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Re: Flight Computers and Radiation

On Wed, 1 May 2002, Murray Peterson  VK2KGM wrote:

> I thought alpha particles would have been easy to stop - ie paper
> 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.

That is exactly what I did on PCsat.  I cut some lead about 1/16th inch
thick into postage stamp sizes and glued them to the top and bottom of
the EEPROMS and CPUs in the TNCs.  But I did this without any
authorization, because all the experts kept saying "it wont work", high
energy particles will knock even more particles out of any shielding and
you end up with a worse situation".

But hey, the satellite is surrounded by at least 1/4" of aluminum, so it
seems to me that the lead would stop the secondaries from those earlier
collisions.  Besides, space rated chips are some times RAD hardened by the
application of a layer of Tantalum and when I asked why that was better
than lead, the anser I got was because it is denser.  So?  Just
increase the thickness of the lead to match...

I never could get an answer I trusted.  Got lots of opinions and we even
have a small nuclear reactor at the Naval Academy, but I never got anyone
interested in doing some testing.

Even in light of the secondaries, it just seems like it cant hurt?  But
maybe the answer is that all the light stuff IS stopped by the outer
aluminum and the secondaries they produce are stopped by even the
slightest last layer of aluminum.  So the only things that reach the CPU
are the real high-energy particles, and so in that sense, then by having
the higher mass LEAD so close to the chip, you just increased the
probability that the particle will find a nucleas to hit, and then when it
generates the secondaries within a mm of your CPU chip that those
secondaries now have a higher probabllity of hitting something.

But probabilities is probabilities, and it would seem to all depend on an
awful lot of geometry considerations?   de WB4APR, Bob

 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
> regards,
> Murray Peterson
> ----- Original Message -----
> From: "Richard Elling" <Richard.Elling@GoldensRule.com>
> To: "Scott Elliott" <scott@pieh0.co.uk>
> Cc: <amsat-bb@AMSAT.Org>
> Sent: Wednesday, May 01, 2002 2:59 PM
> Subject: Re: [amsat-bb] Flight Computers
> | Scott Elliott wrote:
> |
> | > just a quick question :).
> | >
> | > this radiation, is it the type that can be blocked by lead. like x-rays?
> |
> |
> | The problem is that lead emits alpha particles which cause transient
> | errors in memory.
> |
> |
> | > if it is possible, then the only thing to worry about then is the extra
> | > 50-100kg of weight your going to add :P
> |
> |
> | The bigger problem is the weight.
> |   -- richard
> ----
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de WB4APR@amsat.org, Bob

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