Teach lesson
Radioactivity 1/2: counts and distance
Students use repeated measurements from the remote lab to investigate how the detected count changes as distance increases and whether a straight line describes the pattern well.
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Learning Outcomes
Distinguish detector counts from personal radiation dose.
Use three repeats and a mean to compare measurements that vary.
Change distance while keeping all other variables constant.
Compare changes per millimetre to decide whether a straight line represents the data well.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
How does an invisible signal change?
6 min
Radiation cannot be seen, heard, or smelled. Without a detector, a radioactive
source nearby and the same source farther away could appear exactly alike. The
detector turns that invisible signal into something you can investigate: a
number of counts.
Those numbers have an unusual feature. Even when nothing in the setup changes,
three readings will rarely be identical. Each decay is a random event, so one
reading can be misleading; you need repeats, means, and a pattern across the
whole set of data.
From your classroom, you will control real equipment at the University of
Queensland on the other side of the world. You will move the source nearer and
farther without physically touching it and watch how the detector responds. We
expect fewer events to reach it as distance increases, but the more interesting
question is this: does the count fall at a constant rate, or does the rate of
decrease change with distance? Your four measurements will decide which model
fits better.
The remote lab in a classroom
The equipment returns detector counts. This activity compares those counts; it
does not calculate the radiation dose received by a person.
Before you take any measurements, what pattern do you predict as the source
moves farther away?
Explain your prediction in one sentence. You do not need to know the answer yet.
Design a fair comparison
5 min
You will change only the distance. Keep the source set to Strontium-90, use
no absorber, and keep the duration at 5 seconds with 3 repeats. The repeats let
you calculate a mean instead of drawing a conclusion from one value.
Which procedure allows you to attribute the observed change to distance?
Measure at four distances
17 min
Use paper or a calculator while the lab is open. In TEACH, record the three
readings and the mean for each distance.
Open the Radioactivity lab
Open the lab from this activity and enter basic mode (
Modo básico).Select Strontium-90, no absorber (
Ninguno), 5 s, and 3 repeats.Measure at 15 mm. Record the three counts and calculate the mean.
Repeat exactly the same procedure at 20 mm, 60 mm, and 80 mm.
Do not change any other setting between measurements.
The interface may use Spanish labels: Distancia = distance, Duración =
duration, and Repeticiones = repeats.
Your counts at four distances
Complete exactly four rows, one for each distance. Record the three counts shown by the lab and calculate the mean: (r1 + r2 + r3) / 3. Enter counts, not counts per second. If the interface shows extra rows, leave them blank.
| Distance mm | Count 1 | Count 2 | Count 3 | Mean counts |
|---|---|---|---|---|
Use your three counts at 15 mm. Enter the mean and show the sum divided by
3. Round to one decimal place if needed.
Does a straight line describe the pattern?
9 min
Plot the four points in your notebook: distance on the horizontal axis and mean
count on the vertical axis. The template shows axes only; it does not reveal
the answer.
Axes for plotting your four means
Add a vertical scale that includes your highest mean, then plot all four points.
Which description best matches your four means?
Compare intervals of different sizes fairly. Calculate the mean decrease per
millimetre using this template:
- 15 to 20 mm: [mean(15) - mean(20)] / 5 = ___ counts/mm.
- 60 to 80 mm: [mean(60) - mean(80)] / 20 = ___ counts/mm.
Then state in one sentence whether the two rates are similar or clearly
different. If either value is negative, the count increased across that interval
in your data; do not hide that result, and consider repeating the measurement.
Based on your graph and the two rates, would you use a straight line to
represent the whole interval? Answer in two sentences and cite your results. If
your data are inconclusive, say so and explain what you would repeat.
Review your initial idea
3 min
After analysing the data, what do you do with your initial prediction?
### Before submitting
Check that your work tells the same story from beginning to end:
- you recorded three counts and a mean at each of the four distances;
- you plotted all four points with labelled axes and a scale;
- you compared the two decreases per millimetre, not only the total decreases;
- your conclusion cites results and acknowledges if the data are inconclusive.
Which limitation must you remember when interpreting this activity?