Shielding Considerations

Transcription

1 Copyright 2016 California Institute of Technology. Government sponsorship acknowledged. Shielding Considerations By Insoo Jun and the JPL Natural Space Environments Group Jet Propulsion Laboratory, California Institute of Technology September, 2016 At the 2016 RADECS, Bremen, Germany 1

2 Generic Shielding Design Process External Environments Geometry Modeling Operational Scenario Transport Analysis Define Local Radiation Environment at Device or Material Update geometry modeling Upgrade parts capability Move part Change physical configuration Revisit measurement requirement Modify operation Add shielding... Test results for parts or sensors or materials No Meet performance spec with margin? Yes One iteration completed 2

3 Principles of Radiation Protection Minimize the Time of Exposure Careful selection of trajectory, e.g., Juno Maximize the Distance from the Source Place flight electronics and science instruments away from e.g., MMRTG Not possible for the space radiation environment Use Radiation Hardened Parts or Sensors Cost increase Use Shielding Juno: Perijove Passage through Jupiter s Radiation Environment 3

4 Useful Definitions (1) Flux, φ(e,t)de: Number of particles crossing unit area per unit time in (cm 2 -s) -1 peak flux, dose rate, SEU rate. Fluence, Φ(E)dE: Integral of the flux over a given time interval (i.e., one hour, one year, etc.) in cm -2 mission fluence, total ionizing dose, displacement damage dose. Integral Fluence: Number of particles with an energy E1 or greater I( E E1) E1 ( E ) de 4

5 Useful Definitions (2) Linear Energy Transfer (LET), de/dx: Energy transferred to material as an ionizing particle travels through it in MeV/cm LET is material dependent Mass LET: LET/ρ in MeV-cm 2 /g, where ρ is the density of shielding material Nonionizing Energy Loss (NIEL) Energy transferred to material to displace atoms from their lattice sites NIEL is also material dependent In MeV-cm 2 /g 1 rad = 100 erg/g = 6.25x10 7 MeV/g 1 inch = 2.54 cm = 1,000 mil g/cm 2 = thickness/density of material Ionizing Dose Displacement Dose E E ( E) LET ( E) de ( E) NIEL( E) de 5

6 What s different about Jovian Environment? Expected mission total ionizing doses are substantially higher compared to other planetary or deep space missions Shielding strategy needs system level attention to optimize the use of shielding mass The Jovian radiation environment is dominated by high-energy electrons High-energy electrons can cause increased levels of internal electrostatic discharge Secondary particles from high-energy electrons produce more transients and background noise in detectors and sensors Special attention will be required to mitigate the radiation effects due to the high energy electron environment 6

7 Particle Environment behind Shield The following particles should be included for assessment of sensor/detector performance in terms of transient or background noise levels in the Jovian radiation environment Primary and secondary electrons Bremsstrahlung photons Primary and secondary protons Secondary neutrons due to primary protons or electrons Heavy ions from nuclear fragments or spallation reactions High Bremsstrahlung photon and secondary neutron fluxes due to high energy electrons are a unique concern for shielding design in the Jovian radiation environment 7

8 Ionizing Dose (rads, Si) Shielding Material Consideration High-Z materials can be effective for shielding for TID However, high-z materials will produce more secondary particles: Photons Neutrons -> These will increase DDD, transient, and background noise levels 1E+06 1E+05 1E+04 1E+03 Aluminum, Novice Al & W, Novice Tungsten, Novice 1E+00 1E+01 1E+02 Spherical Shell Shielding Thickness (g/cm^2) Example: Aluminum and Tungsten Shielding in a Jovian Environment Trades should be made to find shield material combinations that optimize the instrument performance and shielding mass 8

9 Integral flux, cm -2 Example: A Simple Geometry Photon Environment Behind Shield 100 MeV Electron Broad Beam 1.E-03 WCu 1.E-04 Aluminum Shield Material Silicon Detector Aluminum Backing Copper Layer 16.3 g/cm2 10 mm cm 0.32 cm 1.E-05 Shield Material 16.3 g/cm2 MCNPX simulation 1.E Photon energy spectra at the silicon detector behind Aluminum or WCu shield MeV High-Z shielding materials will produce more secondary photons than low-z materials 9

10 Example Histogram of Transient Noise TID behind Al and Ta Shielding Region1 Electron beam test with 60MeV on an instrument to quantify the transient noise in the imaging sensor. Regions where improvement could be made by selecting appropriate physics model and processes during the Monte Carlo simulations. Region 1: the implication is that analysis predicts lower dose than the test, which would lead to insufficient shielding. The MCNPX and Geant4 requires different shielding mass to achieve the same dose level. 10

11 Dose-Depth Curve What is a dose-depth curve? How do we use a dose-depth curve? 11

12 rad(si) Example Dose-Depth Curves 1.E+08 1.E+07 JEO 2008 "Reference" Environment 320 mil (or 8 mm) aluminum: JEO: 817 krad JGO: 82 krad 1.E+06 JGO 2008 "Design" Environment 1.E mil (or 2.5 mm) aluminum: JEO: 2900 krad JGO: 900 krad 1.E aluminum spherical shell thickness, mil A dose-depth curve provides the TID level at the center of a spherical shell spacecraft for various aluminum thicknesses 12

13 Mission TID rad(si) How do you use a dose-depth curve? (1) Comparison of TID environments for different missions 1.0E E E E E E+04 MRO Cassini GLL dose through J35 (GIRE) Juno For example, we can immediately say that the Cassini/MRO environments are less severe that the Juno/Galileo environments by just looking at the respective dose-depth curves. 1.0E E Aluminum Spherical Shell Thickness, mil 13

14 rad(si) How do you use a dose-depth curve? (2) With known shielding thickness -> Selection of part capability 1.E+08 1.E+07 1.E+06 1.E+05 JEO 2008 "Reference" Environment JGO 2008 "Design" Environment 100 mil (or 2.5 mm) aluminum: JEO: 2900 krad JGO: 900 krad 320 mil (or 8 mm) aluminum: JEO: 817 krad JGO: 82 krad For example, if a device is located within a 500 mil (or 1.3 cm) thick aluminum box, the JEO dosedepth curve indicates that the part would receive ~500 krad. To meet the RDF=2 requirement, then the device should have a minimum 1 Mrad capability. 1.E aluminum spherical shell thickness, mil 14

15 rad(si) How do you use a dose-depth curve? (3) With known radiation hardness level -> Estimate of shielding thickness required 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 JEO 2008 "Reference" Environment JGO 2008 "Design" Environment 100 mil (or 2.5 mm) aluminum: JEO: 2900 krad JGO: 900 krad 320 mil (or 8 mm) aluminum: JEO: 817 krad JGO: 82 krad aluminum spherical shell thickness, mil For example, if a 300 krad component is to be used, then the shield should be designed for 150 krad to meet the RDF=2 requirement. Based on the JEO dose-depth curve, the shielding thickness required is estimated to be about 2.54 cm or 1 inch of aluminum. 15

16 How do you use a dose-depth curve? (4) Used in geometry sectoring or ray tracing code PWB A Instrument Deck B Suppose we want to compute the TID level at the center of yellow sphere in Box A. A ray tracing code constructs the N rays (uniformly distributed in the 4p steradians) originated from the target point. Each ray will have a characteristic path length, p i. The TID level corresponding to this path length can be read from the dose-depth curve, D i = D(p i ). The total dose is then: D tot = i (D i )/N. 16

17 Survey of Radiation Shielding (Transport) Codes Introduction of widely used radiation shielding tools Primary applications of radiation transport tools 17

18 Ray Tracing Codes Ray tracing codes are useful to perform system level trade studies fast Ray-tracing codes with CAD interface capability would be very useful Tools available: FASTRAD: MEVDP: SIGMA option in Novice: 18

19 Transport Codes Species Transport codes model actual particle interactions in the material (Ray tracing codes do not) It is important to model all particle species when performing transport analyses Electrons Photons Protons Neutrons Heavy Ions Each transport code considers only a specific set of particles Radiation transport analyses will be required to cover a wide range of particle species 19

20 Commonly Available Radiation Transport Codes Electron Photon Proton Neutron Heavy Ion CREME96 creme96.nrl.navy.mil TRIM ITS3.0 www-rsicc.ornl.gov NOVICE MCNP(X) mcnpx.lanl.gov Geant4 geant4.web.cern.ch/geant4/ X X X X X X X X X X X X X X X X X X X X Radiation transport codes not covered: EGS4, CEPXS, HZETRN, SpaceRad, PENELOPE, FLUKA, MARS, etc 20

21 Features of Common Transport Codes Code Primary Application Comments CREME96 Heavy Ion LET Spectra Limited to spherical shell aluminum shielding TRIM ITS3.0 (TIGER) NOVICE MCNP(X) Geant4 Proton or heavy ion beam simulation Electron or photon beam simulation for dose and charging rate profiles Spacecraft level shielding analysis Full 3-D detector/sensor simulation Transients calculation Full 3-D detector/sensor simulation Transients calculation 1-dimensional Only Coulomb interaction Excellent electron/photon physics Extensively benchmarked Adjoint (fast for space environment application) No secondary neutrons Not accurate for secondary electrons Good physics and extensive development history Slow for space application Good physics Many Geant4-based tools are available Slow for space application Comments are based on current JPL experience 21

22 Radiation Management Bound total dose at 3 mil Al Radiation Design Factor of 2 Goal: 300 krad electronic parts Testing, testing, testing Preferred parts & materials database Utilize radiation vault 150 krad environment inside of vault Benefits of packing and self shielding Micro Meteoroid and Orbit Debris protection IESD protection Juno IESD approach Protection circuitry Cable overwrap, minimize floating metal Accommodation Work In/outside the vault decision IESD approach Vault radiation transport analysis structure, tanks, etc. others vault chassis part/circuit Radiation Vault: Shielding Sum is Greater than the Parts Pre-Decisional Information For Planning and Discussion Purposed Only 22

23 Summary Shielding is a key design consideration A traditional shielding design approach may result in ineffective use of overall shielding mass Shielding mass trade at the system level has to be made early in the project A common set of tools between instrument and spacecraft designers is recommended Shield design must consider TID, flux and secondary particles Choice of appropriate tools may depend on specific applications Understanding the underlying physics models used in radiation transport codes is also very important to correctly interpret the results 23

24 Thank You! Questions? 24

25 Backup 25

26 Shielding: Size matters Electrons, protons, heavy ions: charged particles Neutrons and photons: neutral particles ATOM: = Nucleus + Atomic Electrons SIZE: Atom -- ~1 Å or m Nucleus ~1 fm or m FIVE orders of magnitude difference between atomic size and nucleus size So, particles are travelling in almost empty space even within a solid material. Neutrons: occasional direct interactions with nucleus Photon: occasional direct interactions with atomic electrons Charged particles: continuous long-range Coulomb interactions with atomic electrons # of e-/g Z/A Hydrogen The number of electrons in unit mass is the driving factor for shielding charged particles and photons 26