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Re: Radiation in space



On Thu, 25 Feb 1999, Jenkins wrote:

> Date: Thu, 25 Feb 1999 20:06:07 -0800
> From: Jenkins <jenkins@ridgenet.net>
> To: Bob Bruninga <bruninga@nadn.navy.mil>
> Cc: sarex@AMSAT.Org
> Subject: Re: [sarex] Radiation in space
> 
> If parrifin is good, what about parrafin sealed in a plastic container?

Most plastics have the same problem as paraffin in a vacuum, they
evaporate (known as outgassing).  The material that has outgassed usually
condenses some place you don't want it, like on camera lenses.  This is
why material selection is big deal in space applications, even in
'pressurized' vessels like MIR or the crew cabin the shuttle.  Most of the
low outgassing plastics are not practical for making sealed containers
that will pass muster.

There are few electronic devices that have been tested for effects due to
radiation.  Most that have been tested are very low tech and old parts. 
The databases on the web primarily contain parts that some program wanted
to fly them and paid to have tested.  There is no real guide or method of
accurate prediction of how a part will fair in a radiation environment. 
Even minute changes in the manufacturing process of the same part can have
significant effects. 

The smaller the size of the circuits on the chip and the lower the
operation voltage (i.e. any modern chip) increase the probability of an
event.  The smaller the transistors the more that can be zapped by a
charged partical's ion trail.  The lower the operating voltage the lower
the threshold for flipping bits by the charges left behind by the
particals.
   
So the name of the game these days is single event tolerance.  This means
you do the best you can to be able to recover from an event instead of
trying to be immune.  Typically watchdog circuits are used to reset
effected electronics.  This makes designing electronic systems for space
more of a challenge.  It is difficult to simulate the environment even to
test designs on the ground.  That's why it feels like winning the lottery
when your stuff actually works in space.  :)  It is an awsome feeling!

For information on outgassing check out http://epims.gsfc.nasa.gov/og/
For information on radiation check out http://radnet.jpl.nasa.gov/


> Bob Bruninga wrote:
> > 
> > Miles,
> >    I was surprised when I wanted to put LEAD around my CPU in the
> > satellite we are building.  (We had no mass restrictions)  The experts
> > said that lead doesnt really do anything to help against neutrons.
> > Neutrons are "scattered" by Lead, but not slowed down at all.  Further.
> > they said aluminum is worse, when neutrons hit aluminum it generates a
> > splatter of all kinds of particles which actually increase your chances of
> > a CPU upset.
> > 
> >     The best thing that will absorb neutrons is Hydrogen such as water or
> > parrifin.  Apparently the neutron grabs a hydrogen an becomes helium which
> > slows it down.  But I bet Parrafin outgasses and brings its own problems.
> > 
> >    Since we got bumped, we never persued this further.  But I would like
> > to hear from any experts about retro-fit radation hardening of commercial
> > chips in space...
> > 
> >    Oh, they also say that the electrons and Protons are effectively
> > blocked by other materials, they seemed to only think Neutrons
> > were the problem...???
> > 
> > bob, WB4APR
> > 

Ken McCaughey - N3FZX          
e-mail: n3fzx@amsat.org        

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