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Forum: Processes Involving Optical Photons
Re: Question GEANT4 optical processes units (Adam Glaser)
Date: 05 Nov, 2012
From: Gumplinger Peter <Gumplinger Peter>

Hello Adam,

> I am very new to GEANT4 and have primarily been using GEANT4 through
> GAMOS. My research is primarily focused on light transport, tissue
> optics, and diffuse spectroscopy. There are a number of MC codes to
> perform tissue optics simulations, but I see a unique opportunity with
> GEANT4 due to many of its already existing features.

I'll be very happy to hear from you (and learn) how one might increase the functionality of optical photon transport in Geant4. It is now mostly the domain of learned users that must drive the continued development of this physics simulation in G4. I shall be happy to assit you and others in doing so.

> I am creating a physics list with modified optical photon processes from
> those already offered in GEANT4 but have a simple usability question. In
> the tissue optics community, optical photons are generally thought of in
> terms of the wavelength (nm or um) and optical absorption/scattering a
> thought of as attenuation coefficients (inverse length) as opposed to
> attenuation lengths.
> 
> Is there anyway that this code be easily integrated into GEANT4's unit
> definitions (i.e., when expected an energy unit, if GEANT4 sees *nm it
> will automatically convert it to *eV, and if GEANT4 sees an attenuation
> coefficient *mm^-1 in the material properties table for optical
> processes, it will auto convert to *mm)?

As you know the material properties are provided to Geant4 in terms of property tables. The existing code is written such that it expects photon energy (and not wavelength) and attenuation lengths (distance). The input to the tables - to the receiving program - are just numbers. The units are just numbers as well, so I don't know how you (the program) can recognize the units. One would have to add something to the class that holds the units alone - do you see what I mean?

At any rate, I think your proposal is a good one. Many users have had to convert their input 'by hand' before entering the numbers in the program. This would surely make it more convenient.

> Would this need to be addressed in GEANT4 or CLHEP?

In Geant4, more specifically (I think) the class G4MaterialPropertiesTable.

What I envision would be to add two class members that hold the units used for the input. If they are not used (default to zero) the class works as before. If however, they are assigned (with a Set-Method), the AddProperty and AddEntry methods would do your conversion.

Would you be willing to give it a go and propose an implementation to me?

Best regards, Peter

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1 None: Re: GEANT4 optical processes units   (Michael H. Kelsey - 06 Nov, 2012)
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