Scatter interaction flag dependent on production cut

[DavidLeibold]

Description
There are two flags available that denote the number of Compton or Rayleigh interactions that a photon underwent in a phantom, called nPhantomCompton and nPhantomRayleigh. We noticed that the ratio of events marked as scatter by these two flags drops significantly below a certain production cut.

Desktop (please complete the following information):

• OS: Ubuntu 20.04.2 (LTS)
• Gate version or commit hash: v9.1 on vGate
• Geant4 version: 10.7
• Root version: 6.19/02
• compiler version: gcc 9.4.0

Same behaviour is observed in Gate v9.0, by the way.

Minimal example

See the attached main.mac and the GateMaterials.db files.

Expected behavior
The ratio of events flagged by Gate as scatter must not depend on the production cut.

Additional context
To cross check the scatter ratio, we simulated the passing of a pencil beam through a water phantom. Behind the water phantom a CsI detector is placed, and all Hits in that detector volume are stored. The production cut of that detector volume is changed. Any photon that intersects the detector outside a small area around the pencil beam line must be a scattered photon (or a secondary), which in the following will be called "off-centre photons".

This analysis showed that the amount of off-centre photons remained constant for a changing production cut (as expected). However, the number of events flagged as scatter via the nPhantomCompton and nPhantomRayleigh parameters dropped considerably when changing from a production cut of 1 mm to 0.1 mm.

photonID

For a production cut of 10 mm (everything works as expected), all events registered in the Hit file have a value of “1” for the photonID parameter.

For a production cut of 0.01 mm (number of events flagged by Gate is wrong), some events registered in the Hit file have a value of “0” for the photonID parameters, others have value “1”. All events that are correctly flagged as a scatter event have a photonID value of “1”, the ones that are flagged incorrectly all have a photonID value of “0”.

energy spectra

We plotted the energy spectra of the registered photons. First, the total deposited energy for each photon was extracted. Barring different noise due to the different number of events, the shape of the spectra seems to match for the photons that are correctly/incorrectly flagged (see Figure 1 below).

Secondly, we plot the energy spectra of only the first interaction of a photon with the detector (and a production cut of 0.01 mm), now one can indeed see a clear difference between the photons that are flagged correctly/incorrectly (see Figure 2 below). The two sets of events differ by the maximum energy deposited and by their shape as well. The spectrum of deposited energy by correctly flagged events in their first interaction exhibits two peaks, which are in the region of the K-edges of iodine and cesium (33 keV and 36 keV, respectively). Interestingly, the energy spectra for the 1st interaction look the same for a production cut of 0.1 mm (a production cut of 0.01 mm was used in the figures shown).

When changing the production cut in the phantom only (as opposed to the production cut in the detector, as done in the analysis shown in the attached notebook and figures), the number of off-centre photons and photons flagged by Gate as scatter are always in agreement.

Please find attached also a Jupyter notebook that contains the analysis of the data.

Kind regards,
David

Gate_macros_and_Jupyter_nb.zip