EXPERIMENT #7:
MAGNETIC DEFLECTION OF BETA RAYS

Introduction

     The placing of a magnetic field across the path of beta radiation causes a change in the direction of the rays so that they bend. The strength of the magnets and the energy of the beta particles will determine the degree of deflection of the beta particles from the source.

Objective

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

Materials

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. Se the aluminum magnet holder so that the quarter inch hold is in line with the path of the beta particles.
  5. Place the Geiger counter 8 centimeters from the source. In front on the window of the Geiger counter, place a second shield of aluminum with a quarter inch opening in front of its window.
  6. Take a reading with the source and magnet holder in place but without any magnets.
  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 Table 7.1.
  9. Next, place unlike poles of the two cow magnets into the opening of the magnet holder and repeat step 8.

Data

Table 7.1: MAGNETIC DEFLECTION OF BETA RAYS

Background __________ cpm        Sr-90 time=60 s
Number of magnets Trial 1 (cpm) Trial 2 (cpm) Trial 3 (cpm) Average (cpm) Uncertainty +- square root average
0          
1          
2          

  1. Graph the above data and indicate the error of uncertainty.

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?

Going Further

  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.

This document was produced by the Nuclear Science Division of the Lawrence Berkeley National Laboratory.