Download - Simulation Results for SNAP
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Simulation Results for SNAP• Galactic Protons
– Flux vs Energy and Particle Species– Dose vs. Shield Composition– Dose vs. Shield Thickness
• Solar Flares (worst day)– Dose vs. Shield Thickness– Flux vs Energy and Particle Species
• ¾ cm Aluminum shield results
Tom Diehl05/20/2004
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The Rakhno Model• In “V1.0” the total mass included is 276 kg in the
region directly around the focal plane.– Shield is 2 cm thick, 35 to 41 kg aluminum
– Cold Plate was 99 kg molybdenum -> 49 kg.
– Silicon substrate is 5 kg mixture.
– Silicon itself is 200 thick amounting to 107 g.
– Radiator was 69 kg aluminum.
– The rest is deck, optical bench, supports, etc … amounting to 61 kg.
• New version “V1.1” has mass 226 kg.– I use this as “the standard” as it is closer to the present
design in Solidworks.
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MARS 14 Monte Carlo• http://www-ap.fnal.gov/MARS/ provides a link to
documentation. • MARS 14 is a Monte Carlo code for inclusive and
exclusive simulation of three-dimensional hadronic and electromagnetic cascades, muon and low-energy neutron transport in shielding and in accelerator and detector components in the energy range from a fraction of an electronvolt up to 100 TeV. (straight off the web page).
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The “Rakhno” Model
• See FNAL Technical Memo 2221 by Mokhov, Rakhno, Striganov, Peterson “Radiation Load to the SNAP CCD” 09/15/03 for more detail about the original version (V1.0).
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Shielding and Detector Model (Rakhno)
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Galactic Cosmic Rays @ L2• Creme96
provides the flux of nucleons from H to Fe
• Protons are the most abundant. Helium is second most.
• Flux of protons is 4.7/cm2/s.
No geomagnetic shielding.
No trapped particles.
Solar cycle minimum.
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Galactic Protons @ L2• 2 cm aluminum shield
– Dose is 6.90.4 Rad/Yr
• Table of # cm/s vs. KE (in Si) converted to flux (divide by volume)*
• Integrals (#/cm2/s):
– p: 3.410.06
– n: 4.780.08
12.80.9
– e: 0.780.08 /k: 0.350.02
*See next slide
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Not exactly a flux• MARS gives me a table of particles #*cm vs energy
– Number times Path length– Energy is that which it has when it starts in the silicon.
• For instance:
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Effect of Changing the Material• Change the material that
makes up the shield (maintain thickness = 2 cm).
• Lower Z is a little better than higher Z & more weak evidence for “more shield is worse”.
• Carbon fiber mix was a little better than everything but not very different from Aluminum.
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Changing the Material II
• Changed the material but kept the shield mass at 25 kg.
• No strong evidence for an effect
• Little difference in secondary fluences.
Aluminum is good choice from
Shielding POV.
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Dose vs. Shield Thickness (AL)• The shield in the nominal geometry is 2 cm thick
aluminum cone. Is there an optimal thickness?
• Tested the dose in the silicon as change thickness (AL density) of the shield.
• No apparent reason to make the shield 35 kg to reduce the radiation in the silicon (so long as we have thick cold plate and other material around.
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Material vs. No Material• What is the effect of ALL of the material?
• Set all the material to vacuum, except the silicon. Run the simulation using galactic protons & galactic electrons.– Dose (silicon) = 5.30.4 R/y, less than with the
material around the detector.
– Galactic Protons: Ratio Doses (satellite vs. no satellite) Rprotons = (6.9+0.4/5.3+0.4) = 1.3 + 0.1.
– Galactic (Jovian?) Electrons: Relec. = 2.0+ 0.2. But the electrons have ~100x lower flux than the protons and are therefore rather unimportant.
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Galactic Cosmic Rays @ L2• Table of # cm/s vs. KE
(in Si) converted to flux (divide by volume)*
• Integrals (#/cm2/s): – p: 3.450.02– n: ~0
0.260.26– e: ~0 /k: ~0
• Ratio of total charged particle flux compared to 2 cm AL is 1.320.03 • Recall, ratio of Dose
compared to 2 cm AL is 1.30.1
Turn off all material except CCD’s
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Material vs. No Material• What is the effect of ALL of the material?
– A fast cosmic ray simulation using 4.7 charged particles/cm2/s should be scaled up by 1.3.
– Therefore, 6.1 charged particles/cm2/s seems more realistic for the a fast simulation of protons at solar minimum.
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Galactic Cosmic Rays: Solar Max. vs. Solar Min.
• The solar wind impedes the GCR– GCR Flux solar min: 4.7
pr/cm2/s– GCR Flux solar max: 1.7
pr/cm2/s– The benchmark for GCR
calculations is the flux at solar cycle minimum. (shown just prior).
– Didn’t take this any further.Tylka et al.
IEEE Trans. Nucl. Sci.
V44 #6, 2150 (1997).
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Worst Day Solar Flares @ L2• Creme96
provides the flux of nucleons from H to Fe.
• It has other options: solar min./max., worst week, worst day, worst 5 minutes.
• I chose the worst day to start.
No geomagnetic shielding.
No trapped particles.
Solar Flare of Oct 20, 1989 averaged over 18 hours.
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Worst Day Solar Flares @ L2
• 2 cm Aluminum shield resulted in dose of 23242 Rads/day
Integrals (#/cm2/s):
– p: 2900180
– n: 90070
2900450
– e: 6.53.3 /k: 0.320.03
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Worst Day Solar Flares @ L2
• Shield finally does some good.• Dose decreases until as we can’t shield the higher energy protons.• ¾ cm AL is as good as 2 cm AL.
Dose vs. AL Thickness
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¾ CM Aluminum Shield• Dose from Galactic Protons
is 6.78+-0.45 Rads/Yr.
• Dose from Worst Day Protons is 179+-34 Rads/day.
Integrals (#/cm2/s):
– p: 3.420.06
– n: 4.620.09
11.31.0
– e: 0.710.06 /k: 0.350.03
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¾ CM Aluminum Shield• “Ratio” is particle flux
for ¾ cm AL over particle flux for 2 cm AL.
• Ratio of integrals is – p: 1.01+0.02– n: 0.970.02.10– e: 0.910.12 /k: 0.990.08
- Total charged flux is 0.988+-0.029 “ths” of 2 cm case.
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Summary• Galactic Proton Studies
– The shield doesn’t stop most cosmic ray protons.– Aluminum seems like a pretty good choice of material from
shielding considerations. – Dose from galactic protons is ~ 6.9+0.4 R/yr with a 2 cm
aluminum shield.– There is a scaling for simple simulations that accounts for the
secondaries.
• Solar Flare “worst day” study is done.– An aluminum shield has an important effect.– Dose is 232+42 Rads/day on worst day w/ a 2 cm shield.
• ¾ cm vs 2 cm of aluminum– The thinner shield is as effective as the thicker.
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Plan• Along the lines of this talk
– Continue Solar Flare studies. I can find information about the distribution of intensity of solar flares.
– There is some correspondence between #*cm and SEU & dose calculations and I’d like to learn how that works.
– Heavy ions. – Tests for electronics in different places (I don’t expect the
results to be very different elsewhere in the vicinity of the cold plate).
– Write-up these results.
• Use “Nibble” file to improve geometry of the MARS model. I expect that will be suitable as a starting point for the Geant simulation, too.
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Buffer slide
• Stuff here on isn’t part of this talk.
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Shielding and Detector Model (1)
• The trick with MARS is to select the appropriate level of detail.– Include the most important shielding
components. Consideration is given to proximity to detector and components mass.
– Put a lot of effort into detailing the detector.
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Literature Search (partial list)• Some satellites planned-for L2 include NGST,
Herschel/Planck, GAIA …• References: http://astro.estec.esa.nl/GAIA/Assets/Papers/IN_CCD_operations.pdf.Herchel/Planck Project Document, JPL D-19155 (2003).http://www.ngst.nasa.gov/public/unconfigured/doc_0819/rev_02/Transient_Document_Section_1_and_2_V5.
pdf.
• These give me some confidence that I’m on the right track.
• The NGST suggests an additional concern:– that we may activate the shield/cold-plate. This would
produce additional particles on the focal plane.
• I talked to Nikolai Mokhov yesterday. He is not surprised the shielding is making Si dose worse.
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Unshielded / 2 cm aluminum
• unshielded ccd plane over 2 cm AL (only proton spectrum in ccd’s).
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Range vs. Energy in Aluminum• From a talk by
Mike L. in April 2001.
• The 2 cm of AL shield itself is 5.4 g/cm^2.
• log(5.4)=0.73