UTILIZING SPENVIS FOR EARTH ORBIT ENVIRONMENTS ASSESSMENTS: RADIATION EXPOSURES, SEE, and SPACECRAFT CHARGING Brandon Reddell and Bill Atwell THE BOEING COMPANY NASA Systems Houston, TX USA SPENVIS & GEANT4 Workshop Leuven, Belgium October 2005

ABSTRACT The SPENVIS, developed by the Belgium Institute of Aeronomy (BIRA), Brussels, Belgium, has been utilized to generate trapped proton and electron particle spectra for low-earth (LEO), medium- earth (MEO) and geostationary orbits (GEO) and to perform comparisons/benchmarking for several high-energy particle transport codes. A discussion of the proton and electron spectra and transport codes used in this study is presented. Results of the absorbed dose calculations and code comparisons using aluminum shielding are presented and discussed in detail. An example of a single event effects (SEE) analysis for the International Space Station (ISS) internal and external computers is discussed. Additionally, the application of SPENVIS to calculate spacecraft charging is also discussed in relation to the ISS. A comparison of the SPENVIS predicted structure potential with the Boeing ISS Plasma Interaction Model (PIM) prediction and measured data from the Floating Potential Probe (FPP) is shown. 2

Outline of Talk  Application of SPENVIS to generate trapped electron and proton spectra for various near-Earth orbits  Present dose comparisons for benchmarking radiation transport codes  MULASSIS/GEANT4, FLUKA, MCNPX, Shieldose 2, CEPXS  Other SPENVIS options under consideration/or being used by the Boeing ISS Environments Group  Single Event Effects  ISS vehicle structure charging predictions  Comparison with Boeing ISS Plasma Interaction Model predictions and on-orbit data  Conclusions 3

NEAR-EARTH REGIONS  LOW-EARTH ORBIT (LEO) – Typical ISS orbit  ~ 400 km X 51.6 deg  MEDIUM-EARTH ORBIT – Global Positioning Satellite (GPS) Orbit  20,200 km x 55 deg  GEOSTATIONARY ORBIT – 160 deg W long.  ~36,000 km x ~0 deg 4

LEO PROTON & ELECTRON SPECTRA  AP-8MIN PROTON INTEGRAL & DIFFERENTIAL SPECTRA  ISS ORBIT: 400 KM X 51.6 DEG  EPOCH: JUNE 2007  AE-8MAX ELECTRON INTEGRAL & DIFFERENTIAL  ISS ORBIT: 400 KM X 51.6 DEG  EPOCH: JAN

LEO AP-8MIN PROTON INTEGRAL & DIFFERENTIAL SPECTRA 6

LEO AE-8MAX ELECTRON INTEGRAL & DIFF. SPECTRA 7

MEO AE-8MAX ELECTRON DIFFERENTIAL SPECTRUM 8

MEO AP-8MIN PROTON DIFFERENTIAL SPECTRUM 9

GEO AE-8MAX INTEGRAL & DIFF. ELECTRON SPECTRA 160 deg W long. – Epoch: Jan

COMPUTER CODES  MULASSIS / GEANT 4  3-D Monte-Carlo transport code  Hadronic, Electromagnetic, Low-Energy Neutron Physics  FLUKA  3-D Monte-Carlo transport code  Precision Physics Default, with electromagnetic and low energy neutrons  SHIELDOSE 2  Uses pre-calculated, mono-energetic depth-dose data for an isotropic, broad-beam fluence of radiation incident on uniform aluminum plane media.  Proton transport considers Coulomb interactions only (no nuclear interaction), electron transport with bremsstrahlung  MCNPX  3-D Monte-Carlo transport code  For this study, only electron transport considered, including electromagnetic effects.  CEPXS  1-D planar-geometry discrete ordinates code for coupled photon-electron transport 11

LEO Proton Dose Comparisons MeV Proton incident on 1.0 g/cm 2 Aluminum Slab

LEO Electron Dose Comparisons MeV Electron incident on 1.0 g/cm 2 Aluminum Slab

MEO Electron Dose Comparisons 14

GEO Electron Dose Comparisons

GEO Electron Dose Comparisons

17 Single Event Effects

ISS Single Event Upset Observations 18 ISS Orbit

19 ISS Single Event Upset Observations MDM-DRAMMeasured SEU Count/238 days SEFA SEU Count/238 days FOM SEU Count/238 Days Lab-1 40 g/cm Lab-3 40 g/cm P1-2: 10 g/cm S1-1: 10 g/cm

ISS Spacecraft Charging 20

 Various models available to compute floating potential of structure with respect to plasma  SOLARC, PIX2, thin sheath, thick sheath models for solar array and structure  SOLARC gives best correlation to observed floating potential and Boeing ISS PIM results  SOLARC only accounts for solar array charging (i.e. no magnetic induction) 21 ISS Vehicle Charging FPP Unit

22 FPP Potential Data (April 11, 2001) PEAK 1 PEAK 2 PEAK 3 PEAK 4 PEAK 5 PEAK 6 PCU OFF

23 Potential Map for ISS Stage 11A Potential (V) V min = -31.1V V max = -18.3V Corresponding to April 11, 2001 (FPP Peak 1) Conditions

24 ISS Potential Comparison

 For LEO, spacecraft charging is highly dependent on plasma temperature, density, and geomagnetic field  For these reasons, a probabilistic approach must be used to address spacecraft charging:  Models like the International Reference Ionosphere (IRI-2001) are climatological in nature and provide average properties; not the short transients that occur near eclipse exit  The SPENVIS spacecraft charging models allow the user to input plasma temperature and density to compute floating potential – only compute solar array charging  SPENVIS will need to access a plasma database of measured/known values to calculate spacecraft charging 25 Spacecraft Charging

26 Spacecraft Charging Climatological models do not predict extreme plasma conditions properly For predictions, known plasma parameters must used to simulate transient plasma conditions PIM Results

CONCLUSIONS  SPENVIS was utilized to generate trapped electron and proton spectra for LEO, MEO, and GEO  Various computer codes were used to transport these spectra to compute absorbed dose  Dose rates computed in thin silicon detector behind finite & semi- infinite slab aluminum shielding  For all three regimes, the transport codes generally agree to within 10-15% of each other  Some deviations at extremely thin and extremely thick regions  Can infer differences in radiation transport model physics  SPENVIS can be used to study Single Event Effects  SPENVIS needs a choice of GCR models so that the user can generate preferred spectra  i.e. CREME 96 or Badhwar-O’Neil GCR Model  Recommend adding Figure of Merit (FOM) technique to SPENVIS  SPENVIS could be used to study spacecraft charging provided user knows a priori plasma conditions  SPENVIS needs a geomagnetic field induced potential (vxB·L) option/model. This is important for large spacecraft 27