Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: cloud chamber tracks



Ooooh, oooohh, I know this one! I've worked in high energy physics, and did
my Ph.D. in atomic collision physics. As the particle zips through the gas,
it ionizes atoms in its path. The kinematics of the collision gives many of
these ions/free electrons a large momentum in the same direction as the
incoming particle. These "secondary" ions and free electrons have
sufficient energy to cause a vapor trail of their own. Because the component
of their momentum transverse to the original particle trajectory is very
small, the effect of the secondary ions/electrons is to cause the observed
slight broadening of the track. As for why there is a delay between the
particle's passage and the appearance of the track? The ionization of the
gas along the particle path causes gas molecules to start combining into
droplets via electrostatic attractions. The tiny clusters of atoms serve as
nucleation sites for further condensation. The droplets are accumulating
atoms via thermal collisions in the gas. The gas is usually cold, slowing
down thermal processes to a degree which I am too lazy to calculate just
now. Optically, you can't see the track until the liquid droplets become
large enough to scatter visible light. The need for droplet growth accounts
for the time delay.

Vickie Frohne
Benedictine University

The disclaimer is one of the university's responses to a recent lawsuit,
which is a very good university that doesn't deserve such lawsuits. It's not
a choice thing.

-----Original Message-----
From: Wolfgang Rueckner [mailto:rueckner@FAS.HARVARD.EDU]
Sent: Thursday, May 09, 2002 10:41 AM
To: PHYS-L@lists.nau.edu
Subject: cloud chamber tracks


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
It is prohibited for anyone to disclose, copy, distribute the contents of
this message. Benedictine University does not review, edit or censor
E-Mail communications sent out through their System. The University
maintains and enforces polices regarding the acceptable use of its
technology resources. All messages express views solely of the sender,
which are not to be attributed to Benedictine University, and may not be
copied or distributed without this disclaimer.