Re: High Akltitude Neutron Fields

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

Things at altitudes achievable by conventional aircraft are subject to much
higher neutron dose due to high energy proton reactions with air.  Neutrons
in space are either from the sun or a secondary reaction with much lower
probability.  In space, neutrons really don't contribute to total dose.
Remember, it takes 260 thermal neutrons per cm^2-sec to get 1mrem/hour (in
a person) (Lamarsh p445) in space you get on average 6.8 n/cm^2-s (as
pointed out earlier), ie not much.  The trend on studying neutrons has been
a recent one.  Five years ago: space doesn't have neutrons.  Two years ago:
space has some neutrons, but they are insignificant.  This past year: high
altitude secondary neutrons produced form high energy proton interaction
with the atmosphere can be: a)high energy (up to about 10MeV), b)very bad
for aircraft and c)cause SEU (oops).  Also, don't forget that the earth
effectively shields things from solar neutrons if it happens to be in the
shadow at the time (except in the South Atlantic Anomaly).  

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 15:58 05/06/98 -0500, you wrote:
>RadSafers:
>
>The measurements I have recently seen presented indicate that at about
>subsonic jetplane altitudes (30,000 ft.) dose rates UP TO 1 mrem/hr may be
>expected, with more typical values of about 0.5 mrem/hr, This is for
>mid-and high-magnetic latitudes (40 N to the pole), somewhat lower closer
>to the equator. The high value quoted results from a solar flare. These can
>appear in a matter of hours and are a particular problem for supersonic
>aircraft, like the Concorde and some US and Russian clones now being
>considered. The Concordes typically fly at 60,000 ft. and encounter neutron
>radiation fields 2 to 3 times greater than those at subsonic altitudes.(
>The peak of solar activity that follows the 11-year cycle is a minimum for
>cosmic rays, since the solar activity is accompanied by greatly increased
>solar magnetic fields which entrap the lower energy cosmic rays.) The
>neutrons contribute 40 to 50% od the total dose rate, but I am not sure
>which neutron quality factors are used to calculate this proportion. The
>balace, considered mostly to be low LET radiation, is from gamma-rays,
>muons protons and some pions, although I do not think the latter two are,
>strictly, "low LET" radiations. Good data are in fact somewhat sparse,
>although there is now increased experimental activity in this area. Induced
>radioactivity would seem to be a minor problem compared to these particle
>fluxes.
>
> 
>H.B. Knowles, PhD, Physics Consulting
>4030 Hillcrest Rd, El Sobrante, CA 94803
>Phone\Fax (510)758-5449
>hbknowls@ix.netcom.com
>
>