Speed of an alpha particle.

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Boustrophedon
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Speed of an alpha particle.

Post by Boustrophedon » Sun Nov 17, 2019 5:33 pm

An alpha particle emitted from say radioactive decay of radon will have an energy of about 5Mev and a speed of about 5% of the speed of light.

In other words you should not be able to see them move in a cloud chamber, a track should appear instantaneously right across the chamber.

However on videos of cloud chambers on youtube one can quite clearly see the motion and direction of movement of the track.

What gives?

https://www.youtube.com/watch?v=XGNvAEtYZkw&t=3s
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Re: Speed of an alpha particle.

Post by Gfamily » Sun Nov 17, 2019 5:46 pm

Boustrophedon wrote:
Sun Nov 17, 2019 5:33 pm

An alpha particle emitted from say radioactive decay of radon will have an energy of about 5Mev and a speed of about 5% of the speed of light.

In other words you should not be able to see them move in a cloud chamber, a track should appear instantaneously right across the chamber.

However on videos of cloud chambers on youtube one can quite clearly see the motion and direction of movement of the track.

What gives?

https://www.youtube.com/watch?v=XGNvAEtYZkw&t=3s
There seems to be a general flow from the centre to the edge, and I'm not sure whether the eye/brain is interpreting the mix of short and long radial tracks (supplemented with the slower drift) as moving outwards, as opposed to being created along their entire length instantaneously.

For the second sample, I get a feeling that the tracks to the right of the block are formed 'towards' it.

It would be interesting to take a frame by frame sample and see whether the trail is really laid down over successive frames.
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Re: Speed of an alpha particle.

Post by Boustrophedon » Sun Nov 17, 2019 6:18 pm

Slow the video down to 25% and watch. I find the radon at 8:56 quite instructive with tracks going in all different directions.
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Re: Speed of an alpha particle.

Post by shpalman » Sun Nov 17, 2019 6:21 pm

I think you can only get a condensed vapour trail in a certain zone of the chamber, where the temperature is low enough and the vapour is supersaturated. Depending on where it's emitted and the angle at which the particle is moving, its ionization effects might reach this zone first at one end of the trail or the other.

I just made this explanation up, I doubt they slow down enough that you can see them moving before they capture a couple of electrons and thermalize.
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Re: Speed of an alpha particle.

Post by Gfamily » Sun Nov 17, 2019 6:33 pm

Boustrophedon wrote:
Sun Nov 17, 2019 6:18 pm
Slow the video down to 25% and watch. I find the radon at 8:56 quite instructive with tracks going in all different directions.
I think I see what you mean, but I would still like to see a frame by frame analysis.

Taking up from shpalman's suggestion, maybe the speed with which the cloud trace is generated depends on the energy of the α particle, which will be decreasing along the length of the path.
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Re: Speed of an alpha particle.

Post by shpalman » Sun Nov 17, 2019 6:37 pm

Gfamily wrote:
Sun Nov 17, 2019 6:33 pm
Boustrophedon wrote:
Sun Nov 17, 2019 6:18 pm
Slow the video down to 25% and watch. I find the radon at 8:56 quite instructive with tracks going in all different directions.
I think I see what you mean, but I would still like to see a frame by frame analysis.

Taking up from shpalman's suggestion, maybe the speed with which the cloud trace is generated depends on the energy of the α particle, which will be decreasing along the length of the path.
I was thinking of that too but then I wondered if as long as the energy was more than "enough" it would set off exactly the same cascade of ionization.
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Re: Speed of an alpha particle.

Post by Gfamily » Sun Nov 17, 2019 6:48 pm

shpalman wrote:
Sun Nov 17, 2019 6:37 pm
Gfamily wrote:
Sun Nov 17, 2019 6:33 pm
Boustrophedon wrote:
Sun Nov 17, 2019 6:18 pm
Slow the video down to 25% and watch. I find the radon at 8:56 quite instructive with tracks going in all different directions.
I think I see what you mean, but I would still like to see a frame by frame analysis.

Taking up from shpalman's suggestion, maybe the speed with which the cloud trace is generated depends on the energy of the α particle, which will be decreasing along the length of the path.
I was thinking of that too but then I wondered if as long as the energy was more than "enough" it would set off exactly the same cascade of ionization.
I was leaving that as an exercise for the readers ...
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Re: Speed of an alpha particle.

Post by Boustrophedon » Sun Nov 17, 2019 10:06 pm

Gfamily wrote:
Sun Nov 17, 2019 6:33 pm


Taking up from shpalman's suggestion, maybe the speed with which the cloud trace is generated depends on the energy of the α particle, which will be decreasing along the length of the path.
I am starting to thing that is the correct answer.
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Re: Speed of an alpha particle.

Post by jaap » Mon Nov 18, 2019 6:34 am

shpalman wrote:
Sun Nov 17, 2019 6:21 pm
I think you can only get a condensed vapour trail in a certain zone of the chamber, where the temperature is low enough and the vapour is supersaturated. Depending on where it's emitted and the angle at which the particle is moving, its ionization effects might reach this zone first at one end of the trail or the other.
If this were the case, would there not also occasionally be trails that form in the direction opposite that of the particle?

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Re: Speed of an alpha particle.

Post by shpalman » Mon Nov 18, 2019 7:08 am

jaap wrote:
Mon Nov 18, 2019 6:34 am
shpalman wrote:
Sun Nov 17, 2019 6:21 pm
I think you can only get a condensed vapour trail in a certain zone of the chamber, where the temperature is low enough and the vapour is supersaturated. Depending on where it's emitted and the angle at which the particle is moving, its ionization effects might reach this zone first at one end of the trail or the other.
If this were the case, would there not also occasionally be trails that form in the direction opposite that of the particle?
Gfamily wrote:
Sun Nov 17, 2019 5:46 pm
For the second sample, I get a feeling that the tracks to the right of the block are formed 'towards' it.
having that swing is a necessary but not sufficient condition for it meaning a thing
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Re: Speed of an alpha particle.

Post by Boustrophedon » Mon Nov 18, 2019 9:18 am

shpalman wrote:
Sun Nov 17, 2019 6:21 pm
I think you can only get a condensed vapour trail in a certain zone of the chamber, where the temperature is low enough and the vapour is supersaturated. Depending on where it's emitted and the angle at which the particle is moving, its ionization effects might reach this zone first at one end of the trail or the other.

I just made this explanation up, I doubt they slow down enough that you can see them moving before they capture a couple of electrons and thermalize.
OK and given that the sample is resting on the cooled plate, the range of motion of the charged particles is restricted to "upwards" from a cooler region where the track forms quickly to one where it's warmer and forms slower, makes sense.
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Re: Speed of an alpha particle.

Post by shpalman » Mon Nov 18, 2019 9:48 am

Of course I have no idea if it would even be noticeable, but then the thermal velocity of He atoms would be, what, about 400 m/s? (I'm not going to calculate the mean free path, but at thermal velocity the tracks wouldn't form and wouldn't be straight lines even if they did.) Even if the α particles were travelling at "only" 400 m/s a 4 cm track would form in 0.1 milliseconds and I think human vision starts blurring anything which is much faster than 0.1 seconds.

Having said that, the α particles do lose energy pretty quickly, they start with 5 MeV and are down to "zero" after just a few cm of air, are the tracks "thicker" at the beginning?
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Re: Speed of an alpha particle.

Post by Boustrophedon » Mon Nov 18, 2019 10:20 am

If anything they are thicker at the end.
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Re: Speed of an alpha particle.

Post by shpalman » Mon Nov 18, 2019 10:24 am

In that case when they slow down they spend more time in that region so create more of a track.
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