Space applications of the MITS electron-photon Monte Carlo transport code system Page: 2 of 10
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0
E
0
10*1.0
2.0 2.0
Source Energy (MeV)4.0
FIGURE 1. Comparison of the dose measurements of
Van Gunten [5,6] at the front and back wall of an
aluminum cube irradiated on the front wall by a
uniform, isotropic, and monoenergetic flux as a function
of source electron energy with the predictions of a single
adjoint calculation.
A single adjoint calculation was performed for the dose at
the center and just inside a wall, where that wall is exposed to
the electron source (front wall) and where the opposite wall is
exposed to the source (back wall). By symmetry the same
experimental data was obtained by irradiating only one wall.
Figure 1 compares the calculated data with the experimental
data. Not shown, but also obtained from this calculation, are the
predicted doses where any of the four lateral walls are exposed
to the source (these four distributions are the same from the
symmetry of the problem). Thus, a single adjoint calculation
gives the predicted dose at a point for all source energies and all
source surfaces. Obtaining this information would have
required multiple calculations in the forward mode. The
agreement between the calculations and the experimental data
in Fig. 1 is quite good. The run time for the calculation was 4.7
hrs, resulting in a statistical uncertainty that is usually less than
1% but reaches a maximum of 3% in the lowest-energy group.
The edge of the aluminum cube is 20.32 cm and the wall
thickness is 0.1524 cm. The latter is small enough that the dose
is dominated by direct electron deposition over the range of
source energies plotted in Fig. 1. If lower energies or thicker
walls had been chosen, the dose would have been dominated by
bremsstrahlung produced by the source electrons. This would
have been a more severe test of the MITS code, but
experimental data for this situation were not available. An
example of such a "combined radiation effects" adjoint
calculation using the MITS system is given in the following
GPS application (see also Reference [2]).
In contrast to Fig. 1, Fig. 2 shows the more familiar sort of
information obtained from a single forward calculation at 3.0
MeV, namely energy deposition profiles as a function of
distance from the wall exposed to the source along a line
through its center and one through its corner. Obtaining this
information would have required multiple calculations in the
adjoint mode. Agreement with experiment is again very good.0.20
d
E 0.16
0.1
0
a
0.06
0
-0 -- - Beam from Front WailMITS
- Beam from Back Wall MITS
" Beam from Front Wall Exp
Beam from Back Wall Exp-- centerline
- box comer
, center line, data
~ box comer, data
' --1 - , .3i
0.0 6.0 10.0 16.0 20.0
Distance from Front Wall (cm)
FIGURE 2. Comparison of the dose measurements of
Van Gunten [5,6] as a function of distance from the wall
of an aluminum cube irradiated by a uniform, isotropic,
3.0 MeV flux along a line through the center and
through the corner of the wall with the predictions of a
single forward calculation.
Note also the agreement between the forward calculation of the
dose at the extremes of the profile along the center with the two
adjoint results at 3.0 MeV in Fig. 1.
III. GPS APPLICATION
Because of the altitude and inclination at which GPS
satellites are deployed, they are exposed to a very severe natural
electron environment. Components must be sufficiently
radiation hard to survive the mission-averaged total dose. The
MITS code system has the capability of predicting these doses
with a higher degree of confidence than the less accurate mass-
sectoring codes, albeit with a much longer run time. On the
other hand, it is applications such as these for which simulation
by conventional forward Monte Carlo codes like ITS is
precluded-byexcessive run times.
One of the subsystems aboard the GPS satellite is the Burst
Detector Dosimeter (BDD) box. The ACCEPT code of the
M1TS system was employed to calculate the dose at a point
within this subsystem.
A. Geometry
.Figure 3 is a projection of the geometrical model employed
for radiation analysis of the BDD box. It consists of a set of
components inside an (actual) aluminum box. The five external
walls of the box are 0.508 cm thick. The model is described
using the combinatorial geometry (CG) method of the ACCEPT
code and represents all materials by aluminum with different
mass densities so as to preserve the actual masses and
dimensions. The model includes an artificial aray of small
hemispherical shells surrounding the locations of the detectors
[7] where dose is to be calculated. The detector location
selected for the adjoint MITS simulation is indicated in the
figure. The hemispherical shell had inner and outer radii of
0.1067 and 0.1270 cm, respectively.
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Kensek, Ronald P.; Lorence, Leonard J.; Halbleib, John A. & Morel, J. E. Space applications of the MITS electron-photon Monte Carlo transport code system, article, July 1, 1996; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc670912/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.