Inverse Square Law for Radiation

Author: George Lyle
Demonstration Equipment - Teacher's Guide
SED 695B; Fall 2005

Principles illustrated:

  • Inverse Square Law
  • Radioactive Decay

Standards addressed:

California Science Standards: Physics, Motion and Forces

1. Newton's laws predict the motion of most objects. As a basis for understanding this concept:

m.

* Students know how to solve problems involving the forces between two electric charges at a distance (Coulomb's law) or the forces between two masses at a distance (universal gravitation).

 
       
Materials
Explanation of principles involved

Geiger counter with digital readout

Meter Stick

Radiation source capable of 500-1000 counts/minute at 2cm from Geiger counter.

Computer with spreadsheet software, graphing calculator, or good old graph paper and pencil.

A radioactive source emits radiation at a relatively constant rate. These emissions can be recorded as discrete events by a digital Geiger counter. As one moves further from the source, the emitted particles are dispersed and are therefore less likely to strike the geiger counter. Since the area over which the emissions are dispersed is that of an expanding sphere about the source, the radiation intensity follows the inverse square law as one moves away from the source.
 

Procedure:

 
Place Geiger counter flat on table. Place the zeron end of the meter stick against the Geiger counter. Make sure that the radiation source is directly in line with the sampling window.
Center the radiation source at the 2cm mark. Turn the Geiger counter on for one minute and record the number of counts.
Center the radiation source at the 4cm mark. Turn the Geiger counter on for one minute and record the number of counts.
Center the radiation source at the 6cm mark. Turn the Geiger counter on for one minute and record the number of counts.
Center the radiation source at the 8cm mark. Turn the Geiger counter on for one minute and record the number of counts.
Center the radiation source at the 10cm mark. Turn the Geiger counter on for one minute and record the number of counts.
Center the radiation source at the 50cm mark. Turn the Geiger counter on for one minute and record the number of counts. At this distance the radiation received from the source will be almost nil. This will provide a measurement of the background radiation level.

Using your preferred method, plot the counts per minuteversus the source distance. The result should be similar to the curve shown here.

The actual Excel file can be found here.

   

References & Links:

A general explanation of the Inverse Square Law.

Reasonably-priced sources for the digital Geiger counter I use. Be careful! With the current hysteria about nuclear terrorism, some vendors, including well-known science suppliers, are charging over $500 for this same device! Source 1 - Source 2 .