Investigating Science 11–12 · Year 11

Counting background radiation: the irregular pattern of random decay

Module 2: Cause and Effect – Inferences and Generalisations

Practical, model not builtLow risk

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The idea

Radioactive decay is random, so counts in equal time intervals scatter irregularly, yet the scatter follows a Poisson distribution in which the variance equals the mean.

What you need

a Geiger–Müller counter with a count or scaler mode (no radioactive source: background radiation only)
stopwatch if the counter has no timer, spreadsheet

How to do it

Set the counter to count in 10 s intervals and record 60 consecutive counts.
Tabulate how many intervals gave 0, 1, 2, 3 … counts and draw a bar graph.
Calculate the mean m and the variance of the 60 counts.
Compare the bar graph with the Poisson prediction P(k) = mᵏ e⁻ᵐ / k factorial, multiplied by 60.
Look for “streaks” of high or low counts and decide whether they are patterns or chance, then repeat with 60 s intervals.

What you should see

The counts vary irregularly from interval to interval with no trend. The variance is close to the mean: for 60 intervals with a mean near 3 the ratio of variance to mean scatters about 1 with a standard deviation of about 18 % (√(2/59)). With m = 3.0 the Poisson model predicts that 5.0 % of intervals, about 3 of 60, show zero counts and that the standard deviation is 1.73; runs of high counts appear by chance. ANSTO notes that people in Australia receive about 1500 to 2000 µSv a year from natural background radiation, the source of these counts.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Radiation only comes from nuclear power stations or medical machines.
A run of high counts means something has changed.
Random events are spread out evenly.

Safety card

Low riskLearners carry it out

Hazards

fragile end window of the Geiger–Müller tube

Controls

keep the protective cap near the window and do not touch it

Note

No chemicals, heat or radioactive source are used. If a sealed school source is added, follow the school’s radiation management plan and the ASTA standard operating procedure for sealed radioactive sources.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-9 INS11/12-1 INS11/12-2 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities