Even small overlaps might ruin the stepping

Geometry, Fields and Transportation
1

Hi everyone,

we had a problem when calculating the number of particles (geantinos) hitting some volumes.

We created an isotropic homogeneous radiation field inside a sphere. (For space simulations.)

Based on the descriptions about GPS, the following macro seemed appropriate:

/gps/particle geantino
/gps/pos/type Surface
/gps/pos/shape Sphere
/gps/pos/radius 250 mm
/gps/pos/centre 0 0 0 mm
/gps/ang/type cos
/gps/ang/mintheta 0 deg
/gps/ang/maxtheta 90 deg

However when checking the number geantinos hitting some (geometrically) identical volumes with different positions, we’ve got very different hit numbers. (E.g. 1925, 2197, 2092, 692, 755, 766.)

Our geometry was in GDML converted from CAD files, thus there were tiny overlaps between the volumes. (Every volumes’ mother was the world volume, there were no subvolumes.)

Note: our geometry consists of thin Si detectors with the necessary (simplified) electronics+casings. Its in the order of 10-20 cm in total (the smallest components has a size of a few cm), and the maximal overlaps was around 500 microns.

2

After removing every volumes except for which we’ve calculated the number of hits, thus removing any overlaps (the analysed volumes were well separated), everything seemed to be fine.

We’ve got the following hit numbers (for 10 times more simulated geantinos than the previous run):
24783, 24983, 24980, 25055, 24864, 24975

1 Like
3

Great. Looks like you have figured this out and tested it yourself. I used

# Spherical cavity
/gps/pos/type Surface
/gps/pos/shape Sphere
/gps/pos/radius {cavityRadius}
/gps/ang/type cos

so I don’t think you need min/maxtheta - but OK.

It gives you an isotropic homogenous flux. N events gives you a fluenc of…have you worked that one out yet? I would have to dig into deep history to remember how to do that.

John

4

Thanks, it’s OK. The actual fluence is not important at the moment, but I can figure it out when needed. (I’m using geantinos to calculate some shielding values.)

5

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