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Question Geometry 'transparent' to particle 

Forum: Geometry
Date: 23 Jan, 2013
From: <pcosmic>

Hello,

I'm new in Geant4 and I would really appreciate it if someone could help me with a weird problem I'm facing. I'm using Geant4 in Visual Studio 2010 Express and my aim is to simulate the operation of a neutron monitor. In order to make some initial tests I define a very simple geometry where a neutron is moving toward a polyethylene block. The result is the reflection of the neutron, as expected. The problem comes up when I define a second polyethylene block just after the first one. When I do that, the neutron passes through the first block like it is transparent and gets reflected on the second block.

I suspected that the problem may be on the random generator. I checked the seed that is automatically defined and I found that it is always set to 9876. I made some tests and I found that if I manually define the seed to something else (for example CLHEP::HepRandom::setTheSeed(0);) just before the "run/beam on" command, the simulation seems to work correctly. I'm not completely sure if the result is correct but at least the neutron gets reflected on the first polyethylene block.

The problem becomes greater when I define the whole neutron monitor architecture. If I don't define manually the seed then the whole neutron monitor seems to be transparent to the neutron. If I define it then many interactions and creations of secondary particles are noticed in the neutron monitor, as expected.

I don't know what I'm doing wrong. In the geant4 examples the seed is not set. I wonder if there is any incompatibility with the visual studio. Thank you in advance for your help.

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