The PUG-7N-D is a complete fast-slow neutron monitor system. It includes the PUG-7D, a five-decade survey meter with an attached detection system: the MO-1 Moderator, and the PNS-20 Probe. The PUG-7N-D System, or “Neutron PUG” detects fast or slow neutrons by means of a thermal neutron scintillation detector and a moderator. When the detector (PNS-20) or (PNS-20H) is external to the moderator, thermal neutrons are detected and measured by means of counts produced in the scintillator as read on the meter of the PUG-7D instrument.

The PNS-20 probe detects slow Neutrons. The PNS-20H detects slow Neutrons but in a higher range. When the PNS-20 probes are in the moderator they detect fast Neutrons. When they are external, they detect slow Neutrons.

The polyethylene moderator is surrounded by a cadmium shield. The cadmium shield absorbs thermal neutrons and allows fast neutrons to enter the moderator. The moderator “thermalizes” the entering neutrons thus allowing them to affect the scintillator. When the thermal neutron detector is within the moderator, thermal neutrons are excluded by the cadmium layers and the detector measures only fast neutrons which have entered the moderator and been “thermalized”.