using the cloud chamber

[Ken Olum <kdo cosmos phy tufts edu>]

Here are some of my experiences using the cloud chamber.

First of all, it's small, about 25cm square.  Only about 2 people can
look at it at once, so to do anything useful for a class you have to
have the students come up 2 by 2 to look into it.  Cosmic ray tracks
are clearly visible, but not usually spectacular.  The active region
is not very tall, so vertically moving particles give quite short
tracks.  Alpha particles (presumably from radon decay) are quite
spectacular, but the event rate is only one every few minutes or so.

We did not get the projection voltage system, but you can produce
sone of the same effect by rubbing the glass top with silk.  It does
seem to make a difference in the event rate.  Supposedly you should
ground the bottom of the chamber for this technique, but it isn't
conductive.  I used a piece of aluminum foil, but I'm not sure if that
really made any difference.

Apart from the things that come with the cloud chamber, you need
ethanol and dry ice.  The directions call for 8 oz. of ethanol, but I
found that I needed somewhat less.  I didn't have a ready source of
pure ethanol, so I used the "ethyl rubbing alcohol" that you can get
in the pharmacy.  This is not the "denatured alcohol" that we used to
have, but some new product with acetone and even stranger ingredients.
It is only 70% ethanol, and this may have led to a problem.  After a
couple of hours of running the chamber gets ice crystals (presumably
water ice) on the bottom.  They don't interfere with operation, but
they do make it harder to see the tracks.  I poured out the old
ethanol (carefully!) and put in new and that made it work better for
another couple of hours.

Dry ice is not so easy to get.  You don't want to get to get it too
long in advance, or you'll have to get a lot extra to compensate for
sublimation.  Supposedly in an ice chest you can expect 5-10 pounds
per day.  There aren't that many places that sell it, and some of
them are out.  I went to 2 places -- one which was a retailer and sold
me 8 pounds for $12, and one which was a manufacturer (Dry Ice Corp.)
and had a 50 pound minimum but only charged $20 for that amount.  The
instructions ask for a piece 6"x12"x2.5" and then tell you to cut it
up in chunks to go under the bottom of the chamber.  What you'd really
like is more like 9"x9" instead of 6"x12", and then you wouldn't have
to break it up.  I didn't want to slice up a 50 pound block, so I
bought 50 lbs. of pellets, and those worked fine.  The instructions
say that you can run for 8 hours on the amount of dry ice they
recommend, and that seems realistic.

                          Ken