sphere 5um-water.flair (2.1 KB)
My calculation model is as follows：
For a 5um spherical shell, the resulting energy deposition is 130% off：
Did you divide the USRBIN result by the region volume, as it has to be done in the special case of a region mesh (see the manual, note 13 of the USRBIN card)?
You are missing two fundamental points.
First, in case of a region mesh (as the one you’ve asked for), the result given by the USRBIN card is not in GeV/g, rather GeV cm^3 /g, because FLUKA does not know the value of your region volume and so cannot normalize by it. This is what I recalled you above and is indicated in the manual. So, in order to get the average dose in GeV/g over your water layer, it’s definitely necessary to divide your FLUKA result by the volume of the latter.
Second, especially when simulating the 5 micron layer case, the transport thresholds play quite a significant role, and in principle should be lowered as much as possible, not to artificially stop in water secondary electrons that may leave and not deposit all their energy there (in reality, the presence of other materials also matters, since water cannot stay suspended in vacuum). You took the default thresholds of PRECISIO, i.e. 100 keV for electrons, and so got 1.4 E-12 GeV/g as you just calculated above. Making use of the EMFCUT card and setting 1 keV for electrons, which is their minimum limit in FLUKA, one gets 4 to 5 E-13 GeV/g, namely about three times less, due to the increased number of escaping electrons. If you set the electron threshold at 2 MeV, i.e. you do not allow them to leave water (which is an artificial assumption), you will get 4.1 E-10 GeV/g (three orders of magnitude more!), which matches well your MCNP result.
