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Re: Flight Computers and Radiation



> 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".

Well, you're right that added shielding will stop more particles than
it creates, but they're right that the heavier materials like lead are
not necessarily the best.

Every neutral atom consists of a tiny nuclei surrounded by relatively
huge electron shell(s). Any time you crash a charged particle into an
atom, it is almost certainly going to interact with an electron.  So
shielding is all in the lightweight electrons; the heavy protons and
neutrons are just along for the ride. (If you can figure out how to
launch just the electrons, you could have yourself some very
effective, lightweight radiation shielding.)

But all that energy in the incoming particle has to go somewhere.
That "somewhere" are the electrons in the shielding, which are excited
into higher energy states (essentially, higher altitude orbits).
These states are unstable, so the electrons eventually give up their
excess energies and drop back down to their original states.

This excess energy is released in the form of "secondary" photons. At
the energies we're talking about, that can mean X-rays or even gamma
rays.

This is not good, as X-rays and gamma rays can also fry electronics.

But you have a choice. The secondary photon wavelengths depend on the
allowable electron quantum energy states, and those in turn depend on
the atomic number of the shielding material. Low atomic number
materials tend to produce less energetic photons than high atomic
number materials.

So if you're building an X-ray tube for a dentist's office, then by
all means use a high atomic number material like tungsten (lead melts
too easily) and slam 40-60 keV electrons into it. But if you're trying
to shield sensitive electronics, pick a low atomic number material if
you can.

Unfortunately, low atomic number materials tend to be less dense, so
they don't pack as many shielding electrons per cubic centimeter as
you might like. If you have the volume, don't worry about it; use
aluminum. Otherwise, make a tradeoff. I suspect tantalum is both
affordable and close to optimum, which would explain why it is
commonly used.

Phil


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