AAWSAP DIRD, Positron Aerospace Propulsion, March 2010

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Antimatter
Antimatter appears in the form of fundamental particles that have their sign of electric
charge reversed from their matter counterpart. For example, the positron, e+, is the
antiparticle to the electron, e·. According to the CPT theorem,4 properties of matter and
their counterpart antimatter particles are identical. This has been tested in the
laboratory to an accuracy of roughly 1 part in 10 million.
Antimatter is appealing for aerospace uses because its specific energy by annihilation is
180 MJ/µg, or 10 orders of magnitude larger than chemical energy, as shown in Figure
1.
1.00E+03 ~--------------­
1.00E+02 +-------------­
1.00E+01 +-------------­
1.00E+O0 -+-------------­
1.00E-01 -+-----­
Energy Density
_OOE-02 ----­
MJ/µg
1.00E-03 ----­
_OOE-04 +------­
1.00E-05 +-----­
1.00E-06 -+-----­
1.00E-07 -+-----­
1.00E-08 -1-----.,----
Olerrical
Fission
Fusion
Fbsitron
Annihilation
Figure 1. Specific Energy for Chemical, Nuclear, and Antimatter Materials
In the presence of matter, the positron binds with an electron to form a short-lived
atom called positronium (Ps). Depending on the relative spin orientations of the
positron and electron, Ps has a mean lifetime in a vacuum of 125 picoseconds (para-Ps,
spins antiparallel), or 142 nanoseconds (ortho-Ps, spins parallel).
From quantum number conservation, para-Ps decays into two gamma rays of equal
energy, 511 kiloelectronvolts (keV), whereas ortho-Ps decays into three gamma rays
whose energies add up to 1022 keV. Hence, when Ps self-annihilates, there is 100
percent conversion of mass into electromagnetic energy given by Einstein's famous
equation, E = mc2, where c is the speed of light.
Although the energies of positron annihilation gamma rays are on the nuclear scale,
they have none of the undesirable features associated with nuclear energy. First, the
annihilation evolves rapidly (nanoseconds) and is controllable under predictable
electromagnetic forces. There is no long-term inertia as with nuclear reactors.
Consequently, positron-generated thrust can be throttled.
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