物理輻射與放射性 DSE、CE 選擇題（第 3 頁）

21.

Which one of the following is not a safety precaution for handling radioactive sources ?

22.

An insulated metal sphere carries positive charges. Which of the following will discharge the sphere ?

(1)bringing an alpha source near the sphere

(2)touching the sphere momentarily with a finger

(3)bringing a negatively charged metal rod near the sphere (but without touching it)

23.

The background count rate recorded by a Geiger-Muller counter is $80$ counts per minute. When a radioactive source is placed closely in front of the Geiger-Muller tube, the count rate recorded is $560$ counts per minute. After $6$ hours, the count rate drops to $140$ counts per minute. Find the half-life of the source.

24.

Which of the following statements about $\alpha$ particles is incorrect ?

25.

Which of the following diagrams correctly shows the directions in which $\alpha$ , $\beta$ and $\gamma$ radiations are deflected in a uniform electric field produced by two charged metal plates ?

Four diagrams A to D showing alpha, beta and gamma tracks between a positive plate on the left and a negative plate on the right

26.

Which of the following statements about $\beta$ particles is incorrect ?

27.

Which of the following can be deflected by both an electric field and a magnetic field ?

(1)

\alpha

particles

(2)

\beta

particles

(3)

\gamma

rays

28.

The activity of a radioactive isotope falls to $\frac{1}{16}$ of its initial value in one hour. Find the half-life of the isotope.

29.

Which of the following cannot travel through a vacuum ?

30.

Which of the following statements about X-rays is/are correct ?

(1)X-rays consist of fast moving electrons.

(2)X-rays can blacken photographic films.

(3)X-rays can be used to detect weapons hidden in luggage.