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Re: gamma rays and chips



Hi Bob/Guys,
              With "KiwiSAT" in mind I'll watch the thread with 
considerable interest.   Good logic to me!
  Please count me in with any off bb feedback!
 Rgds
Fred
ZL1BYP  

Date sent:      	Fri, 29 Nov 2002 09:04:07 -0500 (EST)
From:           	Bob Bruninga <bruninga@usna.edu>
To:             	Daniel Schultz <n8fgv@usa.net>
Copies to:      	amsat-bb@AMSAT.Org
Subject:        	[amsat-bb] gamma rays and chips

> On Fri, 29 Nov 2002, Daniel Schultz wrote:
> 
> > In space, most of the radiation that you will measure is caused by
> > trapped protons and electrons in the Van Allen belt. There are very
> > few gamma rays, although solar events may produce high energy
> > x-rays. The effects of charged particles hitting the outside of the
> > satellite will produce x-rays and gammas by the Bremstralung effect.
> 
> Which brings me back to PCsat's TNC's in orbit.  All the Pros said you
> cannot shield things in space with lead and that in some cases it can
> make things worse (by increasing the probabiliy of the above efffect).
>  But then when I investigated replacing our $3 ROM with a SPACE
> HARDENED $3,000 part (with one year lead-time), I asked what made it
> so different.  And the answer was "testing" and it had a layer of
> Tantalum (externally applied) over the chip die.
> 
> When I asked whats the difference between Tantalum and Lead for that
> same purpose, they said Tantalum is better for less mass.  Duh, 1g on
> a 12,000g satellite doesnt seem to be that big a deal...
> 
> So, since I didnt have a year to wait, didnt have $3000, but did have
> some Lead, I used some scissors and cut a 1/2" square of Lead out of
> 1/16th inch thick sheet and Epoxied it onto the top and bottom of the
> ROMS and CPU in both TNC's.  And then used Kapton Tape in case the
> epoxy failed.
> 
> My theory is that when the experts say that Shielding with Lead may
> increase the secondary emissions effects, they may be thinking about a
> lot of lead as an exterior box shield or something.  Then yes, you
> probably have increased the probability of a high energy particle
> causing an avalanche of other particles because of the tremendous
> surface area.
> 
> But my thinking is... ok, the normal aluminum skin of the spacecraft
> also generates such avalanche conditions, then wouldnt having a small
> chip of Lead over my CPU then help stop the resulting secondary X-rays
> and lower energy particles?  I mean, it couldnt hurt...
> 
> And, I never could get one of the experts to tell me "the conditions"
> or the "geometry" under which the old-experts-tale of increased
> crosssection might be "worse".  Its like asking "experts" about NiCd's
> and memory effect...  Half of the experts vehemently defend completely
> opposite theories..
> 
> So, anyway, PCsat has lasted 14 Months in orbit with its little chips
> of lead... and we still have no clue whether it was a good idea or a
> bad one. I am certainly open to and expert opinion...
> 
> de WB4APR, Bob
> 
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