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starship-design: Antimatter Annihilation Products

Hi all

Steve wrote, on 2/4 at 18:49 EST (quoting me),

>>Antimatter conversion efficiency = 1.0

[That was really just a value picked to make the analysis easier
to check; it was qualified by "Unfortunately, the analysis
reported there (in my 4/4/96 note) was for only the case with 100
percent conversion of antimatter energy to exhaust kinetic energy
... Subsequent analysis, which I haven't yet reported, has
expanded the calculation to cover antimatter conversion
efficiencies less than 100 percent."]

>That's a common misconception.  Unfortunately the annihilation
>products of matter-antimatter reactions aren't all photons...
>My understanding (bolstered by some half-remembered statements by
>Robert Forward) is that the practical conversion efficiency is
>more like 0.8.

I have before me Robert Forward's report "Antiproton Annihilation
Propulsion," AFRPL TR-85-034, September 1985 (done under US Gov't
contract, so it should be in the public domain, David). It may not
be the best currently available information, but should be a good
reference point from which to judge progress.  He said at that
time (on p. 109, et seqq):

"When an antiproton annihilates with a proton, the predominant
reaction products (98%) are pions.  A recent survey of the
literature [1984 reference] found that on the average there are
3.0 charged pions, 1.5 neutral pions, 0.05 charged kaons, 0.03
neutral kaons, and 0.02 prompt gamma rays. ... The neutral pions
have a lifetime of only 90 attoseconds and almost immediately
convert into two high-energy gamma rays." 

The charged pions are stopped by matter and deposit their energy
(about 2/3rds of the total) locally as heat.  The energy of the
gamma rays is harder to capture.

He continued:

"In many applications of the use of antiprotons for energy storage
and propulsion, consideration is being given to annihilation of
the antiprotons with heavier nuclei than protons.  Since a neutron
has the same baryon number as a proton and a free neutron will
spontaneously decay into a proton, a neutron can be considered as
an 'excited state' of a proton.  Thus, antiprotons, *will*
annihilate with a neutron as well as a proton inside a heavy
nucleus.  Since the neutron has a neutral charge and charge must
be conserved in the annihilation process, the reaction products
from the annihilation of an antiproton in a heavy nucleus will
produce different numbers of the various types of charged and
uncharged pion and kaon particles.

"Annihilation inside a heavy nucleus has the potential for
increasing the efficiency of an antiproton annihilation propulsion
system, since the neutral pions are absorbed in the nucleus
instead of decaying into gamma rays.  The annihilation reaction
will 'heat up' the nucleus as well as cause spallation fission of
charged nuclear fragments."


"An ideal reaction would be one where the energy from the
antiproton causes the nucleus to break up into doubly charged
alpha particles..."

Beryllium 9?  Carbon 13? ... (We could accelerate alphas.)

I hope these quotes will help put us all on the same page.  If
anyone has more information, please share it with us.

[And after we convert antimatter energy to usable energy, we
still have to convert the usable energy to exhaust kinetic
energy (via some kind of engine/electrical generator powering an
accelerator), so the "antimatter conversion efficiency" which is
the overall efficiency used in the calculation of the starship
properties reported in my 2/4/97 note will be even lower than

Rex Finke   <DotarSojat@aol.com>