These probes for the 6150AD are used for measurements in hard-to-reach places or to extend the measuring range.
6150AD-15: Measuring range 1 mSv/h to 10 Sv/h, energy range 65 keV to 3 MeV.
6150AD-18: Measuring range 1 µSv/h to 10 mSv/h, energy range 65 keV to 1.3 MeV.