PURPOSE: Demonstrate radioactivity, the Geiger counter, and show some differences between alpha, beta, and gamma rays.

DESCRIPTION: Three sources are provided: a Po-210 source, mainly ~5 MeV alphas; Tl-204 and Sr-90 sources, producing a number of electron energies around 1-2 MeV; and a chunk of natural uranium ore with a large number of low-energy gamma rays. In addition, Cs-137 (0.662 MeV) and Co-60 (1.1 and 1.3 MeV) gamma-ray sources are available.

The alpha particles have a range in air of about 2 or 3 cm; you must place the source close to the Geiger tube to observe the alphas. Inserting a piece of paper in the alpha beam stops them! The betas have a longer range in air, and are mostly unaffected by passing through a piece of paper. A thin lead sheet stops the betas, but some counting remains due to the presence of a some gammas in the beta source. The gammas are unaffected by the paper or the thin lead sheet, but can be stopped by a lead brick!

SUGGESTIONS: Click HERE for a series of questions regarding how alphas, betas, and gammas interact with different types and thicknesses of materials.

See Question of the Week #144 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 7D10.10) See Demonstration Reference File for information about this experiment and data on the full spectra of these sources.

EQUIPMENT: Geiger tube with audio amplifier and loudspeaker, alpha, beta, and gamma sources.

SETUP TIME: None.