In browsing the Internet I found a question that a physics
teacher asked and the answer provided by Jack Uretsky. This
was not found in in the Phys-L archive; the items are not
dated. I am pasting them for two reasons: (A) to show an
example of misunderstanding (this is not important,
questions are not always asked clearly) and (B) to show
how I understand the operation of a diffusion cloud chamber.
A) Note that the teacher was asking about a diffusion cloud
chamber while Jack's answer referred to an expansion cloud
chamber. Information about commercially available diffusion
cloud chambers can be found at:
That gadget seems to be worth having; like a Geiger
counter a diffusion chamber shows that nuclear processes
are constantly occurring in our environment. Unlike old
expansion chambers they react to charged particles
constantly (rather than only for short intervals after
each expansion).
Question: I am a junior high Science teacher. For the
past two years I have tried to help my students build
cloud chambers. We use small plastic dishes with
black felt on the bottom and a black paper ring around
the inside edge. We use Coleman lantern mantles and
91% isopropyl alcohol. We place this in a Wendy's
salad container along with vermiculite and dry ice.
And we wait, and wait. The room is darkened. The
children use flashlights to see into the chamber. And
they see nothing. I know I have seen this experiment
work. It has worked for me in the past...but not these
last two years. What am I doing wrong? Does anyone
have any suggestions? Help! I would really like to do
this -- but with success.
Answer: Have you tried it with a radioactive source? It
is a difficult construction; I know, I have tried it. Usually
you have to compress and expand the chamber, and
you see tracks only on the expansion. But it the expansion
is not "smooth" then the cloud is turbulent and will not
hold tracks. Please let me know if you get one working.
Jack L. Uretsky
B) I wish I had a good description of the principles
of operation at home. Then I would not bother Jeff
Radtke, the president of a small company that make
these instruments available (see the URL above). He
helped me and corrected some of my misconceptions.
It is good when a supplier is ready to help. Let me
summarize what I learned from Jeff. Please correct
me, if necessary, and share your understanding.
How would you answer the teacher above?
a) The diffusion chamber is a sealed box; the "floor"
is cold, about -60 C (due to dry ice), while the "ceiling"
is warm, about 20 C. The vertical gradient of temperature
implies a gradient of density; density at the cold bottom
is slightly different than at the warm top. About 500 ml
of alcohol, in felt near the top, will last for about 8 hr
of continuous operation. The amount of air and the amount
of alcohol inside remain constant. Air is always a gas but
alcohol evaporates (from felt) and condenses (at the floor,
and along the tracks of charged particles). Thus the amount
of liquid alcohol in the original dish decreases while the
amount of the liquid alcohol at be bottom increases.
b) The air is pure and that is why condensation of alcohol
occurs only at the bottom. The layer above the bottom is
constantly supersaturated. Here condensation can take place
only on ions formed along the tracks of charged particles.
In Jeff's chambers the supersaturated layer is about one
inch thick. That is where tracks of charged particles can
be observed through the transparent ceiling. This is
possible because the chamber is illuminated and light is
scattered by tiny droplets forming along the tracks.
c) Our archive <http://lists.nau.edu/archives/phys-l.html>
has several interesting messages on the subject. I was not
aware that a vertical electrostatic field (positive is at
the transparent "ceiling" and grounded negative is at the
"floor") is created to make visible tracks of particles
traveling above the supersaturated layer. Particles
traversing above the supersaturated volume do form ions
along their paths but the air is not supersaturated and
no condensation occurs on these ions. The electric field
pushes columns of positive ions into the supersaturated
region (before they disperse) and particles condense on
these columns. That is how the number of tracks is
increased; some of the particles would not form visible
tracks without the electric field.
d) The existing electric field is responsible for making
two tracks from a single track (that would exist in the
supersaturated layer without the electric field). As time
progresses columns of positive ions drift down while
columns of negative electrons move up (much more rapidly).
Condensation takes place on positive and on negative columns.
Diffusion, superimposed on drifting, is responsible for
the fact that tracks become wider and wider. If you do
not have a cloud chamber ask for the free video at the
above URL.
e) In one of his messages Jeff wrote: "There were several
papers published describing theoretical aspects in the
1950's. One of the better was written by R.P. Shutt and
published in Review of Scientific Instruments. R SHUTT,
A THEORY OF DIFFUSION CLOUD CHAMBERS, RSI, VOL 22,
#10, p.730-736, 1951. These articles do not describe the
projection field; this may be an original [his own]
contribution."
Once again, I would like to thank Jeff Radtke for the
video, and for answering my questions. I am sharing my
understanding of his explanations. A teacher wants to
teach, even after retiring.
P.S.
I have two unanswered questions. To avoid additional
complications let us assume that E=0. Another assumption
is that nothing enters or escapes the chamber.
1) Jeff mentioned that the density near the cold bottom
is greater than near the warmer top. How can this be
explained?
2) Is there a vertical gradient of pressure in this vessel?
At what side is P higher?