Transmission of N-atoms (T N ) through small diameters tubes (1.5 and 3 mm internal diameter (i.d) and 9, 50 and 80 cm length for silicone tubes, 1.5 mm i.d and 6.5 cm length for stainless steel tubes) has been measured in late N₂ and Ar-N₂ flowing afterglows of microwave plasmas in continuous and pulsed gas injection at a flow rate of 1 and 3 Standard liter by minute (Slm), a gas pressure from 2 to 4 Torr for N₂ and 20 Torr for Ar-1%N₂ and a plasma power from 150 to 300 Watt. From the experimental T_N values, it is deduced the γ-destruction probability inside the tube walls as being y = (1-1.6)×10^-3 for the silicon tubes and y = (1.6-2)×10^-2 for the stainless steel tubes.