物理輻射與放射性 DSE、CE 選擇題（第 2 頁）

11.

Graph of count rate (counts per minute) against time (minute): decays from about 350 at 0 to about 50 at 20 minutes

Susan performs an experiment in which a radioactive source is placed closely in front of a GM counter. The graph shows the variation of count rate with time. What is the half-life of the radioactive substance ?

12.

Some dangerous substances are stored in a metal container inside a wooden box as shown in the figure. What metal should be used for the container and what type of substance is stored ?

Photograph of a wooden box with a hinged lid and a radioactive warning label

	Metal used	Type of substance stored
	Iron	Radioactive
	Iron	Flammable
	Lead	Radioactive
	Lead	Flammable

13.

A radioisotope $X$ has a half-life of $2$ days while another radioisotope $Y$ has a half-life of $1$ day. Initially there are $N$ undecayed atoms of $X$ and $8N$ undecayed atoms of $Y$ . After how many days will $X$ and $Y$ have the same number of undecayed atoms ?

14.

Source in a lead castle in a magnetic field pointing into the paper, with Geiger-Muller tubes at positions P and Q

A radioactive source is placed in front of a uniform magnetic field pointing into the paper as shown above. If a high count rate is recorded at positions $P$ and $Q$ , what kinds of radiation have been detected ?

	$P$	$Q$
	$\gamma$	$\alpha$
	$\gamma$	$\beta$
	$\beta$	$\alpha$
	$\beta$	$\gamma$

15.

Different absorbers are placed in turn between a radioactive source and a Geiger-Muller tube. Three readings are taken for each absorber. The following data are obtained :

Absorber	Count rate / $\mathrm{s^{-1}}$
–	200	205	198
Paper	197	202	206
5 mm aluminium	112	108	111
25 mm lead	60	62	58
50 mm lead	34	36	34

What type(s) of radiation does the source emit ?

16.

Which of the following statements about $\alpha$ particles and $\gamma$ rays is/are correct ?

(1)They can both be deflected by a magnetic field.

(2)

\alpha

particles have a stronger ionizing power than

\gamma

rays.

(3)They are emitted with almost the same speed in radioactive decay.

17.

Two decay graphs: activity of P falls from 800 Bq to 400 Bq at 10 min; activity of Q falls from 800 Bq to 200 Bq at 10 min

The figures above show the variation of the activities of two radioactive sources $P$ and $Q$ with time. Find the ratio of the half-life of $P$ to that of $Q$ .

18.

Which of the following particles cannot be deflected by a magnetic field ?

19.

The initial activity of a radioactive isotope is $2000$ Bq. After $4$ hours, the activity of the isotope drops to $125$ Bq. Find the half-life of the isotope.

20.

Which of the following statements about $\alpha$ particles and $\gamma$ rays is correct ?