5.2 Analyzing Radioactivity
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5.2
Analyzing radioactive decay
a.
Radioactivity
1.
Radioactivity is the spontaneous disintegration of an unstable nucleus accompanied by the emission of energetic particles or photons.
Figure 3: Diagrammatic representation of the ionization b.
Radioactive detectors
i. 1. 2.
Cloud Chamber A cloud chamber is used to show the path of ionising radiation. The tracks of the radioactive emission are formed in the same way as as the condensation trails behind a high-flying aeroplane when the air in the sky is cold and damp enough. Different tracks are formed for each radioactive emission.
3.
Figure 4: Tracks of radioactive emissions ii.
Geiger-Muller Tube
1.
A Geiger-Muller tube (GM tube) is a very versatile, sensitive and useful detector of radiation. It operates at a voltage of about 450 V. Radiation enters the GM tube through the mica window and ionises the argon gas. A pulse of current is produced. This pulse of current is counted by a scaler or a ratemeter. The scaler gives the number of counts over a certain period of time. The ratemeter gives the count rate in counts per second or counts per minute. The GM tube can detect alpha particles, beta particles and gamma rays.
2. 3. 4. 5. 6. 7.
1
iii.
Photographic Film
1.
A photographic film found in badges worn by the staff at radiation laboratories is developed at the end of every month. The degree of darkening of the photographic film indicates the amount of radiation received.
2. iv.
Spark Counter
1. 2.
4.
A spark counter uses the ionising effect of radiation. A high voltage is applied between the gauze and a wire below the gauze, and adjusted until it is just below the voltage required to produce sparks. When a radioactive source is brought near, the radiation ionises the air between the gauze and the wire, and sparks are produced. Spark counters are suitable for detecting alpha particles.
c.
Characteristics of radioactive emissions
3.
Figure 5: Natural characteristics of radioactive emissions 1.
There are three kinds of radioactive emissions: • Alpha particles • Beta particles • Gamma rays
2
Alpha decay Beta decay Gamma decay An alpha particle is A beta particle is emitted A gamma ray photon is emitted emitted The proton number is The proton number is The proton number and reduced by two and the increased by one and the nucleon number are nucleon number is nucleon number is unchanged reduced by four unchanged Table 1: Radioactive emissions
Figure 6: Range of radioactive emissions in air
Figure 7: Penetrating powers of radioactive emissions
Figure 8: The effect of a magnetic field on the radioactive emissions
Figure 9: The effect of an electric field on the radioactive emissions
3
Table 2: Nature and characteristics of radioactive emissions
d.
Radioactive Decay
1.
The process of a nucleus changing to a more stable nucleus while emitting radiation is called radioactive decay. The nucleus before the decay is called the parent nuclide and the product of the decay is the daughter nuclide. Radioactive decay is named according to the type of radioactive emitted. Type of the radioactive decay
2. 3. 4.
4
i.
Alpha decay
Alpha decay usually happens to the heavier unstable nuclei.
Figure 10: An alpha decay 238 92
ii.
U Æ234Th + 4He 90
2
Beta decay
Beta decay usually occurs for nuclei that have an excess of neutrons.
Figure 11: Beta decay
iii.
Gamma decay
Gamma decay occurs when an unstable nucleus releases its excess energy in the form of high frequency electromagnetic waves called γ-rays.
e.
Radioactive Decay Series
Bismuth-214 goes through a series of 5 decays to become a stable lead-206.
5
f.
Half-life
The half-life of a radioactive nuclide is the time taken for the number of undecayed nuclei to be reduced to half of its original number.
Questions: 1.
A pupil investigates the penetrating power of radiation from a radioactive source.
The table shows the results. Background count Count with source only
25 counts per minute 630 counts per minute Count with source and paper 630 counts per absorber minute Count with source and aluminium 180 counts per absorber 3 mm thick minute The source emits A alpha and beta-particles. B beta-particles and gamma-rays. C beta-particles only. D gamma-rays only.
2.
(a)
14
C is a radioactive isotope of carbon which decays by β-particle emission with a half-life of 6000 years. (i) What is a β-particle? (ii) A radioactive sample contains 5000 atoms of 14C. On the axes below, plot the graph of the number of 14C atoms in the sample over the next 18 000 years. [3]
6
(b)
A radioactive source emits β -particle and γ -rays. The following figure shows how the two types of radiation are deflected when traveling in a vacuum through a uniform magnetic field is applied at right angles to the plane of the paper. (i) Explain why the γ -rays are not deflected (ii)
Explain why the β -particles are deflected [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 βparticles.
(ii)
Describe how you would demonstrate that the radioactive emission from the source is random. State one safety precaution that you would take when handling any radioactive source. [8]
(iii)
(b)
A radioactive isotope of radon (Rn-220) is represented as 220 86 Rn. 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-particles. (i)
State the number of neutrons in a nucleus of Rn-220.
7
(ii)
The nuclear equation that represents the decay of Radon220 is written as
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. [5] 4.
A smoke detector contains a radioactive source that emits a -particles.
The figure shows the structure of a simple smoke detector. The α-particles ionize 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. [1] (b) Explain why a source that emits β -particles is not used in this detector. [1] (c) State how a current is produced in the air between the plates. [1] (d)
The radioactive source that emits α -particles contains Americium-241. A nucleus of Americium-241 is represented as 241 95 Am. It decays into a nucleus of Neptunium-237. The chemical symbol for Neptunium is Np. Write down the nuclear equation that represents the emission of an α-particle from a nucleus of 241 95 Am . [2]
5.
A teacher counts the number of particles emitted from a radioactive source. (a) State the name of a detector able to detect particles from a radioactive source.
8
(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 below table
(i) (ii)
6.
(a) (b) (c)
State and explain which source has the shortest half-life. 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, 2. source B. [5]
Some atoms are radioactive. Explain what is meant by radioactive. [2] Some hospital equipment is sterilized using gamma-rays. State two properties of gamma-rays that make them suitable for this use. [2] Explain why radioactive sources should only he handled at a distance from the body. [2]
Answers: 1.
B
Since a -particles are totally absorbed by paper and there is no change in the count rate with the paper absorber, a -particles are not present. Since α-particles are totally absorbed by the aluminium absorber, β -particles are present since there is a significant decrease in count-rate. Since the final count-rate does not fall to background count, γ -rays are present.
2.
(a)
(i) Fast-moving electron
9
(b)
(i) (ii)
γ-rays are neutral. β-particles are negatively charged.
3.
A Geiger-Muller tube is placed near to the radioactive source. When a piece of paper is placed between the source and the detector, there is a large drop in the count rate. It shows that α-particles are emitted. Then a metal plate of thickness about 3 mm is placed between the source and the detector. There is no significant change in the count rate showing that β particles are not emitted. (ii)
(iii)
The count rate from a long half-life radioactive source is measured in short time intervals. The count rates would show random fluctuations which demonstrate the random nature of decay. The radioactive source should be handled using long forceps.
b)(i) Number of neutrons = 220 - 86 = 134 220 4 216 (ii) 86 Rn → 2 α + 84 Po
10
4.
a. b. c. d.
A helium nucleus consisting of two protons and two neutrons. β-particles have weaker ionizing effects. The electric field between the plates produces forces on the ions in the air causing the ions to move. Since the ions carry charges, the moving charges results in a current. 241 237 4 95 Am → 93 Np + 2 α
5.
a. b.
Geiger-Muller tube. i. Source B. 1 ii. Source A; ( ) 3 .160 = 20 min −1 2 1 6 Source B; ( ) 1600 = 25 min −1 2
6.
a.
The nucleus of the atom is unstable and undergoes transformation in its composition by emitting a combination of α-particle, β-particle and γ–rays. This produces that is more stable than its parent. 1. Ionisation powers. 2. Strong penetrative powers. Some radioactive sources may decay to produce a radioactive gas. If this gas is inhaled, the living cells in the body may damaged.
b. c.
11
Analyzing radioactive decay
a.
Radioactivity
1.
Radioactivity is the spontaneous disintegration of an unstable nucleus accompanied by the emission of energetic particles or photons.
Figure 3: Diagrammatic representation of the ionization b.
Radioactive detectors
i. 1. 2.
Cloud Chamber A cloud chamber is used to show the path of ionising radiation. The tracks of the radioactive emission are formed in the same way as as the condensation trails behind a high-flying aeroplane when the air in the sky is cold and damp enough. Different tracks are formed for each radioactive emission.
3.
Figure 4: Tracks of radioactive emissions ii.
Geiger-Muller Tube
1.
A Geiger-Muller tube (GM tube) is a very versatile, sensitive and useful detector of radiation. It operates at a voltage of about 450 V. Radiation enters the GM tube through the mica window and ionises the argon gas. A pulse of current is produced. This pulse of current is counted by a scaler or a ratemeter. The scaler gives the number of counts over a certain period of time. The ratemeter gives the count rate in counts per second or counts per minute. The GM tube can detect alpha particles, beta particles and gamma rays.
2. 3. 4. 5. 6. 7.
1
iii.
Photographic Film
1.
A photographic film found in badges worn by the staff at radiation laboratories is developed at the end of every month. The degree of darkening of the photographic film indicates the amount of radiation received.
2. iv.
Spark Counter
1. 2.
4.
A spark counter uses the ionising effect of radiation. A high voltage is applied between the gauze and a wire below the gauze, and adjusted until it is just below the voltage required to produce sparks. When a radioactive source is brought near, the radiation ionises the air between the gauze and the wire, and sparks are produced. Spark counters are suitable for detecting alpha particles.
c.
Characteristics of radioactive emissions
3.
Figure 5: Natural characteristics of radioactive emissions 1.
There are three kinds of radioactive emissions: • Alpha particles • Beta particles • Gamma rays
2
Alpha decay Beta decay Gamma decay An alpha particle is A beta particle is emitted A gamma ray photon is emitted emitted The proton number is The proton number is The proton number and reduced by two and the increased by one and the nucleon number are nucleon number is nucleon number is unchanged reduced by four unchanged Table 1: Radioactive emissions
Figure 6: Range of radioactive emissions in air
Figure 7: Penetrating powers of radioactive emissions
Figure 8: The effect of a magnetic field on the radioactive emissions
Figure 9: The effect of an electric field on the radioactive emissions
3
Table 2: Nature and characteristics of radioactive emissions
d.
Radioactive Decay
1.
The process of a nucleus changing to a more stable nucleus while emitting radiation is called radioactive decay. The nucleus before the decay is called the parent nuclide and the product of the decay is the daughter nuclide. Radioactive decay is named according to the type of radioactive emitted. Type of the radioactive decay
2. 3. 4.
4
i.
Alpha decay
Alpha decay usually happens to the heavier unstable nuclei.
Figure 10: An alpha decay 238 92
ii.
U Æ234Th + 4He 90
2
Beta decay
Beta decay usually occurs for nuclei that have an excess of neutrons.
Figure 11: Beta decay
iii.
Gamma decay
Gamma decay occurs when an unstable nucleus releases its excess energy in the form of high frequency electromagnetic waves called γ-rays.
e.
Radioactive Decay Series
Bismuth-214 goes through a series of 5 decays to become a stable lead-206.
5
f.
Half-life
The half-life of a radioactive nuclide is the time taken for the number of undecayed nuclei to be reduced to half of its original number.
Questions: 1.
A pupil investigates the penetrating power of radiation from a radioactive source.
The table shows the results. Background count Count with source only
25 counts per minute 630 counts per minute Count with source and paper 630 counts per absorber minute Count with source and aluminium 180 counts per absorber 3 mm thick minute The source emits A alpha and beta-particles. B beta-particles and gamma-rays. C beta-particles only. D gamma-rays only.
2.
(a)
14
C is a radioactive isotope of carbon which decays by β-particle emission with a half-life of 6000 years. (i) What is a β-particle? (ii) A radioactive sample contains 5000 atoms of 14C. On the axes below, plot the graph of the number of 14C atoms in the sample over the next 18 000 years. [3]
6
(b)
A radioactive source emits β -particle and γ -rays. The following figure shows how the two types of radiation are deflected when traveling in a vacuum through a uniform magnetic field is applied at right angles to the plane of the paper. (i) Explain why the γ -rays are not deflected (ii)
Explain why the β -particles are deflected [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 βparticles.
(ii)
Describe how you would demonstrate that the radioactive emission from the source is random. State one safety precaution that you would take when handling any radioactive source. [8]
(iii)
(b)
A radioactive isotope of radon (Rn-220) is represented as 220 86 Rn. 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-particles. (i)
State the number of neutrons in a nucleus of Rn-220.
7
(ii)
The nuclear equation that represents the decay of Radon220 is written as
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. [5] 4.
A smoke detector contains a radioactive source that emits a -particles.
The figure shows the structure of a simple smoke detector. The α-particles ionize 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. [1] (b) Explain why a source that emits β -particles is not used in this detector. [1] (c) State how a current is produced in the air between the plates. [1] (d)
The radioactive source that emits α -particles contains Americium-241. A nucleus of Americium-241 is represented as 241 95 Am. It decays into a nucleus of Neptunium-237. The chemical symbol for Neptunium is Np. Write down the nuclear equation that represents the emission of an α-particle from a nucleus of 241 95 Am . [2]
5.
A teacher counts the number of particles emitted from a radioactive source. (a) State the name of a detector able to detect particles from a radioactive source.
8
(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 below table
(i) (ii)
6.
(a) (b) (c)
State and explain which source has the shortest half-life. 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, 2. source B. [5]
Some atoms are radioactive. Explain what is meant by radioactive. [2] Some hospital equipment is sterilized using gamma-rays. State two properties of gamma-rays that make them suitable for this use. [2] Explain why radioactive sources should only he handled at a distance from the body. [2]
Answers: 1.
B
Since a -particles are totally absorbed by paper and there is no change in the count rate with the paper absorber, a -particles are not present. Since α-particles are totally absorbed by the aluminium absorber, β -particles are present since there is a significant decrease in count-rate. Since the final count-rate does not fall to background count, γ -rays are present.
2.
(a)
(i) Fast-moving electron
9
(b)
(i) (ii)
γ-rays are neutral. β-particles are negatively charged.
3.
A Geiger-Muller tube is placed near to the radioactive source. When a piece of paper is placed between the source and the detector, there is a large drop in the count rate. It shows that α-particles are emitted. Then a metal plate of thickness about 3 mm is placed between the source and the detector. There is no significant change in the count rate showing that β particles are not emitted. (ii)
(iii)
The count rate from a long half-life radioactive source is measured in short time intervals. The count rates would show random fluctuations which demonstrate the random nature of decay. The radioactive source should be handled using long forceps.
b)(i) Number of neutrons = 220 - 86 = 134 220 4 216 (ii) 86 Rn → 2 α + 84 Po
10
4.
a. b. c. d.
A helium nucleus consisting of two protons and two neutrons. β-particles have weaker ionizing effects. The electric field between the plates produces forces on the ions in the air causing the ions to move. Since the ions carry charges, the moving charges results in a current. 241 237 4 95 Am → 93 Np + 2 α
5.
a. b.
Geiger-Muller tube. i. Source B. 1 ii. Source A; ( ) 3 .160 = 20 min −1 2 1 6 Source B; ( ) 1600 = 25 min −1 2
6.
a.
The nucleus of the atom is unstable and undergoes transformation in its composition by emitting a combination of α-particle, β-particle and γ–rays. This produces that is more stable than its parent. 1. Ionisation powers. 2. Strong penetrative powers. Some radioactive sources may decay to produce a radioactive gas. If this gas is inhaled, the living cells in the body may damaged.
b. c.
11
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