# Cloud Chamber

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1. Mar 1, 2015

### Garlic

1. The problem statement, all variables and given/known data

I'm in high school. I chose my yearly project in physics, and wanted to make a cloud chamber. The subject is "deflection of charged particles in a magnetic field", so I must set a magnet alongside the cloud chamber. I've prepared for it, bought isopropyl alcohol and gathered the Americium 241 source from a smoke detector. The chamber will look like this. [Picture]
My question is, what kind of magnets are strong enough to deflect the particles? Will a small neodymium magnet be sufficient?
Americium 241 is an alpha radiation emitter.

2. Relevant equations
-none-

3. The attempt at a solution
I have no idea how strong a magnet can be in order to deflect it clearly.

Note: I know the dangers of radiation and chemicals like isopropanol, I take full responsibility of my actions.

Note 2: It is my first post, so please tell me if I made a mistake in posting- or a grammar mistake.

2. Mar 1, 2015

### matteo137

The main decay channel of Americium 241 is via emission of alpha particles.
Alpha particles have mass $m=6.64*10^{-27} \,\text{kg}$, charge $q=3.2 *10^{-19} \,C$, and you can estimate the speed to be $v=15\,000 \,\text{km}/\text{s}$.

Knowing that the radius of curvature $R$ for the trajectory of a charged particle in a magnetic field $B$, is
$$R=\dfrac{2 m v}{q B}$$
you can put in the equation a value for $R$, such that a bent trajectory can be easily seen inside your box, and extract a lower bound for $B$.

P.S. keep in mind that your $B$ is not uniform throughout the box...

3. Mar 2, 2015

### Garlic

Thank you very much

http://bit.ly/1DuDBvh

I found a text explaining magnetic fields of a neodymium magnet.
Quotation from the pdf:
"The material is defined by its maximum energy product. The symbol N42 of a magnet means that the maximum value of negative prouct of the magnetic field B and H of 42.10^6 GOe (Gauss-Oersted) = 334 kJ/m^3. The letter N means that the magnets remain operational under 80° celcius.
The magnetisation is very strong: whereas AlNiCo shows the remanence of nearly 0.2 T it is 1.3 T with Neodymium magnets. One should note that the magnetic field B is the same only in the middle of a long bar magnet. On pole surfaces it is nearly half as big. (image 1: https://keep.google.com/media/1293mkHnP5rgfYLSoVtIAowcCicy-pA )
A neodymium magnet develops to a second magnet or a piece of iron in small distances up to 50 Newton per cm^2 polearea. Magnets from 2cm^2 Pole-areas can cause painful injuries.
Because of the 1/r^3-dependence strenght of a dipolar field of neodymium magnets, more than 1mT (a dangeorus magnetic field that is capable of harming electronics) magnetic fields occur if the magnet is nearly more than 15 centimeters long."

I tried to translate as best as I could.
So, I want to do something like this (image 2: https://keep.google.com/media/1gSvUjipL9dUSa-6BmN2laEGgm7C4XQ )

1) So how can I calculate the B field in of a particle that moves in the middle (between two magnets)?

2) Does Lorentz force effect the particle? Is the reason of its curvature because of Lorentz force? (image 3: https://keep.google.com/media/1jzwKLOEK-5iPhR8GfsNBAn9NorFbTg )

4. Mar 2, 2015

### andrevdh

As far as I know alpha particles will travel quite a short distance in air before it interacts with the gas molecules
and cause subsequent ionizations. These will more likely be observed over longer distances. You migh also
observe cosmic particles occasionally shooting through the fog. So creating a magnetic field over a larger
region migh be a better option. Since you are experimenting this could be part of the investigation.

Here is a website that gives more info on how to construct and use a cloud chamber (without a magnetic field)
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p087.shtml#summary

Here is some info on the interaction of a charged particle and a magnetic field
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/forchg.html#c1

Hope this helps!

5. Mar 2, 2015

### matteo137

Garlic:

2) yes, it is just Lorentz force.
1) The question does not make any sense. The B field is given by the magnet, not by the particle. If you go to serious magnet manufacturers they usually quote some surface field

6. Mar 2, 2015

7. Mar 8, 2015

### Garlic

Sorry for the wait..
1st :
2nd:
3rd:

I want to add something, in a bbc documentary, I saw they used a magnet to make it have a curved trail. (link: )

matteo137 , I think I wrote the first question wrong. And I found somewhere that says N42 class neodymium magnets have a magnetic field of 1.3 T in the mittel. (see image 1)

andrevdh , thanks, I think I'm gonna buy a search coil to calculate the magnetic field.

8. Mar 8, 2015

### Garlic

I have found some magnetic field meters, but they seem too expensive (which I'm going to use one time), is the search coil you mentioned something I can build myself?

9. Mar 8, 2015

### BvU

Hi Garlic, nice and ambitious project !

2nd link (picture) this is for electric fields. Curved trajectory and field direction are in the same plane.
In a magnetic field the force is perpendicular to the velocity and to the field.

3rd link: As you saw in the documentary, they kept the tracks close to the magnet (like 1 cm or so). Two magnets at 15 cm doesn't help much. And at 1 or 2cm apart they obstruct the view.

As Andrev posted, alpha particles don't travel far (see e.g. here) and as you calculated (post #2), the radius of curvature will be pretty big for the 241Am. You might ask teacher if the school (or a nearby university) has a $\beta$ emitter for you to use (but they'll be all over you about radiation hygiëne).

Fortunately, looking at cosmic radiation is exciting enough by itself.
Best thing to do: discuss with teacher.

10. Mar 9, 2015

### andrevdh

Garlic, you might be overthinking the project.
Just begin by doing it and learning as you go.
Here is a nice link to info on a search or field coil - look at the bottom of the page in the Investigation:
https://books.google.co.za/books?id=r8I1gyNNKnoC&lpg=PT290&ots=qQjI78pJ3r&dq=measuring magnetic field using a search coil&pg=PT290#v=onepage&q=measuring magnetic field using a search coil&f=false
http://www-outreach.phy.cam.ac.uk/camphy/index.htm

Last edited: Mar 9, 2015
11. Mar 9, 2015

### Garlic

The only beta emitter I found available is tritium/radium painted watch hands. Well, I'll try to contact universites nearby. Will they trust me if I say I'll put the radioactive material in a lead pig? Or is more protection necessary?

12. Mar 9, 2015

### BvU

I don't know if they will trust you, but they might trust your teacher. It'll be a nice enterprise, so give it a try !
And otherwise: If the source can't come to your chamber, perhaps your chamber can go to the source ...

As I said earlier, cosmics are easier and also quite fascinating. Bit harder to do quantitively (they go really fast....)

Well, as @andrevdh says: just begin and see where you can come ! Good luck !

13. Mar 9, 2015

### Garlic

Thanks..

14. Mar 10, 2015

### andrevdh

See this document page 27 for info and notes on handling the radioactive source safely

Last edited by a moderator: May 7, 2017
15. Apr 1, 2015

### Garlic

Today was the last day to give the project to the teacher, I have successfully done the project.
I really want to thank you to all of you guys for your answers.. I really appreciate it.

16. Apr 1, 2015

### BvU

Good ! Congrats !

17. Apr 2, 2015

### andrevdh

Any photos, posters, videos, results or notes on your project?
We would like to see them.

18. Apr 19, 2015

:)