Pre-Phase A Report - Lisa - Nasa

4.2 Noises and error sources 87
code employs Monte Carlo particle ray-tracing techniques to follow all particles (incident
and generated) through three-dimensional representations of the LISA spacecraft geometry,
taking into account all significant interactions. Table 4.3 shows the interactions that
are currently modelled by GEANT. A more detailed discussion of the use of GEANT for
this type of application can be found in [99].
The geometric model constructed for LISA is summarised as follows: the 4 cm gold cubical
test body is surrounded on all faces by 25 µm gold electrodes mounted on a cubicallysymmetric
15 mm shell of quartz, enclosed in a 5 mm thick titanium vacuum housing,
surrounded by 1 cm of carbon-epoxy representing the spacecraft structure. On two opposing
faces of the cube, quartz windows (7 mm diameter) have been inserted in the
titanium, quartz, and electrode layers, representing the access windows for LISA’s laser
beams. Although this is a somewhat simplified representation, it does contain the key elements
for accurate modelling, namely, a three-dimensional description of the intervening
layers of material between the proof mass and the exposed spacecraft outer surface.
Figure 4.5 shows the computed net charging of the LISA proof mass as a function of
incident particle energy. To produce this curve, GEANT was used to analyse the effect of
isotropic particles striking the outer spacecraft walls, for a range of energies from 100 MeV
Figure 4.4 Cosmic ray differential
energy spectrum (reproduced
from [97]). The hydrogen
spectrum has been multiplied
by a factor of 5 to avoid
clutter. For each species, the
upper envelope indicates the
solar minimum spectrum, the
lower envelope indicates the solar
maximum spectrum. The
shaded area indicates the range
of the solar modulation over a
solar cycle.
Corrected version 2.08 3-3-1999 9:33