Cloud chambers for idiots

["Phil Hypes" <laradcon@HOTMAIL.COM>]

First of all, I'd like to thank all those individuals who provided advice 

and shared their experiences with cloud chambers.  It was very helpful and 

greatly appreciated.



Now, having finally gotten a cloud chamber demonstration to work, I thought 

I'd provide some complete, concise directions for anyone else who wants to 

try this.  It went over very well in the class I taught over the last three 

days.



1.  Get yourself one or more small glass jars.  You need to be able to 

provide an air-tight seal.  Pyrex makes a 2 cup (500ml) glass bowl that 

comes with an air-tight plastic lid.  These worked well for me.



2.  Glue two 1" x 1" patches of felt to the bottom of the jar.  These will 

hold the alcohol that will form the vapor.



3.  Get yourself a radium dial hand, or some other radium-painted object.  I 

had absolutely no luck with uranium ore.  This stands to reason, when you 

consider the specific activity of radium vs. uranium.



4.  Take the plastic lid that came with the pyrex bowl and place it upside 

down on a countertop or table.  Put the radium source in the middle of the 

lid.



5.  Put 5 to 7 drops of ethyl alcohol on each of the felt pads.  Isopropyl 

alcohol will not work, at least it didn't for me.  I actually used 80 proof 

vodka, although I would rather have used something purer.  My students got a 

kick out of my using vodka.



6.  Invert the bowl (opening facing down) and put it on the lid, making sure 

to get a good seal between the lid and the bowl.  Wait 5-10 minutes for a 

good vapor to form inside the bowl.  Placing your hand on the top of the 

bowl can help the vapor to form.  You will not be able to see the vapor.



7.  Put the bowl (still upside down) onto a block of dry ice.  The plastic 

lid should be in contact with the dry ice.  Wait a few minutes (probably 

around 5 minutes).



8.  Dim the lights in the room, and shine a flashlight in through the side 

of the bowl toward the radium source.  You should see tracks appearing in 

the volume immediately around the source.  They will be about an inch long.  

These are the alpha tracks.  Longer, thinner tracks that loop and twist are 

beta or gamma tracks.  If you can't see any tracks, move yourself and the 

flashlight around the bowl; the viewing will be better from some directions 

than from others.  The chamber will continue to function for several 

minutes, depending on how much vapor you have, and whether any moisture 

builds up on the source and attenuates the alpha particles.



Good luck; this is a very rewarding demonstration, and everyone I've done it 

for has been really impressed.  It does a wonderful job of taking 

theoretical lectures on types of radiation and linking them to actual 

physical reality.



Phil Hypes

Los Alamos Radiation Consultants

laradcon@hotmail.com

505.920.9712





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