# Presentation: 196169

## MARS Simulation Results

- 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
- 3⁄4 cm Aluminum shield results

- Tom Diehl
- 05/20/2004

## 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.

## Shielding and Detector Model (Rakhno) 

## 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.

## Galactic Protons @ L2

- 2 cm aluminum shield
- Dose is 6.90.4 Rad/Yr

- Table of  # cm/s vs. KE (in Si) converted to flux (divide by volume)*
- Integrals (#/cm2/s):
- p: 3.410.06
- n: 4.780.08
- 12.80.9
- e: 0.780.08
- /k: 0.350.02

- *See next slide

## 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:

## 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.

## 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.**

## 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.

## 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) = 5.30.4 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.

## Galactic Cosmic Rays @ L2

- Table of  # cm/s vs. KE (in Si) converted to flux (divide by volume)*
- Integrals (#/cm2/s):
- p: 3.450.02
- n: ~0
- 0.260.26
- e: ~0
- /k: ~0
- Ratio of total charged particle flux compared to 2 cm AL is 1.320.03

- Recall, ratio of Dose compared to 2 cm AL is 1.30.1

- Turn off all material except CCD’s

## 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.

## 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).

## 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.

## Worst Day Solar Flares @ L2

- 2 cm Aluminum shield resulted in dose of 23242 Rads/day

- Integrals (#/cm2/s):
- p: 2900180
- n: 90070
- 2900450
- e: 6.53.3
- /k: 0.320.03

## Worst Day Solar Flares @ L2

- Shield finally does some good.
- Dose decreases until as we can’t shield the higher energy protons.
- 3⁄4 cm AL is as good as 2 cm AL.

- Dose vs. AL Thickness

## 3⁄4 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: 3.420.06
- n: 4.620.09
- 11.31.0
- e: 0.710.06
- /k: 0.350.03

## 3⁄4 CM Aluminum Shield

- “Ratio” is particle flux for 3⁄4 cm AL over particle flux for 2 cm AL.
- Ratio of integrals is
- p: 1.01+0.02
- n: 0.970.02
- .10
- e: 0.910.12
- /k: 0.990.08
- Total charged flux is 0.988+-0.029 “ths” of 2 cm case.

## 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.
- 3⁄4 cm vs 2 cm of aluminum
- The thinner shield is as effective as the thicker.

## 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.

## Buffer slide

- Stuff here on isn’t part of this talk.

## 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.

## Literature Search (partial list)

- <date/time>

- Some satellites planned-for L2 include NGST, Herschel/Planck, GAIA ...
- References:
- _[http://astro.estec.esa.nl/GAIA/Assets/Papers/IN_CCD_operations.](http://astro.estec.esa.nl/GAIA/Assets/Papers/IN_CCD_operations.pdf)__[pdf](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.](http://www.ngst.nasa.gov/public/unconfigured/doc_0819/rev_02/Transient_Document_Section_1_and_2_V5.pdf)__[pdf](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.

## Unshielded / 2 cm aluminum

- unshielded ccd plane over 2 cm AL (only proton spectrum in ccd’s).

## 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