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I have seen it said that because high-energy GCR particles cause a spray of secondary particles when they pass through material, there is more radiation behind even quite thick shielding that there is if there is no shielding at all. A paper by Vanessa Aulessa was used to produce the graph below in a lecture by Rob Mueller

gcr dose starts at 0.015 Sv, peaks at 0.13 Sv at 180 g/cm2, tails off to 0.02 Sv at 500 g/cm2

But I have also been shown graphs that show cosmic radiation starting to drop as soon as it hits shielding. The graph below from a paper by Donald Rapp (7th page) shows that version:

gcr starts at 60 cSv, drops to an average of 40 cSv after 10 g/cm2, tails off to less than 30 cSv after 50 g/cm2

This slide presentation by Martha Clowdsley comes to the same conclusion, and lists the transport models used.

I have seen both models referred to elsewhere. Why do they apparently contradict each other?

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    $\begingroup$ these kinds of cascade calculations and dosimetry estimations are pretty - really - complicated and there can be substantial differences between definitions, spallation yield models, and other assumptions. The second paper may not be peer reviewed by physicists knowledgable in this kind of modeling (two reviewers, one is also the editor!), I wouldn't even bother looking at it again. This is tricky modeling and I'd stick to sources that come from well-accepted models that have passed peer review many times. Unfortunately that may require some paywall mitigating tricks. $\endgroup$
    – uhoh
    Mar 30, 2017 at 1:36
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    $\begingroup$ @uhoh - i have hoped to get clarity somehow from true experts, because i realize this is not something i can simply bone up on. Wait... Donald Rapp, i see now, is rather controversial... $\endgroup$
    – kim holder
    Mar 30, 2017 at 2:44
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    $\begingroup$ The portion of the video that is relevant starts shortly after where it opens when the link is clicked. I do recommend the rest of it though as generally very interesting. It isn't all that complex - it is an introduction - but it covers a lot of ground from a real hands-on perspective. $\endgroup$
    – kim holder
    Mar 30, 2017 at 2:53
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    $\begingroup$ I added another reference for the second graph, that one by a researcher out of NASA Langley. $\endgroup$
    – kim holder
    Mar 30, 2017 at 3:09
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    $\begingroup$ Significant difference: amount of cosmic radiation vs radiation dose absorbed by human body. For the most energetic cosmic rays human body is entirely transparent, so they pass harmlessly. But slow them just enough with a shield just thick enough... $\endgroup$
    – SF.
    Mar 30, 2017 at 9:16

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Those two graphs are not showing the same thing. The lecture slide is misleadingly labeled, but the original paper makes clear that the table is only showing neutrons, mostly produced by shield spallation. That does go up with added shielding (no shielding means no spallation) but the overall dose including other sources goes down.

See figure 7 in the second paper for a graph showing the various components of the radiation dose as shield thickness is increased.

enter image description here

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  • $\begingroup$ The Aulessa paper mentions 'light nucleons', so protons too. But i was frustrated at the lack of citation about that, because it seems to come from elsewhere. Rapp's paper is much more specific and focused, however it hard for me to reconcile that a NASA lecture specifically about long-term habitats on the Moon would not get that right. Could there be other factors here? They really got something so central so wrong? $\endgroup$
    – kim holder
    Mar 30, 2017 at 0:53
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    $\begingroup$ Looking at that plot for a while, I get more and more confused by the source of high Z ions. What is the actual source of Z>= 10 (Neon and heavier) ions that decrease with tens of centimeters of shielding? Are they energetic, like >100 MeV and produced outside the shielding, or in the shielding, or are they low energy and produced inside the body as recoil from lighter particles? $\endgroup$
    – uhoh
    Mar 30, 2017 at 1:00
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    $\begingroup$ @kimholder Other particles are mentioned, but the points on the graph correspond directly to a table that is more specifically labeled. It isn't so much wrong as incomplete - and the secondary particles may be the most relevant as any shield capable of blocking them won't be letting anything else through either. $\endgroup$
    – Quentin Clarkson
    Mar 30, 2017 at 5:59
  • $\begingroup$ @uhoh See the radiation sources section of Rapp - The GCR has a relatively small number of larger particles, but being large and energetic you don't want to get hit by them. $\endgroup$
    – Quentin Clarkson
    Mar 30, 2017 at 6:01
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    $\begingroup$ Rapp appears to put the neutron peak earlier, but that isn't unreasonable given all the assumptions that can differ in a complex model. The big difference is in goals - Rapp is designing for a mars mission and just needs to get a two year mission below the allowable lifetime dose, so he can dismiss the 5m shield as having diminishing returns, especially when that shield and supporting structure would weigh twice as much on Mars. Aulessa is aiming for Earth surface level doses, so a 1m shield is insufficient and detailed effects of the first meter of shilding can be ignored. $\endgroup$
    – Quentin Clarkson
    Mar 30, 2017 at 21:25

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