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Forum: Processes Involving Optical Photons
Re: None Bug in the production of Cherenkov photons? (Erik Dietz-Laursonn)
Date: 16 Mar, 2015
From: Gumplinger Peter <Gumplinger Peter>

> 2) When simulating a material with a refraction index that is rising
> linearly with rising energy (in my case from 1.125 at 1eV to 1.8 at
> 15eV), everything is fine and the result is a Cherenkov spectrum which
> shows a decreasing number of photons with decreasing energy/refraction
> index and which stops at the energy then the refraction index falls
> below the critical value (n < 1/beta).

This is a normally dispersive medium. The index of refraction must not change linearly with rising energy so long as it is monotonically.

> 3) When simulating the refraction index the other way round (in my case
> decreasing from 1.8 at 1eV to 1.125 at 15eV), one would expect the same
> results as before (just the other way round). But the simulation shows a
> different behaviour: For small beta values (the refractive index falls
> below the critical value), no photons are produced. For larger beta
> values (the refractive index is always above the critical value),
> photons are produced, but they are distributed like in the case of an
> energy-independent refraction index.

I don't know of any transparent medium with the property dn(e)/de < 0

> I looked into the G4Cerenkov.cc and I think there is mistake in the
> usage of G4MaterialPropertyVector::GetMinValue() and
> G4MaterialPropertyVector::GetMaxValue():
> The names of the functions suggest that they return the minimal/maximal
> property (-> refractive index) value that is specified in the
> G4MaterialPropertyVector. In that case, the line "if (nMax <
> BetaInverse) -> no photons generated" would be correct. (No Cherenkov
> photons, if the maximal refractive index is below the critical value)
> But actually, these functions just return the first/last entry and not
> the maximal/minimal one

You are correct. I agree that the naming of the methods suggests differently. Yet the implementation simply takes the last entry in a table with increasing energy entries as the maximum. For a normally dispersive medium the index of refraction will be maximal at the highest energy entry.

> (I think the G4MaterialPropertyVector class was
> change in the last few years. Maybe before that, the functions did what
> there names suggest.).

No, the code has always been like this - even already in the old Geant3 source.

I don't know how realistic it is to generate Cerenkov light in material with absorbtion bands etc.

Perhaps, I should clarify in the documentation that the generation of Cerenkov photons in Geant4 is for 'normally dispersive media'.

To find the maximum in a list of numbers is certainly more time consuming than to simply take the last one. I am not sure that doing so for the vast majority of cases where it's not really required warrants changing the code.

Do you have a use case?

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1 None: Re: Bug in the production of Cherenkov photons?   (Erik Dietz-Laursonn - 15 Apr, 2015)
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