© 2015 The Aerospace Corporation

Geant4 Simulations of Space Radiation Sensors at The Aerospace Corporation

M. D. Looper, J. E. Mazur, T. P. O’Brien, & J. B. Blake

Physical Sciences Laboratory

Space Sciences Department

mark.d.looper@aero.org

310-529-3406

August 28, 2015

Modeling Response of Sensors at Aerospace: Outline

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• Van Allen Probes – Magnetic Electron/Ion Spectrometer (MagEIS)

– Part of Energetic Particle, Composition, Thermal Plasma (ECT) suite

• Van Allen Probes – Relativistic Proton Spectrometer (RPS)

• Lunar Reconnaissance Orbiter – Cosmic Ray Telescope for the

Effects of Radiation (CRaTER)

– Modeling of lunar “albedo” from cosmic-ray impact

• Example dosimeter calculations (RPS, CRaTER), other calculations

Geant4 Geometry Models of MagEIS

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Early design of sensor (2010)Mixed STEP/GDML & primitivesFukui DAWN renderer

Low/Med sensor head, as builtOnly Geant4 primitives (G4Box, …)Screen capture of VRML2 viewer

MagEIS Magnetic Electron Spectrometers

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Low: 20-240 keV0.3 mm Si550 Gauss

Med: 80-1200 keV3.0 mm Si1600 Gauss

High: 0.8-4.8 MeV0.3 + 3-9 mm4800 Gauss

Instrument paper: Blake et al., Space Sci. Rev. 2013, doi:10.1017/s11214-013-9991-8

Background correction: Claudepierre et al., J. Geophys. Res. 2015, doi:10.1002/2015JA021171

Proton sensor mounted with High, not discussed here

Idealized Response of Low Sensor Head

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Tracked non-interacting charged geantinos through uniform field from slit to detector plane (nominal angular distribution from collimator opening)

Implicitly deposits all energy in pixel, response is pure geometry (and statistics)

Background pixel 0 is not used in flight

Realistic Magnetic Field, No Scattering

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Still non-interacting charged geantinos traced from slit to detectors, but with “magnetic circuit” analysis using LORENTZ code

“Flaring” of field at edges of magnet steers some electrons toward the walls, reducing response

Add Pixel Scattering

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Now following electrons through realistic field, but with no baffles or magnets to scatter off of, and only for ideally collimated incident particles

Reduces response as particles scatter off detectors without depositing all their energy (10 keV energy deposit threshold to be counted here)

Add Collimator Shaft

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Now simulating electrons incident isotropically on top of collimator, so some particles out of the nominal acceptance “cone” can scatter into the slit and be counted

General increase toward higher energies, which “scrub” some energy off in the collimator and are counted in each pixel’s passband

Full Geometry: Low

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Finally, we add in the chamber with magnets and baffles, so electrons can scatter off them (before, electrons hitting them left the simulation volume, same effect as if absorbed)

Some decrease due to baffles blocking edges of pixels, but also increase from lower energy electrons scattering into higher pixels

Full Geometry: Med

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The sequence of changes to the response with increasing realism of the simulation is about the same for the Med head

50 keV threshold here; actual response uses passband (well above 50 keV for Med), but have to include a threshold for an integrated plot like these

Full Geometry: High

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High sensor has four pixels, no background pixel

Increased response toward higher energies due to scattering off collimator and shaft is more prominent

500 keV threshold (energy deposited in rear detector stack) is implicit in this plot of integral response

Realistic (As-Flown) Front-Detector Thresholds in High

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High sensor has coincidence required between 300m front detector and rear detector stack (3.0-9.0 mm)

Energetic electrons deposit around 90 keV in 300m front detector; previous plot had 30 keV threshold, but real sensors require 80-95 keV, which rejects a significant fraction of events

A Closer Look: Pixel 5 of Low Unit

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This plot shows intensity of response as a function of energy deposit as well as incident energy (integrating along vertical cuts gives curves as in previous plots)

This particular plot is for uniform field, only scattering in the detector plane (not collimator, not chamber)

Nominal response

Scatteredout of pixelScattered

into pixel

A Closer Look: Pixel 0 of High Unit

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This plot shows response as before in the rear detector of pixel 0 of the High sensor head

Higher energy primary electrons result in a different balance of non-nominal response

This particular plot is for uniform field, full geometry, 30 keV front detector threshold

Scattered offof collimator

Escaping from sidesof rear detector stack

How Are We Doing?

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Simulations of sensor response gave improved agreement of Med and High unit fluxes with REPT values

Simulated (solid lines) and measured (dashed lines) energy deposit spectra in Low are displaced

Now investigating cause: calibration of energy deposit, field strength, detector position, …

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Relativistic Proton Spectrometer (RPS)

• Real sensor images at left, Geant4 above

• Designed for maximum background rejection

• Four 20mm diameter Si SSDs for collimation

• Eight 23mm SSDs to measure energy deposit

• Cherenkov radiator assembly (CRA) with MCP– Black paint on small end– Shaped for directional total internal reflectionMazur et al., Space Sci. Rev., 2012

doi:10.1007/s11214-012-9926-9

SSDA Output to Determine Energy of Protons

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Simulations in top two plots show how averaging two lowest SSD energy deposits from D1-D8, rather than all eight, tightens up curve to read off proton energy (idea due to Norm Katz). Data at right shows cuts to reject “spur” of backward-going protons, though highest energies in both directions merge toward lower left corner of plot.

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RPS – Sample of Observations

A

B

A: High-energy forward protons

B: Protons below Cherenkov threshold, with scintillation and delta rays in CRA

C: High-energy backward protons

D: Delta rays from protons missing SSDs

E: Forward alphas

F: Backward alphas

G: Van Allen Belt electrons

H: Calibration pulser

C

E

F

G

D

H

D D

Particle by particle tracking enables us to sort out the many species/energies contributing to the observations.

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RPS – Simulation of Protons (Van Allen Belts, GCRs)

A

B

C

F

D

D

Simulation of protons, with spectrum drawn from observations. Paint reflectivity, scintillation in radiator are tuned to place features in the right places relative to observations; cuts to isolate forward protons are in white.

A: High-energy forward protons

B: Protons below Cherenkov threshold, with scintillation and delta rays in CRA

C: High-energy backward protons

D: Delta rays from protons missing SSDs

Distribution in L of CRA Values at Minimum Ionization

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Region A is inner zone, E is “hang time” in cosmic rays at apogee, C and D are inner zone with some cosmic rays. Region B is peculiar; it is too far out in L for minimum-ionizing relativistic protons to be trapped.

Spectra of CRA Values at Minimum Ionization

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Note inner-zone regions C and D have more low-light events, from several-MeV electrons. A and E are dominated by forward and backward protons, but note peak around 500 counts is relatively more prominent in B, and noticeably broader and shifted toward the right.

CRA Values from Protons at Minimum Ionization

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This is a simulation of protons with average of minimum two SSD values falling in the tight minimum-ionizing band. There is just a tight strip of saturated values of Cherenkov output, from relativistic particles going all the way through the crystal.

CRA Values from Electrons at Minimum Ionization

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A much broader range of electrons produce many different values of CRA counts within the SSDA cuts, including “supersaturated” values with multiple relativistic delta rays. We are investigating electrons from p-m-e decay as possible source.

Cosmic Ray Telescope for the Effects of Radiation

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• Six silicon solid-state detectors

• D1-D6 from zenith to nadir

• Odd numbers 149m, evens 1mm

• Above is cutaway Geant4 model of sensor head

• Thick detectors measure low LET and thin detectors measure high

Photo courtesy of NASA

To space

To moon

D1/2 D3/4 D5/6

CRaTER – Galactic Cosmic Ray (GCR) Protons

p+p+

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Keeping track of particles through stack helps us understand subsets of sensor response.

CRaTER – GCR Protons Entering Through Side

D1/2 D3/4 D5/6

p+

e-

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Logging incidence location/direction of particles lets us identify backgrounds to measurement.

CRaTER – Sample of Observations

Looper et al., Space Weather, 2013doi:10.1002/swe.20034

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Lunar “Albedo” Protons from GCR Impact

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Modeling to quantify “albedo” protons on previous slide. Note “limb brightening” from forward showers to right of plot. Map of variations over lunar surface: Wilson et al., J. Geophys. Res., 2012, doi:10.1029/2011JE003921.

Effect of Water in Deep Regolith on Proton Production

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This is a difference plot between simulations of albedo from dry regolith and regolith with 5% water by weight, down to tens of meters. Note 1% suppression at small angles, 0.5% enhancement at large angles.

Effect of Shallow Hydrated Layer on Proton Production

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Model of production of protons from albedo neutrons coming from deep regolith “filtered” through shallow hydrated layer. Note unphysical-looking steps as models handed off one to another in FTFP_BERT_HP physics list.

Effect of Shallow Hydrated Layer on Proton Production

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Other Sensors: Teledyne Microdosimeters

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Besides complex multi-element scientific sensors, we fly (and model) single-chip dosimeters.

De-lidded Teledyne part (licensed from Aerospace design) and Geant4 geometry shown at left.

Flown in electronics boxes of RPS and CRaTER, and a variety of other satellites.

Mazur et al., Space Weather, 2011, doi:10.1029/2010SW000641

Mazur et al., Space Weather, 2015, doi:10.1002/2015SW001175

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Other Simulation Tasks – Neutron Conversion Foils

Another recent project involved selection of a foil to enhance neutron detection in a silicon microdosimeter. Here is a thin aluminum foil over the detector; up to about 14% of neutron energy can go to recoil Al or Si nuclei and be detected. Around 10 MeV, start getting additional energy from disruption of target nuclei.

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Polyethylene Neutron Conversion Foil

Half of the atoms in polyethylene are hydrogen, so kinematics allow nearly all of neutron energy to go to a proton and thence be detected. (Gd absorbs neutrons and emits gammas, B absorbs neutrons and emits alphas and lithium ions, but all at lower energies than were of interest for this project.)

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Other Simulation Tasks – Probe Limits of Thin Layers

Geant4 Is An Indispensible Tool for Radiation Sensors

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• Sensors almost never operate as simply as a conceptual design would suggest

• Scattering, production of secondary particles can significantly change sensor response

• Geant4 allows us to isolate and quantify non-ideal response

• Realistic response calculations enable more accurate measurements

• Comparison of simulation results with observations can help uncover processes or populations not thought of before launch

• Incorporating detailed simulations of expected response in design phase well before launch can reduce surprises

• Plenty of work still to do on all of the projects reported here…

© 2015 The Aerospace Corporation