A Deterministic Electrons/Photons, Protons/Trapped Heavy Ions (C-O-S) Transport Suite for the Study of the Jovian System

A Langley Research Center (LaRC) developed deterministic suite of transport
codes which describe the transport of electrons/photons, protons/heavy ions in condensed
media is used to simulate the effects and exposures from spectral distributions typical of
electrons, protons and carbon-oxygen-sulfur (C-O-S) trapped heavy ions in the Jovian
radiation environment. The particle transport suite consists of a coupled
electrons/photons deterministic transport algorithm (CEPTRN) and a light/heavy ion
deterministic transport algorithm (HZETRN). The primary purpose for the development
of the transport suite is to provide a means to the spacecraft design community to rapidly
perform numerous repetitive calculations essential for electrons/protons/heavy ions
radiation exposure assessments in complex space structures. Several favorable
comparisons have been made with statistically oriented Monte Carlo (MC) calculations.
ITS V.30 (integrated Tiger series) MC package was used for electrons/photons transport
verfication in the Jovian environment [1]. FLUKA and HETC-HEDS were used for
protons/heavy ion transport verification at low Earth orbit (LEO) [2] as Jovian heavy ion
counter (HIC) spectra were not available to the authors. The proton/heavy ion models
have been extensively validated in LEO by comparing to measurements aboard the
International Space Station (ISS) and the Space Transportation System (STS) [3]. The
transport suite verifications versus MC packages have indicated that for typical space
environment spectra at LEO or at Galilean satellites, the particle transport accuracy has
not been compromised at the expense of computational speed.

The radiation environment of the Galilean satellite Europa is used as a
representative boundary condition to show the capabilities of the transport suite. While
the CEPTRN/HZETRN suite can directly access the output electrons/protons spectra of
the Jovian environment as generated by the Jet Propulsion Laboratory (JPL) Galileo
interim radiation electron (GIRE) model of 2003 [4], for the sake of relevance to the
upcoming Europa Jupiter system mission (EJSM4) workshop, the authors have opted to
directly use the tabulated 105 days at Europa mission fluence energy spectra [5] to
produce the corresponding depth-dose curve in silicon behind an aluminum shield. The
transport suite can also accept ray-traced thickness files from a computer-aided design
(CAD) package and calculate the total ionizing dose (TID) at a specific target point. The
dependency on a CAD generated ray-traced file is inherent in the deterministic nature of
the transport formalism. In that regard, using a low-fidelity CAD model of the Galileo
probe generated by the authors, the transport suite was verified versus aluminum
equivalent shell simulations for orbit G1–I32 of the Galileo extended mission (1996-
2001) [6].

Beyond computing the traditional aluminum-silicon depth-dose curve as a
standard shield-target combination, for a limited number of candidate shielding materials
such as tantalum (Ta), copper (Cu) and tungsten (W), the transport suite is used to
evaluate the particle flux and TID dose due to electrons, protons and C-O-S trapped
heavy ions at Europa (Rj=9.4) as a function of latitude, longitude and altitude.
Furthermore, the fast execution time of the transport suite allows a design engineer to
swap shield materials to perform Z-grade optimization on multi-layered shields.
The Jovian radiation environment will be presented first. An explanation of the
particle transport formalism as implemented in the transport suite will follow. Validation
results versus Galileo probe measurements will also be shown followed by an
explanatory description on how shield designers can go about performing Z-graded
studies or final shield optimization.