Ecco, BobHiggins: I have verified that especially NaI scintillator tube/electronics (even that it is in aluminium shell) can be affected by EMI to a very high extent.
I have also everything shielded, using top quality cables but it does not matter. The peak(s) can exceed background noticeably and you might not be able to check, that it is really noise or radiation.
I have performed many tests in this area and found that it can be filtered well, but it is really not good, because measurement can be flawed in any way. The peaks are moving and even that there is single frequency, there might be more peaks close together.
Noise that is generated from my reactors is not blocked even by 1cm thick aluminium block - only attenuated. Unfortunately there is also radiation that comes from the process and also some from the heater.
So personally I would not trust any electronic detection system in vicinity of the reactor before very carefull investigation.