Defense Intelligence Reference Document Positron Aerospace Propulsion

UNCLASSIFIED/ ,'P81l 8PPll!l*L tl!II!! t!IHLY
Lifetime shortening owing to cyclotron
radiation of the electron and positron
gyrating in the magnetic field are not
included in this model. Because this
process is proportional to B2, magnetic
fields should be small-less than 0.1 Tesla
based on our computations of the effect.
This, in turn, renders the atom very large,
with up to 1-micrometer elongation. If
this can be verified in the laboratory, the
first of the two conditions laid out in the
previous section will be satisfied.
Next, it is important to identify a storage
medium with the largest possible voids.
Silica aerogel is a promising material that
was developed by NASA because of its
extremely low density and excellent
thermal insulating properties. It is
composed of strands of SiO2 (silica)
;.: ,,·l
B
E----_,; I i, l., ,
,, -.,
' '
--: --
i' a. u.
grains suspended in a gel that has been
dried and expanded by injection of gases
to a very-low-density configuration of
large voids within an irregular lattice of
silica strands (Figure 22). Silica aerogel is
Figure 21. Computer Simulation of Ps Atoms in a
5-Tesla Magnetic Field and 100 V/cm Electric Field
(1 a.u. = 0.052 nm) (courtesy University of
Bielefeld, Germany) 74
available commercially with typically 20-nanometer average voids.
produced silica aerogel with up to 1-micrometer void sizes.
Recent research has
Experiments in Japan and at Positronics
Research LLC 33 with low-energy positrons
in silica aerogel show a high efficiency
(~35 percent) for making Ps through the
interaction of the positron with silica
grains. High radiation exposure from
positrons implanted in the material result
in it becoming "paramagnetic,"
permanently at low temperatures, with a
high density of "dangling bonds"
containing very loosely bound electrons
that explains the high efficiency for Ps
formation. It therefore serves a dual role
as source and storage medium for Ps.
What lifetimes might be expected working Figure 22. TEM of Silica Aerogel (courtesy
with this material? Lawrence Berkeley Laboratory)
The Ps decay rate in a porous material is given by: 75
/.. = k'/(R - r') + /.. + /..
T q
26
UNCLASSIFIED/ ,<EiOAt OEiEiIQI.«1k W&li &•~LY
( 2)