Modifying the CRaTER-μ microdosimeter by adding a 5 mm thick aluminum casing emulates the ICRU sphere. The top panel shows the absorbed dose rate dependence on the modulation parameter ϕ in the range between 400 MV and 1500 MV. Two additional configurations corresponding to 3.3 mm and 8.45 mm of aluminium shielding are shown to present the general trend. The bottom panel compares the absorbed dose rate that an ICRU sphere would suffer to the measurement and to the absorbed dose rate in the modified microdosimeter. Measuring absorbed dose rate in silicon using such a modified microdosimeter is sufficient to know the absorbed dose rate that a full ICRU water phantom would suffer in open space.