Geiger–Müller counter simulation

Seng Kwang Tan AI, IP4 19 Nuclear Physics, Javascript, Simulations

A Geiger-Muller (GM) counter is an instrument for detecting and measuring ionizing radiation. It operates by using a Geiger-Muller tube filled with gas, which becomes ionized when radiation passes through it. This ionization produces an electrical pulse that is counted and displayed, allowing users to determine the presence and intensity of radiation.

Svjo-2, CC BY-SA 3.0, via Wikimedia Commons

This simulation (find it at https://physicstjc.github.io/sls/gm-counter) allows students to explore the random nature of radiation and the significance of accounting for background radiation in experiments. Here’s a guide to help students investigate these concepts using the simulation.

Exploring Background Radiation

Q1: Set the source to “Background” and start the count. Observe the count for a few minutes. What do you notice about the counts recorded?

A1: The counts recorded are relatively low and vary randomly. This reflects the background radiation which is always present.


Q2: Why is it important to measure background radiation before testing other sources?

A2: Measuring background radiation is important to establish a baseline level of radiation. This helps in accurately identifying and quantifying the additional radiation from other sources.


Investigating a weak source (e.g. banana) as a Radiation Source

Q3: Change the source to “Weak Source ” and reset the data. Start the count and observe the readings. How do the counts from the banana compare to the background radiation?

A3: The counts from the weak source are higher than the background radiation. Did you know that bananas contain a small amount of radioactive potassium-40?


Q4: How do the counts per minute (CPM) for the weak source vary over time? Is there a pattern or do the counts appear random?

A4: The counts per minute (CPM) will increase for the first 60 seconds and stabilise slightly after that. The counts per minute for the weak source vary over time and appear random, reflecting the stochastic nature of radioactive decay.


Exploring a strong source, e.g. Cesium-137 Source

Q5: Set the source to “Strong Source” and reset the data. Start the count and observe the readings. How do the counts from the strong source compare to both the background radiation and the weak source?

A5: The counts from the strong source are significantly higher than both the background radiation and the weak source.


Understanding the Random Nature of Radiation

Q6: By looking at the sample counts, can you predict the next count value? Why or why not?

A6: No, you cannot predict the next count value because radioactive decay is a random process. Each decay event is independent of the previous ones.


Q7: How can you use the background radiation measurement to correct the readings from the sources?

A8: You can subtract the average background CPM from the CPM of the sources to get the corrected readings, isolating the radiation from the specific sources.


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