Hand machined mechanical pencils

Americium 241 Source Holder (08/06/11)

The construction of this holder involves handling a radioactive source. Avoid touching the source directly and keep skin at least 5cm away from the front of the source, since this is the maximum range of the α particles. Because you're reading this, I assume you already have some knowledge of radioactive sources and are capable of handling them safely.

Safety rant over. I came across an old smoke detector in the attic a few days ago and thought it would be a nice little project to make a holder for the Americium 241 source inside the detector. Am-241 sources like these are pretty safe to handle, since they are relatively low activity and only emit α particles, which have quite a short range in air (around 5cm). Nevertheless, I made sure to always handle the source itself with tweezers and pliers, until it was mounted in the aluminium holder.

The source consists of a tiny disc of Americium 241 (about 2mm in diameter), which has been gold-plated and mounted in a small steel "button", 6mm in diameter. There is a 3mm spigot on the back of this, which enabled the source to be mounted on the smoke detector PCB. In the holder I made, the little spigot is held by a friction fit in a 3mm hole. An aluminium cap screws on the end to keep the source covered. The photos below should explain everything.

Original source and PCB mount

Rear view

Source removed, front view

Source removed, rear view

Holder and cap

Closeup showing split. This provides enough spring to hold the spigot on the source tightly.

Source mounted in hole

With cap


Warning label

Finished source. Overall size is 12mm dia., 45mm long

10/06/11: See the effect this source has on a spark radiation detector.

20/12/13: Also see this being detected with a Geiger-Müller tube.