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Forum: Processes Involving Optical Photons
Re: Question trouble with optical properties meaning
Date: 22 May, 2012
From: Gumplinger Peter <Gumplinger Peter>

Hi Ester,

Please be advised that any and all novice and basic examples are for pedagogical purposes only - showing how to write code - and that details of the implementation such as material properties, in particular, must be checked and rechecked by an application developer. In some cases, they may be the best of what is known, coming from some seasoned simulation application, but in many cases the actually numerical values are mere place holders; e.g. the source code of an example must not be construed as replacing a material reference book! (this is also true for extended examples and even advanced examples should be thoroughly scrutinized in this respect)

> My first problem is the dependence on energy of the index of refraction:
> I tried to use a few formulas reported for example in "Bashkatov, Water
> refractive index in dependence on temperature and wavelength: a simple
> approximation", but I get totally different results from those in
> exampleN06. Can anyone tell me where those numbers come from?

If I recall correctly, the Super-Kamiokande simulation. Now, I am not saying that these are the official numbers SK is presently using, or have used in the past in their simulation. I am not a SK collaborator. You may find a publication by SK detailing their water's optical properties which they have surely measured.

> Then there is rayleigh scattering length, which is calculated, I
> suppose, using Einstein-Smoluchowski formula, that expresses a relation
> between diffusion coefficient and temperature, therefore to get the
> scattering coefficient we use:
> 
> D = 1/(mu'_s + mu_a)
> 
> where mu_a is the inverse of the abs length previously discussed, and
> mu'_s = (1 - g)mu_s, right?
> 
> So what do I get from Einstein-Smoluchowski; mu_'s or mu_s? Or am I
> completely wrong?

In G4 the bulk absorption and the Rayleigh scattering are two independent processes. The Rayleigh scattering attenuation coefficient is calculated from the Einstein-Smoluchowski fromula which is (9.118) in my edition of J.D. Jackson's Classical Electrodynamics book.

> mu'_s = (1 - g)mu_s, right?

What's your little g?

> The last parameters are related to Heyney-Greestein approximation of Mie
> scattering: I couldn't find any publication for Mie scattering length,
> and I really don't get the meaning of the three parameters
> MIEHG_FORWARD, MIEHG_BACKWARD, and MIE_FORWARD_RATIO. I know they are
> the parameters which enter in formula (13.5) of the Physics reference
> manual, relative to the final differential cross section for Mie
> Scattering, but still can't get their meaning. I mean: isn't any
> material characterized just by ONE anisotropy factor, that tells about
> any preferential direction of emission? Then, what are g_f and g_b?

Please, see: http://cern.ch/geant4/UserDocumentation/UsersGuides/PhysicsReferenceManual/fo/PhysicsReferenceManual.pdf

and the references therein and:

http://www.hep.princeton.edu/~mcdonald/neutrino/Milagro_Simulation.ppt

The implementation is thanks to Xin Qian (Kellogg Radiation Lab of Caltech) based on work from Vlasios Vasileiou (University of Maryland) using the Henyey-Greenstein phase functions approach.

g_f and g_b are respectively the asymmetry factors for forward scattering and backscattering and the ratio is the partitioning coefficient between forward and backward scattering.

Best regards, Peter

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1 Question: Re: trouble with optical properties meaning   (esther - 24 May, 2012)
(_ None: Re: trouble with optical properties meaning   (Vlasios Vasileiou - 29 May, 2012)
(_ Question: Re: trouble with optical properties meaning   (esther - 07 Jun, 2012)
(_ Feedback: Re: trouble with optical properties meaning   (Gumplinger Peter - 07 Jun, 2012)
(_ 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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