Question Constellation Sign up for free AQA · GCSE · Combined Science · Higher
Measured count rate and source activity
Lanthanum-140 is a radioactive isotope. Activity is the rate at which a radioactive source decays.
Activity is the rate at which a radioactive source decays.
A teacher measured the count-rate from a sample of lanthanum-140 using a Geiger-Muller (G-M) tube.
Explain why the count rate was less than the activity of the sample of lanthanum-140.
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Marking points
- 1 Mention that radiation from the sample spreads out in all directions.
- 2 Mention that only some of the emitted radiation enters the G-M tube.
- 3 Mention that even some radiation entering the G-M tube is not detected.
- 4 For full credit, give two valid reasons, or link incomplete detection in the G-M tube to gamma radiation being weakly ionising.
Full-mark answer
The radiation is emitted in all directions, so only some of it enters the G-M tube. Also, not all the radiation that enters the tube is detected.
Why this answer loses marks
The G-M tube was not working properly.
The question asks for the normal physical reason why count rate is less than activity, not a fault in the equipment.