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Question Cos-squared theta distribution using GPS 

Forum: Particles
Date: 22 Jul, 2017
From: Nan Ma <Nan Ma>

Hi everyone,

I am trying to simulate cosmic ray neutrons but my scoring result does not match the angular spectrum I set up.
I start with assigning a cos-squared theta distribution to the neutron source in vacuum using macro file as following:

## Define the shape of the source
/gps/particle neutron
/gps/pos/type Plane
/gps/pos/shape Rectangle
/gps/pos/rot1 -1 0 0
/gps/pos/rot2 0 1 0
/gps/pos/halfx 0.0001 m
/gps/pos/halfy 0.0001 m
/gps/pos/centre 0 0.35 0 m
#
## Set up the histogram for theta using "/gps/hist/point theta(radian) cos^2"
/gps/ang/type user
/gps/hist/type theta
/gps/hist/point 0.000 1.0000
/gps/hist/point 0.017 0.9997
/gps/hist/point 0.035 0.9988
/gps/hist/point 0.052 0.9973
(omit the rest of the histogram, but I have it to go all the way to theta = 1.571 radian (90 degrees) )

And I set up a scorer following this tutorial http://geant4.slac.stanford.edu/tutorial/mcgill06/HandsOn3/HandsOn3.htm#_Installation
such that I have a hemisphere centered at the source with a radius of 0.5 meter. There are 90 rings scoring the incoming sphere surface current on the hemisphere, so each corresponds to the angle theta with an increment of 1 degree. However, the scoring result does not match the cosine-squared distribution. Here is part of the result of 1e4 neutrons on target:

Theta Scorer 1
0 45.2475
1 8.11681
2 4.85017
3 3.33102
4 2.60347

Can anyone help me out? Any ideas and advice will be greatly appreciated. Thank you!

Nan

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1 Question: Re: Cos-squared theta distribution using GPS   (Vahagn Ivanyan - Jan 02, 18:32)
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