Dear Andrea,
Thanks for providing the additional information on the experimental measurement you want to address with your simulation.
The settings required for the correct scoring of air kerma have been discussed in other posts on this forum, e.g. post1, post2.
There are different approaches to evaluate the air kerma in a FLUKA simulation. Looking at your input file, the DOSE deposited in your detectors regions is scored with USRBIN by Region (remember to normalize the results with the volume of the region). Since the kerma is the “kinetic energy released in matter” you must make sure that no kinetic energy in form of electrons and positrons can escape the scoring region.
To do so, you can set the transport cutoff for e-/e+ to a high value with EMFCUT card. It is good practice to do the same then with the PROD-CUT cutoff for e-/e+ but for this you will need to create a new AIR material for the detectors regions and apply the cut on this new material.
Hope this help,
Giuseppe