Alpha Radiation is produced by radioactive decay eg from Americium

Radioactive americium 241 from a smoke detector is a potent alpha radiation emitter with an energy of 5.38 MeV. Alpha particles are helium nucleii (2 protons and 2 neutrons).
“Continue reading” for radiation counts, sources and shielding effects.

These particles are relatively heavy and have poor penetrating power. 90% are blocked by a sheet of paper or the outside dead layer of your skin. The photo is of the americium source from a smoke detector on my Geiger counter which reads 80,180 counts per minute.

The Am-241 in the form of americium dioxide is used in many smoke detectors and has a typical dose of 1 microcurie (37 kBq) (= 0.28 mcg). The half life is 432.2 years and it decays into neptunium 237 with half life 2.14 million years.

Now, 8 years after this photo was taken in 2006, the Geiger count should have dropped from 80,180 to 79,157 which is around 1.2% less. This should have formed 3.3 ng of neptunium. Be careful, about 60 kg of neptunium is critically fissionable ie a bomb. Hence, if you have 20 billion smoke detectors, don’t put them on the same shelf.

So, you probably have some neptunium in your home. Check your domestic smoke detector out by opening it up as if to change the battery and you should see the radioactive warning sign. You won’t be able to do this if they are wired in to a central alarm system.

The americium sample (1 mCurie) shows a count of 80,180 CPM (counts per minute) when placed directly on the counter. The majority of the alpha radiation is blocked by 1 thickness of paper with the count dropping to 1250 CPM.

The americium sample (1 mCurie) shows a count of 80,180 CPM (counts per minute). The majority of the alpha radiation is blocked by paper with the count dropping to 1250 CPM. Thin aluminium sheet drops the count to 750 CPM. There is some gamma radiation from the americium which may be accounting for some of this.