How to use GPS and sample particles' positions from subtracted volume?

Particles, Track, Event, Run and Biasing
1

Hey Everyone,

I am able to sample my particles from regular shapes (sphere, rectangle etc, via GPS defined in macro). How can I randomly sample particle’s positions from a custom physical volume defined in DetectorConstruction?

My physical volume of interest is detector volume/shape subtracted from a large sphere; to replicate random generation of events around the detector. Also let me know if there is a better way to implement this? I know I can take care of this in post analysis but wondering if G4 can already take care of this?

2

https://geant4-userdoc.web.cern.ch/UsersGuides/ForApplicationDeveloper/html/GettingStarted/generalParticleSource.html

you could sample from a larger volume (e.g., the whole sphere you subtract the detector shape from), and then apply /gps/pos/confine to the physical volume.

3

@weller : Thanks, based on your suggestion I tried the following. To make it simplistic, I am interested in sampling/confining the events to 1-cm annular region (subtracted volume) of two spheres:

//Subtraction of two spheres, giving 1-cm annular region
G4VSolid*
Sphere_out = new G4Sphere("Sphere_out",0.*cm, 10.*cm,0.0*deg,360.0*deg,0.*deg,180.*deg); // outside sphere

G4VSolid* Sphere_in = new G4Sphere("Sphere_in",0.*cm, 9.*cm,0.0*deg,360.0*deg,0.*deg,180.*deg); // inside sphere

G4SubtractionSolid *Sphere_net = new G4SubtractionSolid("SphereNet",Sphere_out,Sphere_in); // sample subtracted volume
fSS_sphere_logic = new G4LogicalVolume(Sphere_net,fWorldMater,"SphereNet");
                   new G4PVPlacement(0,                         
                                     G4ThreeVector(0.,0.,0.0*cm),
                                     fSS_sphere_logic,
                                     "SphereNet_Phys",
                                     fWorldLogical,
                                     false, 
                                     0,
                                     checkForOverlaps);

Then I use the following in macro, note that the dimension/position of this sphere matches to the outer sphere as you mentioned.

/gps/pos/type Volume
/gps/pos/shape Sphere
/gps/pos/centre 0. 0. 0. cm
/gps/pos/radius 10 cm
/gps/pos/confine SphereNet_Phys

Plotting the initial position of each event, however, still gives me a 10-cm sphere, not the annular region. What am I doing wrong?

positions
positions794×1367 108 KB

4

hm, that’s something for the experts then. maybe it only checks if it is within the outmost envelope of the phys volume!?

if it is just the hollow sphere, have you tried /gps/pos/inner_radius ? (or confine to a hollow sphere with min radius >0, no need for subtraction there anyways) in principle confining to the subtraction solid would be useful…

„Sets the inner radius [default 0 cm] for annuli. The units can only be micron, mm, cm, m or km.“
Don’t know if for the (hollow) sphere it works the same way.

5

@weller: Yeah, so it is just sampling everything that is inside of the outer envelope. I wanted to restrict that to region between inner and outer envelope.

I tried Annulus with gps/pos/inner_radius but that’s a planar source! It acts like a ring, not a spherical shell. There isn’t a volumetric version of that as far as I see.

I will wait if anyone else has got any suggestions to take care of this.

6

this is the code that is executed when generating a confined primary position:

no trace of any envelope, just the Navigator checking if the point is inside or outside. So even with a hollow physical volume, this should work.

are your image slices or projections? :wink:

7

Thanks for sharing. The images are starting position of each event that I exported in post-analysis.