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Re: [Phys-l] Electron vs. Alpha particle...



In reading Hugh's message I realized I made a bog mistake. I treated electrons as if they were positively charged. Negative electrons are attracted, toward alpha particles. For large impact parameters their trajectories bend toward alpha particles, not away from it. The concept of the minimal approach is still applicable, for this kind of scattering. For small impact parameters the alpha particle and the electron, in the classical model, fuse forming a neutral pint-like particle.

But suppose we model an alpha particle as a cloud of positive charge, of Radius R. in the CM frame of reference the alpha particle (cloud) is at rest while the electron it traveling toward it. The kinetic energy of relative motion increases when the distance between the particles decreases, and vice versa. Yes, I am thinking about cases which the electron can penetrate the cloud. Hugh is right, classical model is not useful.

"Scattering" of very energetic electrons (hundreds of MeV) on atomic nuclei "positive clouds" i provided useful information sizes of such clouds, about half a century ago. But that has nothig to do with the original question. I am sorry to forget that electrons are negative.

Ludwik
==========================

On Jan 12, 2011, at 6:31 PM, Hugh Haskell wrote:

At 15:38 -0700 01/12/2011, Jeff Loats wrote:

In discussing Rutherford scattering I ask students to use the simple case of
an alpha particle colliding head on with an electron at rest. The idea is to
use conservation of energy and momentum to show that in a classical
"billiard ball" model, the alpha particle can ignore electrons in its path
to a good approximation.

This term a curious student asked some great questions about what would
happen when such a collision took place.

This exceeded my knowledge a bit, so I thought I would ask here.

What would happen if an alpha particle was fired head-on at an
electron.

(I know the question is posed in an incorrect "billiard ball" fashion.)

Any insights?


Of course, Rutherford scattering isn't between electrons and alpha
particles, it's between alpha particles and gold nuclei (or other
heavy bound nuclei, which enables one to treat the system without
having to worry about the heavy nucleus recoiling. Since, in this
case both particles are positively charged, it is possible to treat
the problem, at least to first order as a classical interaction,
using Newton and Coulomb to do the physics. Make the interacting
particles of opposite charge, as you suggest, makes the interaction
much more complex, and quantum mechanics cannot be avoided in the
problem for the most part, except in a rather narrow range of
energies where a classical gravitation-type interaction can be
reasonably achieved. But at low energies we run into the problem that
there is a minimum-energy bound state (the ionized helium atom), and
at high energies the electron and alpha particle will interact at the
nuclear level, and the individual particles lose their distinct
identities.

In general, though, to answer your main question: it depends.
Kinematically, it's no different from firing an electron at an alpha
particle. Since the force between the two is attractive, The energy
of the interacting pair is important. If it is low (about 25 eV or
less--that is, the initial KE of the incoming electron) one of the
options is that the electron is captured into a bound state and the
pair become a singly ionized helium atom. But that's not guaranteed,
even at low energies there is an elastic scattering channel that is
also available. At higher energies the situation becomes more
complex, with possible Raman scattering channels, high order Rydberg
channels and even nuclear channels opening up, as well as elastic
channels.

Electron-proton scattering has been extensively studied for many
years, and more details should be available in intermediate level
atomic scattering theory texts and monographs.

It's really hard to think about electron-proton or electron-alpha
scattering in billiard ball terms, since the force between them is
long range and attractive, so a quantum solution is probably going to
be the only realistic possibility.

Since low energy atomic scattering theory was the subject of my
dissertation, I can testify that the mathematics of these processes
are daunting. My understanding is that the experiments are not
trivial, either.

What is usually studied is electron-atom scattering, since that
allows one to treat the forces between the electron and the atom as a
finite-range process, in the sense that the net Coulomb forces among
the particles (which can loosely be thought of as similar to the Van
Der Waals forces that chemists are fond of) drop off much faster than
an inverse square rate. That simplifies the mathematics considerably.
I made the mistake of studying electron-ion scattering, in which the
net force is pure Coulomb, and thus "long-range" in any practical
sense, and quickly found that I was dealing with versions of
Schroedinger's equation whose solutions had to be expressed in terms
of confluent hypergeometric functions. I ended up learning a lot more
about numerical analysis than I did about physics.

Hugh
--

Hugh Haskell
mailto:hugh@ieer.org
mailto:haskellh@verizon.net

It isn't easy being green.

--Kermit Lagrenouille
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Ludwik

http://csam.montclair.edu/~kowalski/life/intro.html