I am currently working on calculating the air kerma from an X-ray source based on a measured spectrum taken at a distance of 30 cm. However, I’ve encountered a discrepancy of approximately 10% when comparing my calculated air kerma to the experimental values.
I have a couple of questions regarding the simulation:
Should I set the production cutoff for electrons to 1 keV? I intend to ensure that electrons can be generated within the scoring volume. By implementing a higher transport cutoff, they would deposit their energy in line with the kerma definition.
what should be my transport cut-off if my max energy is 60 keV? Can I set 1 Mev as all will deposit??
Would lowering the electromagnetic field fix (EMFFiX) significantly impact my results?
I have attached the relevant inputs for your reference. I would greatly appreciate your insights or any suggestions if I am overlooking something in my calculations.
You can already find some guidance in this and this which is looking at a very similar problems to yours. A discrepancy of 10% with respect to experimental values doesn’t sound too bad. A discrepancy of 10% doesn’t actually sound too bad.
To reply to each of your questions:
As discussed in the EMFCUT entry of the manual: particles with energy higher than production thresholds and lower than transport cutoffs are not transported but still generated (their energy is deposited at the point of production). It is suggested to avoid this situation unless it is really necessary, as particle generation demands a considerable computer time and partially offsets the gain due to a higher transport cutoff. You can indeed check if your result is actually sensitive to changing the production thresholds to low values compared to just keeping them the same as the transport thresholds.
You can set the electron/positron thresholds to just below the max energy, making sure that no generated electrons/positrons leave the simulation volume or cell of the USRBIN scoring. You can check the range of electrons using the estar database.
I don’t believe changing or even using EMFFIX will impact your results, you want to dump the energy of the electrons and positrons on the spot anyway so changing the step length should not matter.