>  > Hi Chris,
>>  Tantalum might be a possibility, especially as it is available in many
>>  forms.  The machine we are dealing with is a therapy machine
>>  generating X-rays at 6 MV.
>>  Steve

Ahh, 6 MeV. Yes, you'll need a lot of lead/tungsten/etc. The mfp of 
Pb at 6 MeV is around 6 cm, for W closer to 3.5.  How about using DU 
for shielding?  That should give the RoHS goons something to worry 
about. Hey, at least it ain't lead. (The mfp at 6 MeV is 2.5 cm so it 
is slightly better)


>It just occured to me that maybe I haven't made it clear that this is not a
>regular x-ray machine as most people think of them.  The device is a linear
>accelerator, which accelerates electrons to the desired energy, smashes them
>into a metallic target generating high-energy x-rays, and then collects,
>focuses,and collimates the x-rays into a beam that is used to radiate
>cancerous tumors accurately.  The procedure is usually referred to as
>external beam therapy.  The beam that is generated is the one that will
>subject our electronics to x-radiation after having passed through the
>patient.

Of course the vast majority of the x-rays are at lower energies, with 
peaks at the characteristic energies for the target material. In 
fact, chances are that most of the damage is being done by lower 
energy x-rays anyway, as the higher energies are zipping through the 
Si without causing damage. Well, most of them. You might find that 
you only need to shield up to a few hundred kV?  Insert hand waving 
here. Or are the lower energies already being attenuated by the 
machine/patient?


-- 

---
Chris Smolinski
Black Cat Systems
http://www.blackcatsystems.com
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