Message-Id: <l03102801b18f80d2c6af@[209.142.17.175]>
Date: Mon, 25 May 1998 14:29:51 -0700
To: pinhole@exploratorium.edu
From: Ron Wong <ronwong@inreach.com>
Subject: Cloud chambers, Thorium, & mantles
A. Cloud chambers
I lost the thread, but not too long ago a pinholer remarked that he/she was
having some trouble getting his/her cloud chamber to work having been
reduced to lining the bottom of the chamber with paper towels rather than
the paper blotter that was apparently recommended.
Although it might work, I think that the use of either would only be
partially effective if lining the bottom of the chamber was all that was
proposed.
If this is the typical diffusion cloud chamber and not the classical Wilson
cloud chamber like that at the Exploratorium, then the process involves
creating a range of supersaturation vertically across the chamber. One way
this can be achieved is by securing a layer of black velvet on *both* the
top (a paper towel or blotter paper should also work here) and the bottom
of the chamber (the black velvet on the bottom makes it easier to see the
tracks). Soak the upper piece of velvet with alcohol and place the whole
thing on a slab of dry ice. The alcohol vapor will then diffuse from the
"warm" upper plate down to the colder lower plate. Somewhere in between is
a region where you will get the optimum results for observing radioactive
tracks. Raising or lowering the radioactive sample in the chamber will
allow you to find where this region is.
If the chamber has an opening at the top, you might consider shaving a
little dry ice over the opening so that dry ice particles fall into the
chamber. This allows the students to see the effects of cloud seeding and
is a good way of introducing them to what will happen in the chamber when,
instead of particles of dry ice, ions appear as the radioactive particles
race through the chamber.
B. Owen asked:
> We used those knit coleman lantern "bulbs" (?) which, for some reason,
>are treated with a radioactive substance. Anyone know why???
The "bulbs" are known as mantles in the trade. They are used in propane as
well as gas lanterns and can be found in many homes that use propane as
their primary source of fuel.
Thorium oxide can be raised to very high temperatures without breaking
down. The mantles are coated with it and it acts like a binder - holding
together the ash that is created when you initially burn off the mantle
prior to using it in your lantern for the first time.
The problem is that the working mantle is very fragile (as users of these
types of light sources are well aware) and, under the right circumstances,
can be ingested or inhaled if the mantle is accidentally reduced to a fine
powder.
Although the level of activity is said to be low, having radioactive
material lodged in your body is not considered very good for you in the
long term. Supposedly, this is the reason Coleman no longer uses thorium
oxide in its mantles (whether this is true or not I don't know). Other
manufacturers still do. Read the information on the package carefully the
next time you need a one. One way or the other, you will want to make the
right choice.
ron