XPost: sci.physics   
      
   In article <79cf0a8.0310171314.1a9d8696@posting.google.com>, sbh   
   rris@ix.netcom.com (Steve Harris  sbharris@ROMAN9.netcom.com) writes:   
   >mmeron@cars3.uchicago.edu wrote in message news:...   
   >> >Then you have the problem that heavy ion biological effects are the   
   >> >most poorly studied of all known radiation effects, and here they're a   
   >> >major player.   
   >>   
   >> Only if you shield very poorly.  The reason heavy ion biological   
   >> effects are the most poorly studied, is because heavy ions are the   
   >> easiest to block.  It is only when their kinetic energy, per nucleon,   
   >> exceeds their binding energy (again, per nucleon), that spallation   
   >> effects become a bitch.  But you don't encounter many heavy ions in   
   >> this range.   
   >   
   >COMMENT:   
   >   
   >You do in interplanetary space and on the Martian surface---that's the   
   >whole point. The fraction of hadron radiation in space goes down as   
   >some power loaw, but it's not absolute fractions that are important as   
   >energy fluxes (fluences). For (extra galactic) cosmic rays (which will   
   >cause more radiation than a quiet sun), in order to look at all   
   >significant biological dosing, you have to integrate all the way up   
   >from channels of 10 MeV per nucleon to 100 GeV per nucleon.   
      
   You got some confusion here.  There are protons in all possible   
   energies.  There is little if any evidence for any significant amount   
   of heavy ions at hig energy (heavy ion is an ion with *Z > 1*).   
   Proton at 1000 Gev has an energy of 1000 GeV pre nucleon.  This *does   
   not* mean that you've heavy ions at high energies.   
      
   > All above   
   >binding energy. As I said, 20% of your total radiation dose on the   
   >Martian surface is spallation neutrons.   
   >   
   Certainly since all protons above 10 MeV or so will give   
   you spallation.  Nothing to do with "heavy ions".   
   >   
   >> >For every phase you have that 3-element neutron sheild design, but you   
   >> >have to do it THREE times, and in three separate ways.   
   >> >   
   >> Not really.  A neutron shield is a neutron shield.  One which is   
   >> designed for the highest energy neutrons you expect to face is good   
   >> for all the rest, as well.  The problem with neutron shields is that   
   >> there is really no way to make them compact.  You need sufficient   
   >> thickness for moderation.   
   >>   
   >> So, basically, you've to plan on sufficient thickness to stop the fast   
   >> protons, followed by a sufficient thickness, from this point, to   
   >> moderate the neutrons.  And the you've to absorb the thermal neutrons   
   >> and provide sufficient thickness, from this point, to absorb most of   
   >> the resulting gammas.  That's quite bulky.   
   >   
   >COMMENT:   
   >   
   >Reread my message. You need three neutron sheilds because you need to   
   >make them out of three different sets of materials, and they need to   
   >do the job against two different radiation environments.   
      
   No you don't yyou need a primary shield which is thick enough to stop   
   all (or nearly all) charged particles).  Then you need a moderation   
   layer thick enough to slow all neutrons in generated within the first   
   layer (or neutrons incoming into the first layer) to thermal or near   
   thermal velocities.  Then you need something to absorb these neutrons   
   and absorb the gammas generated by the neutron absorption.   
      
   Mati Meron                      | "When you argue with a fool,   
   meron@cars.uchicago.edu         |  chances are he is doing just the same"   
      
   --- SoupGate-Win32 v1.05   
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