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• RADIOACTIVITY AND NUCLEAR ENERGY

P.1

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HKCEE PAPER I

2011 7

It is known that plutonium-238 (238

94 Pu) decays by emitting one particle.

(a) Write a nuclear equation for the decay of plutonium-238. Use the symbol Y as the daughter

nucleus. (2 marks)

(b) A sample of plutonium-238 is put in a cloud chamber. Some tracks are seen.

(i) Describe the tracks that are seen. (1 mark)

(ii) No tracks can be seen when the sample is covered by a piece of paper. Explain.

(2 marks)

(c) Plutonium-238 can be used in heater units in spacecrafts for outer space missions. It is

known that the power of the heater unit is directly proportional to the activity of

plutonium-238 contained. Each heater unit has a power of 2 W when it is newly

manufactured. How long can a newly manufactured heater unit last if the minimum

power output required is 0.25 W ?

Given : half-life of plutonium-238 = 87.7 years (3 marks)

2009 7

Radon-222 (Rn-222) has a half-life of 3.8 days and undergoes a radioactive decay series as shown in

Figure 8 to become a stable nuclide Lead-206 (Pb-206).

Figure 8

(a) Estimate the mass of undecayed Rn-222 after 15.2 days if its initial mass is 1 10-5

g.

(2 marks)

(b) State the nuclear radiation emitted in the process (1) indicated in Figure 8. (1 mark)

(c) Write down the nuclear equation for process (2) indicated in Figure 8. (2 marks)

(d) Determine the total number of particles and the total number of particles emitted in the radioactive decay series from Rn-222 to Pb-206. (4 marks)

2008 12

(a) A teacher places a radioactive source 1 cm in front of a Geiger-Muller tube (GM tube) and

measures the count rate. When he inserts a piece of paper between the radioactive source and

the GM tube, he finds that there is no significant change in the measured count rate. State the

conclusion about the type of radiation emitted from the radioactive source. (1 mark)

The teacher then conducts another experiment to investigate the deflection of radiations inside

a magnetic field as shown in Figure 2.3. the GM tube can be rotated from 0o to 180

o around

the magnetic field. Figure 24 shows the count rate recorded at different angles with or

without the magnetic field.

Atomic number 82 83 84 85 86

Mass number

206

214

216

222

218

220

Rn

Po

(1)

Pb

Pb

Po

Bi (2)

• RADIOACTIVITY AND NUCLEAR ENERGY

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Figure 23

Figure 24

(b) Estimate the count rate due to the background radiation. (1 mark)

*(c) Using the result in Figure 24, explain why it can be concluded that the radioactive source

emits and rays. (5 marks)

(d) Estimate the count rate due to each type of radiation at = 90o without the magnetic field. Write the answer in Table 3. (2 marks)

Table 3

Type of radiation Count rate / counts per minute

0

0o

GM tube connected

to counter

region with magnetic field

pointing into paper

90o

180o

in a lead box

Recoded count rate / counts per minute

250

200

150

100

50

0 50 90 180

/ o

with magnetic field

without magnetic field

• RADIOACTIVITY AND NUCLEAR ENERGY

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2007 8

In a physics lesson, a teacher uses the apparatus shown in Figure 13 to find the range of particles in

the air. Describe the procedures of the experiment. (5 marks)

Figure 13

2006-8

Workers of nuclear plants are required to wear film badges (see Figure 14) to monitor their exposure to

radiation. Inside the film badge, an opaque plastic bag is wrapped around a sheet of photographic film.

Aluminium and lead sheets are also placed inside the badge (see Figure 15) so that the types of

incoming radiation can be distinguished.

(a) What type(s) of radiation can be detected by the badge? (1 mark) (b) Why is an opaque plastic bag used to wrap the photographic film? (1 mark) (c) The film of three workers John, Mary and Ken were developed. Table 3 shows the degrees of

blackening on different regions of the films inside the badges which they wore.

Regions on the film Degree of blackening (0 5)

(0 = not blackened; 5 = most blackened)

John Mary Ken

Beneath the open window 5 5 5

Beneath the 1 mm aluminium sheet 5 3 4

Beneath the 3 mm aluminium sheet 5 1 2

Beneath the 5 mm lead sheet 4 0 0

Table 3

*(i) Based on the results in Table 3, explain which type(s) of radiation John and Mary are

definitely being exposed to respectively. (4 marks)

(ii) Give on reason why different degrees of blackening were recorded on the films of Mary

and Ken. (1 mark)

(d) Suggest one hazard of exposure to ionizing radiations. (1 mark)

• RADIOACTIVITY AND NUCLEAR ENERGY

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2005 7

Read the following passage about Iodine-131 therapy and answer the questions that follow.

Iodine-131 is a radioisotope which emits and radiations. It can be used for thyroid cancer treatment.

A patient suffering from thyroid cancer will first undergo an operation to have the thyroid gland

removed. However, some thyroid tissue may remain in the neck of the patient or may be carried in the

blood stream to other parts of the body. Iodine-131 is then used to trace and get rid of the remaining

thyroid tissue in the body.

Iodine-131 therapy consists of two stages. In Stage 1, the patient will take a low does of Iodine-131 to

trace the remaining thyroid tissue. A detector is placed near the patient to monitor the activity of the

radiation coming from the patient.

In case any remaining thyroid tissue is spotted in State 1, the patient will then take a higher dose of

Iodine-131 in Stage 2. The iodine will be absorbed by the thyroid tissue and the radiation emitted can

kill the cancer cells.

Special hospital rooms are designed for patients who receive Stage 2 of the therapy. The rooms have

metallic shielding in the doors and reinforced walls. Inside the rooms, there are plastic covers on the

furniture, doors, handles and switches.

Source: Iodine-131 Therapy, The Ohio State University Medical Center, 2003.

(a) Explain why, in Stage 1, radiation from the patient cannot be detected by the detector. (1 mark)

(b) In Stage 2, which kind of radiation is more effective in killing caner cells? Explain you

(c) State one special feature of the hospital rooms designed for patients receiving State 2 of the

therapy and explain its function. (2 marks)

2005 8

Carol performs an experiment to measure the half-life of a radioactive source. She places a Geiger-

Muller tube in front of the source and the following results are obtained:

Time t / hour 0 10 20 30 40 50 60 70

Count rate / counts per

minute

400 225 154 119 107 105 100 102

Table 1

(a) Plot a graph of the count rate against time in Figure11. (4 marks)

(b) Estimate the background count rate. (1 mark)

(c) Estimate the corrected count rate at t = 0.

Hence, or otherwise, estimate the half-life of the source. (2 marks)

2004 9

Figure 15

Figure 14

slot

electrode

americium source

to an alarm circuit

• RADIOACTIVITY AND NUCLEAR ENERGY

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Figure 14 shows a smoke detector. The circuit inside the detector is shown in Figure 15. A small

amount of the radioisotope americium-241 ( Am24195 ) is placed between two electrodes. The two

electrodes are connected to a battery and an alarm circuit. The detector has slots in it to allow air flow.

(a) An americium-241 nucleus decays by emitting an -particle to form a daughter nucleus neptunium (Np), with a half-life of 432 years.

(i) Write down an equation for the decay of an americium-241 nucleus. (2 marks)

(ii) Find the number of neutrons in the daughter nucleus. (1 mark)

(b) Under normal conditions, a small current flows in the circuit inside the detector. However,

when smoke particles enter the detector, the current drops significantly. This triggers the

alarm to sound.

*(i) Explain why a current flows between the electrodes under normal conditions.

(4 marks)

(ii) Suggest one possible reason why the current drops when smoke particles enter the

detector. (2 marks)

(c) Explain why it is preferable for the radioactive source used in smoke detectors to have a long

half-life. (2 marks)

(d) Carbon-14 ( C146 ) is a radioisotope which decays by emitting particles and has a half-life of

5700 years. Explain whether this source is suitable for use in smoke detectors or not.

(2 marks)

(e) People are concerned about the biological hazards of radiation. If you are the manufacturer of

the described smoke detector, how would you explain to the public that using the detector will

not pose any health hazard? (2 marks)

2003 9

In April 1986, a disastrous nuclear accident happened at the Chernobyl Nuclear Station. A large

amount o

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