[Simulation question]: Issue with Generating Parallel Beam from a Ring Source in TOPAS

[SFR1993]

Simulation question

Hi everyone,

I am new to TOPAS simulation and have encountered an issue with generating a parallel beam from a ring source. Here’s how I defined the source:
#######################

Source Geometry

s:Ge/RingSourceVolume/Type = "G4Torus"
s:Ge/RingSourceVolume/Parent = "World"
s:Ge/RingSourceVolume/Material = "Vacuum"
b:Ge/RingSourceVolume/IsActiveSource = "True"
d:Ge/RingSourceVolume/RTor = 8 um
d:Ge/RingSourceVolume/RMax = 0.01 um
d:Ge/RingSourceVolume/TransZ = 10 um
Source

s:So/RingGammaSource/Type = "Volumetric"
s:So/RingGammaSource/Component = "RingSourceVolume"
sc:So/RingGammaSource/ActiveMaterial = "Vacuum"
s:So/RingGammaSource/BeamParticle = "gamma"
d:So/RingGammaSource/BeamEnergy = 1.25 MeV

s:So/RingGammaSource/BeamPositionDistribution = "Flat"
s:So/RingGammaSource/BeamAngularDistribution = "None"

s:So/RingGammaSource/BeamDirectionType = "FixedDirection"
u:So/RingGammaSource/BeamDirectionX = 0
u:So/RingGammaSource/BeamDirectionY = 0
u:So/RingGammaSource/BeamDirectionZ = -1

i:So/RingGammaSource/NumberOfHistoriesInRun = 1000
#######################
However, when I ran the simulation, the beam radiated in all directions instead of forming a parallel beam.
I also tried this configuration:

######################

Source Geometry

s:Ge/RingSource/Type = "TsCylinder"
s:Ge/RingSource/Parent = "World"
sc:Ge/RingSource/Material = "Vacuum"
dc:Ge/RingSource/RMax = 8 um
dc:Ge/RingSource/RMin = 7 um
dc:Ge/RingSource/HL = 0.01 um
dc:Ge/RingSource/TransZ = -10 um
b:Ge/RingSource/IsActiveSource = "True"

Source Setting

s:So/GammaSource/Type = "Beam"
s:So/GammaSource/Component = "RingSource"
s:So/GammaSource/BeamParticle = "gamma"
dc:So/GammaSource/BeamEnergy = 1.25 MeV

s:So/GammaSource/BeamPositionDistribution = "Flat"
s:So/GammaSource/BeamPositionCutoffShape = "Ellipse"
dc:So/GammaSource/BeamPositionCutoffX = 8 um
dc:So/GammaSource/BeamPositionCutoffY = 8 um
s:So/GammaSource/BeamAngularDistribution = "None"
s:So/GammaSource/BeamDirectionType = "FixedDirection"
uvc:So/GammaSource/BeamDirection = 3 0 0 -1

b:So/GammaSource/OverrideDirection = "True"
b:So/GammaSource/OverridePosition = "True"
b:So/GammaSource/Verbosity = "True"

i:So/GammaSource/NumberOfHistoriesInRun = 1000
#########################
However, beams is emitted from the entire surface of the circle, not just from the ring!
Could anyone help me figure out what I might be doing wrong? I’d really appreciate any advice or suggestions.

Thanks in advance!
Looking forward to your response.

TOPAS-nBio category

Geometry

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