Date: Tue Jul 31 14:16:43 2007
Author: Bill Norwood
Subject: Cloud Chambers, HS Teachers' Workshop
Regarding the Starbucks cloud chambers (google brings them up) and their use
in our High School Teachers' "make and take with you" workshop:
I was unable to see cosmic ray tracks, but the chamber worked very nicely
with uranium ore. I found that one wants to use just a thin/small piece of
the ore in order that the cooling of the ore not take too much time and in
order that the convection it caused not disrupt the experiment too much.
But, the really nice find (relating to the acknowledged find of poor
contrast in the "original" Starbucks cup experiment) was that since such a
thin bottom as aluminum foil would work, why would not a thin plastic
bottom. I tried it, and it did, and that produced much improved contrast. It
was a piece of black garbage bag. Since the material is so thin the thermal
conductivity is almost not a factor.
One sees tracks in 2-5 minutes, and it's still working an hour later if you
make sure that there is still good contact with the dry ice.
On request I provided the following troubleshooting list:
Checklist for when no particle tracks are seen:
1. Is the source active?
2. Room lights on?
3. Looking or illuminating from the wrong angle?
4. Good contact with dry ice? (If the dry ice evaporates enough so that
the rim of the cup rests against the Styrofoam surface below the dry ice,
then that will decrease the contact area and contact force between the dry
ice and the thin plastic bottom of the cloud chamber, and there will be less
heat transfer.) Sometimes when there is plenty of dry ice, just gently
pressing down on the cup will increase the heat flow through the thin
5. Too much water in methanol.
6. Carbon dioxide from dry ice has gotten inside the cloud chamber. (This
could happen if there is a hole in the thin plastic covering the bottom of
the cloud chamber.)
7. Alcohol vapor leaking out of cloud chamber too fast. (This might happen
if the plastic is not reasonably securely sealed all of the way around the
cup with the rubber band.)
Physics Elementary Labs
U of MD Physics
College Park MD 20742