Re: cloud chamber tracks

[Jeff Radtke <jr@REFLECTIONIMAGING.COM>]

If you are observing tracks in a diffusion cloud chamber, rather than
an expansion chamber, I believe that I can provide some explanation.

A diffusion cloud chamber produces a supersaturated region near the
bottom.  This is a horizontal layer typically one to five cm thick.
The vertical dimension of the chamber is about eight times this layer
thickness. So, 8x more background events occur outside the sensitive
layer than within.

The typical and most readily obtainable environment in the chamber is
STP air saturated or supersaturated with ethanol vapor.  It is most
likely that the ions formed will be N2+ and O3-.  If there is a
vertical electric field present, these species will drift up or down.
Mobility of N2+ in N2 is 1.9 cm2 V-1 s-1.  Mobility of O3- in 80/20
N2/O2 is 2.2 cm2 V-1 s-1.  (The Mobility and Diffusion of Ions in
Gases, McDaniel and Mason)  If we assume a 17 V cm-1 field, drift
velocities are 32 and 37 cm s-1.  (Commonly recommended "clearing
field" strength)  This gives about a 30 ms delay for each vertical
cm in particle trajectory, and some time for ions to diffuse apart.

Since chamber walls and top are typically insulators, one is probably
unlikely to prevent some non zero electric field without introducing an
electrode at the chamber top.  We recommend intentionally creating an
downward field of 170 V cm-1, to drift the positive ions down into the
sensitive layer.  (The electrons diffuse before forming relatively heavy
ions, so positive ion tracks are sharper.)  We call this a "projection
field," and with it one sees an abundance of tracks from only natural
background radiation in our oversized diffusion cloud chambers.  Cosmic
rays which traverse our 50 x 50 cm Lecture Hall Chamber frequently
present a clearly discernible time difference in the appearance of
droplets on each end of the track.

We distribute a free video on CD-rom which shows tracks from natural
and artificial sources.  Stepping through the alpha demonstration,
I just noticed that some tracks form first at the end away from the
source.

Jeff Radtke
http://www.cloudchambers.com


Wolfgang Rueckner wrote:
> If you observe the formation of cloud chamber tracks, it appears that
> the track "grows" in the direction of motion of the charged particle
> that caused it.  Here's my question:  Since these are high speed
> particles and traverse the cloud chamber in what must be microseconds
> or less, why doesn't the entire track appear simultaneously?  Why
> does it (at least appear to) get seeded at one end and grow in the
> direction of the particle motion on a time scale of milliseconds or
> longer?  Wolfgang