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Activity from Decay Curve
Figure 16 shows how the number of atoms of a radioactive element in a sample varied with time.
Activity is the rate at which a source of unstable nuclei decays.
Determine the activity of the radioactive sample at 300 seconds.
Give the unit.
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Marking points
- 1 Uses a valid tangent at 300 s to estimate the instantaneous gradient.
- 2 Calculates the magnitude of the tangent's gradient, accounting for the ×10^23 axis multiplier.
- 3 Obtains an activity in the allowed range 6.5 × 10^20 to 7.6 × 10^20.
- 4 Gives the unit Bq or becquerel.
Full-mark answer
Draw a tangent to the curve at 300 s and calculate the magnitude of its gradient. A suitable tangent gives a decrease of about 7.1 × 10^20 atoms each second, so the activity is approximately 7.1 × 10^20 Bq.
Why this answer loses marks
I read the curve value at the stated time and called that the activity.
Activity is a rate, not the number of atoms. Draw a tangent at the stated time and use the magnitude of its gradient with the correct unit.