[Gate-users] Absorption length in optical transport

[Vesna Cuplov]

Hello,

I did not see this issue while working with optical photons.
I did a test in Gate v6.2 and Geant4.9.5.p01. I use a phantom made of a
material that is assigned an absorption
length of 2mm. The tracking verbose shows correct values for the step
length which average is ~2mm. This is
correct.

The absorption length is the mean (average) path photons will take before
they are absorbed. The GetMeanFreePath
method in G4OpAbsorption returns the mean free path that is stored with
ABSLENGTH in the Materials.xml. But it would
be completely unphysical if they all went exactly 2mm before they get
absorbed. The actual step length these photons
travel is sampled in Geant4 from an exponential in
G4VDiscreteProcess::PostStepGetPhysicalInteractionLength (). The
value of the actual physical step is: -log(G4UniformRand()) x MeanFreePath.

You can take a look at the Physics Reference Manual of Geant4, Chapter3
(Particle Transport).

I hope this helps,
vesna



On Tue, Jan 8, 2013 at 10:11 PM, Abdul Fattah MOHAMAD HADI <
Abdul-Fattah.Mohamad-Hadi at subatech.in2p3.fr> wrote:

> **
> Hello,
>
> In principle, the optical absorption length is the average distance
> traveled by the photon before being absorbed by the  medium. This
> absorption length must be given by the user of GATE in the file
> "Materials.xml". But I think that there is bug in considering the user
> entered absorption length by Geant4. I noticed this problem with GATE 6.1
> compiled with geant4.9.4.p01. In fact, I added the optical properties of
> the the the liquid xenon in the "Materials.xml" file with absorption length
> of  about 35 cm, but in the tracking of Geant4 I see that the track length
> of optical photons before absorption is  of  random values between 1 and 11
> cm !.
>
> Abdul Fattah
>
> Vesna Cuplov wrote:
>
>
> The optical photon travels in the medium through steps. The step length is
> calculated from the probability of interaction based on
> the physics process (absorption, scattering, ....) cross section. The
> process that is applied is the one that produced the minimum
> step length.
>
>
>
> On Mon, Jan 7, 2013 at 9:35 PM, Jenny Nilsson <jenny.nilsson at radfys.gu.se>wrote:
>
>>  Hello,
>> Yes I know. But if, for example, the absorption length is 300 cm then all
>> optical photons will not travel 300 cm before being absorbed, the beam
>> intensity will be 1/e after one absorption length.
>>
>> What I'm interested in is when in the calculations is the distance an
>> optical photon travels determined. Is the distance sampled from a
>> probability distribution before the actual transport begins?
>>
>>  Best regards
>> Jenny
>>
>

-- 
Dr. Vesna Cuplov
CEA - Service Hospitalier Frederic Joliot
Orsay, France
http://v-cuplov.net
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