STK SEET Radiation

The STK SEET Radiation Environment computes the expected dosage rate and accumulated dose (ionizing energy deposition) due to energetic electron and proton particle fluxes for a range of shielding thicknesses, and also computes the energetic proton and electron fluxes for a wide range of particle energies. The STK SEET Radiation Environment incorporates the following models from Air Force Research Laboratory’s AF-GEOSpace program version 2.1P: APEXRAD, CRRESRAD, CRRESELE, CRRESPRO, NASAELE, and NASAPRO. For dose computations in combination with the CRRES or NASA flux models, the SHIELDOSE-2 radiation-transport model is used to estimate ionizing energy deposition (dose).

The CRRES proton and radiation database models (CRRESPRO and CRRESRAD) are categorized according to whether the data were collected before or after the large solar storm of March 24, 1991. Data collected after the storm is considered to be more active than data observed before the storm. Note that orbits with apogees near geosynchronous altitudes may show a higher dose rate using the average model than using the active or quiet models because the average model uses the period between 19 March 1991 and 31 March 1991 during which a strong solar proton event occurred (Kerns and Gussenhoven, 1992).

For additional background information, consult: SEET: Space Environment and Effects Tool for STK (PDF).

For additional information and recommendations on using the ShieldDose2 (SD2) model, which is used by STK to compute radiation dose from particle flux behind passed shielding materials and thicknesses of those shielding materials, see: ShieldDose2 Model.

For additional information on the NASA AE-8 implementation, see: AE8/AP8 IMPLEMENTATIONS IN AE9/AP9, IRBEM,AND SPENVIS (PDF).

Options for STK SEET Radiation CRRES Activity

Option Description
Proton Options are:
  • Active - model based on data during higher solar activity reference obtained from 31 March 1991 to 12 October 1991 (after the large 24-Mar-1991 storm).
  • Quiet - model based on data during lower solar activity from 27 July 1990 to 19 March 1991 (before the large 24-Mar-1991 storm).
Radiation

Options are:

  • Active - model based on data during higher solar activity reference obtained from 31 March 1991 to 12 October 1991 (after the large 24-Mar-1991 storm).
  • Average - model based on the average of data taken over the entire CRRES mission.
  • Quiet - model based on data during lower solar activity from 27 July 1990 to 19 March 1991 (before the large 24-Mar-1991 storm).

Options for NASA Electron and Proton Activity

Option Description
Activity

Options are:

  • Solar Min - Sets values for the NASA models NASAELE and NASAPRO appropriate for the minimum of the solar cycle.
  • Solar Max - Sets values for the NASA models NASAELE and NASAPRO appropriate for the maximum of the solar cycle.

The CRRESELE database model returns electron flux at discrete energies in the range 0.5-6 MeV, reporting electron fluxes for a specific set of energy levels only (0.65, 0.95, 1.60, 2.00, 2.35, 2.75, 3.15, 3.75, 4.55, 5.75 MeV). Similarly, the CRRESPRO database model computes proton flux at discrete energies in the range 1-100 MeV, reporting proton fluxes only for a specific set of energy values only (1.5, 2.1, 2.5, 2.9, 3.6, 4.3, 5.7, 6.8, 8.5, 9.7, 10.7, 13.2, 16.9, 19.4, 26.3, 30.9, 36.3, 41.1, 47.0, 55.0, 65.7, 81.3 MeV).

The NASAELE electron-flux model computes electron flux using the NASA AE-8 radiation belt models at energies between 0.04 and 7.0 MeV, and the NASAPRO proton-flux model computes proton flux using the NASA AP-8 radiation belt models at energies between 0.1 and 400 MeV. Unlike the CRRESELE and CRRESPRO database models, you may specify the energy levels for the NASA models.

Options for NASA Model Energy Levels

Option Description
NASA Energies

Options are:

  • Default -The default energy levels will be those from the CRRESS models.
  • Custom - Levels between 0.04 and 7.0 MeV may be specified for the NASA electron-flux model, and levels between 0.1 and 400 MeV for the NASA proton-flux model.

    • When using the NASA models, a minimum of three (3) electron energy levels are needed to compute electron and Bremsstrahlung dose and dose rates, and at least three (3) proton energies are needed to compute proton rates and doses.

References

AF-GEOSpace User’s Manual Version 2.1 and 2.1P, AFRL/RVBS, 29 Randolph Rd, Hanscom AFB, MA 01731 (2006).

Kearns, K.J. and M.S. Gussenhoven (1992), CRRESRAD Documentation, PL-TR-92-2201, Phillips Laboratory, Hanscom AFB, MA, ADA 256673.

ShieldDose2 (SD2) Dose Model

The ShieldDose2 (SD2) dose model is used in STK SEET to compute radiation dose from particle flux behind passed shielding materials and thicknesses of those shielding materials. SD2 is generally considered to be valid for shielding thicknesses between 10-3 g/cm2 (0.15 mils of Aluminum) and 50 g/cm2 (7,300 mils Al). For the model to accurately compute dose at a given shielding thickness, the inputs to the model must include fluxes for an appropriate energy range. In particular, for accurate dose predictions at small shielding thicknesses, the input particle fluxes must extend to low enough energies that the range in aluminum of the lowest energy particles is less than the smallest desired shielding thickness. The lower limit of 10-3 g/cm2 corresponds to the range of 0.025 MeV electrons and 0.3 MeV protons.

The Scenario->SEET Radiation page supports the use of default or custom energy thresholds with the NASA flux models (AE-8, AP-8). When the default setting is used, the NASA models produce flux at the same energy thresholds as the CRRES flux models:

  • The CRRES electron model supports the following fixed energy thresholds, corresponding to the HEEF instrument channels: 0.65, 0.95, 1.6, 2.0, 2.35, 2.75, 3.15, 3.75, 4.55 and 5.75 MeV. The 0.65 MeV lower limit for electrons corresponds to a shielding thickness of about 34 mils Al; therefore these energies should not be used for dose predictions for thicknesses less than 34 mils.
  • The CRRES proton model supports the following fixed energy thresholds, corresponding to the PROTEL instrument: 1.5, 2.1, 2.5, 2.9, 4.3, 5.7, 6.8, 8.5, 9.7, 10.7, 13.2, 19.4, 26.3, 30.9, 36.3, 41.1, 47.0, 55.0, 65.7 and 81.3 MeV.

The NASA models support broader energy ranges than do the CRRES models, as they utilize data from additional satellite instruments. The NASA electron model supports energy thresholds from 0.04 through 7.0 MeV. The NASA proton model supports energy thresholds from 0.1 through 400 MeV.

Recommendations for Computing Accurate Dose Measurements

The following settings are recommended for computing dose measurements:

  1. Make the following changes on the Scenario->Basic->SEET Radiation properties page:
    • Set NASA Energies to Custom.
    • Add the following values to the Electron Values list:
      0.04, 0.10, 0.20, 0.5 and 7.0 MeV.
    • Add the following values to the Proton Values list:
      0.1, 0.2, 0.5, 1.0, 100.0, 200.0, 400.0 MeV.
  2. On the <STK Object>->Basic->SEET Radiation properties page:
    • Do not use CRRES electron and proton flux models in scenarios that will be used to compute dose measurements (note that STK SEET contains the CRRESRAD models which contain measured doses under specific shielding thicknesses).
    • Do not use shielding thicknesses less than 1 mil Al (approximately 0.07 g/cm2).
  3. To generate a graph of the dose measurements, you will need to create custom graphs from the SEET Radiation Flux and the SEET Radiation Electron Flux graph styles in the Report & Graph Manager.

When reporting the combined dose rate, we recommend using a step size higher than 5 seconds. At smaller step sizes, the values can show higher frequency content that is warranted by the information of the data set itself.