Difference in low energy physics g4.9.0 -> g4.9.2

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Hi,

I have been working for several months with geant4.9.0.p01. Recently I installed geant4.9.2.p02, together with its new data libraries and I see some significant differences between both versions that I do not quite understand. To show the differences, I have to first explain a bit how is the system that I am simulating.

I am simulating a detector to measure the polarization direction of hard X-rays (50 keV to 500 keV). The instrument consists basically of a plastic scintillator target divided into almost 2000 scintillator bars. Each incoming X-ray photon interacts with the target exciting one or more of the scintillator elements.

Since the direction of scattering of the photons in the target depends on their polarization direction, it is possible to measure the polarization of the incoming flux by looking at the angular distribution of the hits. To do so, for each incoming particle we select the two largest energy depositions and calculate the azimuthal angle between the positions of the two corresponding scintillator elements (defined respect to the X-axis of the detector).

So, with geant4 I defined my detector and I used the low energy electromagnetic libraries that describe polarized low energy Compton effect, low energy photoelectric effect, and e- multiple scattering. I have an input file that contains the information on the incoming photons (their energy, momentum, initial position, and polarization direction). Then I introduce this input file in my simulation, compiled either using g4.9.0 or g4.9.2. The detector geometry is exactly the same in both cases, only the version of geant4 is changing. The software used to analyze the output file is also the same in both cases. This software first of all sums per each event all the energy depositions inside the same bar, and then orders the hits in descendent value of deposited energy. Then it calculates the distribution of scattering angles between the two largest energy depositions.

What I observe is the following: the angular distribution of the two largest energy depositions per event is changing dramatically from g4.9.0 to g4.9.2. The amplitude of this modulation curve increases far above the statistic uncertainty. The attached figure 1 shows the modulation curve obtained with g4.9.0 (blue line) and g4.9.2 (red line).

I have looked at many other parameters (energy released, distance between the selected hits, position of the hits, etc, and I do not see anything that justifies the striking difference in this angle.

Looking event per event at the verbose output of both simulations I observed that in the old version very often the energy deposited in the step was directly deposited by the incoming photon, whether in the g4.9.2 version I always see a recoil electron that deposits the energy in the step. I am not sure if this is at all related with the difference in the final output. But it seems to me that some difference must have been introduced that changes the energy distribution along the multiple interactions of the photon in the detector. Maybe some cross sections for Compton scattering or for multiple scattering have been modified, so that the two largest energy depositions are now more correlated with the incoming photon polarization.

I would like to understand which of the changes between g4.9.0p01 and g4.9.2p02 could explain the difference on my output. I would be very thankful if somebody could give me some indication.

Thank you very much in advance, Estela.

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2009/10/19/11.45-54520-fig1.pdf