Mismatch between deposited energy and optical photons

Physics Processes, Models and Cross Sections
1

Hi guys,
there is something I can’t understand concerning optical photons production and deposited energy.
I have a detector composed of a tower of many plastic scintillators:

  • each plane is square-shaped with 1 cm thickness
  • each plane includes two PMTs placed at the opposite corners.

In my simulations, I’m storing as an output the energy deposited in each plane, and the photo-electrons collected in each PMT (scintillator efficiency, attenuation length, PMT quantum efficiency, etc. are set according to the datasheets of the materials/components).

I have run several simulations with monochromatic proton energies (ranging from 40 MeV to 250 MeV), impinging in a specific point close to the plane’s center.
For each energy and for each plane, I have the distributions of p.e. and deposited energies. I’m fitting those distributions with a gaussian and using the mean values to obtain a p.e. vs MeV plot (on the Y-axis there is the geometric mean of the photo-electrons counted in the two PMTs of the same plane, on the X-axis the deposited energy in that plane). The resulting plot is fitted with a Birk’s function [0]*x/(1+[1]*x, with [0] and [1] free parameters. As you can see from the first figure (left), there are a lot of points where the p.e. number seems to be smaller than expected.

Investigating those points I discovered that they are always related to the plane in which the particle is stopped (i.e. where the Bragg peak is present and the energy loss is higher). In fact, if I remove the plane where the particle is stopped for each energy, the result is the one in the second figure (right):

Schermata da 2021-09-03 17-21-07
Schermata da 2021-09-03 17-21-071029×296 21.7 KB

Can someone pls explain to me why with higher energy loss, the collected photo-electrons don’t seem in agreement with the expectation? I am neither a geant4 expert nor the person who wrote the code I’m using… is there any parameter that I should check to solve this? I’m thinking for example about some thresholds related to the generation of optical photons …
Thank you very much for any help.

ps. Geant-4 version is 4.10.01.p03

2

It’s a complicated geometry and analysis so it’s difficult to say much, at least for me. You may need to break the problem down.

Some random observations:

  • how (what process) generates the photoelectrons? You have protons that generate optical photons via scintillation–then what?
  • optical photons are generated and transported without regard to energy conservation
  • how do you get the error bars in the plot? There’s really only 1 point that doesn’t fit in the left plot.
  • how do you know you’re collecting all the photoelectrons?