I did some research on this. James Mahaffey who is a nuclear engineer talks about this a bit in his book 'Atomic Adventures.': "Professional-grade Geiger counters, such as the Eberline E-120, are available as surplus, but they are usually sold without the probe assemble, which is what wears out when in constant use in a nuclear power facility. The probe is the expensive part, and installation of a new one will require calibration."