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Forum: Processes Involving Optical Photons
Re: None the local time goes back -- the true step length is negative (khoi nguyen)
Re: Feedback Re: the local time goes back -- the true step length is negative (Gumplinger Peter)
Re: None Re: the local time goes back -- the true step length is negative (khoi nguyen)
Re: None Re: the local time goes back -- the true step length is negative (khoi nguyen)
Re: Feedback Re: the local time goes back -- the true step length is negative (Gumplinger Peter)
Date: 14 Aug, 2012
From: khoi nguyen <khoi nguyen>

hi Peter,

sorry for the very late answer.

the problem was due to the way how i set the property WLSABSLENGTH.

*** short explanation : keyword : spline fit.

*** long explanation : the plot in the attachment should help explain this.

1. i used, say 100 test points, as input to feed the abslength array. within 5sigma around my meanEnergy, the abslength should be gaussian-distributed, outside of that range, i set the value very high just to prevent those photons from undergoing WLS effect. i set the property in 2 ways :

    i. propTab->AddProperty("WLSABSLENGTH",E,absLength,n)

   ii. propTab->AddProperty("WLSABSLENGTH",E,absLength,n)->SetSpline(true)

2. after filling the property, i extracted the abslength with a finer energy array, say with 200 energy-values (i.e. some energies fall in between the energies i had in the 100-point input-array) :

    i. without spline fit --> linear interpolation used --> fine !

   ii. with spline fit --> here comes the problem when my energy is ~5sigma away from the meanEnergy.

--> reason : spline fit fails at the 5sigma tail, sometimes generating negative abslength.

the reason why i didn't see this when testing with optical photons only (mentioned in my very first post in the thread) was that i used an energy falling well within the 5sigma range.

when i turned on scintillation process (again, i used a test emission spectrum), the photons' energy are "more random" and some are around the 5sigma tail. that's why i had the problem.

cheers khoi

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1 None: Re: the local time goes back -- the true step length is negative   (khoi nguyen - 14 Aug, 2012)
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