Magnetic Deflection of Beta Rays

When a magnetic field is applied across the path of beta radiation, the beta particles are deflected due to their electric charge. The degree of this deflection depends on both the strength of the magnetic field and the energy of the beta particles.

Objective   

To deflect the path of beta radiation by means of magnetism.

Procedure

1. Turn on the Geiger counter with the scale set to x1 and allow it to warm up for a few minutes.

2. Take background activity measurements.

3. Place the Sr-90 (beta source) into the wooden block.

4. Place the Geiger counter eight centimeters from the source. In front on the window of the Geiger counter, place an aluminum with a quarter inch hole in front of the Geiger window.

5. Place the magnet holder so that when a magnet is inserted in it, the path of the beta particles is between the Geiger Counter and the source.

6. Take a reading with the source and magnet holder in place but without any magnets near them.

7. Place one cow magnet into the magnet holder so that a magnetic field crosses the path of the beta particles.

8. Record the counts per minute for three trials in the table below.

9. Next, place unlike poles of the two cow magnets into the opening of the magnet holder and repeat step eight.

10. Again record the counts per minute for three trials in the table.

Questions:

1. Does the addition of one magnet have any effect on the beta particles? Explain.

2. Does the amount of bending of beta particles increase or decrease when two magnets are used in the experiment? Explain.

3. What results would you expect if gamma particles were used? If alpha particles were used?

Optional: 

1. How are magnets used in a cyclotron? (For more information on cyclotrons, take a look at the 88" Cyclotron home page at LBNL.)

2. Carefully remove the Geiger counter, and with your hands, rotate it until you determine the maximum deflection of beta particles.

3. Repeat the above experiment using gamma rays.