Radioactive Decay (Edexcel GCSE Physics): Flashcards
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Define radioactive decay.
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Define radioactive decay.
The process by which an unstable nucleus emits radiation to become more stable.
Why can an atomic nucleus be unstable?
Because of an imbalance of forces within the nucleus, for example due to it being too large or having too many or too few neutrons.
Is radioactive decay a predictable or a random process?
It is a random process. It is not possible to know exactly when a particular nucleus will decay.
State the five types of radiation that can be emitted from an unstable nucleus.
Alpha (α), beta-minus (β⁻), beta-plus (β⁺), gamma (γ) and neutrons.
An alpha particle is the same as a ______ nucleus.
An alpha particle is the same as a helium nucleus, consisting of two protons and two neutrons.
What are beta-minus and beta-plus particles, and what charge does each carry?
Beta-minus: a fast-moving electron, charge −1. Beta-plus: a fast-moving positron, charge +1.
True or False?
Gamma rays and neutrons both carry no electric charge.
True.
Gamma rays are electromagnetic waves with no charge, and neutrons are neutral particles with no charge.
Define ionisation.
The process by which an atom loses an electron due to radiation, giving the atom a net positive charge.
Which type of radiation is the most ionising, and why?
Alpha radiation, because it has a charge of +2 and is the heaviest type of radiation.
What material is used to stop alpha radiation?
Paper. Alpha is the least penetrating type of radiation.
What thickness of aluminium is needed to stop beta radiation?
A few millimetres of aluminium. Beta can pass through aluminium thinner than this.
How can gamma radiation be reduced?
By several metres of concrete or several centimetres of lead. Gamma passes straight through paper and aluminium.
Out of alpha, beta, and gamma radiation:
_____ has the lowest penetrating power and the highest ionising power
_____ has the highest penetrating power and the lowest ionising power
Out of alpha, beta, and gamma radiation:
Alpha has the lowest penetrating power and the highest ionising power
Gamma has the highest penetrating power and the lowest ionising power
True or False?
Alpha particles have a range of several metres in air.
False.
Alpha particles only travel a few centimetres in air before being absorbed.
Which type of radiation has the shortest range in air, and why?
Alpha, because it is the most strongly ionising and so reacts with air molecules very quickly.
Which type of radiation has an almost infinite range in air?
Gamma. It is not absorbed by air, although it becomes less intense with distance.
Define background radiation.
The radiation that exists around us all the time.
List four natural sources of background radiation.
Radon gas from rocks and soil, cosmic rays from space, carbon-14 in biological material and radioactive material in food and drink.
List four man-made sources of background radiation.
Medical sources (for example X-rays), nuclear waste, nuclear fallout from weapons and nuclear accidents.
Why is radon gas particularly dangerous?
It is an alpha emitter, and is dangerous if inhaled into the lungs in large quantities.
Where do cosmic rays come from, and what do they produce when they collide with molecules in the air?
They come from the sun and other high energy events such as supernovae. Collisions with air molecules produce gamma radiation.
Define corrected count rate.
The count rate due to a radioactive source alone, found by subtracting the background radiation reading (with no source present) from the reading taken with the source present.
To find the ______ count rate, the background reading (with no source) is subtracted from the reading taken with the source present.
To find the corrected count rate, the background reading (with no source) is subtracted from the reading taken with the source present.
True or False?
Potassium-40 in bananas is a man-made source of background radiation.
False.
Potassium-40 in bananas is a natural source of background radiation, found in food and drink.
Define dose (of radiation).
The amount of radiation received by a person, measured in sieverts (Sv).
Name two devices used to detect and measure radiation.
Photographic film and a Geiger-Müller tube.
How does photographic film detect radiation?
It becomes darker when it absorbs radiation. The more radiation absorbed, the darker the film when developed.
On a radiation film badge, which type of radiation is absorbed by the paper section, and which is absorbed by the aluminium section?
Alpha radiation is absorbed by the paper. Beta radiation is absorbed by the aluminium.
How does a Geiger-Müller tube detect radiation?
Each time it absorbs radiation, it sends an electrical pulse to a counting machine, producing a clicking sound or a count rate.
The dose of radiation received by a person is measured in ______.
The dose of radiation received by a person is measured in sieverts (Sv).
True or False?
The further a Geiger-Müller tube is from a radioactive source, the higher the count rate detected.
False.
The further away from the source, the lower the count rate detected.
What is beta-minus decay?
In beta-minus decay, a neutron in the nucleus changes into a proton and an electron (a beta-minus particle) is emitted.
What is beta-plus decay?
In beta-plus decay, a proton in the nucleus changes into a neutron and a positron (beta-plus particle) is emitted.
What happens to the atomic number and mass number of a nucleus during beta-minus decay?
The atomic number increases by 1. The mass number stays the same.
What happens to the atomic number and mass number of a nucleus during beta-plus decay?
The atomic number decreases by 1. The mass number stays the same.
Nuclei with too many neutrons undergo beta-_____ decay, while nuclei with too many protons undergo beta-_____ decay.
Nuclei with too many neutrons undergo beta-minus decay, while nuclei with too many protons undergo beta-plus decay.
Carbon-14 (
) decays by beta-_____ emission to form nitrogen-14 (
).Carbon-14 (
) decays by beta-minus emission to form nitrogen-14 ().True or False?
An electron and a positron both have a mass number of 0.
True.
Electrons and positrons have negligible mass compared to protons and neutrons, so both have a mass number of 0.
What is the atomic number of an electron and of a positron?
Electron: −1. Positron: +1.
Define alpha emission.
The emission of an alpha particle, consisting of two protons and two neutrons, from a large unstable nucleus.
What happens to the atomic number and mass number of a nucleus during alpha emission?
The atomic number decreases by 2. The mass number decreases by 4.
Why does a nucleus emit gamma radiation?
Because it has excess energy, usually following a previous decay, and needs to lose it.
What happens to the atomic number and mass number of a nucleus during gamma emission?
Neither changes, because a gamma ray has zero atomic number and zero mass number.
What happens to the atomic number and mass number of a nucleus during neutron emission?
The atomic number stays the same. The mass number decreases by 1.
A gamma ray has high energy but causes ______ change to the mass or charge of the nucleus.
A gamma ray has high energy but causes no change to the mass or charge of the nucleus.
True or False?
Neutron emission decreases the mass number of a nucleus by 1 but leaves the atomic number unchanged.
True.
No protons are lost during neutron emission, only a neutron, so the atomic number is unaffected while the mass number decreases by 1.
How does the effect on atomic number and mass number differ between alpha emission and gamma emission?
Alpha emission changes both the atomic number (−2) and the mass number (−4). Gamma emission changes neither.
Define nucleon number (A).
The nucleon number (A) is the total number of protons and neutrons in the nucleus.
Define proton number (Z).
The proton number (Z) is the total number of protons in the nucleus.
What happens to the nucleon number and proton number of the daughter nucleus during alpha decay?
The nucleon number decreases by 4 and the proton number decreases by 2 compared with the parent nucleus.
What happens to the nucleon number and proton number of the daughter nucleus during beta-minus decay?
The nucleon number stays the same as the parent, and the proton number increases by 1.
What happens to the nucleon number and proton number of the daughter nucleus during beta-plus decay?
The nucleon number stays the same as the parent, and the proton number decreases by 1.
During gamma decay, the nucleon number and proton number of the daughter nucleus are both the ______ as the parent.
During gamma decay, the nucleon number and proton number of the daughter nucleus are both the same as the parent.
True or False?
The parent nucleus is the nucleus that remains after a radioactive decay has happened.
False.
The parent nucleus is the nucleus that decays; the daughter nucleus is the nucleus that remains after the decay.
In a nuclear equation, what two quantities must balance on both sides of the equation?
The sum of the nucleon (mass) numbers and the sum of the proton (atomic) numbers must each balance on both sides.
What is meant by the random nature of radioactive decay?
The random nature of radioactive decay means it cannot be predicted when a particular nucleus will decay.
What evidence from a Geiger-Muller (GM) tube supports the random nature of radioactive decay?
The count rate is found to fluctuate irregularly and cannot be predicted, since each count represents the decay of an individual, randomly-occurring nucleus.
Why is rolling a dice a good analogy for radioactive decay?
Both are random processes - you cannot know when a specific outcome (a decay, or a six) will occur, but you can determine its probability.
True or False?
The rate of radioactive decay is affected by surrounding conditions, such as temperature.
False.
The rate of decay is unaffected by surrounding conditions; only chance determines when a nucleus decays.
During one half-life, a particular nucleus has a ______ % chance of decaying.
During one half-life, a particular nucleus has a 50 % chance of decaying.
Will the number of undecayed nuclei in a sample ever fall to exactly zero? Explain your answer.
No. Even after a large number of half-lives, there is still a small probability that a particular nucleus has not decayed, so the number remaining never quite falls to zero.
Define a source of radiation.
A source of radiation is an object containing radioactive nuclei.
Define activity.
Activity is the rate at which the unstable nuclei from a source of radiation decay.
Define the Becquerel (Bq).
1 Becquerel is equal to 1 nucleus in a source decaying in 1 second.
What happens to the activity of a radioactive isotope over time, and why?
The activity decreases over time, because the number of undecayed nuclei of that isotope decreases as it decays.
A source of radiation with an activity of 2000 Bq means ______ nuclei decay every second.
A source of radiation with an activity of 2000 Bq means 2000 nuclei decay every second.
A source has an activity of 2000 Bq. How many unstable nuclei decay in 2 minutes?
240 000 nuclei (activity × time period = 2000 Bq × 120 s = 240 000).
True or False?
Activity and count rate are the same thing.
False.
Activity is the rate at which unstable nuclei decay, whereas count rate is the rate at which radioactive emissions are detected, measured in counts per second.
Define half-life.
Half-life is the time taken for half of the undecayed nuclei in a sample to decay, or for the activity of a source to fall to half its original level.
Carbon-14 has a half-life of 5700 years. What percentage of a carbon-14 sample remains after two half-lives (11 400 years)?
25 % of the original carbon-14 remains.
True or False?
The half-life of an isotope changes as a sample gets older.
False.
The half-life is constant for a particular isotope, no matter how much of the sample remains.
The time taken for the activity of a sample to fall from 100 % to 50 % is the ______, the same length of time as falling from 50 % to 25 %.
The time taken for the activity of a sample to fall from 100 % to 50 % is the half-life, the same length of time as falling from 50 % to 25 %.
A radioactive sample's activity falls from 8 × 10⁷ Bq to 2 × 10⁷ Bq in 12 hours. What is the half-life?
6 hours (the activity halves twice in 12 hours: 8 × 10⁷ → 4 × 10⁷ → 2 × 10⁷ Bq).
Outline the procedure for determining the half-life of a sample from its activity.
Measure the initial activity, A₀; measure how the activity changes with time; find the time taken for the activity to fall to half its original value - this is the half-life.
A sample contains 2 million undecayed atoms. After a year, 500 000 atoms remain undecayed. What is the half-life?
6 months (the sample has undergone two half-lives in one year: 2 million → 1 million → 500 000 atoms).
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