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Forum: Processes Involving Optical Photons
Re: Question trouble with optical properties meaning
Re: Feedback Re: trouble with optical properties meaning (Gumplinger Peter)
Re: Question Re: trouble with optical properties meaning
Re: None Re: trouble with optical properties meaning (Vlasios Vasileiou)
Re: Question Re: trouble with optical properties meaning
Date: 07 Jun, 2012
From: Gumplinger Peter <Gumplinger Peter>

Hi Esther,

> 1) As far as it concerns Rayleigh scattering, which is the name with
> which I enter the Rayleigh attenuation lengths in the material
> properties table? I mean, for absoprtion I have something like:
> 
> myMPT->AddProperty("ABSLENGTH", PhotonEnergies, Absorption, nEntries);
> 
> What word do I have to use for Rayleigh scattering in place of
> "ABSLENGTH"?

"RAYLEIGH"

http://www-geant4.kek.jp/lxr/source/processes/optical/src/G4OpRayleigh.cc#L236

> 2) When I enter Rayleigh scattering attenuation lengths through a
> material properties table, can I use Rayleigh scattering cross section
> or do I have to stick to Einstein-Smoluchowski approximation?

As with ABSLENTH, RAYLEIGH is also a length - you give the mean free path and not the scattering cross section.

> (I think
> if I give those values directly I can use whichever formula I prefer,
> but I just want to make sure I'm doing things right).

Yes, if you provide the Rayleigh scattering length it'll be used as provided.

> 3) I want to make some trials using alternatively only Rayleigh
> Scattering or only Mie scattering, so I need to deactivate one of them
> at the time.

If you wrote the code that attaches processes to optical photons, you could comment out one or the other process from the call list and recompile (or some other logic that allows you to switch) If you use G4OpticalPhysics builder and messenger, you can:

/optics_engine/selectOpProcess OpRayleigh /optics_engine/setOpProcessUse false

or OpMieHG.

> For Mie-HG scattering I think it is enough not to enter its parameters
> in the material properties table: right?

I don't think that is a good idea....

> But what do I have to do for Rayleigh scattering? Because the manual
> says: for a medium named water (which I'm currently using) and no
> rayleigh scattering attenuation length specified by the user, the
> program automatically calls the RayleighAttenuationLengthGenerator...

Yes, that's true. Actually the name 'Water' is your choice. You could use as your water material the Nist definition, G4_WATER, or just name your water 'WATER' and you would not run into this problem. In fact, if you use 'Water' the program assumes that you don't want to provide RAYLEIGH which could be considered convenient.

> Do I just have to write something like
> 
> inactivate/rayleigh
> 
> in my main.cc file? If so, what is the exact syntax?

In the command file, after:

/run/initialize

you can add:

/process/inactivate OpRayleigh

> 
> 4) What happens if I decide to activate both scatterings (Rayleigh and
> Mie) at the same time? Does Geant4 has a cut in energy, a threshold with
> which to choose which one of the two processes suits best for each
> particular case?

No, there is nothing in the G4 kernel that cross checks whether processes are compatible with each other. That's the user's responsibility. You define this by the scattering length (as a function of energy/wavelength). If a process is not applicable at a certain energy you give DBL_MAX.

Hope this helps, Peter

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1 Ok: Re: trouble with optical properties meaning   (esther - 08 Jun, 2012)
(_ Question: Re: trouble with optical properties meaning   (Aram Teymurazyan - 04 Feb, 2013)
(_ None: Re: trouble with optical properties meaning   (Vlasios Vasileiou - 06 Feb, 2013)
(_ None: Re: trouble with optical properties meaning   (Aram Teymurazyan - 06 Feb, 2013)
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