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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: 18 Apr, 2016
From: Svetlana Shasharina <Svetlana Shasharina>

Hi Michael,

Several questions:
1.  What is the counting problem?  I do not see the problem in the fact 
that we count gammas entering the hollow sphere even if they originated 
in it.  Or maybe you refer to the fact that gamma fluence becomes zero 
as as soon as I add Va volumes (with matching boundaries) outside and 
inside the hollow sphere?
2.  A more general question.  How close can I make volumes to each 
other?  For example, if I have a double shielding of Al and Ta, can I 
make one extend to x=x0 and the other to start from x=x0?
3.  What does Geant4 if it sees overlapping (not just in boundary) 
volumes?  Is it when it sends messages about the stuck tracks?  Such 
simulations take forever but they do end and produce results.  Can I 
believe them?

Thank you,
Svera

On 4/17/16 6:30 PM, Michael H. Kelsey 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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