60-Co 1.25 MeV gamma-ray irradiation with Geant4-DNA

I am new to Geant4, and I need to reproduce the results from a paper. The paper is “Monte Carlo simulation of ionizing radiation induced DNA strand breaks utilizing coarse grained high-order chromatin structures.” with authors Ying Liang, Gen Yang, Feng Liu, and Yugang Wang [doi:10.1088/0031-9155/61/1/445].

I need to simulate the radiation track structures.

In the paper they indicate that the interaction of primary particles as well as secondary particles with liquid water is done using Geant4-DNA processes.
They build a target box filled with liquid water. Incident particles are generated and interact with the target box. All the events in the scoring region (a cubic box) are recorded. The output data, including particle name, process name, event location, and energy deposition are written in a data file.

In the supplementary data file they give the instructions for the simulation, which I will quote:
“For simulation of a given radiation field, we first build a cubic target box filled with liquid water. This target box defines the simulation boundary, particles escape this volume will no longer be traced. Parallel incident particles are generated at random positions on the upper surface of the target box. The incident particles travel down perpendicularly and interact with the water in the target box. Then we define a scoring region around the center of the target box. All events in the scoring region are recorded. The scoring region is a cubic box. The corresponding side length of the scoring region is 2.5 μm. Dimension of the target box varies with radiation qualities and particle energies. For example, when simulating 60-Co 1.25 MeV gamma-ray irradiation, the size of the target box is 3 mm so that high energy Compton electrons could slow down and electron equilibrium could be established in the scoring region.”

I read the documentation of the extended examples in Geant4 and I think that the dna/microdosimetry [extended/medical/dna/microdosimetry] example is the one that resembles the most to the paper simulation parameters.

I was able to simulate the incident particles starting point and direction of movement.
But I am not able to obtain any gamma interactions with water. When I select gamma as the primary particle, I just obtain a straight line that does not interact with the water in the target box.

I believe the problem is in the PhysicsList.cc file, because I could be omiting some particles or processes, but since I am new to Geant4 I have no clue on where the mistake is.

If anyone knows a better example for me to start with, or if you could help me modifying the microdosimetry one I would appreciate it.