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Forum: Geometry
Re: None What is considered an overlap in geometry? (Svetlana Shasharina)
Re: Idea Re: What is considered an overlap in geometry? (Michael H. Kelsey)
Date: 17 Apr, 2016
From: Svetlana Shasharina <Svetlana Shasharina>

It is all doable with simple shapes. How do I place inner/outer vacuum if I have tessellated solids from CAD?  

iSveta

> On Apr 17, 2016, at 6:30 PM, Michael H. Kelsey <kelsey@slac.stanford.edu> wrote:
> 
> 
> *** Discussion title: Geometry
> 
>> On Sun, 17 Apr 2016 19:28:21 GMT, Svetlana Shasharina wrote:
>> 
>> I have been doing simple test runs calculating fluence of gammas on a
>> hollow Al sphere irradiated by electrons. To my surprise, the gamma
>> fluence to the sphere was not zero. I assume now that gammas come from
>> the sphere from the inside and get into the sphere again (if I make the
>> sphere solid, the fluence becomes 0).
>> 
>> If I add Va volumes just outside of the sphere (1mm off compared to 30cm
>> radius of the sphere) and just inside the sphere (1mm off again), I see
>> that almost all gammas coming from the sphere enter the vacuum volumes
>> (the fluence from the sphere is close to the sum of fluences to the
>> vacuum shapes). The number is the same as in the simulation with just Al
>> sphere. If I make vacuum volumes match the sphere radius, the gamma
>> fluence comes to zero.
>> 
>> Is it ok to make one shape have outer radius be equal to the inner
>> radius of another shell? Why the results are so different compared to
>> the case with a little offset?
> 
> How are you counting fluence? From what you describe, it sounds like
> you're using a stepping action to count boundary crossings from World
> into your Al sphere. That would exactly explain what you see: the hollow
> sphere by itself has _World_ in the inner hollow, because that hollow
> isn't part of the aluminum ball. When you place some other material into
> that hollow region, it's a sibling volume to the Al ball, with a
> possibly matching surface.
> 
> To avoid your counting problem without having to write different code,
> you can do what you've done, or you can make your Al sphere solid, and
> then place another vacuum sphere inside it as a daughter volume.
> 
>  -- Michael Kelsey
> 
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