Theory and Modeling (High Performance Simulation)

Theory and Modeling (High Performance Simulation)

Mater. Res. Soc. Symp. Proc. Vol. 929 2006 Materials Research Society 0929-1101-01 Radiation Shielding Analysis for Various Materials in the Extreme Jovian Environment William Atwell 13100 Space Center Blvd., The Boeing Company, Mail Code: HB 2-30, Houston, TX, 77059-3556 ABSTRACT Earlier particle experiments in the 1970s on Pioneer-10 and -11 and Voyager-1 and -2 provided Jupiter flyby particle data, which were used by Divine and Garrett to develop the first Jupiter trapped radiation environment model. This model was used to establish a baseline radiation effects design limit for the Galileo onboard electronics. Recently, Garrett et al. have developed an updated Galileo Interim Radiation Environment (GIRE) model based on Galileo electron data. In this paper, the GIRE model was utilized to generate trapped proton and electron spectra as a function of Rj (Rj = radius of Jupiter = -71,400 km). Using these spectra and a highenergy particle transport codes (MCNPX and HZETRN), radiation exposures and dose effects for a variety of shielding materials (Al, polyethylene [PE], and Ta plus several other elemental materials for "Graded-Z" portion of the paper) and thicknesses are presented for the Icy Moon, Europa, Ganymede, and Callisto for several orbital inclinations. In addition, an in-depth discussion and absorbed dose calculations are presented for "Graded-Z" materials and several computer codes were utilized for comparison purposes. We find overall there is generally quite good agreement between the various computer codes utilized in the study: MCNPX (Monte Carlo) vs. HZETRN (deterministic) for slab shielding and the comparison of "Graded-Z" shielding using the CEPXS, MCNPX, NOVICE, and NASA JPL codes. Finally, we conclude that the merits of using "Graded-Z" materials that include PE, due to cost and weight, should aid future Jupiter mission planners and spacecraft designers. INTRODUCTION During nearly eight years in the Jupiter magnetosphere, the Galileo spacecraft made a number of major scientific discoveries while making 34 orbits of Jupiter. The mission was truly a marvelous scientific and engineering success. However, the spacecraft was constantly beset with radiation-induced anomalies from the extremely harsh radiation environment that required ground support personnel to expend untold hours at considerable expense in correcting and overcoming the radiation-induced systems problems. Low on attitude control capability and to

prevent the spacecraft from colliding with one of Jupiter's Icy Moons (Europa, Ganymede, and Callisto), orbital maneuvers were performed and Galileo impacted Jupiter during its 35 th orbit on September 21, 2003.The Jupiter Icy Moons Orbiter (JIMO) program, as originally conceived, is a scientific mission to return to Jupiter and to further extend the discoveries made by Galileo, especially whether the Icy Moons harbor oceans beneath their icy surfaces. Additional information on the JIMO mission is located at the website URL: http://www.jpl.nasa.gov/jimo/. Unfortunately, funding for the JIMO program was canceled. The previous incarnation, NASA/JPL's Europa Orbiter Project, was cancelled last June because its price tag had exceeded well over a billion dollars. The project was renamed the X2000 Advanced Avionics Project to keep technology development alive until a new mission could be designed. Shortly thereafter, the National Research Council's Space Studies Board completed a Solar System Exploration Survey (SSES) and issued a report (http://www.nas.edu/ssb/newfrontiersfront.html) entitled, "New Frontiers in the Solar System - An Integrated Exploration Strategy." The report noted that a Europa orbiter mission is still a high priority among planetary scientists, though noted that its price tag (and enabling technologies) is a recurring problem. Nevertheless, the SSES recommended a hypothetical mission, dubbed Europa Geophysical Explorer (EGE) that would fall under a new mission category called "Flagship." Note that, as of this moment, EGE does not exist, but is really only a "placeholder" mission concept until NASA can decide whether to resurrect (and fund) a Europa orbiter. One of the many challenges facing spacecraft designers is to mitigate the radiation effects to onboard systems. To overcome or minimize this obstacle would be a tremendous cost savings. It is the intent of this paper to assist future spacecraft and systems designers to properly identify the systems and parts that will perform satisfactorily for missions operating in the Jupiter magnetosphere. In this paper the intense radiation environment, which has been modeled based on trapped proton and electron data obtained during the Voyager 1 & 2 and Pioneer 10 & 11 missions of the 1970's and more recently the Galileo Energetic Particle Detector [EPD] 1 data, has been produced in terms of proton and electron spectra for three of the four Icy Moons (Europa, Ganymede, and Callisto) as a function of orbital inclination. The radiation effects to onboard spacecraft systems are strictly related to the amount and type of material shielding the system of interest. Depth dose calculations utilizing both Monte Carlo and deterministic methodologies are presented for a baseline, reference material, aluminum, several other shielding materials, and approaches, such as the use of "Graded-Z" materials. These other approaches include the application of low- and high-z materials and applying strategies that incorporate multi-disciplinary optimization (MDO) techniques. RADIATION ENVIRONMENTS The updated GIRE (Galileo Interim Radiation Electron) model 2 was used to generate both integral and differential trapped electron particle spectra for four orbital inclinations, 0 deg, 30 deg, 60 deg and 90 deg, for the Jupiter Icy Moons: Callisto, Ganymede, and Europa. For Europa, trapped proton spectra were generated in the orbital region of ± 0-28.19 deg. It is worth mentioning that the GIRE model differs from the original Divine-Garrett Jupiter radiation

model 3. The GIRE model includes recent Galileo Energetic Particle Detector 1 electron data in the region L [Mcllwain parameter] 4 = 8-16 Rj, where Rj = radius of Jupiter (-71,400 km) and the electron and proton energies have been extended to 1000 MeV (1 GeV). Also, the proton environment is the same in both models. The GIRE has a built-in Offset Tilted Dipole magnetic field model option, which was utilized in the particle spectra calculations reported herein. The particle fluxes also vary as a function of longitude at each Icy Moon. There is approximately a factor of two in particle intensity between 110.8 W longitude and ±90 deg (i.e., 20.8 deg and 200.8 deg W longitude). For the calculations discussed below, the "worst case" 110.8 deg W longitude has been used. Callisto Fig. 1 shows the trapped electron integral and differential spectra at Callisto as a function of orbital inclination. The update GIRE was used to generate daily particle spectra where the particle energies range from 0.1 MeV to 1000 MeV (1 GeV). Electron Energy, MeV 1. Callisto trapped electron integral and differential spectra using the GIRE model. Ganymede Fig. 2 shows the daily, trapped electron integral and differential spectra at Ganymede as a function of orbital inclination. Europa Fig. 3 shows the daily-trapped electron integral and differential spectra at Europa as a function of orbital inclination, and Fig. 4 shows the daily-trapped proton integral and differential spectra at Europa as a function of orbital inclination.

1.E+14 Ganymede g 0 deg-lnt A 0 deg-diff 30 deg-lnt 30 deg-diff 60 deg-lnt 60 deg-diff 90 deg-lnt 90 deg-diff 10 100 1000 Particle Energy, MeV 2. Ganymede trapped electron integral and differential spectra using the GIRE model. Europa - Electrons 0 deg-lnt 0 deg-diff 30 deg-lnt 30 deg-diff 60 deg-lnt 60 deg-diff 90 deg-lnt 90 deg-diff Electron Energy, MeV 100 1000 Europa trapped electron integral and differential spectra using the GIRE model.

Europa - Protons lnt-0 deg Diff-Odeg lnt-15deg Diff-15deg lnt-28.19deg Diff-28.19deg Europa trapped proton integral and differential spectra using the GIRE model. ABSORBED DOSE CALCULATIONS - MCNPX CODE & ALUMINUM SHIELDING In this section the absorbed dose calculations are presented using the trapped particle spectra discussed in the previous section. Callisto electron absorbed doses Using the MCNPX code 5 and aluminum shielding thickness, the electron absorbed dose rate per day for Callisto is shown as a function of orbital inclination in Fig. 5. Ganymede electron absorbed doses Using the MCNPX code and aluminum shielding thickness, the electron absorbed dose rate per day for Ganymede is shown as a function of orbital inclination in Fig. 6. When comparing the electron dose rates for Callisto and Ganymede (Figs. 5-6), we observe almost an order of magnitude dose rate increase at Ganymede, and it is seen in a later section that Europa is ~2 orders of magnitude larger.

Callisto Electron Dose Rate Al Shielding - MCNPX 20 30 40 50 Aluminum Thickness, g/cm 2 5. Callisto electron absorbed dose rate using the MCNPX code and aluminum shielding for four orbital inclinations. Ganymede Electron Dose Rate Al Shielding - MCNPX 1.E-07 20 30 40 50 Aluminum Thickness, g/cm 2 6. Ganymede electron absorbed dose rate using the MCNPX code and aluminum shielding for four orbital inclinations.

Europa electron and proton absorbed doses Jupiter's magnetosphere exhibits a particle trapping structure similar to earth, i.e., protons are trapped in an "inner belt" region, and this is such the case at Europa. Figs. 7-8 show the electron and proton absorbed dose rates, respectively, as a function of orbital inclination using the MCNPX code and aluminum shielding. It is noted that the proton trapping extends to approximately ±30 degrees latitude. Europa Bectron Dose Rate Al Shielding - MCNPX 10 20 30 40 50 Aluminum Thickness, g/cm 2 Europa electron absorbed dose rate using the MCNPX code and aluminum shielding for four orbital inclinations. Europa Proton Doses Al Shielding - MCNPX 10 Al Thickness, g/cm 2 8. Europa proton absorbed dose rate using the MCNPX code and aluminum shielding for three orbital inclinations.

ABSORBED DOSE CALCULATIONS - HZETRN CODE & Al, PE & Ta SHIELDING In this section the NASA Langley Research Center (LaRC) HZETRN code 6 was utilized to perform similar absorbed dose calculations shown in the previous section, but to include Al, PE and Ta shielding materials. When we compare the MCNPX and HZETRN codes using Al shielding, excellent agreement is observed. Callisto electron absorbed doses i) / day!s)> O o 8Q 1 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1..E-07 1.E-08 Callisto - Electron Dose (LaRC) 10 15» PE-0 deg -a PE-30 deg e PE-60 deg K PE-90 deg -A AI-60deg ~-*~~AI-90deg m Ta-0 deg ^ Ta-30deg - Ta-60deg -A Ta-90 deg Material Shield Thickness, g/cm 2 9. Callisto electron absorbed dose rate using the LaRC code and Al, PE and Ta shielding for four orbital inclin< itions. Ganymede electron absorbed doses 1.E+02 Ganymede - Electron Doses (LaRC) Material Shield Thickness, g/cm 2 10. Ganymede electron absorbed dose rate using the LaRC code and Al, PE, and Ta shielding for four orbital inclinations. 10