Americium-241 source foils (gold-coloured items) mounted flush with the interior walls of a Pyrotronics® model F3/5A ionization technology smoke detector manufactured in 1971. Chamber made of aluminium with bare interior and Aquadag® coated exterior. Original activity of all source foils in the unit was 80 µCi of Am-241. The source foils in the reference chamber account for approximately 20 µCi Am-241 of the 80 µCi total. Note that only the area of each foil that is not covered by the punched aluminum retaining slots contains active material. Each foil can be trimmed of the ends and not lose any activity (this is a deliberate result of the manufacturing process which centers the Am-241 material on the billet).