In this experiment you will use a piece of Uranium Ore as your radioactive source. The radioactive decays that you will register with the Geiger counter are mostly beta-decays and also some gamma decays. Here is where they come from:

Uranium has sixteen isotopes, all of which are radioactive. Naturally occurring uranium nominally contains 99.28305% by weight 238U, 0.7110% 235U, and 0.0054% 234U. So we'll be registering mostly the decay products of the  238U isotope.

 

 

 

238U goes through a series of decays indicated above. The alpha decays will not be registered by the Geiger counter, because the alpha particles are easily absorbed in material. The graph below shows the average energy of the particles emitted in the decay series of  238U.  For reference, the range of a 5 MeV alpha particle in air is 4.368E-03 g/cm2. This number has to be divided by the density of air 1.25 kg/mto get the distance that an alpha particle will travel in air before it looses all its kinetic energy. So, an alpha particle originating from a decay in the  238U series will travel only about 3.5 cm before being stopped. The electrons from the beta decay interact much less with the material, hence their range is larger than that of the alpha particles. In air, it is about 0.2 g/cm2 for a 0.5 MeV electron. These electrons will make the Geiger counter click!