Radioactive Decay Flashcards

All 39 cards in this deck

Name the five types of radiation that can be emitted from unstable nuclei.

Alpha ($\alpha$), beta-minus ($\beta^-$), beta-plus ($\beta^+$ / positron), gamma ($\gamma$) and neutron radiation.

True or false? Alpha, beta and gamma emissions are all ionising and are all emitted by unstable nuclei.

True. Stable nuclei do not give radioactive emissions, and all of these emissions are ionising.

What is an alpha particle?

A helium nucleus — 2 protons and 2 neutrons.

What is a gamma ray?

Electromagnetic radiation emitted from the nucleus.

What material will stop alpha radiation?

A sheet of paper; it is also absorbed by a few cm of air.

What material will stop beta radiation?

A few mm of aluminium.

What material is needed to absorb most gamma radiation?

Thick lead or several metres of concrete.

True or false? Gamma is the most ionising of alpha, beta and gamma radiation.

False. Alpha is the most ionising and gamma the least; gamma is the most penetrating.

What is background radiation?

The low-level ionising radiation that is around us all the time, from natural and man-made sources in the environment.

Give two origins of background radiation from the Earth.

Radon gas from rocks (e.g. granite) and radioactivity in food and drink; also building materials.

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

Cosmic rays from the Sun and other stars.

True or false? With no radioactive source nearby, a Geiger counter reads zero.

False. It still records a count because of background radiation.

How does a Geiger–Müller tube measure radioactivity?

It detects each particle or ray that enters it, and the counter displays the count, giving the count rate.

How does photographic film detect radiation?

The film darkens (fogs) when exposed to ionising radiation — the more radiation, the darker it goes.

How do you correct a measured count rate for background radiation?

Measure the background count rate with the source removed, then subtract it from the count rate measured with the source in place.

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

An electron emitted from the nucleus.

Describe what happens inside the nucleus in $\beta^-$ decay.

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

Describe what happens inside the nucleus in $\beta^+$ decay.

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

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

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

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

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

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

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

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

Neither changes. The nucleus just rearranges and loses energy as gamma radiation.

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

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

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

The total mass number (top numbers) and the total charge / atomic number (bottom numbers).

How is an alpha particle written in a nuclear equation?

$^{4}_{2}\text{He}$ (mass number 4, charge $+2$).

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

$^{0}_{-1}\text{e}$ (mass number 0, charge $-1$).

How is a positron written in a nuclear equation?

$^{0}_{+1}\text{e}$ (mass number 0, charge $+1$).

What is the activity of a radioactive source?

The number of nuclei that decay each second.

What is the unit of activity of a radioactive isotope?

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

What happens to the activity of a radioactive source as time passes?

It decreases, because fewer undecayed nuclei are left to decay.

What is the half-life of a radioactive isotope?

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

How do you find the half-life from a graph of activity (or count rate) against time?

Read the starting activity, halve it, and read off the time taken for the activity to fall to that value. That time is the half-life.

True or false? After two half-lives, all of the radioactive nuclei in a sample have decayed.

False. A quarter of the original undecayed nuclei are still left.

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

It cannot be predicted which nucleus will decay next, or when a particular nucleus will decay.

A student measures the activity of the same source four times and gets 21, 23, 19 and 22 Bq. What does this show?

That decay is random — the activity varies with no pattern.

True or false? Because decay is random, nothing about a radioactive sample can be predicted.

False. For a very large number of nuclei, half-life lets the activity be predicted.

What is ionising radiation?

Radiation that can remove electrons from atoms, leaving them as charged ions.

True or false? The half-life of a radioactive isotope depends on how much of it you start with.

False. The half-life of an isotope is constant, whatever the starting amount or activity.

What is radioactive decay?

An unstable nucleus randomly giving out radiation (e.g. alpha, beta or gamma) and so becoming more stable.