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Forum: Processes Involving Optical Photons
Re: Question Questions on how use the optical photon (Filippo Ambroglini)
Date: 23 Jan, 2014
From: Gumplinger Peter <Gumplinger Peter>

Dear Filippo,

> I this forum tread
> http://hypernews.slac.stanford.edu/HyperNews/geant4/get/opticalphotons/517.html
> I have found a list of all possible parameter that can be used but in
> any part of the Application guide (the pdf version) I have found an
> explanation of FASTCOMPONENT and SLOWCOMPONENT.

At http://geant4.web.cern.ch/geant4/UserDocumentation/UsersGuides/ForApplicationDeveloper/html/ch05s02.html#sect.PhysProc.Photo

Section 5.2.5.2 reads:

In GEANT4 the scintillator can have a fast and a slow component. The relative strength of the fast component as a fraction of total scintillation yield is given by the YIELDRATIO. ..... It is sufficient to specify in the user's DetectorConstruction class a relative spectral distribution as a function of photon energy for the scintillating material.

Perhaps I could add "..... a relative spectral distribution (FAST/SLOWCOMPONENT) as a function...."

> Looking at the example in the application guide I have made the
> hypothesis that with this parameter we are going to set the Q.E. of the
> material as a function of the various energy is that correct?

Thus, the numbers that go into the arrays for COMPONENT are just the relative strength of the emission spectrum for each bin over the whole spectrum. They don't need to be absolute Q.E.

> My calorimeter is made by several layer of scintillator in the between
> of the layer there will be carbon finer support structure and each layer
> will be packed with "white teflon" except a couple of small area in
> which we will put the APD.

> 1) I have to define something for the carbon finer support structure in
> order to avoid the waste of time on propagating optical gamma on these
> structure

Optical photons will not propagate in any material that you didn't define an index of refraction for. So, when you add the support structure, let's assume it's made from Al and you haven't given Al an RINDEX, op. photons will just 'die' on the surface of the Al. Now, that's not necessarily what you want because your optical system is 'closed' with reflectors and ultimately with detectors. So, you'll define surfaces that enclose your optical system. With these in place, no optical photon will leak into the support structure.

> or I have to take care at the stepping action level like it
> has been done on the LXeSteppingAction.cc?

Sorry, don't know what you refer to.

>     There is way to set for the optical surface of the scintillator this configuration 
>     OpSurface -> SetType(dielectric_dielectric);  
>     OpSurface -> SetModel(unified);
>     OpSurface -> SetFinish(polishedbackpainted);

Yes, you'd want that to define the surface of your scint. layers with the white teflon. Thus, you don't have to implement the teflon itself - as explained in the manual.

>     except for the area where I put the APD detector in which I would like to have 
>     a simple polished surface that will be contiguous with the APD? 

As you may know from reading the manual, OpSurface (G4OpticalSurface) encapsulates the optical properties of a surface. Where that surface actually is in your geometry you need to define with either G4LogicalBorderSurface or G4LogicalSkinSurface. In your case, you must define two G4LogicalBorderSurfaces; one polishedbackpainted between the scint. layer and the volume behind the teflon, and a second one that is between the scint. layer and your APD volume. Next you declare your APD is a 'metal' and thus your OpSurface should be of type 'dielectric_metal'. In addition, you give the APD a reflectivity and, if you like, an 'efficiency' - as is done in the G4 examples.

Peter

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