IGCSE · 5 · multiple choice

Nuclear physics · Question 6

Nuclear physics · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand: demanding
Marks: 1
Topics: 1
Answer: Complete

Fig. 6.1A cut-through view of the ionisation chamber of a smoke alarm. Two horizontal metal plates face each other across a small air gap, and a radioactive source is fixed to the underside of the upper plate so that it irradiates the gap. A wire from the upper plate leads to a cell and a wire from the lower plate leads to the alarm, so the two plates and the air gap between them form part of one complete series circuit. An arrow shows smoke drifting sideways into the gap.

multiple choice1 mark

A smoke alarm contains a source that ionises the air in a small gap, allowing a tiny current to flow. Smoke entering the gap reduces the current and sets off the alarm. Which source is most suitable?

A
an α-emitter with a half-life of several hundred years
B
an α-emitter with a half-life of a few hours
C
a γ-emitter with a half-life of several hundred years
D
a β-emitter with a half-life of a few days

Your working or explanation

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Answer overviewCorrect answer: A — an α-emitter with a half-life of several hundred years

Answer and reasoning

1 mark

How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

1
The correct choice is A: an α-emitter with a half-life of several hundred years
2
Two properties have to be right at once. Alpha is by far the most strongly ionising and is stopped by the alarm's own casing, so it is both effective and safe; and a long half-life means the alarm keeps working for years. B has the right radiation but would stop working almost immediately.

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