Behavior of DigitizerAdderActor across multiple sensitive volumes?

[jwonl]

I am writing hoping that you can help me understand the role of group_volume in DigitizerAdderActor.

I am trying to create a simulation for a two-detector Compton camera, where each detector is pixellated. I want to use DigitizerAdderActor in combination with the DigitizerHitsCollectionActor, in order to collect all singles event across different detector pixels. Pixellated detectors are modeled with gate.geometry.utility.get_grid_repetition. The question is, when the DigitizerAdderActor's group_volume attribute is the mother volume containing these two pixellated detectors, I get singles with positions that are energy weighted across the entire mother volume, not within each individual pixel. However, when the group_volume is one of the two detectors' name, it works as intended, meaning that I get energy weighted singles from both detector pixels. Below is the minimum reproducible code.

import opengate as gate
import uproot
if __name__ == "__main__":
    sim = gate.Simulation()
    sim.random_seed = 42

    sim.g4_verbose = True
    m = gate.g4_units.m
    cm = gate.g4_units.cm
    mm = gate.g4_units.mm
    sec = gate.g4_units.second

    MeV = gate.g4_units.MeV
    Bq = gate.g4_units.Bq
    
    # --- geometry (toy example) ---
    sim.volume_manager.add_material_database("./GateMaterials.db")
    world = sim.world
    world.size = [100.0 * cm, 100.0 * cm, 100.0 * cm]  # cm
    world.material = "G4_AIR"
    world.color = [1, 1, 1, 1]

    cc_lower  = gate.geometry.volumes.BoxVolume(mother=world, name="cc_lower", size=[120.0*mm, 40.0*mm, 120.0*mm], material="G4_AIR")
    cc_lower.translation = [0.0 * mm, -31.0 * mm, 0.0 * mm]
    sim.add_volume(cc_lower)
    cc_lower.color = [1, 1, 0, 1]

    lower_absorber = gate.geometry.volumes.BoxVolume(
        mother=cc_lower,
        name="lower_absorber",
        #size=[100.0*mm, 6.0*mm, 100.0*mm], 
        size=[1*mm, 6*mm, 1*mm],
        material="G4_Si"
        )
    

    lower_absorber.translation = gate.geometry.utility.get_grid_repetition(size=[100, 1, 100], spacing=[1 * mm, 1*mm, 1 * mm], start=[-49.5 * mm, -12.5 * mm, -49.5 * mm])
    sim.add_volume(lower_absorber)
    lower_absorber.color = [1, 0, 0, 1]

    lower_scatterer = gate.geometry.volumes.BoxVolume(mother=cc_lower,
                                                    name="lower_scatterer",
                                                    size=[1*mm, 6*mm, 1*mm],
                                                    material="G4_Si"
                                                    #   size=[100.0*mm, 6.0*mm, 100.0*mm],
                                                    #   material="CZT"
                                                    )
    lower_scatterer.translation = gate.geometry.utility.get_grid_repetition(size=[100, 1, 100], spacing=[1 * mm, 1*mm, 1 * mm], start=[-49.5 * mm, 12.5 * mm, -49.5 * mm])
    sim.add_volume(lower_scatterer)
    lower_scatterer.color = [1, 0, 0, 1]
    
    sim.physics_manager.physics_list_name = 'G4EmStandardPhysics'

    sim.physics_manager.set_production_cut("world", "electron", 0.01 * mm)
    sim.physics_manager.set_production_cut("world", "gamma", 0.01 * mm)

    sim.physics_manager.set_production_cut("cc_lower", "electron", 0.01 * mm)
    sim.physics_manager.set_production_cut("cc_lower", "gamma", 0.01 * mm)

    
    # --- visualization switches ---
    sim.visu = True           # turn on visualization
    sim.visu_type = "qt"      # native Geant4 Qt + OpenGL viewer

    source = sim.add_source('GenericSource', 'point_source')
    source.position.type = "point"
    source.particle = 'gamma'
    source.position.translation = [0 * mm, 0 * mm, 0 * mm]
    source.direction.type = "iso"
    source.energy.type = "mono"
    source.energy.mono = 0.4405 * MeV
    source.activity = 1e5 * Bq

    sim.run_timing_intervals = [
    [0* sec, 1 * sec],

]
    
    lower_hits_actor = sim.add_actor('DigitizerHitsCollectionActor', 'Lower_Hits')
    lower_hits_actor.attached_to = ['lower_scatterer', 'lower_absorber']
    lower_hits_actor.output_filename = 'lower_test_hits.root'
    lower_hits_actor.attributes = ['UnscatteredPrimaryFlag', 'EventKineticEnergy', 'KineticEnergy', 'PostKineticEnergy', 'PreKineticEnergy', 'TotalEnergyDeposit', 'TrackVertexKineticEnergy', 'PostPosition', 'PrePosition', 'Position',
                    'GlobalTime', 'PostStepUniqueVolumeID', 'PreStepUniqueVolumeID', 'RunID', 'ThreadID', 'TrackID', "TrackVolumeName", "EventID", 'ParentID', 'ParentParticleName', 'ParticleName', 'ParticleType', 'TrackCreatorProcess']
    
    stats = sim.add_actor('SimulationStatisticsActor', 'Stats')

    adder_lower = sim.add_actor("DigitizerAdderActor", "Singles_lower")
    adder_lower.output_filename = 'test_singles_lower.root'
    adder_lower.input_digi_collection = "Lower_Hits"
    adder_lower.policy = "EnergyWeightedCentroidPosition"
    adder_lower.group_volume = cc_lower.name

    sim.run(start_new_process=True)
root_file = uproot.open("/Users/xxx/Research/opengate/scripts/test_singles_lower.root")
singles_tree = root_file["Singles_lower;1"]
singles_data = singles_tree.arrays(library="pd")

singles_data.groupby("EventID").nunique().max()

Basically, the above code yields singles that are all aggregated, such that singles_data.groupby("EventID").nunique().max() value just gives 1. However, when I change from cc_lower.name to lower_scatterer.name, I get singles in each pixels of not only the lower_scatterer, but lower_absorber as well. What is the role of group_volume and why does it act radically differently depending on how I set it?