"Step Length of Energy Deposit are Illegal" error

_Geant4 Version:_11.1
_Operating System:_Windows Subsystem for Linux
_Compiler/Version:_gcc 13.1
_CMake Version:_3.16.3

Hi! I was trying to simulate a Wavelength Shifting Fiber of cylindrical shape which has two PMTs attached to it on both ends. The geometry is shown in the bottom picture. However, each time I run the simulation, I get the following error at least more than 10 times for each run:

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : TRACK001
issued by : G4VParticleChange::CheckIt()
Step length and/or energy deposit are illegal
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------

My Geometry:

It doesn’t affect my compilation or the simulation, but I want to understand the reason and effect of this error on the results. My WLS layer thickness is about 25 micron and the absorption lengths of the material are 0.5 - 15 micron, depending on photon’s energy. However, if I make the absorption length shorter to make more photons get absorbed by the WLS layer, the frequency of the error increases and very surprisingly, I get less photons in my PMTs although this time a large number of photons were absorbed by the material. Does this have something to do with the error I am getting?

I am not sure about your error. But is your primary particle photons? If so, you may be counted primary particles in your PMTs. So absorbing more in your fiber would reduce the number on your PMT. Are you simulating WLS photon production?

Hi! Thank you again for your reply. Yes, my primary particles are photons and some of them directly make it to the PMTs. Also, yes, I am simulating WLS photon production and the green fiber is the WLS fiber. I was trying to make the absorption length shorter to make more photon getting absorbed in the fiber, but I’m getting this error. Also, with shorter absorption length, it seems like photon are getting re-emitted in the upward direction instead of isotropically, as you can see in the figure (red tracks are the ones that got re-emitted after absorption). Could it be due to the error or you have any thoughts on this?

Is your fiber wrapped in WLS layer? A thickness of 25 microns is very small. That also means that all of your photons will be absorbed at the surface of your detector, and 50% of them will emit away from your detector. Does your WLS layer and the rest of your detector have the same index of refraction? It is possible that most of your photons that emit towards the middle of your detector reflect back out and away.

I am not sure what is causing the error, but it might be due to the small distances you are using for absorption and energy deposition.