[meteorite-list] Cosmic Ray Penetration

[MexicoDoug]

Hello Eric,

First and most importantly, I would not talk about a cosmic ray being 
absorbed and leave the term absorbed for energy in its many form. Keep 
in mind so called 'cosmic rays' are really nano-meteoroids that the 
NOMCOM hasn't gotten around to classifying ;-): particles, ions, and 
for practical purposes just think of Speedy Gonzalez protons and other 
speedsters flying around from random cosmic or high power solar events.

That said you want to know how far a proton (or other small particle) 
will penetrate before its kinetic energy is absorbed by the collision 
and whether that mean penetration depth varies depending on the target 
substrate. I would expect this to depend on the volume fraction of the 
atoms in the substrate, much like asking how far you penetrate a forest 
you accidentally ski into... Is there a difference between metallic 
meteorites vs. stony meteorites? Or metallic meteorites? I would expect 
there is - and I would estimate that based on the packing factors of 
the usually heterogeneous matrix they smash into. Taking a hint from 
the crystallographers, atomic packing is in the range of 33% occupied 
space to 74% occupied space.

So I would estimate that to be the order of magnitude of the 
differences - as a first approximation for a sly proton slipping 
through. Perhaps it would be better to use cross sectional area than 
volume (a "planar packing fraction") so take it with a grain of salt. 
In any case the extreme case is a bit over double and I would expect 
that to be in the ballpark without overanalyzing this.

Not over analyzing this because you also have assumptions at work that 
the cosmic ray particle stream is estimated based on a standard (I 
believe - long time since I thought about this).. And finally, throwing 
up the hands, the normally quoted mean penetration distance is "a few 
meters"! So, you can be confident that by 10-20 meters there's nothing 
much and in 0-3 meters you are bombarding throughout the body. The 
reason we have no activity on earth is not because spallation in the 
atmosphere, plus a major contribution to deflection deflection by the 
Earth's magnetic field.

Good luck, sorry no time to refresh more on this, but I hope that 
helps. The attempt to explain penetration is something I pulled out of 
my ear just now, but I think it's the right concept..
Doug


-----Original Message-----
From: Eric Wichman <eric at meteoritesusa.com>
To: meteorite-list at meteoritecentral.com
Sent: Mon, Jun 27, 2011 6:46 pm
Subject: [meteorite-list] Cosmic Ray Penetration

Hi all, I normally don't ask about these things because I can look it 
up onlin. However, I'm writing an article, am in a time crunch, and 
need a bit of quick help here finding the appropriate information.

I need to know how deeply cosmic rays penetrate into the body of any 
given meteoroid, asteroid or comet. And of course the relationship to 
composition and what effect composition has on the penetration depth. 
Do different types of asteroid absorb cosmic rays at different rates? 
etc...

Thanks for any help on this you guys can provide.

Regards,
Eric

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