Radioactivity

Paper 2

1 Fig-1 illustrates nuclear fission of Uranium-235 in a nuclear reactor.

Fig-1

(a) (i) Describe the process illustrated in Fig.-1.

(ii) Describe how the process of nuclear fusion differs from the process of nuclear fission as
illustrated above.
[5]

(b) The waste products from a nuclear reactor contain isotopes such as Iodine-131 and
Caesium-137. These waste products are highly radioactive and dangerous. They are stored
in sealed metal cans which are placed under water for a few months.
(i) Explain how these isotopes were produced.
(ii) Explain what is meant by the term radioactive.
(iii) Give a reason why metal cans are used and a reason why they are placed under water.
[5]
 (c) Radioactive isotopes are used in medicine and in industry. Describe and explain one use
that is made of a radioactive isotope. In your account, indicate whether the isotope should
have a long or a short half-life. [5]

2 A radioactive source and a detector are used to check the level of fruit juice in a carton.
Cartons of fruit juice pass between the detector and the radioactive source, as shown in
Fig.-2. The radioactive source emits β-particles.

Fig-2

(a) State the name of a suitable detector of the β-particles.

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(b) What is a β-particle?
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(c) Explain why the level of detected radiation falls when a full carton of juice goes past
the detector.
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(d) Explain
(i) why a source emitting α-particles is not used,
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 (ii) why a source emitting γ-rays is not used.
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[2]

3 (a) A radioactive source emits α-particles only.

(i) Describe, with the aid of a diagram, an experiment that demonstrates that the source
emits α-particles but not β-particles.
(ii) Describe how you would demonstrate that the radioactive emission from the source is
random.
(iii) State one safety precaution that you would take when handling any radioactive source.
[8]
(b) A radioactive isotope of radon (Rn-220) is represented as ²²°86Rn. The nucleon number (mass
number) of this nuclide is 220 and the proton number (atomic number) is 86. Radon-220
decays into polonium (Po-216) by the emission of an α-particle.
(i) State the number of neutrons in a nucleus of Rn-220.
(ii) The nuclear equation that represents the decay of Radon-220 is written as
220 … 216

86 Rn → …α + … Po.

Copy this equation and complete it by adding the missing nucleon number and proton
number for the α-particle and the missing proton number for the polonium nucleus.
(iii) During the decay, there is an apparent decrease in mass of 1.14 . 10–29kg. Calculate the
energy released in the decay. [speed of light = 3.0*108 m/s]
[7]
4 A smoke detector contains a radioactive source that emits α-particles.

Fig-3

Fig.-3 shows the structure of a simple smoke detector. The α-particles ionise the air
between the plates. Positive ions and negative ions are created in the air and, as a result, a
current is produced in the circuit. When smoke is present, the current decreases.

(a) State the nature of an α-particle.

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(b) Explain why a source that emits β-particles is not used in this detector.
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(c) The radioactive source that emits α-particles contains Americium-241.
A nucleus of Americium-241 is represented as 24195Am.
Describe the structure of an atom of Americium-241.
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5 A teacher counts the number of particles emitted from a radioactive source, as shown in
Fig-4.

Fig-4

(a) State the name of a detector able to detect particles from a radioactive source.
................................................................................................................................... [1]
(b) The teacher measures the number of particles emitted in 1 minute from three different
sources. The measurements are repeated each hour for four hours.
The results are shown in the table.
 (i) State and explain which source has the shortest half-life.
source with shortest half-life .....................................................................................
reason ................................................................................................................... ...
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(ii) The experiment continues until the time is 6 hours.

For this time of 6 hours, calculate the number of particles emitted in 1 minute from

1. source A,

number = ..................................

2. source B.

number = ..................................
[5]

6 (a) Some atoms are radioactive. Explain what is meant by radioactive.

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(b) Some hospital equipment is sterilised using gamma-rays. State two properties of
gamma-rays that make them suitable for this use.
1. .......................................................................................................................... ...........
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2. .......................................................................................................................... ...........
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(c) Explain why radioactive sources should only be handled at a distance from the body.
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7 Table-1 gives details about some radioactive isotopes.

Table - 1
(a) (i) Uranium-235 has a proton number (atomic number) of 92 and a nucleon number (mass
number) of 235.Describe the structure of an atom of Uranium-235.
[4]
(ii) Uranium-235 and Uranium-238 are isotopes. Explain what is meant by this statement.
[2]
(iii) Uranium-235 can be involved in nuclear fission. Describe what happens to a nucleus in
nuclear fission. [3]
(iv) When the Earth was formed there was about 64 times more Uranium-235 present than
there is now. Use this information to estimate the age of the Earth. [3]
 (b) Radioactive sources are used to detect leaks from pipes underground. A liquid containing the
source is placed in the pipe. Some liquid leaks from the pipe and the radiation it emits can be
detected above ground.
(i) State the most suitable radioactive isotope in Table 11.1 for this purpose.
(ii) Explain why the half-life of the isotope you have chosen and the radiation it emits are
suitable for this purpose. [3]

8 Details of two radioactive sources used by a school are shown in Fig-5.

Fig-5

(a) On Fig-6, the number of Co-60 atoms in source A and the number of Sr-90 atoms in
source B at time t = 0 has been plotted as x.

Fig-6

(i) Plot accurately the number of Co-60 atoms in source A at t = 5, 10, 15 and
20 years. Draw the best line through your points.
(ii) Sketch how the number of Sr-90 atoms in source B changes from t = 0 to 20 years.
[3]
 (b) A detector is placed near source A. It records 4000 counts in one minute from the
source when t = 0.
A piece of lead, which absorbs 99% of gamma rays, is immediately placed between the
source and the detector. Determine
i) the counts in one minute that the detector now records from the source,

counts in one minute = ......................................

(ii) the time t when the detector would measure 10 counts in one minute from the source.

time = ......................................
[3]

9 Hydrogen nuclei fuse together in the Sun. The nucleus of one isotope of hydrogen contains
one proton and has the nuclide notation11 H. Other isotopes of hydrogen have the nuclide
notations 21H and 31H.

(a) State the number of protons and the number of neutrons in a nucleus of each of the two
other isotopes of hydrogen.
2
1H ........................................................................................................................... .........
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3
1H ....................................................................................................................................
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(b) Nuclei may fuse when they come together.
(i) Explain why nuclei do not easily come together.
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(ii) Explain why nuclei are able to come together in the centre of the Sun.
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[4]

10 This question is about the measurement and dangers of radioactivity.

(a) A student measures the radioactive emissions from a source with the apparatus shown in
Fig-7. The counter records the number of particles entering the detector in any specified
time interval.
 Fig-7

The student has a source emitting alpha particles. The source is known to have a half-life of
about one hour. Suggest how the student can measure this half-life.
You should
(i) suggest how background radiation is taken into account,
(ii) suggest what readings should be taken,
(iii) suggest how long the experiment should last,
(iv) explain how the half-life is found from the readings.
[6]

(b) (i) State two precautions the student should take when handling or storing radioactive
sources.
(ii) The effects of radiation on the human body depend on the properties of the radiation.
Explain why a source emitting alpha particles inside the body is more dangerous than
a source of the same activity emitting gamma rays inside the body. [5]

(c) Carbon–14 is used to find the age of objects. A 10 g sample of carbon is taken from a young
plant. Fig-8 shows how the number of counts in one minute from the sample decreases
with time.
 Fig-8

The number of counts per hour from an identical sample of carbon taken from an old piece
of wood is 380.
(i) Use Fig-8 to determine the age of this piece of wood. Explain how you obtained your
answer.
(ii) Explain why it is important to take background radiation into account in this
measurement.
[4]

11 The pie chart in Fig-9 shows the relative contributions made by different sources to
background radiation.

Fig-9

(a) Which source makes the greatest contribution to the background radiation?
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 (b) State one effect of background radiation.
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(c) Where do cosmic rays come from?
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220
(d) The nucleon number (mass number) of 86Rn is 220.
Define nucleon number.
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(e) Radon (22086Rn)
decays by the emission of an alpha-particle.
State the proton number (atomic number) and the nucleon number (mass number) of
the nucleus left after an alpha-particle is emitted from this nucleus.

proton number ……………………..

nucleon number ….………………….

[2]

12 A doctor uses a radioactive isotope, iodine-131, to find the volume of blood in a patient’s body.
Information about iodine-131 is given in Fig-10.

Fig-10

(a) (i) Describe the structure of an atom of iodine-131.

(ii) The radioactive decay equation below shows an iodine-131 nucleus decaying into a
xenon nucleus (Xe). Copy the equation and insert the proton number and the nucleon
number of the xenon nucleus.
153
31 I → Xe + β
[5]
(b) Describe the differences between beta-particles and gamma-rays. [3]

(c) The doctor uses a sample of iodine-131 that initially produces a count rate of 144 000 per
second. The whole sample is injected into the patient’s arm. Nine small samples of blood, each of
volume 2.0 cm3, are taken from the other arm at 2 minute intervals.

Fig-11 shows the count rates from the nine samples.

Fig-11
(i) State two reasons why different count rates are obtained from the nine samples.
(ii) Calculate the average value of the count rates from the last four samples.
This is the average count rate from a volume of 2.0 cm3 of blood.
(iii) Using your answer to (ii), determine the volume of blood in the patient’s body, which
has a total count rate of 144 000 per second.
(iv) Sample number 9 is kept.
The count rate is measured again after 16 days.
Estimate the value obtained, given that the half-life of iodine-131 is 8.0 days.
[6]
 (d) Describe one precaution that the doctor must take when handling this radioactive source.
[1]

13 (a) Some atoms that undergo radioactive decay have a half-life of 6 hours. The count rate near a
sample of these atoms is initially 838 counts/minute. Background radiation near the sample is
18 counts/minute.

(i) Describe the structure of an atom. It may help to draw a diagram. [3]
(ii) Explain what is meant by radioactive decay. State clearly which part of the atom decays.
[3]
(iii) State what is meant by background radiation. [1]
(iv) The equipment is left undisturbed for 12 hours. Calculate the count rate due to the
sample of atoms alone after this time. [2]
 (b) The table shows a radioactive series. Atom A emits a beta-particle and becomes atom B.
Atom B then emits a particle to become atom C.

(i) Calculate the proton number X of atom B and explain how you calculated it. [2]
(ii) State the name of radiation Y and describe the changes that occur in the atom when this
radiation is emitted. [3]
(iii) Using information from the table, explain why atoms A and C are not isotopes of the
same element. [1]

14 Fig-12 is a half-scale diagram of a radioactive source stored in a safe way.

Fig-12

(a) The source emits alpha-particles, beta-particles and gamma-rays.

A teacher handles the box. Explain how the teacher is completely protected from the
alpha- and beta-particles but only partially protected from the gamma-rays.
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 (b) Describe and explain how the teacher should remove the source from the box safely.
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(c) The source is brought near a radiation detector.

(i) Name a suitable detector.

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(ii) Describe how you would use the detector to show that the source emits particles
at random.
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15 A radioactive source undergoes radioactive decay.

(a) Explain what is meant by radioactive decay.

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(b) Gamma-rays from a radioactive source are used to treat some foods.
The gamma-rays kill bacteria in the food so that it stays fresh.
Some people are worried that food treated in this way becomes radioactive. A scientist
tests three food samples. She measures the count-rate of the food before and after
treatment with gamma-rays. The results are shown in the table below. The radioactive
source is not present during any test.

(i) Explain why there is a measured count-rate before the food is treated.
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 (ii) Determine and explain whether the treated food becomes radioactive.
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16 (a) Explain how it is possible for an element to have different isotopes.

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(b) State what is meant by the half-life of a radioactive isotope.

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(c) Fig-13 shows how the number of atoms of a radioactive isotope changes with time.

Fig-13

Determine the half-life of the radioactive isotope. On Fig-13, show how you obtained
Your result.

half-life = ................................................ [2]