From:
bds@rzg.mpg.de (Bruce Scott TOK)
Newsgroups: sci.physics.plasma
Subject:
Re: How much energy for e+P=N?
Organization: Rechenzentrum der
Max-Planck-Gesellschaft in Garching
References:
<3A52AD22.773C576D@iprimus.com.au> <3A550FEE.CE0EF9EA@iprimus.com.au>
<93550o$1gsd$1@saturn.cs.uml.edu>
In article
<93550o$1gsd$1@saturn.cs.uml.edu>,
Russ.Shaw
<rjshaw@iprimus.com.au> wrote:
>
>Hi all,
>
>Assuming
an accelerated electron hits a free proton
>(such as an ionized
hydrogen atom), how much energy
>does the electron need to 'combine'
with the proton?
>
>I assume you'd get a neutron and some
photons.
>Is there a logical way to work out nuclear
>reactions,
or do you just look for a table
>of known reactions?
The
reaction you need is
e-
+ p --> n + nubar
where nubar is an antineutrino. This reaction is what people refer to
as
"inverse beta decay". This is
a weak interaction so the cross
section is very small. It only takes place in Nature in the cores
of
massive stars. When the proton
is part of a composite nucleus, the
process is called "neutron
drip" and usually leads to stellar collapse
since the loss of the
electron depletes the pressure responsible for
resisting gravity. Above the neutron drip density the
equilibrium state
is a mix of electrons, nuclei, and free neutrons.
Normally,
the usual beta decay reaction
n --> p + e- + nubar
dominates because of the chemical
potentials, but at high electron
density the available states are taken up
by free electrons (especially
if the Fermi energy of the electrons is
larger than the neutron/proton
mass difference, which is 1.29 MeV. So the inverse beta decay reaction
has
a threshold in the density.
For more information consult
Shapiro,
S, and Teukolsky, S, Black Holes, White Dwarfs, and Neutron
Stars: the physics of compact objects,
Wiley (1983)
Chapter 2 considers a pure hydrogen star at zero
temperature, where the
e+p reaction is the most important one. Chapter 8 considers the
equation of
state for densities beyond neutron drip.
--
cu,
Bruce
drift
wave turbulence:
http://www.rzg.mpg.de/~bds/