[amsat-bb] gamma rays and chips

[Bob Bruninga <bruninga@xxxxxxxx>]

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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