Radioactive Decay Flashcards

All 54 cards in this deck

What is an alpha particle?

A helium nucleus – 2 protons and 2 neutrons.

What is a beta-minus ($\beta^-$) particle?

An electron emitted from the nucleus.

What is a positron ($\beta^+$)?

A positively charged particle with the same mass as an electron, emitted from the nucleus.

What is a gamma ray?

Electromagnetic radiation emitted from the nucleus; it has no mass and no charge.

What is neutron radiation?

A neutron emitted from an unstable nucleus.

True or false? Alpha, beta-minus, positron and gamma radiation are all ionising radiations.

True. Each can remove electrons from atoms it passes, forming ions.

Which of alpha, beta and gamma radiation is the most ionising?

Alpha.

Which of alpha, beta and gamma radiation is the most penetrating?

Gamma.

What absorber stops alpha radiation?

A sheet of paper (or a few centimetres of air).

What absorber stops beta radiation?

A few millimetres of aluminium.

What absorber is needed to reduce gamma radiation?

Thick lead or several metres of concrete – it is only reduced, never completely stopped.

True or false? Gamma radiation is the most ionising of the three radiations.

False. Gamma is the least ionising but the most penetrating; alpha is the most ionising.

What is background radiation?

The low-level ionising radiation that is around us all the time, from natural and artificial sources.

Name three natural sources of background radiation from the Earth.

Radon gas from rocks, rocks/soil and building materials, and food and drink (living things).

What is the source of background radiation that comes from space?

Cosmic rays, mainly from the Sun and other stars.

Name two artificial (man-made) sources of background radiation.

Medical uses such as X-rays and nuclear medicine, and fallout from nuclear weapons testing or nuclear power.

True or false? Most background radiation comes from man-made sources.

False. Most background radiation comes from natural sources such as radon gas, rocks and cosmic rays.

What are the two methods for detecting and measuring radioactivity you must know?

Photographic film and a Geiger–Müller (GM) tube.

How does photographic film detect radiation?

The film becomes darker (fogged) the more radiation it absorbs.

What does a Geiger–Müller tube connected to a counter measure?

The count rate – the number of counts (clicks) detected per second.

What are the steps to measure the count rate of a source using a GM tube?

e.g. background is 20 counts/min, tube with source reads 320 counts/min

1. Measure the count rate with no source present: 20 counts/min.
2. Measure the count rate with the source in place: 320 counts/min.
3. Subtract the background: $320 - 20 = 300$ counts/min from the source.

True or false? Photographic film shows the total radiation received over a period of time rather than an instant reading.

True. A GM tube gives an immediate count rate; film records a cumulative dose.

Describe the process of beta-minus ($\beta^-$) decay inside the nucleus.

A neutron becomes a proton plus an electron, and the electron is emitted.

Describe the process of beta-plus ($\beta^+$) decay inside the nucleus.

A proton becomes a neutron plus a positron, and the positron is emitted.

True or false? The electron emitted in beta-minus decay comes from the atom's electron shells.

False. It is created in the nucleus when a neutron changes into a proton.

Which type of beta decay turns a proton into a neutron?

Beta-plus ($\beta^+$) decay, which emits a positron.

True or false? In beta decay the number of particles in the nucleus stays the same.

True. One nucleon changes type, so the mass number is unchanged.

What happens to the mass number and atomic number when a nucleus emits an alpha particle?

Mass number decreases by 4; atomic number decreases by 2.

What happens to the mass number and atomic number when a nucleus emits a beta-minus particle?

Mass number stays the same; atomic number increases by 1.

What happens to the mass number and atomic number when a nucleus emits a positron ($\beta^+$)?

Mass number stays the same; atomic number decreases by 1.

What happens to the mass number and atomic number when a nucleus emits a gamma ray?

Neither changes; the nucleus only loses energy.

What happens to the mass number and atomic number when a nucleus emits a neutron?

Mass number decreases by 1; atomic number stays the same.

Why is gamma radiation often emitted straight after a nucleus has undergone alpha or beta decay?

The nucleus rearranges itself and loses its excess energy as gamma radiation.

What two quantities must balance on both sides of a nuclear equation?

Mass number (nucleon number) and charge (atomic number).

What are the steps to balance a nuclear equation?

e.g. $^{99}_{42}\mathrm{Mo} \rightarrow \;\mathrm{Tc} + ^{\;0}_{-1}\mathrm{e}$

1. Make the mass numbers total the same on each side: $99 = A + 0$, so $A = 99$.
2. Make the atomic numbers total the same: $42 = Z + (-1)$, so $Z = 43$.
3. Write the nuclide: $^{99}_{43}\mathrm{Tc}$.

How is an alpha particle written in a nuclear equation?

$^{4}_{2}\mathrm{He}$ (or $^{4}_{2}\alpha$).

How is a beta-minus particle written in a nuclear equation?

$^{\;0}_{-1}\mathrm{e}$.

How is a positron written in a nuclear equation?

$^{0}_{+1}\mathrm{e}$.

How are a gamma ray and a neutron written in a nuclear equation?

Gamma ray: $^{0}_{0}\gamma$; neutron: $^{1}_{0}\mathrm{n}$.

What does it mean to say that radioactive decay is a random process?

You cannot predict which nucleus will decay next or when any particular nucleus will decay; each has the same chance.

True or false? Knowing an isotope's half-life lets you predict when a particular nucleus will decay.

False. Half-life only predicts the average behaviour of a very large number of nuclei.

Why can half-life be used to make useful predictions even though decay is random?

Because samples contain a very large number of nuclei, so their average (overall) activity is predictable.

True or false? The count rate from a GM tube fluctuates slightly even when the source and detector are unchanged.

True. Decay is random, so the number of decays each second varies.

What is the activity of a radioactive source?

The number of nuclear decays per second.

What is the unit of activity of a radioactive isotope?

The becquerel (Bq); 1 Bq = 1 decay per second.

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

It decreases, because fewer undecayed nuclei are left; it halves every half-life.

What shape is a graph of activity against time for a radioactive source?

A decay curve – steep at first, then gradually levelling off, never quite reaching zero.

True or false? The activity of a source falls by the same number of becquerels each second.

False. It falls by half each half-life, so the drop per second gets smaller as activity decreases.

What is the half-life of a radioactive isotope?

The time taken for half the undecayed nuclei to decay, or for the activity of a source to fall by half.

What fraction of the original nuclei remains undecayed after $n$ half-lives?

$\left(\frac{1}{2}\right)^n$

e.g. after 4 half-lives, $\frac{1}{16}$ remains.

What are the steps to find the half-life from a graph of activity against time?

e.g. a source starts at 80 Bq

1. Halve the starting activity: $80 \div 2 = 40$ Bq.
2. Read across from 40 Bq to the curve, then down to the time axis.
3. That time is the half-life.

What are the steps to find the activity remaining after a given number of half-lives?

e.g. 800 Bq after 3 half-lives

1. Start with the initial activity: 800 Bq.
2. Halve it once per half-life: 400 Bq, 200 Bq, 100 Bq.
3. Activity after 3 half-lives = 100 Bq.

What are the steps to give the net decline of a source as a ratio?

e.g. after 3 half-lives

1. Find the fraction remaining: $\left(\frac{1}{2}\right)^3 = \frac{1}{8}$.
2. Write it as remaining : original.
3. Net decline $= 1 : 8$.

True or false? The half-life of a particular isotope is always the same, whatever the size of the sample.

True. Half-life is a fixed property of the isotope.