http://www.mineoro.com/treasure/explanations.htm#a10
Although Esteban is an expert in Mineoro detectors he lacks essential information which Mineoro off course does not provide.
The ionic chamber, altough important, is not the essential factor which makes the detectors work. There's much more than that. The pictures above are nice and provide an educational insight regarding the chamber, although the one shown above lacks some components found in the FG series such as infra red sensor and some more things.