Radioactive alpha source mounted in approved holder with a black plastic shaft, threaded for installing in cloud chamber 512000.
Activity: 3.7 kBq
Half-life: 432.6 years
The americium source is designed to emit as “pure” alpha radiation as possible - but nothing in the world is perfect, and the details are worth knowing.
Am-241 alpha decays into countless different excited states in Np-237 which again decays by gamma emission. Most important gamma energies are 13.9 keV (42 %), 26.3 keV (2,4 %) and 59.5 keV (36 %), where the percentages are relative to the number of alpha decays.
In other words, quite a bit of low-energy gamma radiation is emitted from the source, but as a normal Geiger tube has very low gamma sensitivity, it will still be alpha radiation that dominates the observations. In a cloud chamber, the gamma radiation will not be noticed at all.
The daughter nuclide Np-237 is radioactive but with a very low activity: The activity of Np-237 from this americium source will grow very slowly from zero, but will never be able to exceed 1 Bq (one decay per second).