Mike,

The Geiger Muller tube has an extremely thin end-window to let the Alpha's in. I'm not sure what material it is but generally they are mica or mylar (and fragile as heck). The inside surface of the window is coated with something matt black, probably aquadag. The wall appears to be ferrous and "blued" on the outside. The other end has a ceramic insulator with glass evacuation nipple. The tube is opaque so its internal construction is unknown, I assume the anode is a thin wire up the centre, dunno how it is stabilised.

The gamma emissions from the Americium are detectable right through the assembled ion chamber of the smoke detector, but they are quite feeble, about double the background radiation with the Geiger tube right against the PCB under the ion chamber. The chamber roof and walls are a thick piece of tinned steel that appears to have been stamped into shape and perforated to allow airflow. The floor is just a piece of plastic and the PCB it is mounted on.

The Americium source itself is a small base metal disk (Steel I assume?) about 5 mm in diameter. The back-side emits only gamma radiation and is greyish in colour. The "hot" side is is bright andy shiny, probably Silver or Aluminium plated over the Americium itself. The source button is crimped into some kind of tinned metal disk with two tabs which were soldered into the PCB.

The smoke detector ion chamber had both the source and outer shell connected to resistors in the circuit, but a third annular electrode surrounded the Alpha beam and was suspended about 3 mm above the source. One of the pins from the 16 pin IC was soldered directly to that electrode, not making contact with anything else, I presume to offer very low leakage and maximise the fairly feeble ion current into the detector IC.

Regards,

Alan

Alan, Your site is truly fascinating. I have worked with RA detection for many years as a tech for an oilfield company but, only recently began my own experiments. Please tell me how you derive alphas from a smoke detectors Am241 source. I thought the steel envelope would block the alphas.

Regards,

Mike (AD5DT)