# EXPERIMENT #7: MAGNETIC DEFLECTION OF BETA RAYS

## Introduction

Placing a magnetic field across the path of beta radiation makes the beta particles bend because the particles are charged. The strength of the magnets and the energy of the beta particles will determine the degree of deflection.

## Objective

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

## Materials

• Geiger counter
• Wooden block (to hold the source)
• Aluminum shield with hole (for Geiger counter)
• Two cow magnets
• Magnet holders
• Sr-90 (beta source)

## 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 Table 7.1.
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 Table 7.1.

## 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.