Pre-Phase A Report - Lisa - Nasa

4.2 Noises and error sources 89
charge rate (normalised)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
H+
He++
−0.1
0 500 1000 1500 2000
incident energy (MeV)
charge rate (normalised)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
−0.1
2000 4000 6000 8000 10000
incident energy (MeV)
Figure 4.5 LISA proof-mass charging versus incident particle energy. (The
normalisation is described in the text.) For each species, there are two closelyspaced
curves which differ only by the inclusion of low-energy electrons (see
text).
the proof mass. Therefore, the normalised plot contains information on the combined
shielding power of the structure and the proof mass.
It turns out that the peak of the proton curve corresponds to the energy at which incident
protons actually come to rest in the proof mass, having passed through the spacecraft
structure. At higher energies, the charging is reduced but remains positive, and is due
mostly to secondary protons stopping in the proof mass (most of the primary protons pass
right through). At even higher energies (> 2000 MeV), the charging actually becomes
negative, and is dominated by secondary electrons which stop in the proof mass (most
protons, primary and secondary, pass right through). The curve for helium follows a
similar trend, but is shifted to higher energies (by virtue of their larger size, helium nuclei
require more energy to penetrate a material).
GEANT is only valid for energies in excess of 10 keV. To check whether low energy
(≤ 10 keV) electrons unmodelled by GEANT could significantly affect the charge deposition,
the ITS Monte Carlo code [100] was used to analyse the transport of low energy
electrons in the near vicinity (1 µm) of surfaces. This explains the two closely-spaced
curves for each species in Figure 4.5 . The slight difference between the curves represents
the inclusion of the effects of low-energy electrons. It is seen that the differences only
become significant at high incident particle energies (∼ 1000 MeV) when the electrons are
generated in large quantities.
The ‘raw responses’ in Figure 4.5 can now be ‘folded in’ with the cosmic ray spectra
(Figure 4.4) to yield the desired result, namely, the charge rates due to cosmic rays. The
Corrected version 2.08 3-3-1999 9:33