Hello Federico,
In addition to Thomas’ answers which well describes some critical aspects in the setup of the biasing scheme for your current study, I would like to propose you another approach which does not require any biasing technique.
I used the input file you have provided (thanks!) to do a quick simulation. After removing the SOURCE card which was interfering with the angular description of the photon beam, I have scored the current of photons entering the detector region TK30_Vol: ~2.5E-03 particle/primary reach the detector while many are lost due to the aperture of the beam and the presence of the collimator.
This implies that 1 primary out of ~400 would reach the detector itself (and possibly deposit energy), and therefore a lot of simulation time is actually spent in tracking uninteresting photons.
What I would suggest is to follow a 2-steps simulation (more details at this link) instead.
The idea would be the following:
- Step 1. Starting with the 300 keV beam, the phase space of the photon reaching the aperture of the collimator is scored for the different Al thicknesses of interest.
- Step 2. Starting with primary the scored photon phase space, you track them to score the energy deposition in the detector.
As usual in these two steps simulation approach, attention should be paid to the normalization of the results of the second step. More information in the link posted before.
Let me know if this could be a valuable solution to your problem,
Giuseppe