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Forum: Geometry
Re: Question Creating a helix in Geant 4 (Lech Wiktor Piotrowski)
Re: None Re: Creating a helix in Geant 4 (John Allison)
Re: None Re: Creating a helix in Geant 4 (Lech Wiktor Piotrowski)
Re: None Re: Creating a helix in Geant 4 (Vladimir Grichine)
Re: None Re: Creating a helix in Geant 4 (Lech Wiktor Piotrowski)
Date: 12 Sep, 2013
From: John Apostolakis <John Apostolakis>

Dear Lech,

It may be that you need to write a solid to describe the shape that you need.

Creating a new shape for Geant4 is neither a small nor a very large undertaking - either cobbled-together using similar solids or creating one from scratch.

I suggest to consider whether an existing solid, such as the torus, may offer a reasonable approximation for your measurement - or at least  an estimate which could be useful.  Of course you would need to adapt what you measure, e.g. by only firing optical photons in a particular direction or using a revised distribution of directions.

If this is challenging, or only the full detailed solid can be used, then you should consider creating a class to describe this solid.  The Toolkit developer's guide provides information on how to do this - and you can use an existing solid such as the Tubs or Torus as a starting point in creating the methods which are required.  In this case, of course, you will need to determine
  - whether a position is inside the solid
  - how a straight line 'ray' intersects the surface of the solid - potentially this can be the same underlying method, with a few small customisations for starting inside or outside the solid, or it could be two different methods;
  - the DistanceToIn/Out( G4ThreeVector ) methods can return a very aproximate answer, so long as it is alway conservative (ie an underestimate)
  - the CalculateExtent method is critical, as the Navigator relies on the values it returns.

If you go this way, and create a solid of your own, we can share some programs from our test suite to help you check its self-consistency.

Best regards,
John Apostolakis

On 9/5/13 7:48 AM, Lech Wiktor Piotrowski wrote:

*** Discussion title: Geometry

In my case I test a wavelenghshifting fiber around a plastic scintilator
cylinder. I have to find out the optimal pitch of the helix - the denser
it is the bigger the area for photons to hit, but the longer the fiber
and more absorption in it. So there is an optimum, and the helix is not

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1 None: Re: Creating a helical solid in Geant 4   (Lech Wiktor Piotrowski - 13 Sep, 2013)
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