University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2002—pp. 57-60
FLUER: FLUKA Executing under ROOT—A Monte Carlo Simulation Tool for Space Science
Lawrence S. Pinsky (UH), Thomas L. Wilson (NASA-JSC), Victor Andersen (UH), and Anton Empl (UH)
Abstract
Many different areas in space science require the ability to simulate the transport of
radiation through spacecraft. Numerous sophisticated radiation transport codes exist and
have been used for many years in high energy particle physics; however, the input and
output infra-structure of these codes are not well suited for the treatment of most space
science problems. The FLEUR project is designed to bridge the gap between existing physics
codes and the needs of space scientists. An overview of the FLEUR project are presented
here, along with examples where the results of the FLEUR project have been used to analyze
existing problems in space science and astrophysics.
DETAILED SIMULATIONS OF RADIATION TRANSPORT ARE desirable in many areas of space science from assessing radiation risk for astronauts to designing and interpreting the data from instruments studying galactic cosmic rays. The particle physics community has developed several sophisticated radiation transport codes for the interpretation of experimental data and design of future experiments and sophisticated software packages for the analysis and visualization of both real and simulated particle data. Unfortunately for space scientists, these codes have generally been tailored for accelerator-based problems where mono-energetic particles of a single type enter the area of interest in a well collimated beam. In a typical problem in space science, on the other hand, a spacecraft will be bathed in particles with a large range of energies and species, coming in from all directions. This has traditionally meant that applying the sophisticated physics codes to describe a particular space science problem has been difficult. The purpose of the FLEUR project is to provide the tools necessary for significant improvements in the ability of space scientists to use two existing software packages in the simulation and analysis of space science data. The package is the radiation transport code FLUKA,1,2 which is one of the most complete and sophisticated radiation transport codes available today. The second package is ROOT,3 an object-oriented-physics analysis package being developed at CERN.
Particle Generation
In space, energetic charged particles can be encountered from a number of sources; these
include ever present galactic cosmic rays, as a well as particles from transient sources
such as the sun that can generate large, sporadic bursts of energetic particles. Both
sources of particles can include nuclei from hydrogen up to and beyond the mass of iron.
Although the majority of such particles are protons, in many cases in space physics it is
not possible to simply neglect the heavier nuclei, since, among other things a significant
fraction of the radiation dose for an astronaut is due to these heavier particles. In
addition, cosmic rays have energies that span approximately 15 decades. These two facts
mean that one important capability for simulating particles in space is the ability is to
produce primary particles for transport that are sampled from either observed or modeled
distributions of the particles as a function of both energy and species. This requires us
to use Monte Carlo techniques to produce particle lists suitable for transport by FLUKA.
An example of this procedure is illustrated in Fig. 1. A model cosmic ray spectrum is
shown in the top panel of Fig. 1. The bottom shows a histogram of the energies of 100,000
events sampled from this spectrum using Monte Carlo methods. These events can then serve
as the inputs to be transported by FLUKA.
Figure 1. Model energy spectrum of galactic cosmic ray protons (top panel) and histogram of 100,000 events randomly generated from that model distribution (bottom panel)
Geometry Specification and Event Visualization
The construction of the transport geometry for most accelerator based experiments is
facilitated by the fact that experimental setups have a high degree of symmetry and many
repeated elements. This is almost never true in space science where a spacecraft can have
thousands of unique and individual parts. This makes both the construction of the geometry
and visualization of particle interactions within the geometry very difficult in space
science applications. In order to ameliorate these problems, much effort in FLEUR has gone
into tools for geometry construction and visualization, taking advantage of already
existing facilities within ROOT.
Examples
In the course of the development of FLEUR, we have applied the resulting software to
different existing problems that demonstrate many of the capabilities of FLEUR. These
problems include the calibration of the MARIE instrument aboard Mars Odyssey and the
simulation of prompt neutrino production by the interaction of high energy cosmic rays
with the lunar surface. Illustrative results of these simulations that rely heavily on the
FLEUR infra-structure are given below.
MARIE
MARIE is a charged particle telescope aboard Mars Odyssey, designed to assess the
radiation environment at Mars as a tool for planning an eventual manned mission to the
planet. Because of its design, MARIE can be triggered both by particles that enter the
lightly shielded "front" side of MARIE as well as particles that have passed
through the bulk of the spacecraft’s mass, thus losing a significant amount of
energy. Therefore, calibration of the MARIE data to yield results of astrophysical
interest such as particle spectra or fluxes requires detailed simulation of the particle
transport through the spacecraft.
The Spacecraft Model
The above mentioned complications in the way that MARIE operates mean that to completely
simulate the response of MARIE it is necessary to specify the distribution of mass both in
front of and behind the instrument. A representation of the spacecraft geometry, as it
currently stands, is shown in Fig. 2. MARIE is visible as the box near the center of the
geometry. The small square and circular elements within MARIE are the detector elements
that are the source of the data returned by MARIE.
Figure 2. Simulation geometry of the Odyssey spacecraft and MARIE instrument for use with FLUKA, generated with tools developed as part of the FLEUR project. The visualization uses existing capabilities of ROOT. MARIE is the box in the left center of this figure.
Simulated Particle Events
After the geometry was constructed, simulation of particle events could begin. In
interplanetary space, there is a constant, spatially isotropic background of GCRs that
MARIE measures. In order to simulate the forward moving GCRs, particle positions were
randomly generated on a hemisphere centered in front of MARIE. The particles were assigned
random initial directions, subject only to the constraint that they be moving inward into
the hemispherical region. Energies were assigned using a Monte Carlo sampling of a model
GCR spectrum, as demonstrated in a previous section. The simulated interaction of a 50 MeV
proton with the detectors in MARIE is shown in Fig. 3. The proton enters from the left and
passes through several of MARIE’s detectors, before finally suffering a nuclear
interaction in a detector that produces several photons and an electron, the paths of
which can be seen exiting MARIE. The results of the energy deposited within two of
MARIE’s detectors for ~1000 protons that triggered the instrument are shown in Fig.
4. The information in diagrams such as this is currently being used to calibrate the
signals being produced by MARIE.
Figure 3. Example of a simulated particle event within MARIE
Figure 4. Energy deposited in two of MARIE’s six detectors from approximately 1000 simulated events. The large clump of events in the lower left is due to the fact that particles with energies greater than a few hundred MeV all deposit energies near the minimum deposited by any particle.
Neutrino Production on the Moon
Neutrinos are produced in copious amounts by various interesting astrophysical processes.
The possibility of observing these neutrinos from an Earth-based observatory is hampered
by the fact that cosmic rays entering Earth’s atmosphere produce large numbers of
neutrinos. Essentially, the sky on Earth is "bright" when observed in neutrinos.
It has thus been suggested that the Moon, since it lacks a significant atmosphere, might
make a good site for a neutrino observatory. One important consideration in this regard is
the neutrino albedo of the Moon, attributed to the prompt decay of charmed mesons produced
by the interaction of high energy galactic cosmic rays with the lunar regolith, since at
high neutrino energies, these particles probably dominate the neutrino background on the
Moon. Simulation of these high energy interactions is a natural use for the results of
FLEUR.
The impact of a 100 GeV proton with the lunar surface is shown in Fig. 5. The cylinder is the simulated lunar regolith. The proton enters the regolith from above and quickly produces a shower of other particles. In this figure, neutrinos are shown as black lines, while particles of all other types are shown as light gray.
Figure 5. Particle cascade produced by cosmic ray impact on simulated lunar regolith
Figure 6 shows the spectrum of neutrinos produced by cosmic ray protons in the energy range 100 GeV-10 TeV. Neutrinos with energies above ~100 GeV are primarily produced by charmed meson decay.
Figure 6. Simulated spectrum of prompt neutrinos produced in lunar regolith
References
Publications
Andersen, V. et al. "The FLUKA Code for Space Applications: Recent
Developments," Proc., World Space Congress 2003. (In press.)
Wilson, T. L. et al. "ATIC Backscatter Study Using Monte Carlo Methods in FLUKA &
ROOT," in Calorimetry in Particle Physics. R.-Y. Zhu,
ed. Singapore: World Scientific, 2002. 95-100.
Presentations
Andersen, V., K. Lee, L. Pinsky, W. Atwell, T. Cleghorn, F. Cucinotta, P. Saganti, R.
Turner, and C. Zeitlin. "Monte Carlo Simulation of the Response of the MARIE,"
28th International Cosmic Ray Conference, Tsukuba, Japan, July 31-Aug. 7, 2003. (Accepted.)
Andersen, V., K. Lee, L. Pinsky, W. Atwell, T. Cleghorn, F. Cucinotta, P. Saganti, R.
Turner, and C. Zeitlin. "Monte Carlo Simulations of the Response of the MARIE
Instrument," 34th Annual Lunar and Planetary Science Conference, League City, TX,
March 17-21, 2003.
Wilson, T. L., V. Andersen, and L. S. Pinsky. "Lunar Regolith Albedos Using Monte
Carlos," 34th Annual Lunar and Planetary Science Conference, League City, TX, March
17-21, 2003.
Wilson, T. L., V. Andersen, and L. S. Pinsky. "Prompt Neutrino Production by the
Lunar Surface," 28th International Cosmic Ray Conference, Tsukuba, Japan, July
31-Aug. 7, 2003. (Accepted.)
Funding
"Analysis of Data from the Mars ’01 MARIE Experiment," Aug. 1,
2001-July 31, 2002, NAG9-1347, $25,894.
"Determining the Radial Dependence of Particle Intensities from Coronal Mass
Ejections," Jan. 1, 2002-Dec. 31, 2003, ARP, $103,000.
Investigative Team UH PI: Lawrence S. Pinsky, Ph.D., Professor and Chair NASA-JSC PI: Thomas L. Wilson, Ph.D. UH PDAF: Victor Andersen, Ph.D. Doctoral Student: Kerry Lee |
PDF (K)
Table of Contents
Institute for Space Systems Operations - Y2002
Annual Report
Copyright © 2003
|
|
