|Message: Re: the local time goes back -- the true step length is negative||Not Logged In (login)|
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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 :
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.
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