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Cambridge IGCSE0625

Radioactivity

Physics 0625 Chapter Notes

What this chapter covers

Radioactivity - Detection of radioactivityRadioactivity - The three types of nuclear emissionRadioactivity - Radioactive decayRadioactivity - Half-lifeRadioactivity - Safety precautions
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Radioactivity notes

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In this chapter

  1. 1Background radiation
  2. 2Sources of background radiation
  3. 3Detectors, counters and count rate
  4. 4Corrected count rate
  5. 5Spontaneous and random emission
  6. 6The nature of alpha, beta and gamma
  7. 7Penetrating ability and ionising effect
  8. 8Deflection in electric and magnetic fields
  9. 9Why alpha is more ionising
  10. 10Unstable nuclei and radioactive decay
  11. 11Decay changes the element
  12. 12Why some isotopes are radioactive
  13. 13What decay does to the nucleus
  14. 14Decay equations: α-decay
  15. 15Decay equations: β-decay and γ-emission
  16. 16Half-life
  17. 17Counting half-lives
  18. 18Decay curves
  19. 19Background radiation in half-life data
  20. 20Which radiation for which job
  21. 21Which half-life for which job
  22. 22Effects of ionising radiation on living things
  23. 23Moving and using radioactive materials safely
  24. 24Storing radioactive materials and waste
  25. 25Reducing exposure: time and distance
  26. 26Reducing exposure: shielding

1. Background radiation

Background radiation is ionising nuclear radiation that is always present in the environment.

It is detected even when there is no source present. A detector with no source nearby still gives a reading.

Examiner insight

“The most important point of the concept of background radiation is that it is detected even when there is no source present.” “Misconceptions included confusing background radiation with count rate and suggesting it was the radiation used to operate the counter.”

Worked example 11 mark

Complete the sentence about radiation. Ionising nuclear radiation that is always present in the environment is known as ... radiation.

  1. 1

    background (radiation) [1]

2. Sources of background radiation

The sources that make a significant contribution are:

radon gas (in the air), rocks and buildings, food and drink, and cosmic rays.

Radon gas from rocks is a natural source. Medical X-rays, nuclear weapons testing and radioactive waste from power stations are due to human activity.

Examiner insight

“Common errors included references to the Sun, suggesting confusion with infrared radiation and stating the ground as a source, perhaps selecting this from Fig. 10.1.” “Candidates are advised to use the most significant sources of background radiation listed in the syllabus where possible, rather than brief answers such as medical or industrial.”

Worked example 11 mark

Some sources of background radiation are natural and other sources are due to human activity. Which source is natural?

A medical X-rays · B nuclear weapons testing · C radioactive waste from power stations · D radon gas from rocks

  1. 1

    D: radon gas from rocks. [1]

3. Detectors, counters and count rate

Ionising nuclear radiation can be measured using a detector connected to a counter.

The count rate is the number of counts in each second or each minute. It is measured in counts / s or counts / minute.

Examiner insight

“Candidates needed to remember to add the unit to the numerical answer. Some candidates omitted the unit.”

Worked example 11 mark

The equipment used to measure ionising nuclear radiation includes a detector. What is the detector connected to?

A an ammeter · B a counter · C a force meter · D a timer

  1. 1

    B: a counter. [1]

4. Corrected count rate

A detector near a source counts the source and the background together.

Measure the background count rate on its own, then subtract it from the count rate with the source. The result is the corrected count rate, due to the source alone.

corrected count rate = measured count rate - background count rate

Examiner insight

“A common error was to add the background count rate at the end, but this was unnecessary as the question only asked for the count rate due to the source.”

Worked example 11 mark

A radioactive isotope has a half-life of 8.0 days. A detector close to a sample of this isotope gives a count rate of 200 counts per minute. Without the source, the background count is 20 counts per minute. What is the count rate due to the source after 8.0 days?

A 80 counts per minute · B 90 counts per minute · C 100 counts per minute · D 110 counts per minute

  1. 1

    source alone at the start = 200 - 20 = 180 counts per minute

  2. 2

    after one half-life of 8.0 days: 180 ÷ 2 = 90 counts per minute

  3. 3

    B: 90 counts per minute. [1]

Recap

  • Background radiation is ionising nuclear radiation always present in the environment.
  • It is detected even when no source is present.
  • Sources: radon gas, rocks and buildings, food and drink, cosmic rays.
  • A detector connected to a counter measures ionising nuclear radiation.
  • Count rate is in counts / s or counts / minute.
  • Extended: corrected count rate = measured count rate minus background count rate.

5. Spontaneous and random emission

Radiation is emitted from the nucleus of a radioactive atom. The emission is spontaneous, and its direction is random.

A nucleus does not become more likely to decay as time passes.

Examiner insight

“Candidates struggled to recall the necessary facts about the random nature of radioactive decay.” “Many candidates had the misconception that the probability of a nucleus decaying increases over time and therefore chose option D.”

6. The nature of alpha, beta and gamma

Alpha (α), beta (β) and gamma (γ) are the three emissions from a nucleus. In this course β means β–, not β+.

An α-particle is a helium nucleus. It contains two neutrons and has a charge of +2. A β-particle is an electron, with a charge of –1. γ-radiation is an e.m. wave with no charge.

The nature of an emission is what it is. A charge is only one of its properties.

Examiner insight

“Candidates found this question quite challenging with many failing to identify +2 as the charge on an alpha particle. The most common error was to simply say it was positive, i.e. not giving the relative size of the charge.” “Only a few candidates knew that beta particles are electrons. A common answer was negative charge.”

Worked example 13 marks

Table 9.1 shows some features of three types of radioactive emission. The table is incomplete.

Complete Table 9.1 by adding the missing information.

type of radioactive emissionnature of radioactive emissioncharge on radioactive emission
alpha (α)helium nucleus
beta (β)–1
gamma (γ)0
  1. 1

    alpha (α) charge: +2 [1]

  2. 2

    beta (β) nature: electron [1]

  3. 3

    gamma (γ) nature: e.m. wave [1]

7. Penetrating ability and ionising effect

Alpha particles are stopped by paper. Beta particles can penetrate paper and thin aluminium, but not thicker aluminium. Gamma radiation is the most penetrating. It needs thick lead, many cm of it, or concrete at least 1 m thick.

Alpha particles are less penetrating and more ionising than beta particles. Gamma radiation is the most penetrating, but it is not the most ionising.

Examiner insight

“A significant number of candidates indicated that lead was needed to stop gamma radiation. Lead on its own was insufficient. There needed to be an indication that it should be thick lead (i.e. at least 10 cm or many cm of lead).” “A common error was to indicate that gamma radiation was the most ionising.”

Worked example 11 mark

Americium-241 decays by emitting alpha particles. Americium-242 decays by emitting beta particles.

Compare the ability of alpha particles and beta particles to penetrate paper.

  1. 1

    alpha is stopped (by paper) AND beta can penetrate (paper) [1]

8. Deflection in electric and magnetic fields

α-particles and β-particles are charged, so both are deflected in an electric field and in a magnetic field. γ-radiation has no charge and is not deflected.

Between two charged plates, an α-particle curves towards the negative plate. A β-particle is negatively charged, so it curves the other way, towards the positive plate, and it is deflected more than an α-particle. γ-radiation is undeflected. A path starts to curve as soon as the particle enters the field.

In a magnetic field the path of a charged particle is circular. The deflection is at right angles to both the magnetic field and the velocity of the particle. It is not towards a magnetic pole. α-particles and β-particles are deflected in opposite directions.

Examiner insight

“A common misconception was to continue the path of alpha particles undeflected until they were a long way through the electric field.” “Some candidates identified that opposite charges experience an electrostatic force without clearly stating that the beta particles were negatively charged.”

Worked example 13 marks

Figure 9.1 shows a beam of α-particles and γ-radiation entering an electric field between two charged metal plates P and Q.

The beam of α-particles and γ-radiation in Figure 9.1 is replaced by a beam of β-particles.

State and explain the path of the β-particles in the electric field. Use ideas about electrostatic force in your answer.

  1. 1

    statement: β-particles are deflected towards the positive plate (the path curves upwards) [1]

  2. 2

    explanation: β-particles are negatively charged [1]

  3. 3

    charged particles in an electric field experience a force [1]

9. Why alpha is more ionising

Alpha particles are more ionising than beta particles because they have more kinetic energy.

Alpha particles also have a greater magnitude of charge: +2 for an α-particle against –1 for a β-particle.

Examiner insight

“The majority of candidates knew that alpha particles were less penetrating and/or more ionising than beta particles, but could not explain that this was due to alpha particles having more kinetic energy than beta particles.” “Few candidates made a clear statement that alpha particles have a greater magnitude of charge than beta particles.”

Recap

  • Emission from the nucleus is spontaneous and random in direction.
  • α = helium nucleus, +2. β = electron, –1. γ = e.m. wave, no charge.
  • Penetrating: α stopped by paper, β passes paper but not thicker aluminium, γ needs thick lead.
  • α is more ionising and less penetrating than β. γ is not the most ionising.
  • Extended: α curves towards the negative plate, β curves more and towards the positive plate, γ is undeflected.
  • Extended: in a magnetic field the deflection is at right angles to the field, with α and β in opposite directions.
  • Extended: α is more ionising than β because it has more kinetic energy and a greater charge.

10. Unstable nuclei and radioactive decay

Radioactive decay is a change in an unstable nucleus. The nucleus can emit α-particles or β-particles and/or γ-radiation.

The changes are spontaneous and random. Spontaneous means the decay does not depend on external conditions, including temperature. Random means unpredictable.

To explain changing count readings, say that the decay is random. Saying that the numbers vary is too vague.

Examiner insight

“Many candidates had the misconception that the probability of a nucleus decaying increases over time and therefore chose option D.” “Many candidates, even those who were strong on most questions, failed to appreciate the random nature of radioactive decay.”

11. Decay changes the element

During α-decay or β-decay, the nucleus changes to that of a different element.

γ-emission is not one of these. Only α-decay and β-decay change the element.

Examiner insight

“A substantial number of candidates chose option B and were unaware that beta decay changes the substance into a different element, as specified in syllabus section 5.2.3.2.”

Worked example 11 mark

Which statement about radioactive decay by α, β and γ emission is correct?

A The only type of radioactive decay that changes a nucleus of one element to that of another element is α-decay.

B The only types of radioactive decay that change a nucleus of one element to that of another element are α-decay and β-decay.

C The only types of radioactive decay that change a nucleus of one element to that of another element are α-decay and γ-emission.

D The only types of radioactive decay that change a nucleus of one element to that of another element are β-decay and γ-emission.

  1. 1

    α-decay changes the nucleus to that of a different element

  2. 2

    β-decay changes the nucleus to that of a different element

  3. 3

    γ-emission does not, so the only types are α-decay and β-decay: option B [1]

12. Why some isotopes are radioactive

Isotopes of an element may be radioactive. This is due to an excess of neutrons in the nucleus and/or the nucleus being too heavy. Not every isotope is radioactive.

Examiner insight

“Only the strongest candidates gained full credit here, recognising that the stable isotope has fewer neutrons because unstable isotopes usually have an excess of neutrons or are too heavy.” “A significant proportion of candidates of all abilities thought that isotopes must be radioactive/unstable.”

Worked example 12 marks

Carbon-14 and carbon-11 are different isotopes of carbon used in medicine. State the similarity and the difference between the composition of the nuclei of carbon-14 and carbon-11.

  1. 1

    similarity: the same number of protons [1]

  2. 2

    difference: carbon-14 has 14 - 11 = 3 more neutrons [1]

13. What decay does to the nucleus

Each kind of decay increases the stability of the nucleus. It reduces the number of excess neutrons.

In β-emission a neutron changes into a proton and an electron. The electron is emitted. The nucleus has one more proton and one fewer neutron.

neutron → proton + electron

Examiner insight

“Many candidates gave only half an answer, stating either that the number of protons increased by one or that the number of neutrons decreased by one.” “Common misconceptions were that the nucleus didn't change or became positively charged.”

Worked example 11 mark

A carbon-14 nucleus decays into a nitrogen (N) nucleus by the emission of a beta particle (β-particle). State the change that occurs in the carbon-14 nucleus during the decay.

  1. 1

    a neutron changes into a proton (plus an electron, which is emitted) [1]

14. Decay equations: α-decay

In nuclide notation the nucleon number is written above the proton number. An α-particle has nucleon number 4 and proton number 2.

In a decay equation the nucleon numbers balance and the proton numbers balance.

^(A)_ZX → ^(A-4)_Z-2Y + ⁴₂α

Examiner insight

“A common mistake was to use the number of neutrons as the nucleon number for Am.” “Weaker candidates used the neutron numbers in place of mass numbers or got the mass number and atomic number the wrong way up.”

Worked example 11 mark

A nucleus of radon, Rn, emits an α-particle and changes into a nucleus of polonium, Po. The equation shows the change.

^(a)₈₆Rn → ²¹⁴_bPo + ⁴₂α

What are the values of a and b?

A 210, 84 · B 210, 88 · C 218, 84 · D 218, 88

  1. 1

    nucleon numbers balance: a = 214 + 4 = 218

  2. 2

    proton numbers balance: 86 = b + 2, so b = 84 (option C) [1]

15. Decay equations: β-decay and γ-emission

A β-particle has nucleon number 0 and proton number -1. In β-emission the proton number goes up by one and the nucleon number does not change.

γ-radiation is a wave and has no mass or charge. A γ-emission changes neither number.

^(A)_ZX → ^(A)_Z+1Y + ⁰₋₁β

^(A)_ZX → ^(A)_ZX + γ

Examiner insight

“The most common incorrect answers were to give the atomic number (proton number) of protactinium as 89 or the equivalent number for the beta-particle as 1 rather than –1.” “Common mistakes included using the neutron number as the nucleon number, omitting the nucleon number for beta and repetition of the Sr symbol on both sides of the equation.”

Worked example 12 marks

A carbon-14 nucleus decays into a nitrogen (N) nucleus by the emission of a beta particle (β-particle). Complete the nuclear equation for the decay.

¹⁴₆C → ? + ?

  1. 1

    proton number goes up by 1, so 6 + 1 = 7 and the nucleus is ¹⁴₇N [1]

  2. 2

    β-particle: ⁰₋₁β, so 14 = 14 + 0 and 6 = 7 + (-1) [1]

Recap

  • Radioactive decay is a change in an unstable nucleus. It is spontaneous and random.
  • The nucleus can emit α-particles or β-particles and/or γ-radiation.
  • α-decay and β-decay change the nucleus to that of a different element.
  • Extended: an isotope may be radioactive through an excess of neutrons and/or being too heavy.
  • Extended: decay increases stability and reduces excess neutrons. In β-emission a neutron changes into a proton and an electron.
  • Extended: balance nucleon numbers and proton numbers. α is ⁴₂α, β is ⁰₋₁β.

16. Half-life

The half-life of a particular isotope is the time taken for half the nuclei of that isotope in any sample to decay.

The count rate or activity of the sample falls to half in the same time.

To use it, count how many half-lives fit into the time given. Then halve once for each half-life.

n = t/(t_1/2)

amount left = start × (1/2)ⁿ

Examiner insight

“The time for half the nuclei in a radioactive sample to decay was often expressed poorly with weaker candidates stating that it was the time for half the nucleus (or atom) to decay.” “Another common mistake was to state that it is the time for the substance to halve, without making clear that it is the mass of radioactive isotope that halves.”

Worked example 13 marks

Iodine-125 has a half-life of 60 days. A radioactive source contains 40 mg of iodine-125. Calculate the mass of iodine-125 that remains in the source after 180 days.

  1. 1

    180 ÷ 60 = 3, so 3 half-lives [1]

  2. 2

    40 → 20 → 10 → 5, or 40 × 1/2 × 1/2 × 1/2, or 40 ÷ 8 [1]

  3. 3

    mass remaining = 5.0 mg [1]

17. Counting half-lives

Each half-life halves what is left. After two half-lives a quarter is left, not none.

The number of half-lives is the time divided by the half-life. To find a time, multiply the number of half-lives by the half-life.

You can also work backwards from the amount left to the number of halvings.

Examiner insight

“Many candidates seemed to think that if the half-life of astatine-210 was eight hours, the astatine-210 would have completely disappeared after 16 hours.” “However, many then divided the half-life time by 3 instead of multiplying.”

Worked example 12 marks

Scientists discover an ancient wooden object. The object contains 1.0 × 10⁻¹⁰ g of carbon-14. The object contained 8.0 × 10⁻¹⁰ g of carbon-14 when it was made. Carbon-14 has a half-life of 5700 years. Estimate the age of the ancient wooden object.

  1. 1

    8.0 → 4.0 → 2.0 → 1.0, so 3 half-lives [1]

  2. 2

    age = 3 × 5700 = 17 100 ≈ 17 000 years [1]

18. Decay curves

A decay curve is a smooth curve with a negative gradient of decreasing magnitude: steep at first, then flatter. It does not reach the time axis. It is not a straight line.

To find the half-life, read the time for the quantity to fall to half of its starting value. The half-life is not half of the maximum time shown on the axis.

Examiner insight

“Few candidates recognised the shape of a graph showing the decay of a radioactive isotope, with many candidates choosing a straight line graph.” “A significant number of weaker candidates thought that the half-life was half of the maximum time shown on the horizontal axis.”

Worked example 13 marks

A tree contains 16 μg of carbon-14 when it dies. Carbon-14 has a half-life of 5700 years. Sketch a graph to show how the mass of carbon-14 in the tree decreases during the first 15 000 years after the tree dies. Show your working and label the axes with suitable values for one point on the graph.

  1. 1

    smooth curve with a negative gradient of decreasing magnitude, starting from 16 μg on the mass axis [1]

  2. 2

    curve which does not reach the time axis [1]

  3. 3

    value of 8 μg at 5700 years on the graph, or 4 μg at 11 400 years [1]

19. Background radiation in half-life data

A detector close to a source gives the total count rate: the source and the background radiation together. Data and decay curves can be given with the background not subtracted.

To find the half-life: subtract the background from the starting value. Halve what remains. Then add the background back before you read the time from the graph or table.

Some candidates ignore the background count and simply halve the starting count. Others subtract it and halve what remains, but do not add it back. Both give the wrong half-life.

source count rate = total count rate - background count rate

Examiner insight

“A large majority of candidates totally ignored this information, whereas, they should have subtracted the background count from both the initial measured count rate and the measured count rate when a half-life has been reached.” “The most common mistake was to correctly subtract the background count (20) from the initial count (180 – 20 = 160), halve what remains (160 / 2 = 80) but not to add the background count to this before reading the half-life from the graph.”

Worked example 11 mark

Oxygen-15 is used in hospitals. The count rate from a detector placed close to a sample of oxygen-15 was recorded over a period of 15 min. The background count rate is 20 counts / min. The graph shows the total count rate from the detector. What is the half-life of this sample of oxygen-15?

A 2.0 min B 2.4 min C 2.8 min D 7.5 min

  1. 1

    source at the start = 180 - 20 = 160 counts / min

  2. 2

    half of this is 80, so the detector reads 80 + 20 = 100 counts / min

  3. 3

    the graph falls to 100 counts / min at 2.0 min: A [1]

20. Which radiation for which job

The radiation is chosen by how far it penetrates and how much the material absorbs it. Alpha is stopped by paper. Beta can penetrate paper.

A smoke alarm uses alpha: it is highly ionising and easily stopped by the smoke particles. A gauge for the thickness of aluminium strips uses beta: beta particles pass through thin strips but not thicker strips, so the amount of absorption depends on the thickness.

Gamma rays are used to sterilise equipment and in the diagnosis and treatment of cancer. Radiation is also used to irradiate food to kill bacteria.

Examiner insight

“They were able to give alpha as the radiation source, explaining that it is suitable because it is highly ionising or easily stopped by the smoke particles.” “Some weaker candidates confused X-rays with γ-rays or stated that γ-rays were used in chemotherapy.”

Worked example 11 mark

Americium-241 decays by emitting alpha particles. Americium-242 decays by emitting beta particles. Compare the ability of alpha particles and beta particles to penetrate paper.

  1. 1

    alpha is stopped by paper AND beta can penetrate paper [1]

21. Which half-life for which job

The half-life must suit the job. A source used for years, as in a thickness gauge, needs a half-life in years. Otherwise the activity falls quickly and the source needs replacing too often.

To trace a leak in a water pipe, a very short half-life decays too quickly for the leak to be detected. A very long half-life would contaminate the water supply.

Examiner insight

“Stronger candidates stated that an isotope with a short half-life would decay too quickly for the leak to be detected.” “Very few candidates understood that the half-life should be in years, with many giving an answer of seconds or days.”

Worked example 12 marks

A factory uses a beta source to monitor the thickness of aluminium strips all day, every day. The source can have a half-life of 5 hours or 5 years. State which half-life is suitable and give a reason.

  1. 1

    a half-life of 5 years [1]

  2. 2

    otherwise the activity would become low very quickly, so the source would need replacing too frequently [1]

Recap

  • Half-life = time taken for half the nuclei of that isotope in any sample to decay.
  • Number of half-lives = time ÷ half-life. Halve once for each half-life.
  • A decay curve is a curve, not a straight line. It does not reach the time axis.
  • Half-life from a graph: the time for the value to fall to half, not half the axis.
  • Extended: subtract the background, halve, then add the background back.
  • Extended: alpha is stopped by paper and beta can penetrate paper. The radiation chosen links to penetration and absorption.
  • Extended: half-life must suit the job. Years for a long-running gauge. Too short and it decays before it can be used.

22. Effects of ionising radiation on living things

Ionising nuclear radiations can damage living things. The effects include cell death, mutations and cancer.

The damage happens because the radiation is ionising.

Examiner insight

“Many candidates gained partial credit for identifying one of the effects of radiation on the human body, but very few went on to explain that this was due to the ionising capability of the radiation.”

23. Moving and using radioactive materials safely

Do not touch a radioactive material. Use gloves or tongs to prevent contact. Keep it at a distance.

Examiner insight

“This was most often for the use of gloves or tongs (to prevent contact), the use of lead containers for storage or to keep a distance away.” “Answers such as wearing protective clothing or goggles were too vague.”

Worked example 11 mark

A student is doing an experiment with radioactive materials. Which piece of advice is not useful?

A Do not spend too long doing the experiment · B Keep any material not being used at a distance · C Take frequent sips of water · D Wear gloves

  1. 1

    C: sips of water do nothing to reduce the risk. A, B and D all reduce it. [1]

24. Storing radioactive materials and waste

Store radioactive sources in lead containers.

Radioactive waste is hazardous. It must be stored securely and safely. It can need storing for a long time.

Examiner insight

“Stronger candidates referred to the need for storing radioactive waste securely or safely and the cost involved.” “Some candidates simply referred to disposal of radioactive waste or sending it to landfill.”

Worked example 12 marks

State one advantage and one disadvantage of the use of nuclear energy to generate electricity compared with the use of light from the Sun (solar cells).

  1. 1

    advantage: is available 24 hours per day [1]

  2. 2

    disadvantage: produces hazardous / dangerous waste [1]

25. Reducing exposure: time and distance

Two precautions cut the radiation a person receives. Reduce the exposure time. Increase the distance between the source and living tissue.

A longer distance lowers the amount of radiation reaching the person. Moving closer is not a precaution.

Examiner insight

“However, the majority of weaker candidates thought that reducing the distance between a radioactive source and a person using it was also a precaution and chose option A.” “Good explanations explained that increasing distance lowered the amount of radiation reaching staff or that shielding absorbed radiation etc.”

Worked example 11 mark

Safety precautions are needed when a person is using a radioactive source. What is not a precaution for a person using a radioactive source?

A Keep as far from the source as possible while using it · B Keep a transparent shield between the person and the source · C Keep the time for which the source is being used as short as possible · D Keep the source switched off when the radiation is not needed

  1. 1

    A is distance, B is shielding and C is exposure time. These are the three precautions.

  2. 2

    D is the one that is not a precaution. [1]

26. Reducing exposure: shielding

Put a shield between the source and living tissue. The shield absorbs the radiation.

Lead is used as a shield. A lead apron shields the wearer by absorbing some of the radiation. Lead does not repel or reflect radiation.

Examiner insight

“Candidates did not understand that lead absorbs the radiation from a radioactive source.” “A common, correct answer was to suggest wearing a lead apron as the apron absorbs X-rays.”

Worked example 11 mark

A radiographer works in an area where there is a radioactive source. The radiographer wears a lead apron. How does the apron act as a safety precaution?

A It increases the distance between the radiographer and the source · B It reduces the time that the radiographer is exposed to radiation · C It shields the radiographer by absorbing some of the radiation · D It stops radiation reaching the radiographer by reflecting the radiation

  1. 1

    C: it shields the radiographer by absorbing some of the radiation. [1]

Recap

  • Ionising radiation can cause cell death, mutations and cancer.
  • Use gloves or tongs. Keep sources at a distance. Store them in lead containers.
  • Radioactive waste is hazardous and must be stored securely.
  • Extended: reduce exposure time, increase distance, and use shielding.
  • Extended: a shield such as lead absorbs radiation. It does not repel it.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what is meant by background radiation.1 mark
  2. A student wants to measure the ionising radiation from a rock. Name the two pieces of equipment she needs.2 marks
  3. State two sources that make a significant contribution to background radiation.2 marks
  4. A student says, ‘Background radiation only comes from space.’ Explain why she is wrong.2 marks
  5. A detector with no source nearby records 150 counts in 5.0 minutes. Calculate the background count rate in counts / minute.2 marks
  6. A counter records 7200 counts in one hour. Give the count rate in counts / s.2 marks
  7. With no source present, a detector in a laboratory records 24 counts in 30 s. In a basement room, with no source present, it records 150 counts in 2.0 minutes. A student says the basement has the greater background count rate. Is she right?3 marks
  8. A detector near a source gives a count rate of 85 counts / minute. The background count rate is 15 counts / minute. Calculate the corrected count rate.2 marks
  9. A museum tests two old watches, X and Y, with the same detector and counter. With no watch present, the counter records 100 counts in 5.0 minutes. With watch X in place, it records 660 counts in 3.0 minutes. With watch Y in place, the count rate is 7.0 counts / s. Show that the count rate due to watch Y is twice the count rate due to watch X.4 marks
  10. A factory checks a counter with a test source. The count rate due to the test source alone is known to be 6.0 counts / s. With no source present, the counter records 480 counts in 10 minutes. The test source is then put in place. Calculate the number of counts the counter should record in 5.0 minutes.4 marks
  11. Describe the emission of radiation from a nucleus.2 marks
  12. State the nature and the charge of a beta particle.2 marks
  13. State the charge on an alpha particle and say what an alpha particle is.2 marks
  14. Put alpha, beta and gamma radiation in order of penetrating ability, least penetrating first.3 marks
  15. A student says that a sheet of lead stops gamma radiation. Correct the statement. Then name one other material that can be used, with its thickness.2 marks
  16. State which of alpha and beta is more ionising and which is more penetrating.2 marks
  17. Which of alpha, beta and gamma are deflected in both a magnetic field and an electric field? Explain why the other one is not.3 marks
  18. Explain why alpha particles are more ionising than beta particles. Do not answer in terms of penetration.2 marks
  19. A source emits two types of radiation in one narrow beam. In a vacuum, the beam passes between two charged plates and reaches two detectors. Detector X is in line with the beam. Detector Y is closer to the negative plate. Both detectors record radiation from the source. A sheet of paper is then placed between the source and the plates. Detector Y now records nothing from the source. The reading on detector X does not change. (a) Identify the radiation reaching X and the radiation reaching Y. Give the evidence for each. (b) The paper is removed and the charges on the two plates are swapped. State and explain what each detector now records.6 marks
  20. A beam of α-particles, β-particles and γ-radiation travels from left to right across a page, in a vacuum. It enters a region where a magnetic field points into the page. The β-particles are deflected towards the bottom of the page. (a) State what happens to the α-particles and to the γ-radiation in this region. (b) A student says that the magnetic field must point towards the bottom of the page, because that is the way the β-particles move. Explain why the student is wrong. (c) The magnetic field is replaced by an electric field between two charged plates, one above the beam and one below it. The β-particles are still deflected towards the bottom of the page. State which plate is positive and the direction in which the α-particles are now deflected.6 marks
  21. State what is meant by saying that radioactive decay is random.1 mark
  22. A student heats a radioactive source to make it decay faster. State what happens to the decay and explain why.2 marks
  23. Which of α-decay, β-decay and γ-emission change a nucleus to that of a different element?2 marks
  24. A student counts the particles from a source for four periods of 10 s. The counts are 14, 9, 12 and 11. Explain why the counts are different.2 marks
  25. State two reasons why an isotope of an element may be radioactive.2 marks
  26. A nucleus contains 20 protons and 25 neutrons. It emits a β-particle. State the change that occurs in the nucleus, and the numbers of protons and neutrons in the nucleus after the emission.3 marks
  27. Complete the decay equation: ²²⁶₈₈Ra → ^(a)_bRn + ⁴₂α. Give a and b.2 marks
  28. Strontium-90 decays by β-emission to yttrium, Y: ⁹⁰₃₈Sr → ⁹⁰_bY + ⁰₋₁β. Find b.2 marks
  29. A hospital uses a radioactive isotope of iodine, I. Each nucleus contains 53 protons and 78 neutrons. The nucleus emits a β-particle and becomes a nucleus of xenon, Xe. The xenon nucleus then emits γ-radiation. (a) Write the decay equation for the β-emission, using nuclide notation. (b) Determine the number of neutrons in the xenon nucleus. (c) State the proton number and the nucleon number of the nucleus after the γ-emission. (d) A stable isotope of iodine has 74 neutrons in each nucleus. Suggest why the isotope used in the hospital is radioactive.6 marks
  30. A rock contains thorium, ²³²₉₀Th. A thorium nucleus changes into a stable nucleus of lead, ²⁰⁸₈₂Pb, through a series of decays. Each decay in the series emits either an α-particle or a β-particle. (a) Determine the number of α-particles and the number of β-particles emitted in the series. (b) State the number of neutrons that change into protons during the series.6 marks
  31. State what is meant by the half-life of an isotope.1 mark
  32. Isotope X has a half-life of 12 days. A sample contains 96 g of isotope X. Calculate the mass of isotope X left after 36 days.2 marks
  33. An isotope has a half-life of 2.5 hours. A sample contains 160 g of the isotope. Calculate the time taken for the mass of the isotope to fall to 10 g.2 marks
  34. The table shows the mass of a radioactive isotope in a sample as it decays. Determine the half-life of the isotope. Then calculate the mass of the isotope in the sample at 20 hours. | time / hours | 0 | 4 | 8 | 12 | 16 | | --- | --- | --- | --- | --- | --- | | mass / mg | 200 | 100 | 50 | 25 | 12.5 |3 marks
  35. A detector near a source reads 340 counts / min. The background count rate is 20 counts / min. The half-life of the source is 6.0 hours. Calculate the detector reading after 12 hours.3 marks
  36. The table shows the total count rate from a detector near a radioactive source. The background count rate is 20 counts / min. Calculate the half-life of the source. | time / min | 0 | 10 | 20 | 30 | | --- | --- | --- | --- | --- | | total count rate / counts / min | 220 | 120 | 70 | 45 |3 marks
  37. A smoke alarm contains a source that emits alpha particles. Explain why alpha particles are suitable for a smoke alarm.2 marks
  38. A gamma source is used to find a leak in an underground water pipe. Source A has a half-life of 10 minutes, source B 8 hours and source C 40 years. Which source is suitable? Explain your choice.3 marks
  39. A hospital technician tests a radioactive isotope. With no isotope near the detector, it records 150 counts in 5.0 minutes. With the isotope close to the detector, it records 670 counts / min at the start and 110 counts / min 4.5 hours later. (a) Calculate the half-life of the isotope. (b) Calculate the count rate the detector records after a further 3.0 hours.5 marks
  40. A factory makes aluminium foil. A beta source is on one side of the foil and a detector is on the other side. With no foil in place, the detector records the total count rates in the table. The background count rate is 40 counts / min. (a) Determine the half-life of the source. (b) The source is replaced when the count rate due to the source alone falls to 100 counts / min. Calculate the time after which it is replaced. (c) Another beta source has a half-life of 20 days. Explain why it is not suitable for this factory. | time / years | 0 | 10 | 20 | 30 | 40 | | --- | --- | --- | --- | --- | --- | | total count rate / counts / min | 840 | 606 | 440 | 323 | 240 |7 marks
  41. State two effects of ionising nuclear radiation on living things.2 marks
  42. A student says that nuclear radiation damages cells because it is dangerous. Name the property of the radiation that causes the damage.1 mark
  43. State two precautions a student should take when using a radioactive source in an experiment.2 marks
  44. A school keeps its radioactive sources in a lead container. State what the lead does to the radiation.1 mark
  45. State one problem with the waste from a nuclear power station.1 mark
  46. A teacher picks up a radioactive source with tongs. She does not pick it up by hand. Explain why.2 marks
  47. A technician uses a radioactive source. State how each of the following reduces her exposure to radiation. (a) Working for a shorter time. (b) Moving further from the source. (c) Placing a lead screen between her and the source.3 marks
  48. A nurse works near a patient who has a radioactive source in their body. Explain two safety precautions she can take.4 marks
  49. A vet injects a horse with a radioactive material for a scan. For the next day the horse gives out ionising radiation. A trainee writes three rules for the stable staff. Rule 1: Stand close to the horse so that each visit is over quickly. Rule 2: Stay behind the lead-lined door when you can, because lead repels the radiation. Rule 3: Share the visits between all the staff. Do not let one person do every visit. (a) Rule 1 joins a correct precaution to a mistake. Identify each one and explain the mistake. (b) Rewrite the reason given in Rule 2 so that it is correct. (c) Explain how Rule 3 reduces the risk to each member of staff.5 marks
  50. A worker must tighten a bolt beside a radioactive source. A detector placed where she stands counts the radiation arriving from the source. It records 240 counts / min at 0.50 m from the source and 15 counts / min at 2.0 m from the source. Method X: she uses a short spanner at 0.50 m. The job takes 90 s. Method Y: she uses a long tool at 2.0 m. The job takes 12 minutes. (a) Calculate the number of counts the detector records during the job for each method. (b) State which method is safer. Explain your answer in terms of exposure time and distance. (c) A lead screen between the source and the worker halves the count rate. Calculate the number of counts for method Y with the screen in place. Explain how the screen reduces the count rate.6 marks

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