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RE: Flight Computers and Radiation

> However I guess the question is really how much attenuation
> of the radiation
> in needed and how much weight of materials is required to
> stop that. maybe a
> small piece of lead could be adheared to the top of the memory chip or
> microprocessor and underneath it - maybe in the IC socket.
> 2mm of lead will
> stop basically all alpha and beta particles and attenuate
> gamma rays by ">
> 60 db". However neutrons are an other problem - hydrocarbons
> are better at
> attenuating these - maybe a suitable polymer?
> - my 2 cents worth

Many of our satellites  have flown shielding for CPU's and especially
memory. For example, as far back as Phase-3A (the one that an early Ariane
dumped into the Atlantic in 1980) we used a brass outer box to shield the
CPU & RAM, and further surrounded critical chips with Tantalum foil. AO-10
and -13 and -40 all used Tantalum. Tantalum has better stopping power per kg
than other materials.

In addition to the shielding, the spacecraft use chips that are as radiation
tolerant as possible. For example, most of the 74AC series logic chips are
known to be good to ~ .5 megarad. In contrast, any part that uses a charge
pump (like the DC/DC voltage converters in MAX-232 chips and a LOT of CPU
chips) are known to fail at ~20-30 krad. To verify designs we have often
used (both human and veterinary) medical (e.g. Cancer) Cobalt-60 sources.
These have had their aperture opened up to the "The patient is already dead.
Let's do the cremation now." settings. In one test we did in VE6-land, a
module was subjected to 1-3 krad/night dosage. Again we found that
microprocessors with charge pumps inside croaked at ~25 krad. This would
have been OK at LEO, but for a satellite like AO-40 flying thru the Van
Allen belts would not have been good.


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