so, how does that help it become a gas? =S

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tiny-tim
Homework Helper
… If so, how does that help it become a gas? =S
Hi merry!

As Bob S says, the ions seed bubbles in the bubble chamber when the pressure is lowered.

Did you look at the other link I gave … http://en.wikipedia.org/wiki/Cloud_chamber ?
The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation) …

Consider what happens with boiling water where bubbles of steam form within the liquid and rise to the surface.These bubbles don't form everywhere in the water each one needing a "seed" so the water is able to evaporate.The seed could be something like a jagged bit of the container or a particle of dust.A similar thing happens in the bubble chamber but it is the ions that form the seeds.
An analogous thing happens if you drop something like sand into a glass of beer.You will see a line of bubbles because the sand grains act as seeds for the CO2 to come out of solution.In fact it is said that Glaser,the inventor of the bubble chamber,got the idea for his invention after puzzling over and then working out where the bubbles came from in his beer.He won the nobel prize for his work.Cheers.

Born2bwire
Gold Member
Consider what happens with boiling water where bubbles of steam form within the liquid and rise to the surface.These bubbles don't form everywhere in the water each one needing a "seed" so the water is able to evaporate.The seed could be something like a jagged bit of the container or a particle of dust.A similar thing happens in the bubble chamber but it is the ions that form the seeds.
An analogous thing happens if you drop something like sand into a glass of beer.You will see a line of bubbles because the sand grains act as seeds for the CO2 to come out of solution.In fact it is said that Glaser,the inventor of the bubble chamber,got the idea for his invention after puzzling over and then working out where the bubbles came from in his beer.He won the nobel prize for his work.Cheers.
And they told me that using the beam line's cryogenics to keep my beer cold had nothing to contribute to high energy physics.

Charged particles ionize matter per the Bethe-Bloch equation. The charged particles generally lose energy at about 2 MeV per gram/cm^2. These ions seed bubbles in the bubble chamber when the piston lowers the pressure on the liquid hydrogen. Ions in cloud chambers also are the seed for water droplets in the saturated gas. See
http://en.wikipedia.org/wiki/Bethe_formula
Thanks for the analogies Dadface; they help a lot =D

Now I have two questions:
a) How is the liquid ionised by the charged particles? Do they knock out electrons or smthn?
b) Is the evaporation caused by the energy releases by the charged particles upon ionizing the liquid?
This concept is new to me. I have learnt about atoms getting ionised in chemical reactions... but the whole idea of fast moving charged particles ionizing atoms is new to me.

Can someone explain this stuff please =D

Btw, thanks everyone for all the help! XD

Hi merry!

As Bob S says, the ions seed bubbles in the bubble chamber when the pressure is lowered.

Did you look at the other link I gave … http://en.wikipedia.org/wiki/Cloud_chamber ?
Yes, I did... But I got lost at the ionising part =S

And they told me that using the beam line's cryogenics to keep my beer cold had nothing to contribute to high energy physics.
Donald Glasser won the Nobel Prize in physics for discovery of the bubble chamber. He was an assistant professor at the Univeristy of Michigan 1949-1959, and made the dicovery while contemplating bubbles in cold glasses of beer. So there is still hope for the rest of us. See
http://prola.aps.org/abstract/PR/v87/i4/p665_1