[Gate-users] optical photon transmittance

[Vesna Cuplov]

Dear Devin,

GATE inherits the Optical Absorption physics process from GEANT4. In that
code, the absorption length is the average distance traveled by a photon
before being absorbed by the medium. It is the mean free path that is
returned by the G4 GetMeanFreePath method.

AttenuationLengthVector = aMaterialPropertyTable->GetProperty("ABSLENGTH");

AttenuationLength = AttenuationLengthVector->Value(thePhotonMomentum);
==> From the propertyvector of ABSLENGTH provided by the user,
Value(thePhotonMomentum) is an interpolation to get the attenuation lenght
of that specific photon energy.

Take a look at sections 3.2.1 and 3.2.2 of the following document:

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

These sections show how the mean free path and cross section are related
and how the defined distance (step) to the next interaction
is sampled in a Monte Carlo method from the given mean free path (or cross
section).

What the OpticalAbsorption code from Geant4 provides (either calculated
internally by the code or from a database) is the
average/mean/expectation-value of the distance a particle will travel
before undergoing the interaction.

I hope this helps,

Vesna

Laboratoire Imagerie Moléculaire In Vivo (IMIV)
UMR 1023 Inserm/CEA/Université Paris Sud - ERL 9218 CNRS
CEA/I2BM/Service Hospitalier Frédéric Joliot
4 place du Général Leclerc
91400 Orsay


On Mon, Jul 13, 2015 at 4:19 PM, Devin VanElburg <vanelbud at tbh.net> wrote:

> Hello all,
>
> I am attempting to factor in the transmittance of optical photons through
> different light guides. I suppose I will have to change the absorption
> length parameter in the Materials.xml file, but it's not quite the same. I
> want to be clear with how GATE uses the absorption length. So:
>
>       <propertyvector name="ABSLENGTH" unit="m" energyunit="eV">
>         <ve energy="1.84" value="50"/>
>         <ve energy="4.08" value="50"/>
>       </propertyvector>
>
> Does this mean that for an optical photon of 1.84eV, it will travel about
> 50m before it is absorbed? Is this an average depth value? Is it based on a
> Gaussian probability distribution?
>
> Transmission of optical materials is normally given as a fraction of the
> incident light, at a given wavelength of light and for a given thickness of
> material. For example, say 92% transmittance of 405nm photons for a 1.0mm
> thick lead glass lightguide.
>
> Can anyone offer any insight as to how GATE uses these ABSLENGTH values?
>
>
>
> Cheers,
> Devin Van Elburg
> ------------------------
> Physics w/ c/ Biomedical Physics
> Lakehead University
>
>
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