Exp 4: Radioactivity

PHYS 47100 - Spring 2026

This experiment will explore some nuclear physics and radioactivity.

Experiment 1: Characterize the Geiger-Müller Tube

From the instrument manual:

"If a radioactive sample is positioned beneath a tube and the voltage of the GM tube is ramped up (slowly increased by small intervals) from zero, the tube does not start counting right away. The tube must reach the starting voltage where the electron “avalanche” can begin to produce a signal. As the voltage is increased beyond that point, the counting rate increases quickly before it stabilizes. Where the stabilization begins is a region commonly referred to as the knee, or threshold value. Past the knee, increases in the voltage only produce small increases in the count rate. This region is the plateau we are seeking. Determining the optimal operating voltage starts with identifying the plateau first."

A sample inside the detector

Instructions:

Turn on the counter instrument (red button on the back) and make sure it's all connected to the computer. The icon for the software is the little yellow STX icon on the desktop. Start it up.

Place a sample of Co60 on a tray and place it in the chamber.

In the Experiments menu, you will find one called Plateau. Clicking this will bring up the set up screen. Choose some parameters, like this.

Setting up the Plateau Experiment

Then you can press Run and it should work it's way through the sequence of HV values, and record the counts. You shouldn't see any counts at 700V, but then they will start to increase, until the numbers reach a sort of flat region, called the plateau.

You should see a graph like this forming.

The incoming data

For the subsequent experiments, you'll want to pick a HV value on the lower side of the plateau region.

Save this data set (use the Save As command) and prepare a simple plot to include in your report.

Experiment 2: Counting

Next, we will do some counting statistics for the radioactivity events. And event consists of the emissions and detection of particles (alpha and/or beta) from the radioactive sample.

Begin by measuring the background radiation. Do a sequence of runs with the following parameters: Time: 5 seconds, and Runs = 150, without a sample present in the chamber. You can choose the HV setting based on your prevous plateau measurement. Save your data.

Then do a set of runs with the Co60 sample present. Set your Time to 1 second and the number of runs at 750. Save your data.

Counts from a Cobolt 60 sample

The data taken here will be used to verify the Poisson statistics of the radioactive process. See below

Experiment 3: Halflife

Finally, we will create a sample of a very short lived radio isotope.

You will make a sample of Cs/Ba-137m Isotope which has a very short halflife, one that we can measure in a few minutes. Here are the instructions on how to do that.

Instructions

Making a sample

Instructions:

Using your HV setting from Exp 1, do a background measurement: In the preset Menu, set Runs to 0 and Time to 30. Save the data.

Then, make the sample and add about 10 drops to the metal planchet and do 30 more runs at 30 seconds each. You should see the counts decaying fairly rapidly. You can then do a fit to this curve and use that to figure out the half life of this sample.

After you are done, you can dump your remaining liquid in the sink in the back room and rinse off the metal planchet and return it.

To submit