Monte Carlo Simulation of collimated Sr90 source
A small monte carlo simulation based on GEANT4 has been used to model the energy loss of the electrons from the Sr90 source in the fiber scintillatior. The basic geometry is a copper collimator of 4mm thickness with a slit of 0.5mm width and 2mm or 5mm length. The fiber scintillator is simulated with 1mm diameter with cladding thickness of 6% of the total fiber diameter. The initial energy spectrum of the electrons from the Sr90 source is modeled with a 6th order polynomial giving the event weight. Electrons are generated randomly on a 5mm by 5mm area with a polar angle between zero to 55 degree and azimuthal angle from 0 to 360 degree. The result is shown in the following two plots for a 2mm slit and 5mm slit. The energy spectrum have a 100keV cut on the energy in the trigger scintillator. The screen shot of one simulated event is shown to visualize the geometry of the fiber, collimator and trigger scintillator used in the simulation.
In the example above the collimator thickness is set at 4mm. In the fiber test stand the actual thickness of the collimator is 3.31mm however. This has a substantial influence on the result as is shown below: