Hi all,

I have a question about some options in the gps class.
If I write:

/gps/pos/shape Circle
/gps/pos/centre 0. 0. 0. mm
/gps/pos/radius 1. mm
/gps/pos/sigma_r .2 mm
/gps/direction 0 0 1

what is the physical meaning of sigma_r? How can I imagine my beam? And what changes if I write /gps/pos/sigma_x(or y)?

Thanks

mkelsey
January 20, 2023, 7:25pm
#2
The “sigma” arguments represent the “spread” of the beam, as a Gaussian distribution in the given direction.

Hi @mkelsey ,

Thanks. Ok, I imagined the answer was this one.

So, if I put sigma_r =0 I can simulate an ideal collimated beam. Is it right?

If sigma_r is different from 0 I should imagine my radius r in the range [r - sigma, r + sigma]. Is it right?

mkelsey
January 20, 2023, 8:23pm
#5
Yes, exactly so.

You can imagine it that way, but not exactly. Most (68%) of your beam will be contained in that annulus, but there will be tails (16% each) out to larger and smaller radii. The distribution will be a Gaussian, centered on the `/gps/pos/radius`

value.

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