Oops, saw two more messages in this thread. I suspect that the observation
of any delays in the formation of the beginning and end of the track depends
strongly on the thermal conditions in the cloud chamber. I've seen the
effect, but it might not always be noticable. I don't have a lot of
experience with cloud chambers, so I'm guessing. The density of ionisation
really is greater near the end of the track, especially for alphas. This is
because some of the secondary ions created by the alpha have enough energy
to make secondary (tertiary?) ions of their own, & etc. The passage of one
particle results in a "shower" of ions and electrons along the track. For
alphas and protons, at least, the kinematics favors showers near the end of
the track. If the secondary ions/electrons in the track are accelerated by
an electric field, one gets an "avalanche" of secondaries. Geiger counters
work by collecting the "avalanche" in the form of an electronic pulse. In
the cloud chamber, one just sees a widening of the track. I suppose that one
could learn something about the particle by the degree of widening. I also
suspect that local variations in the thermal characteristics of the cloud
chamber would make this type of quantitative analysis difficult to
impossible. Maybe it was done in the past, but as far as I know cloud
chambers aren't used much in research anymore. They're still "cool" to look
at, though.
For Brian W., this discussion doesn't really apply to gas bubbling in bubble
chambers, which are different "beasts" than cloud chambers. For droplet
formation in cloud chambers, I'm not an expert but because each droplet must
scoop up a large quantity of vapor in order to become visible, it could take
some time. Just to get an idea, one liter of liquid nitrogen occupies around
200 liters at atmospheric pressure when vaporized. (I didn't do the
calculation...this is just a "rule of thumb" number.) Each droplet will have
to acquire many, many times its own volume of gas in order to become
visible.
Vickie
I didn't write the disclaimer. Blame the lawyers.
-----Original Message-----
From: kowalskil [mailto:kowalskil@MAIL.MONTCLAIR.EDU]
Sent: Thursday, May 09, 2002 3:01 PM
To: PHYS-L@lists.nau.edu
Subject: Re: cloud chamber tracks
I can understand your explanation of the delay between
the moment at which an ion track is formed and the
moment at which it becomes visible (formation of tiny
droplets by condensation). But the question was about
a delay (I do not remember seeing it when I watched
tracks many years ago) which presumably exists
between the moment at which the beginning of the
track appears and the end of the track appears.
1) Are such delays really observed with naked eyes?
2) If so then how to explain them?
In the case of alpha particles, often used to demonstrate
tracks) the density of ionization near the end of tracks
is always much larger than at the beginning. Can this
lead to differences in "condensation times?" My suspicion
is that delays are due to the temperature non uniformity,
This would be easy to demonstrate by using more than
one source in the same chamber.
Ludwik Kowalski
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