A Geiger–Müller pancake detector enclosed in lead box. This is the GM detector. Typically has a 5 CM diameter window.
The 5 cm diameter GM detector detector window is left exposed from the lead box.
A 20 cm long 5 cm internal diameter Aluminum tube wrapped in lead feeds into the GM detector window.
The 20 cm length of the tube is filled with paraffin or high density polyethylene (HDPE). This is the neutron moderator.
One end surface of the neutron moderator is coated with silver. Vaporization of a silver wire may be used to coat the surface. Silver leafing foil may also be used.
The silver end is placed against the GM detector window.
Natural Silver consists of two isotopes, 107Ag and 109Ag, with 51.839% being 107Ag.
107Ag (~39 Barns) neutron activation product is 108Ag, half life 142.2 seconds (2.37 minutes).
109Ag (~89 barns) neutron activation product is 110Ag, half life 24.6 seconds.