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RE: starship-design: Hull Materials

Hi Ken,

You wrote:

>>Which brings us to deceleration, NOW the shields need to be at the BACK 
>>of the ship, not the front...
>This is especially problematic when you consider that you need holes in 
>the back for the engines to spew out propellant.  How could we 
>conceviably have shielding there??

One could wonder if shielding is necessary for an engine that spews out a
wall of particles which have velocities of say 25% of the cruise velocity.
The engine would probably have enough shielding against its own destuctive
forces, thus also against the few particles that get through "the wall of fire".

>... Even the electron that is kicked out by photo ionization
>will have a small stopping distance compared to the K-alpha xrays.
>Alpha particles will be even less of a concern.  The energies of 
>the secondary k-alpha xrays are determined by the Z of the atom; from 
>50eV in Li to 100KeV in Uranium.  That's why you want to have the 
>higher-Z materials on the outside.  If you dump lots of energy into k-
>alphas in an outer Tungsten layer, for example, you can attentuate the 
>secondary 60KeV xrays in further lower-Z layers without worrying about 
>creating more high energy xrays.  If the last part of the shield is 
>Deuterium ice, the worst secondary radiation will be the 500eV k-alpha 
>line from oxygen, which isn't horrible.

Does one really need low Z-materials on the inside? High Z-atoms do have
other (lower) electron shells. So in theory high Z-materials should do
absorb both high and lower energy X-rays.

>Another problem is that ionization cross-sections start dropping once 
>the xray gets much more energetic than the k-alpha energy.  You start 
>dumping energy into compton scattering, in which the atom only partially 
>absorbs the x-ray, as well as electron-postiron pair-production over 
>1.5MeV.  The strong 500KeV gamma line that might result from positron 
>annihilation could be a problem, but I suppose that's still better than 
>the orginal MeV-pluse xrays.

I think you mean the strong 1000 KeV gamma line (512+512=~1000) ;)
But indeed every decrease in energy of the gamma-rays makes it easier to
stop them further on.
Do you know how if proton radiation will increase the amount of high energy
gamma rays? (A proton at 0.3c has an energy of 45MeV.) While a proton likely
doesn't collide with an big (positive) atom core, it may radiate
Bremsstrahlung when it is path is bent due to the Coulomb force.