Atoms and nuclear radiation

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Apuntes de la lección

Unstable Nuclei and Radioactive Decay

Some atomic nuclei are unstable because of an imbalance in the forces within the nucleus.
Isotopes can be unstable because of their large size or because they have too many or too few neutrons.
For example, carbon-14 is unstable because it has two extra neutrons compared to stable carbon-12.
Unstable nuclei emit radiation to become more stable; radiation can be a high-energy particle or wave.
As radiation moves away from the nucleus, it transfers energy away, reducing the overall energy of the nucleus and making it more stable.
The process of emitting radiation is called radioactive decay.
Radioactive decay is a random process: it is not possible to know exactly when a particular nucleus will decay.

Types of Radiation

α (α) particles are the same as a helium nucleus: 2 protons and 2 neutrons, with a charge of +2.
β-minus (β⁻) particles are fast-moving electrons emitted from the nucleus, with a charge of -1.
β-plus (β⁺) particles are fast-moving positrons (positive electrons), with a charge of +1.
Gamma (γ) rays are electromagnetic waves with no charge and the highest energy of all EM waves.
Neutrons (n) are neutral particles (no charge) that can also be emitted from unstable nuclei.
α and β particles can be affected by an electric field because they have a charge; gamma rays and neutrons are unaffected.

Penetrating power of alpha, beta, and gamma radiation through different materials

Properties of Radiation

Penetrating power: α is stopped by paper, β by a few mm of aluminium, gamma is reduced by several cm of lead or metres of concrete.
Ionising power: α is the most ionising, gamma is the least ionising.
Range in air: α travels a few cm, β a few tens of cm, gamma has an infinite range (but intensity decreases with distance).
The more ionising the radiation, the shorter its range in air because it reacts with air particles sooner.
β particles are only stopped by aluminium if it is a few mm thick; thinner aluminium may not stop them.

Background Radiation

Background radiation is the radiation that exists around us all the time.
Natural sources include: radon gas from rocks and soil, cosmic rays from space, carbon-14 in biological material, and radioactive material in food and drink.
Man-made sources include: medical sources (X-rays, CT scans, tracers, therapy), nuclear waste, fallout from nuclear weapons, and nuclear accidents (e.g. Chernobyl).
Radon gas is an α emitter and is particularly dangerous if inhaled in large quantities.
To find the corrected count rate, subtract the background count rate from the measured count rate.

Detecting Radiation

Photographic film darkens when it absorbs radiation; film badges are worn by radiation workers to monitor exposure.
A Geiger-Müller (GM) tube is the most common radiation detector; it produces an electrical pulse for each radiation absorbed, giving a count rate.
The count rate is the number of decays detected per second; the closer the tube to the source, the higher the count rate.
The dose of radiation received is measured in sieverts (Sv); people normally receive about 3 mSv per year from background radiation.

Beta Decay

β-minus decay: a neutron changes into a proton and an electron; the electron is emitted.
In β-minus decay, the mass number stays the same and the atomic number increases by 1.
β-plus decay: a proton changes into a neutron and a positron; the positron is emitted.
In β-plus decay, the mass number stays the same and the atomic number decreases by 1.
β decay forms a new element because the atomic number changes.

Beta decay

Nuclear Transformations and Equations

α emission: the nucleus loses 2 protons and 2 neutrons, so the mass number decreases by 4 and the atomic number decreases by 2.
β-minus emission: the mass number is unchanged and the atomic number increases by 1.
β-plus emission: the mass number is unchanged and the atomic number decreases by 1.
Gamma emission: no change to mass or atomic number; the nucleus loses excess energy.
Neutron emission: the mass number decreases by 1 and the atomic number is unchanged.
In nuclear equations, the sum of mass numbers and the sum of atomic numbers must balance on both sides.

Activity and Half-Life

Activity is the rate at which unstable nuclei decay, measured in becquerels (Bq); 1 Bq = 1 decay per second.
Half-life is the time taken for half the undecayed nuclei to decay, or for the activity to halve.
Half-life is constant for a particular isotope and can range from fractions of a second to billions of years.
After one half-life, 50% remains; after two, 25%; after three, 12.5%, and so on.
The number of undecayed nuclei never quite reaches zero because decay is random.
To find half-life from a graph, find the time for the activity to fall to half its initial value.

A half-life graph showing activity falling from A0 to A0/2 to A0/4

Random Nature of Decay

Radioactive decay is random: there is an equal probability of any nucleus decaying, and it cannot be predicted which or when.
The rate of decay is unaffected by surrounding conditions (e.g. temperature, pressure).
The dice analogy helps: you cannot predict a single roll, but you can predict probabilities over many rolls.
Count rate from a GM tube fluctuates irregularly, providing evidence for the randomness of decay.

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Preguntas de práctica

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1.When carbon-14 decays it emits a β-minus particle. A β-minus particle is
Easy
- Aan electron
- Ba neutron
- Ca positron
- Da proton
2.Which type of radiation is the most ionising?
Easy
- Aα
- Bβ
- CGamma
- DNeutron
3.Which type of radiation can penetrate a few millimetres of aluminium?
Easy
- Aα
- Bβ
- CGamma
- DAll of the above
4.Which type of radiation is a type of electromagnetic radiation?
Easy
- Aα
- Bβ
- CGamma
- DNeutron
5.Which type of radiation has a negative charge?
Easy
- Aα
- Bβ-minus
- CGamma
- DNeutron
6.Cobalt-60 decays to nickel-60 through the process of radioactive decay: 60/27 Co → 60/28 Ni + X. What is particle X?
Medium
- Aα particle
- Bβ-minus particle
- CGamma ray
- DNeutron
7.Match each type of radiation with its correct penetrating power.
Medium
- α
- β
- Gamma
- Stopped by paper
- Stopped by a few mm of aluminium
- Reduced by several cm of lead
8.Radioactive decay is a random process.
Easy
True or false?

Más preguntas

9.Which of the following are natural sources of background radiation? (select all that apply)
Medium
- ARadon gas
- BCosmic rays
- CMedical X-rays
- DNuclear fallout
- ECarbon-14 in biological material
10.Place the types of radiation in order of increasing penetrating power (least to most penetrating).
Medium
- α
- β
- Gamma
11.Which of the following statements about the charge of nuclear radiation are correct? (select all that apply)
Medium
- Aα particles have a charge of +2
- Bβ-minus particles have a charge of -1
- CGamma rays have a charge of +1
- Dβ-plus particles have a charge of +1
- ENeutrons have a charge of -1
12.Match each type of radiation with the material that will stop it.
Medium
- α
- β-minus
- Gamma
- A few centimetres of lead
- A few millimetres of aluminium
- Paper
13.Place the types of radiation in order of increasing range in air (shortest range first).
Medium
- α
- β-minus
- Gamma
14.A student measures the count rate from an unknown source with and without different absorbers. Which conclusions could be correct? (select all that apply)
Hard
- AIf paper reduces the count rate to background, the source emits α radiation
- BIf a few mm of aluminium reduces the count rate to background, the source emits β radiation
- CIf lead only slightly reduces the count rate, the source emits gamma radiation
- DIf paper has no effect, the source must emit α radiation
- EIf aluminium has no effect, the source must emit α radiation
15.Gamma radiation is the least ionising type of nuclear radiation.
Easy
True or false?
16.A radioactive source is placed near a Geiger-Müller tube and the count rate is recorded. The source is then moved further away from the tube. What happens to the count rate?
Medium
- AIt decreases
- BIt increases
- CIt stays exactly the same
- DIt becomes zero immediately
17.A worker in a hospital wears a film badge. Which statement best describes how the badge measures the dose received?
Medium
- AThe film becomes darker the more radiation it absorbs
- BThe film becomes lighter the more radiation it absorbs
- CThe film changes colour only when α radiation is present
- DThe film emits a clicking sound when radiation is absorbed
18.Which of the following are man-made sources of background radiation? (select all that apply)
Medium
- AFallout from nuclear weapons
- BCosmic rays from space
- CNuclear waste and accidents
- DRadon gas from rocks
- EMedical sources such as X-rays
19.A radioactive isotope has a half-life of 6 hours. A technician measures an initial count rate of 80 counts per minute. What count rate is expected after 18 hours?
Medium
- A40 cpm
- B20 cpm
- C10 cpm
- D5 cpm
20.A sample of strontium-90 has a mass of 1600 g. Its half-life is 29 years. What mass of strontium-90 remains after 58 years?
Medium
- A800 g
- B400 g
- C200 g
- D100 g
21.A student measures the count rate from a source at increasing distances. The results are: 10 cm → 400 cpm, 20 cm → 200 cpm, 50 cm → 150 cpm, 100 cm → 15 cpm. What is the background radiation count?
Medium
- A15 cpm
- B150 cpm
- C200 cpm
- D400 cpm
22.A Geiger-Müller tube records the count rate from a radioactive source over several one-minute intervals. The readings are 32, 11, 25, 16, 28. Which statements explain these results? (select all that apply)
Medium
- ARadioactive decay is a random process
- BThe count rate fluctuates because each count represents a random decay
- CThe source is losing activity in a predictable pattern
- DThe results show that the half-life is one minute
- EBackground radiation causes the count rate to increase steadily
23.The random nature of radioactive decay means it is impossible to predict exactly when a particular nucleus will decay.
Easy
True or false?
24.Match each type of radiation with its correct range in air.
Medium
- α
- β
- Gamma
- A few centimetres
- A few tens of centimetres
- Infinite
25.Place these types of radiation in order of increasing ionising power (least ionising first).
Medium
- Gamma
- β
- α
26.A student places a radioactive source near a Geiger-Müller tube and records a high count rate. They then move the source further away. What happens to the count rate?
Medium
- AIt increases
- BIt decreases
- CIt stays the same
- DIt drops to zero immediately
27.A nucleus with 11 protons and 13 neutrons undergoes β-minus decay. Which isotope of magnesium (Mg) is formed?
Medium
- A²⁴₁₂Mg
- B²⁴₁₁Mg
- C²⁵₁₂Mg
- D²³₁₂Mg
28.A student uses a Geiger-Müller tube to measure the count rate from an unknown source. With a sheet of paper between the source and the tube, the count rate is unchanged. With a 3 mm thick aluminium sheet, the count rate drops almost to the background level. With a 5 mm thick lead sheet, the count rate is the same as with the aluminium. Which type of radiation is the source emitting?
Medium
- Aα particles
- Bβ particles
- CGamma rays
- DNeutrons
29.A teacher sets up a Geiger-Müller tube connected to a counter and places a radioactive source 10 cm away. She records the count rate for one minute. She then repeats the measurement with the source at 20 cm, 50 cm and 100 cm. Which of the following is the most important control variable in this experiment?
Medium
- AThe distance from the source to the tube
- BThe time for which the count rate is recorded
- CThe type of radioactive source used
- DThe temperature of the room
30.Background radiation must be subtracted from the count rate measured with a source present to find the corrected count rate.
Easy
True or false?
31.A student measures the count rate from a radioactive source at different distances. The results are: 5 cm → 800 cpm, 10 cm → 400 cpm, 20 cm → 200 cpm, 50 cm → 100 cpm. The background count rate is 20 cpm. Which of the following statements are correct? (select all that apply)
Medium
- AThe corrected count rate at 5 cm is 780 cpm
- BThe corrected count rate at 10 cm is 380 cpm
- CThe corrected count rate at 20 cm is 180 cpm
- DThe corrected count rate at 50 cm is 100 cpm
- EThe background count rate must be added to the measured count rate to find the corrected count rate
32.A student measures the background count rate in the laboratory and finds it to be 20 counts per minute. She then measures the count rate from a radioactive source as 320 counts per minute. What is the corrected count rate from the source?
Medium
- A300 counts per minute
- B320 counts per minute
- C340 counts per minute
- D20 counts per minute
33.A radioactive isotope has a half-life of 8 days. A sample initially has an activity of 640 Bq. What is the activity after 24 days?
Medium
- A80 Bq
- B160 Bq
- C320 Bq
- D40 Bq
34.Match each type of radiation with the correct property.
Medium
- α
- β-minus
- Gamma
- Stopped by a few millimetres of aluminium
- Stopped by paper
- Reduced by several centimetres of lead
35.Which absorber would stop α radiation but allow β and gamma to pass through?
Medium
- AA sheet of paper
- BA few millimetres of aluminium
- CSeveral centimetres of lead
- DA metre of concrete
36.The student finds that the count rate is reduced but not zero when a thick lead sheet is placed between the source and the tube. Which type of radiation is still being detected?
Medium
- Aα particles
- Bβ particles
- CGamma rays
- DNeutrons
37.β particles are stopped by a few millimetres of aluminium.
Easy
True or false?
38.Which particle is emitted when a nucleus undergoes β-minus decay?
Easy
- AAn electron
- BA neutron
- CA positron
- DA proton
39.An α particle is the same as the nucleus of which element?
Medium
- AHelium
- BHydrogen
- CLithium
- DCarbon
40.Which of the following correctly describes a β particle?
Medium
- AA fast-moving electron emitted from the nucleus
- BA fast-moving positron emitted from the nucleus
- CA helium nucleus
- DAn electromagnetic wave
41.Complete the table for the particles that make up an α particle.
Medium
An α particle is made up of ____ protons and ____ neutrons.
42.The nucleus of an atom of actinium-225 can be represented by the symbol 225/89 Ac. When actinium-225 emits an α particle, what is the new nucleon number?
Medium
43.When actinium-225 (225/89 Ac) emits an α particle, what is the new proton number?
Medium
44.State one source of background radiation.
Easy
45.Complete the sentences about radioactive decay using the words from the box: reducing, protons, radiation, molecules, random, neutrons, planned, visible light.
Medium
Some isotopes are unstable because of their large size or because they have too many or too few ____. Unstable nuclei can emit ____ to become more stable. As this moves away from the nucleus, it takes some energy with it, ____ the overall energy of the nucleus and making the nucleus more stable.
46.A radioactive source has an activity of 2000 Bq. How many unstable nuclei decay in 2 minutes?
Medium
47.Complete the sentence about the random nature of radioactive decay.
Easy
Radioactive decay is a ____ process, so it is not possible to know exactly when a particular nucleus will decay.
48.A Geiger-Müller tube counts 16 000 decays in 1 hour. What is the count rate in decays per second? Give your answer to 2 significant figures.
Medium
49.A radioactive source has an activity of 500 Bq. How many unstable nuclei decay in 4 minutes?
Medium
50.Complete the sentence about activity and count rate.
Medium
Activity is the rate at which ____ nuclei decay, whereas count rate is the rate at which radioactive emissions are ____.
51.A radioactive source has an activity of 800 Bq. How many unstable nuclei decay in 5 minutes?
Medium
52.A Geiger-Müller tube records 27 000 counts in 45 minutes. Calculate the count rate in counts per second.
Medium
53.State the definition of half-life.
Medium
54.Explain why fluorine-18 must be produced close to the hospital where it is used.
Hard
55.Some tumours inside the body can be treated by using either α radiation or gamma radiation. Explain why the source of α radiation is usually inside the body but the source of gamma radiation can be outside the body.
Hard
56.Describe how a teacher should use a Geiger-Müller (GM) tube to compare the count-rates from two different radioactive rocks.
Hard
57.A patient is injected with a radioactive tracer. Explain why the isotope used must have a half-life that is long enough to be detected but short enough to limit the patient's exposure.
Hard
58.The nucleus of an atom of actinium-225 can be represented by the symbol 225/89 Ac. When actinium-225 emits an α particle, what is the new nucleon number?
Medium
- A265
- B243
- C199
- D221
59.When actinium-225 (225/89 Ac) emits an α particle, what is the new proton number?
Medium
- A78
- B96
- C87
- D105
60.A radioactive source has an activity of 2000 Bq. How many unstable nuclei decay in 2 minutes?
Medium
- A216000 nuclei
- B264000 nuclei
- C288000 nuclei
- D240000 nuclei
61.A Geiger-Müller tube counts 16 000 decays in 1 hour. What is the count rate in decays per second? Give your answer to 2 significant figures.
Medium
- A4.4 decays per second
- B5.4 decays per second
- C6.4 decays per second
- D3.4 decays per second
62.A radioactive source has an activity of 500 Bq. How many unstable nuclei decay in 4 minutes?
Medium
- A120000 nuclei
- B132000 nuclei
- C144000 nuclei
- D108000 nuclei
63.A radioactive source has an activity of 800 Bq. How many unstable nuclei decay in 5 minutes?
Medium
- A264000 nuclei
- B288000 nuclei
- C240000 nuclei
- D216000 nuclei
64.A Geiger-Müller tube records 27 000 counts in 45 minutes. Calculate the count rate in counts per second.
Medium
- A12 counts/s
- B11 counts/s
- C9 counts/s
- D10 counts/s

Tarjetas

What is an α particle?

An α particle is a helium nucleus consisting of 2 protons and 2 neutrons. It has a charge of +2.

What is a β-minus particle?

A β-minus particle is a fast-moving electron emitted from the nucleus. It has a charge of -1.

What is a β-plus particle?

A β-plus particle is a fast-moving positron (positive electron) emitted from the nucleus. It has a charge of +1.

What is a gamma ray?

A gamma ray is an electromagnetic wave with no charge and high energy. It is emitted from a nucleus to lose excess energy.

Which type of radiation is the most ionising?

α radiation is the most ionising because α particles have a charge of +2 and are the heaviest.

Which type of radiation is the least ionising?

Gamma radiation is the least ionising form of nuclear radiation.

What materials will stop α, β and gamma radiation?

α is stopped by paper, β by a few mm of aluminium, and gamma is reduced by several cm of lead or several metres of concrete.

What is the range in air of α, β and gamma radiation?

α travels a few cm, β a few tens of cm, and gamma has an infinite range (it becomes less intense with distance).

What is background radiation?

Background radiation is the radiation that exists around us all the time from natural and man-made sources.

Give two natural sources of background radiation.

Radon gas from rocks and cosmic rays from space are natural sources.

Give two man-made sources of background radiation.

Medical sources (e.g., X-rays) and nuclear fallout from weapons testing are man-made sources.

What is the corrected count rate?

The corrected count rate is the count rate from a source after subtracting the background radiation count.

What is a Geiger-Müller tube used for?

A Geiger-Müller tube is used to detect and measure radiation by producing electrical pulses that are counted.

How does a photographic film detect radiation?

Photographic film becomes darker when it absorbs radiation, with more radiation causing greater darkening.

What is the activity of a radioactive source?

Activity is the rate at which unstable nuclei decay, measured in Becquerels (Bq). 1 Bq = 1 decay per second.

What is count rate?

Count rate is the number of decays detected per second by a detector, often measured in counts per second (cps).

What is half-life?

Half-life is the time taken for half the undecayed nuclei in a sample to decay, or for the activity to halve.

How does the activity of a radioactive sample change over time?

The activity decreases over time, halving with each half-life.

What happens to the nucleus during α decay?

The nucleus loses 2 protons and 2 neutrons, so the atomic number decreases by 2 and the mass number decreases by 4.

What happens during β-minus decay?

A neutron turns into a proton and an electron; the electron is emitted, so the atomic number increases by 1 and the mass number stays the same.

What happens during β-plus decay?

A proton turns into a neutron and a positron; the positron is emitted, so the atomic number decreases by 1 and the mass number stays the same.

What happens during gamma decay?

The nucleus emits a gamma ray to lose excess energy; there is no change in atomic number or mass number.

Why is radioactive decay described as random?

It is impossible to predict exactly when a particular nucleus will decay; each nucleus has an equal probability of decaying in a given time.

What is the effect of radiation on atoms?

Radiation can ionise atoms by knocking out electrons, leaving them with a positive charge.

What is the difference between contamination and irradiation?

Contamination is when radioactive material gets onto or into an object; irradiation is when an object is exposed to radiation but does not become radioactive.

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