RE: neutron flux in space

["Scott D. Kniffin" <Scott.D.Kniffin.1@gsfc.nasa.gov>]

Philip,
I'll do my best given the extreme lack of sleep to give a coherent answer.
Starting with neutrons, as mentioned before, there aren't many, using the
average number and assuming average energy of 10eV over say a three year
mission lifetime, you are looking at something on the order of 10^8 n/cm^2
total fluence.  Given that the skin is most often (though not always)
aluminum or tantalum the activation from these is not that significant, Al:
very short half life, Ta: two states from Ta181 + n = Ta182 are a 15m and a
144day, the capture cross section is tiny (milibarns) and the resonance
integral is fairly large (>500barns).  Now for all other charged particles,
what kinds you see and what the energy spectrum is, is entirely dependent
on the orbit.  For people, we obviously try to put them in the minimum
field that is possible/achievable for obvious reasons.  It also has the
added benefit of keeping the electronics alive longer.  So your big
contributors are protons, and cosmic rays (big charged particles).  With
the cosmic rays, of which there are (thankfully) not many, the interaction
has a lot of possibilities, not all of them radioactive.  Anybody have
numbers for cosmic rays in the human altitudes?  Protons are the big
culprit in comparison.  There are plenty of them zipping off the sun in the
100+MeV range, but these tend to give only one interaction in a given
target so the total activation is not that significant.  The mid range
energies give you the photoactivation mostly in the skin of the craft again
(thankfully).  Electrons: for the sake of this exercise they are mostly the
same as protons.  So in this very drawn out and drowzy way the bulk of your
activation is in the outter skin of the craft spread all over the surface
but primarily on the sunward side.  In terms of dose, what is the dose rate
difference between a check source at say 2 feet vs taking a few check
sources, grinding them up, embeding them in a surface area the size of say
a basketball court or two (complete with lots of self absorbtion) and float
a few feet off the surface?  (Sorry, really bad run on sentence.)  Ok, so
maybe it has to be a fairly large check source, but you get the idea.  In
terms of dose to the person, gammas (xrays) and betas are more of a concern
anyway with protons coming in a close third.  Oh well, I guess I'll just
have to design that navigational defelctor along with that wonder drug that
counteracts the effects of radiation (a la Star Trek).  I hope this makes
as much sense to everybody else as it does to me.  Pass the antihistamines,
please.  

Scott Kniffin

mailto:Scott.D.Kniffin.1@gsfc.nasa.gov
RSO, Unisys Corp. @ Lanham, MD
CHO, Radiation Effects Facility, GSFC, NASA, Greenbelt, MD

The opinions expressed here are my own.  They do not necessarily represent
the views of Unisys Corporation or NASA.  This information has not been
reviewed by my employer or supervisor.  


At 08:48 05/07/98 -0500, you wrote:
>Scott,
>
>I was enjoying the thread (your humor wasn't wasted); however, I'm now
>more curious about an answer to the original question regarding
>activation.  I suspect that the question wasn't meant to be limited to
>strictly neutron activation but also possibly photoactivation, charged
>particle reactions, spallation, etc., all processes that produce
>radioactive atoms.  Would the total activity caused by these events
>still be overshadowed by amounts as small as check sources?
>
>Philip
>
>__________________________
>Philip C. Fulmer, PhD, CHP 
>Carolina Power & Light Company
>Harris Energy and Environmental Center
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>